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Influence of ultraviolet-B radiation on crop-weed competitive interactions Furness, Nancy Heather 2003

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I N F L U E N C E O F U L T R A V I O L E T - B RADIATION O N C R O P - W E E D C O M P E T I T I V E INTERACTIONS by N A N C Y HEATHER FURNESS B. Sc. (Hons), University of British Columbia (1992) Vancouver, Canada A THESIS SUBMITTED IN PARTIAL FULFILLMENT OF THE REQUIREMENTS FOR THE DEGREE OF DOCTOR OF PHILOSOPHY in THE F A C U L T Y OF G R A D U A T E STUDIES (Faculty of Agricultural Sciences) We accept this thesis as conforming to the required standard THE UNIVERSITY OF BRUTISH C O L U M B I A April, 2003 © N A N C Y HEATHER FURNESS, 2003 U B C Rare Books and Special, Collections - Thesis Authorisation Form Page 1 of 1 In p r e s e n t i n g t h i s t h e s i s i n p a r t i a l f u l f i l m e n t of the requirements f o r an advanced degree at the U n i v e r s i t y of B r i t i s h Columbia, I agree that the L i b r a r y s h a l l - make i t f r e e l y a v a i l a b l e f o r reference and study. I f u r t h e r agree that permission f o r extensive copying of t h i s t h e s i s f o r s c h o l a r l y purposes may be granted by the head of my department or by h i s or her r e p r e s e n t a t i v e s . I t i s understood that copying or p u b l i c a t i o n of t h i s t h e s i s f o r f i n a n c i a l gain s h a l l not be allowed without my w r i t t e n permission. Department of l^larvV *5c\€X~iG£, The U n i v e r s i t y of B r i t i s h Columbia Vancouver, Canada http://www.library.ubc.ca/spcoll/thesauth.htrril 4/15/2003 Abstract ii A greenhouse study showed that ultraviolet-B (UV-B; 290-315 nm) radiation effects on seedling growth and morphology differed widely among agricultural weeds, vegetable crops, and broccoli cultivars. Relative UV-B-sensitivity of plants within these groups also differed. These differential responses to U V - B radiation may alter competition among associated species for limited resources. Broccoli (Brassica oleracea L. var. italica cv. Purple Sprouting) and lamb's-quarters (Chenopodium album L.) were chosen for competition experiments based on their growth and morphological sensitivity to U V - B radiation. Greenhouse competition experiments were conducted in 1999 (summer) and 2000 (fall) at ambient (4 kJ m"2 d"1) and above-ambient (7 kJ m"2 d"1) biologically effective U V - B ( U V - B B E ) radiation levels. Broccoli and lamb's-quarters were grown in monocultures (144, 256, and 400 plants m"2) and all binary mixtures in a randomized block design with four replications. Yield per plant declined with increasing species densities; U V - B -effects and treatment interactions were often significant. Inverse yield-density relationships using biomass indicated that in both years broccoli gained in competitiveness relative to lamb's-quarters at above-ambient U V - B radiation. U V - B effects were greater on inter- compared with intraspecific competition. Growth indices (specific leaf weight, leaf weight ratio, leaf area ratio, leaf area index, and shootroot ratio) indicated that morphology and biomass partitioning were influenced by experimental treatments. Total biomass growth was proportional to growth of biomass components and leaf area. Plant densities had greater influence compared with UV-B-effects on allometric adjustments. Direct U V - B radiation and species densities effects were more often detected in broccoli compared with lamb's-quarters. Broccoli height and leaf greenness increased at above-ambient U V - B radiation, while those of lamb's-quarters declined. Over time, inverse yield-density relationships indicated that the contribution of broccoli to interception of photosynthetically active radiation (PAR) decreased at ambient and increased at above-ambient U V - B radiation. Inverse yield-density relationships using overhead canopy coverage showed that lamb's-quarters was always the stronger competitor. Broccoli however, gained in competitiveness relative to lamb's-quarters at above-ambient U V - B radiation in both years. Overall, this study indicated that architectural and physiological plasticity of broccoli and lamb's-quarters, in response to U V - B radiation, influenced ability of each species within this association to compete for PAR. iii I Table of Contents Abstract i i Table of Contents i i i List of Tables vii List of Figures xi List of Appendices xv Symbols and Abbreviations xvii Acknowledgements xix General Introduction 1 1.0 Literature Review 7 1.1 The changing U V - B radiation environment 7 1.2 Plant response to UV-B-enriched environments 8 1.2.1 Damage to the photosynthetic apparatus 8 1.2.2 Damage to plant proteins and membranes 9 1.2.3 Damage to D N A 9 1.2.4 Acclimation and protective mechanisms 10 1.2.5 Growth and morphological responses 12 1.2.6 Variation in inter-and intraspecific response to 13 U V - B radiation 1.3 Mechanisms of UV-B-induced shifts in plant competitive interactions 14 1.3.1 U V - B radiation and interspecific competition 14 1.3.2 U V - B radiation and intraspecific competition 17 1.4 Evaluating competitive impacts on plant growth 17 1.4.1 Replacement series analysis 17 1.4.2 Inverse yield-density relationships 18 1.4.3 Conventional plant growth analysis 20 1.4.4 Allometric relationships 21 1.5 Studying plant response to U V - B radiation under greenhouse 22 conditions iv 2.0 Influence of U V - B radiation on growth and morphology of weed, vegetable, 26 and broccoli cultivar seedlings. 2.1 Abstract 26 2.2 Introduction 26 2.3 Materials and Methods 28 2.3.1 Seed source and plant culture 28 2.3.2 U V - B radiation treatments 30 2.3.3 Growth and morphological response to U V - B radiation 32 2.3.4 Relative seedling sensitivity to U V - B radiation 33 2.3.5 Scanning electron microscopy 34 2.3.6 Leaf greenness measurements 34 2.4 Results 35 2.4.1 Response of weed seedlings to U V - B radiation 35 2.4.1.1 Visual effects 35 2.4.1.2 Effects on growth parameters 35 2.4.1.3 Effects on growth indices 3 9 2.4.1.4 Relative sensitivity to U V - B radiation 41 2.4.2 Response of vegetable seedlings to U V - B radiation 41 2.4.2.1 Visual effects 41 2.4.2.2 Effects on growth parameters 41 2.4.2.3 Effects on growth indices 49 2.4.2.4 Relative sensitivity to U V - B radiation 52 2.4.3 Response of broccoli cultivar seedlings to U V - B radiation 52 2.4.3.1 Visual effects 56 2.4.3.2 Effects on growth parameters 56 2.4.3.3 Effects on leaf greenness 59 2.4.3.4 Relative sensitivity to U V - B radiation 61 2.5 Discussion 61 2.5.1 Response of weed species to U V - B radiation 63 2.5.1.1 Effects on plant morphology, biomass, and leaf area 63 2.5.1.2 Effects on growth indices 65 2.5.1.3 Relative sensitivity of weed seedlings to U V - B radiation 66 2.5.2 Response of vegetable seedlings to U V - B radiation 67 2.5.2.1 Effects on plant morphology, biomass, and leaf area 67 2.5.2.2 Effects on growth indices 68 2.5.2.3 Relative sensitivity of vegetable seedlings to 70 U V - B radiation 2.5.3 Selection of weed and vegetable species for use in competition 70 experiments 2.5.4 Selection of broccoli cultivar for use in competition experiments 71 3.0 Competitive interactions in broccoli (Brassica oleracea L. var. italica cv. 73 Purple Sprouting) and lamb's-quarters (Chenopodium album L.) associations grown at two levels of U V - B radiation. 3.1 Abstract 73 3.2 Introduction 73 3.3 Materials and Methods 76 3.3.1 Seed source and plant culture 76 3.3.2 U V - B radiation treatments 78 3.3.3 Harvesting and data collection 79 3.3.4 Statistical analysis 79 3.4 Results 80 3.4.1 Visual observations 80 3.4.2 Analysis of variance 81 3.4.3 Inverse yield-density relationships 83 3.5 Discussion 92 4.0 Growth indices and allometric analyses of broccoli (Brassica oleracea var. 100 italica cv. Purple Sprouting) and lamb's-quarters (Chenopodium album L.) associations grown at two levels of U V - B radiation. 4.1 Abstract 100 4.2 Introduction 101 4.3 Materials and Methods 104 4.3.1 Plant growth indices 10 5 vi 4.3.2 Allometric analysis 105 4.4 Results 107 4.4.1 Growth indices 107 4.4.1.1 Broccoli 107 4.4.1.2 Lamb's-quarters 116 4.4.2 Allometric analysis 128 4.4.2.1 Broccoli 128 4.4.2.2 Lamb's-quarters 135 4.5 Discussion 142 4.5.1 Growth indices 143 4.5.2 Allometric analysis 148 5.0 Influence of UV-B-induced shifts in leaf and canopy optical properties on 155 broccoli (Brassica oleracea var. italica cv. Purple Sprouting) and lamb's-quarters (Chenopodium album L.) competitive interactions. 5.1 Abstract 155 5.2 Introduction 155 5.3 Materials and methods 158 5.3.1 Canopy interception of PAR 158 5.3.2 Height, leaf number, and leaf greenness 159 5.3.3 Overhead canopy coverage 15 9 5.4 Results 160 5.4.1 PAR interception 160 5.4.2 Height and.leaf number 163 5.4.3 Overhead canopy coverage 163 5.4.4 Leaf greenness 178 5.5 Discussion 184 General Discussion 193 Literature Cited 199 vii List of Tables Table 2.1 Common names, species, cultivars, and seed sources for vegetable crops. 29 Table 2.2 Dates, average minimum and maximum PAR levels and 31 range of night and day temperatures during weed, vegetable, and broccoli cultivar experiments. Table 2.3 Effect of U V - B radiation on height (cm) and leaf area (cm2) of weed species. 37 Table 2.4 Effect of U V - B radiation on growth indices of weed species. 40 Table 2.5 Influence of U V - B radiation on height (cm) of vegetable seedlings. 44 Table 2.6 Influence of U V - B radiation on leaf area (cm2) of vegetable seedlings. 46 Table 2.7 Influence of U V - B radiation on specific leaf weight [SLW (g m" )] 50 of vegetable seedlings. Table 2.8 Influence of U V - B radiation on leaf weight ratio (LWR) of vegetable 51 seedlings. Table 2.9 Influence of U V - B radiation on leaf area ratio [LAR (cm2 g"1)] 53 of vegetable seedlings. Table 2.10 Influence of U V - B radiation on shoot: root ratios (SRR) of vegetable 54 seedlings. Table 2.11 Influence of U V - B radiation on leaf greenness (SPAD units) of the second 60 true leaf of broccoli cvs. Arcadia, Emperor, Everest, Legend, Patriot, and Purple Sprouting. Table 3.1A Population densities of broccoli and lamb's-quarters monocultures and 77 binary associations. Table 3.1B Numbers of broccoli and lamb' s-quarters plants from monocultures 77 and binary associations used in statistical analyses. Table 3.2 A N O V A results (variance ratios) for the effect of U V - B radiation, 82 and broccoli (XB), and lamb's-quarters (XL) densities on broccoli and lamb's-quarters weighted mean biomass per plant (W) in 1999. Table 3.3 A N O V A results (variance ratios) for the effect of U V - B radiation, 84 and broccoli ( X B ) , and lamb's-quarters (XL) densities on broccoli and lamb's-quarters weighted mean biomass per plant (W) in 2000. Table 3.4 Regression parameters and statistics for inverse yield-density 86 Vl l l models (Eqns. 3.1 and 3.2) describing the response of reciprocal biomass per plant ( W 1 ) of broccoli and lamb's-quarters to broccoli and lamb's-quarters population densities in 1999. Table 3.5 Regression parameters and statistics for inverse yield-density models (Eqns. 3.1 and 3.2) describing the response of reciprocal biomass per plant (W"1) of broccoli and lamb's-quarters to broccoli and lamb's-quarters population densities in 2000. Table 3.6 Percent change in regression coefficients and substitution rates for inverse yield-density models describing the response of reciprocal biomass per plant (W"1) of (a) broccoli and (b) lamb's-quarters to broccoli and lamb's-quarters population densities established at 7 compared with 4 kJ m"1 d"1 U V - B B E radiation in 1999 and 2000. 87 91 Table 4.1 A N O V A results (variance ratios) for the effect of U V - B radiation, 108 broccoli (XB), and lamb's-quarters (XL) densities on broccoli plant growth indices weighted by the inverse of the variance of the ratios in 1999. Table 4.2 A N O V A results (variance ratios) for the effect of U V - B radiation, 109 broccoli ( X B ) , and lamb's-quarters (XL) densities on broccoli plant growth indices weighted by the inverse of the variance of the ratios in 2000. Table 4.3 A N O V A results (variance ratios) for the effect of U V - B radiation, broccoli (XB), and lamb's-quarters (XL) densities on lamb's-quarters plant growth indices weighted by the inverse of the variance of the ratios in 1999. 119 Table 4.4 A N O V A results (variance ratios) for the effect of U V - B radiation, broccoli ( X B ) , and lamb's-quarters (XL) densities on lamb's-quarters plant growth indices weighted by the inverse of the variance of the ratios in 2000. 120 Table 4.5 Parameters and statistics for simple bivariate allometric models (Eqn 4.1) between ln(W) and other ln-transformed measures (z) of broccoli grown at two levels of U V - B radiation. Table 4.6 Parameters and statistics for best subset multiple regression models (Eqn. 4.2) of the allometric relationship between plant biomass (W) and root biomass (WR) in broccoli. Table 4.7 Parameters and statistics for best subset multiple regression models (Eqn. 4.2) of the allometric relationship between plant biomass (W) and stem biomass (Ws) in broccoli. Table 4.8 Parameters and statistics for best subset multiple regression models (Eqn. 4.2) of the allometric relationship between plant biomass (W) 129 130 131 132 ix and leaf biomass (WL) in broccoli. Table 4.9 Parameters and statistics for best subset multiple regression models (Eqn. 4.2) of the allometric relationship between plant biomass (W) and leaf area (LA) in broccoli. Table 4.10 Parameters and statistics for best subset multiple regression models (Eqn. 4.2) of the allometric relationship between plant biomass (W) and height (H) in broccoli. Table 4.11 Parameters and statistics for simple bivariate allometric models (Eqn. 4.1) between ln(W) and other ln-transformed measures (z) of lamb's-quarters grown at two levels of U V - B radiation. Table 4.12 Parameters and statistics for best subset multiple regression models (Eqn. 4.2) of the allometric relationship between plant biomass (W) and root biomass (W R) in lamb's-quarters. 133 134 136 137 Table 4.13 Parameters and statistics for best subset multiple regression models (Eqn. 4.2) of the allometric relationship between plant biomass (W) and stem biomass (Ws) in lamb's-quarters. Table 4.14 Parameters and statistics for best subset multiple regression models (Eqn. 4.2) of the allometric relationship between plant biomass (W) and leaf biomass (WL) in lamb's-quarters. 138 139 Table 4.15 Parameters and statistics for best subset multiple regression models (Eqn. 4.2) of the allometric relationship between plant biomass (W) and leaf area (LA) in lamb's-quarters. 140 Table 4.16 Parameters and statistics for best subset multiple regression models (Eqn. 4.2) of the allometric relationship between plant biomass (W) and height (H) in lamb's-quarters. Table 4.17 Summary of standard partial regression coefficients for best subset multiple regression models (Eqn. 4.2) of the allometric relationships between total biomass (W) and root (WR), stem (Ws), leaf (WL) biomass, leaf area (LA) and height (H) per plant in broccoli. Table 4.18 Summary of standard partial regression coefficients for best subset multiple regression models (Eqn. 4.2) of the allometric relationships between total biomass (W) and root (WR), stem (Ws), leaf (WL) biomass, leaf area (LA) and height (H) per plant in lamb's-quarters. Table 5.1 Regression parameters and statistics for inverse yield-density models (Eqn 5.1) describing the effect of broccoli (axr*) and lamb's-quarters (axL) population densities on PAR interception at (a) 4, and (b) 7 kJ m"2 d"1 U V - B B E radiation in 1999. 141 150 151 161 Table 5.2 Regression parameters and statistics for inverse yield-density models (Eqn 5.1) describing the effect of broccoli ( a x B ) and lamb's-quarters (axO population densities on P A R interception at (a) 4, and (b) 7 kJ m"2 d"1 U V - B B E radiation in 2000. 162 Table 5.3 Variance ratios for the effect of U V - B radiation, and broccoli ( X B ) and lamb's-quarters (XL) densities on mean shoot height (H) weighted by the inverse of the variance. Table 5.4 Variance ratios for the effect of U V - B radiation, and broccoli (XB) and lamb's-quarters (XL) densities on mean leaf number weighted by the inverse of the variance. 164 169 Table 5.5 Table 5.6 Table 5.7 Regression parameters and statistics for inverse yield-density models (Eqn. 5.2) describing the response of reciprocal per plant overhead canopy coverage (OCC) of broccoli plants grown for 4 9 1 weeks at 4 and 7 kJ m" d" U V - B B E radiation to broccoli and lamb's-quarters densities in 1999. Regression parameters and statistics for inverse yield-density models (Eqn. 5.3) describing the response of reciprocal per plant overhead canopy coverage (OCC) of lamb's-quarters plants grown for 4 weeks at 4 and 7 kJ m" d" U V - B B E radiation to broccoli and lamb's-quarters population densities in 1999. Regression parameters and statistics for inverse yield-density models (Eqn. 5.2) describing the response of reciprocal per plant overhead canopy coverage (OCC) of broccoli plants grown for 4 weeks at 4 and 7 kJ m"2 d"1 U V - B B E radiation to broccoli and lamb's-quarters densities in 2000. 173 174 175 Table 5.8 Regression parameters and statistics for inverse yield-density models (Eqn. 5.3) describing the response of reciprocal per plant overhead canopy coverage (OCC) of lamb's-quarters plants grown for 4 weeks at 4 and 7 kJ m"2 d"1 U V - B B E radiation to broccoli and lamb's-quarters population densities in 2000. 176 Table 5.9 Variance ratios for the effect of U V - B radiation, and broccoli ( X B ) and lamb's-quarters ( X L ) densities on mean SPAD values for broccoli. 179 Table 5.10 Variance ratios for the effect of U V - B radiation and broccoli ( X B ) and lamb's-quarters (XL) densities on mean SPAD values for lamb's-quarters. 180 xi List of Figures Fig. 2.1 Effects of U V - B radiation treatments on redroot pigweed and green foxtail 36 seedlings. (A) a collapsed epidermal cell (arrow) on the adaxial leaf surface of a redroot pigweed leaf developed under 11 kJ m"2 d"1 of UV-BB E radiation (bar = 20 u,m), and (B) green foxtail seedlings developed under 0, 7, and 11 kJ m"2 d"1 of UV-BBE radiation (left to right). Fig. 2.2 Effect of U V - B radiation on leaf, stalk, and root dry weights of common 38 chickweed, green foxtail, lady's-thumb, lamb's-quarters, redroot pigweed, and shepherd's-purse seedlings. Fig. 2.3 Relative sensitivity of common chickweed, green foxtail, lady's-thumb, 42 lamb's-quarters, redroot pigweed, and shepherd's-purse seedlings to U V - B radiation based on cumulative percent changes in stalk biomass and leaf area for the following contrasts: (A) Control versus U V - B radiation, and (B) Low versus high U V - B radiation dose. Fig. 2.4 Upward cupping of leaves in seedlings of broccoli cv. Purple Sprouting 43 grown at 11 kJ m"2 d"1 of UV-BBE radiation for 6 weeks. Fig 2.5 A Effect of U V - B radiation on leaf, stalk, and root dry weights of (A) beet cv. 47 Cylindra, (B) beet cv. Early Wonder, (C) broad bean cv. Aquadulce, (D) broccoli cv. Purple Sprouting, (E) broccoli cv. White Sprouting, (F) bush bean cv. Contender, (G) cabbage cv. Christmas Drumhead, (H) corn cv. Northern Super Sweet, and (I) cucumber cv. Straight Eight seedlings. Fig2.5B Effect of U V - B radiation on leaf, stalk, and root dry weights of (A) kohlrabi 48 cv. Purple Vienna, (B) leaf beet cv. Perpetual Spinach, (C) lettuce cv. Little Gem Cos, (D) lettuce cv. Lobjoits Green Cos, (E) lettuce cv. Saladin, (F) pac choi cv. Pueblo, (G) radish cv. Mooli Mino Early, (H) spinach cv. Long Standing Bloomsdale, and (I) tomato cv. Cuor Di Bue seedlings. Fig. 2.6 Relative sensitivity of vegetable seedlings to U V - B radiation based on 55 cumulative percent changes in stalk biomass and leaf area for the following contrasts: (A) Control versus U V - B radiation, and (B) Low versus high U V - B radiation dose. Fig. 2.7 Effect of U V - B radiation on the height (cm) of seedlings of broccoli cvs. Arcadia, Emperor, Everest, Legend, Patriot, and Purple Sprouting. 57 xii Fig. 2.8 Effect of U V - B radiation on leaf area (cm2) of seedlings of broccoli cvs. 58 Arcadia, Emperor, Everest, Legend, Patriot, and Purple Sprouting. Fig. 2.9 Relative sensitivity of broccoli cvs. Arcadia, Emperor, Everest, Legend, 62 Patriot, and Purple Sprouting seedlings to U V - B radiation based on cumulative percent changes in stalk biomass and leaf area for the following contrasts: (A) and (C) Control versus U V - B radiation, experiment 1 and 2, respectively, ( B ) and ( D ) Low versus high U V - B radiation dose, experiment 1 and 2, respectively. Fig. 3.1 Inverse yield-density relationships (planar surfaces) for mean inverse total 88 biomass per plant (W"1) of broccoli (left panels) and lamb's-quarters (right panels) grown at 4 kJ m"2 d"1 U V - B B E radiation (top panels) and 7 kJ m"2 d"1 U V - B B E radiation (bottom panels) in 1999. Fig. 3.2 Inverse yield-density relationships (planar surfaces) for mean inverse total 89 biomass per plant (W"1) of broccoli (left panels) and lamb's-quarters (right panels) grown at 4 kJ m"2 d"1 U V - B B E radiation (top panels) and 7 kJ m"2 d"1 U V - B B E radiation (bottom panels) in 2000 . Fig. 4.1 Effect of lamb's-quarters density (plants m"2) on specific leaf weight 110 (SLW) [lamina dry weight/leaf area (g m")] (mean ± SE) of broccoli grown at 4 (open bars) and 7 ( radiation for 4 weeks in 1999. 2 i grown at 4 (open bars) and 7 (shaded bars) kJ m" d" U V - B B E Fig. 4.2 Effect of broccoli (XB) and lamb's-quarters (XL) densities (plants m~2) 111 on specific leaf weight (SLW) [lamina dry weight/leaf area (g m"2)] (mean ± SE) of broccoli plants grown at low (4 kJ m"2 d"1) and high (7 kJ m"2 d"1) U V B B E radiation for 5 weeks in 2000 . Fig. 4.3 Effect of lamb's-quarters density (plants m"2) on leaf weight ratio (LWR) 112 [lamina dry weight/shoot dry weight ] (mean ± SE) of broccoli plants exposed to treatments for 4 weeks in 1999. Fig. 4.4 Effect of broccoli (XB) and lamb's-quarters (XL) densities (plants m"2) 113 on leaf area ratio (LAR) [leaf area/shoot dry weight (cm2 g")] of broccoli grown at low (4 kJ m"2 d"1) and high (7 kJ m"2 d"1) U V B B E radiation for 4 weeks in 1999. Fig. 4.5 Effect of broccoli (XB) and lamb's-quarters (XL) densities (plants m") 114 on the leaf area ratio (LAR) [leaf area/shoot dry weight (cm2 g"1)] of broccoli plants grown at low (4 kJ m"2 d"1) and high (7 kJ m"2 d"1) U V B B E radiation for 5 weeks in 2000 . Fig. 4.6 Effect of broccoli ( X B ) and lamb's-quarters (XL) densities (plants m") 115 on leaf area index (LAI) [leaf area / plant density (cm2 x X ) ] of broccoli plants grown at low (4 kJ m"2 d"1) and high (7 kJ m"2 d"1) U V B B E radiation for 4 weeks in 1999. Xl l l Fig. 4.7 Effect of broccoli (XB) and lamb's-quarters (X L) densities (plants m"2) 117 on leaf area index (LAI) [leaf area / plant density (cm2 x X)] of broccoli plants grown at low (4 kJ m"2 d"1) and high (7 kJ m"2 d"1) U V B B E radiation for 5 weeks in 2000. Fig. 4.8 Effect of broccoli density (plants m"2) on shootroot ratio (SRR) 118 [shoot dry weight: root dry weight] (mean ± SE) of broccoli plants grown at 4 (open bars) and 7 (shaded bars) kJ m"2 d"1 U V - B B E radiation for 5 weeks in 2000. Fig. 4.9 Effect of lamb's-quarters density (plants m 2) on specific leaf weight 121 (SLW) [lamina dry weight/leaf area (g m"2)] (mean ± SE) of lamb's-quarters plants exposed to treatments for (A) 4 weeks in 1999 and (B) 5 weeks in 2000. Fig. 4.10 Effect of lamb's-quarters density (plants m~ ) on leaf weight ratio 122 (LWR) [lamina dry weight/shoot dry weight] (mean ± SE) of lamb's-quarters plants exposed to treatments for 5 weeks in 2000. Fig. 4.11 Effect of broccoli (XB) and lamb's-quarters (XL) densities (plants m"2) 123 on the leaf area ratio (LAR)) [leaf area/shoot dry weight (cm2 g"1)] of lamb's-quarters plants exposed to treatments for 5 weeks in 2000. Fig. 4.12 Effect of broccoli (XB) and lamb's-quarters (XL) densities (plants m") 124 on leaf area index (LAI) [leaf area / plant density (cm2 x X)] of 2 1 2 1 lamb's-quarters plants grown at low (4 kJ m" d") and high (7 kJ m" d" ) U V B B E radiation for 4 weeks in 1999. Fig. 4.13 Effect of broccoli (X B) and lamb's-quarters (XL) densities (plants m") 125 on leaf area index (LAI) [leaf area / plant density (cm2 x X)] of 2 1 2 1 lamb's- quarters plants grown at low (4 kJ m" d") and high (7 kJ m" d") U V B B E radiation for 5 weeks in 2000. Fig. 4.14 Effect of lamb's-quarters density (plants m"2) on shootroot ratio (SRR) 126 [shoot dry weight:root dry weight] (mean ± SE) of lamb's-quarters plants grown at 4 (open bar for 4 weeks in 1999. s) and 7 (shaded bars) kJ m" d" U V - B B E radiation 2 Fig. 4.15 Effect of broccoli (X B) and lamb's-quarters (XL) densities (plants m") 127 on shootroot ratio (SRR) [shoot dry weightroot dry weight] of lamb's-quarters plants grown at low (4 kJ m"2 d"1) and higl (7 kJ m"2 d"1) UVBBE radiation for 5 weeks in 2000. Fig. 5.1 Effect of U V - B radiation and lamb' s-quarters density (plants m"2) 165 on shoot height (mean ± SE) of broccoli plants grown at 4 (open bars) and 7 (shaded bars) kJ m"2 d U V - B B E for 4 weeks in 1999. XIV Fig. 5.2 Effect of broccoli (XB) and lamb's-quarters (XL) densities (plants m") on 166 shoot height of broccoli plants grown at low (4 kJ m"2 d"1) and high (7 kJ m"2 d"1) UVBBE radiation for 5 weeks in 2000. Fig. 5.3 Effect of U V - B radiation and lamb's-quarters densities (plants m") 167 on shoot height (mean ± SE) of lamb's-quarters plants grown at 4 (open bars) and 7 (shaded bars) kJ m"2 d"1 U V - B B E radiation for 4 weeks in 1999. Fig. 5.4 Effect of broccoli (XB) and lamb's-quarters (X L) densities (plants m"2) 168 on shoot height of lamb's-quarters plants grown at low (4 kJ m"2 d"1) and high (7 kJ m"2 d"1) UVBBE radiation for 5 weeks in 2000. Fig 5.5 Effect of broccoli (XB) and lamb's-quarters (X L) densities (plants m" ) 170 on inverse leaf number of broccoli plants in exposed to treatments for 4 weeks in 1999. Fig 5.6 Effect of broccoli (XB) and lamb's-quarters (XL) densities (plants m~) 171 on inverse leaf number of lamb's-quarters plants grown at low (4 kJ m"2 d"1) and high (7 kJ m"2 d"1) UVBBE radiation for 4 weeks in 1999. Fig 5.7 Effect of broccoli (XB) and lamb's-quarters (XL) densities (plants m") 172 on inverse leaf number of lamb's-quarters plants grown at low (4 kJ m"2 d"1) and high (7 kJ m"2 d"1) UVBBE radiation for 5 weeks in 2000. Fig. 5.8 Effect of U V - B radiation and lamb's-quarters density (plants m") 181 on leaf greenness (mean ± SE) (SPAD units) of broccoli plants grown for (A) two weeks and (B) four weeks in 1999. UV-BBE treatment doses were 4 (open bars) and 7 (shaded bars) kJm"2 d"1. Fig. 5.9 Effect of U V - B radiation and broccoli (XB) and lamb's-quarters (X L) 182 9 9 densities (plants m") on leaf greenness (plants m") (SPAD units) of broccoli plants grown for (A) and (C) two weeks, and (B) and (D) four weeks in 2000. Fig. 5.10 Effect of U V - B radiation and broccoli (X B) and lamb' s-quarters (XL) 183 densities (plants m"2) on leaf greenness (SPAD units) of lamb's-quarters plants grown for (A) and C) two weeks, and (B) and (D) four weeks in 1999. Fig 5.11 Effect of U V - B radiation and broccoli density (plants m") on leaf greenness 185 (mean ± SE) (SPAD units) of lamb's-quarters plants grown for (A) two weeks and (B) four weeks in 2000. UV-BBE treatment doses (B) were 4 (open bars) and 7 (shaded bars) kJ m"2 d"1. XV List of Appendices 2.1 Effect of U V - B radiation on leaf, stalk, and root dry weights of broccoli 219 cultivars: A) Arcadia, B) Emperor, C) Everest, D) Legend, E) Patriot, and F) Purple Sprouting seedlings in experiment 1. 2.2 Effect of U V - B radiation on leaf, stalk, and root dry weights of broccoli 220 cultivars: A) Arcadia, B) Emperor, C) Everest, D) Legend, E) Patriot, and F) Purple Sprouting seedlings in experiment 2. 2.3 Response of growth indices of broccoli cultivars to U V - B radiation. 221 2.4 Influence of U V - B radiation on shoot: root ratio (SRR) of broccoli cultivars. 222 3.1 Ambient greenhouse PAR and U V - A levels during the competition 223 experiments. 3.2a Mean values and standard errors for broccoli primary variables in 1999. 224 3.2b Mean values and standard errors for lamb's-quarters primary variables in 1999. 225 3.2c Mean values and standard errors for broccoli primary variables in 2000. 226 3.2d Mean values and standard errors for lamb's-quarters primary variables in 2000. 227 3.3a A N O V A results (variance ratios) for the effects of U V - B radiation, broccoli 228 density (XB), and lamb's-quarters density (XL) on broccoli mean primary variables weighted by the inverse of the variance in 1999. 3.3b A N O V A results (variance ratios) for the effects of U V - B radiation, broccoli 229 density (XB), and lamb's-quarters density (XL) on lamb's-quarters mean primary variables weighted by the inverse of the variance in 1999. 3.3c A N O V A results (variance ratios) for the effects of U V - B radiation, broccoli 230 density (XB), and lamb's-quarters density (XL) on broccoli mean primary variables weighted by the inverse of the variance in 2000. 3.3d A N O V A results (variance ratios) for the effects of U V - B radiation, broccoli 231 density (XB), and lamb's-quarters density (XL) on lamb's-quarters mean primary variables weighted by the inverse of the variance in 2000. 3.4a Regression parameters and statistics for inverse yield-density models 232 (Eqns. 3.1 and 3.2) describing the response of reciprocal root biomass (WR"1) of (a) broccoli and (b) lamb's-quarters to broccoli and lamb's-quarters population densities in 1999. 3.4b Regression parameters and statistics for inverse yield-density models 233 (Eqns. 3.1 and 3.2) describing the response of reciprocal stem biomass (Ws"1) XVI of (a) broccoli and (b) lamb's-quarters to broccoli and lamb's-quarters population densities in 1999. 3.4c Regression parameters and statistics for inverse yield-density models 234 (Eqns. 3.1 and 3.2) describing the response of reciprocal leaf biomass (WL"1) of (a) broccoli and (b) lamb's-quarters to broccoli and lamb's-quarters population densities in 1999. 3.4d Regression parameters and statistics for inverse yield-density models 235 (Eqns. 3.1 and 3.2) describing the response of reciprocal leaf area (LA" 1) of (a) broccoli and (b) lamb's-quarters to broccoli and lamb's-quarters population densities in 1999. 3.4e Regression parameters and statistics for inverse yield-density models 236 (Eqns. 3.1 and 3.2) describing the response of reciprocal root biomass (WR" 1) of (a) broccoli and (b) lamb's-quarters to broccoli and lamb's-quarters population densities in 2000. 3.4f Regression parameters and statistics for inverse yield-density models 237 (Eqns. 3.1 and 3.2) describing the response of reciprocal stem biomass (Ws"1) of (a) broccoli and (b) lamb's-quarters to broccoli and lamb's-quarters population densities in 2000. 3.4g Regression parameters and statistics for inverse yield-density models 238 (Eqns. 3.1 and 3.2) describing the response of reciprocal leaf biomass (WL"1) of (a) broccoli and (b) lamb's-quarters to broccoli and lamb's-quarters population densities in 2000. 3.4h Regression parameters and statistics for inverse yield-density models 239 (Eqns. 3.1 and 3.2) describing the response of reciprocal leaf area (LA" 1) of (a) broccoli and (b) lamb's-quarters to broccoli and lamb's-quarters population densities in 2000. 4.1a Mean values and standard errors for broccoli growth indices in 1999. 240 4.1b Mean values and standard errors for lamb's-quarters growth indices in 1999. 241 4.Ic Mean values and standard errors for broccoli growth indices in 2000. 242 4.Id Mean values and standard errors for lamb's-quarters growth indices in 2000. 243 XVII Symbols and Abbreviations Symbols and Page (First Description abbreviations appearance) a 21 Allometric coefficient (or constant after ln-transformation) aBo or a L O 79 Intercept in yield density models. Inverse of mean yield per plant of broccoli or lamb's-quarters aBB or aLL 79 Intraspecific regression coefficient in yield-density models with broccoli or lamb's-quarters as the target species a B L or a L B 79 Interspecific regression coefficient in yield-density models with broccoli or lamb's-quarters as the target species acc or a w w 19 Intraspecific regression coefficient in inverse yield-density models aCw or a w c 19 Interspecific regression coefficient in inverse yield-density models aCo or aWo 19 Intercept in inverse yield-density models. Inverse of mean yield per plant in the absence of neighbours aXB or aXL 158 Interspecific regression coefficient in inverse yield-density models for PAR ANOVA 81 Analysis of variance P 21 Allometric exponent (or coefficient after ln-transformation) CFC 7 Chloroflurocarbon C 0 2 34 Carbon dioxide Cp 107 Mallows Cp statistic CPD 10 Cyclobutane pyrimidine dimer cv. 3 • Cultivar D 158 Mixture density DNA 7 Deoxyribonucleic acid e 22 Residual variation in yield in allometric models y 22 Non-allometric coefficient H 32 Height I 158 Below-canopy PAR level I 0 158 Above-canopy PAR level k 106 Subscript denoting kth allometric or non-allometric term kJ 26 kilo Joule LA 33 Leaf area LAI 102 Leafareaindex(LA/A) LAR 12 Leaf area ratio ( L A / W S H ) In 22 Natural logarithm LWR 12 Leaf weight ratio (WL/WS H) nm 1 Nanometer 0 3 1 Ozone OCC 155 Overhead canopy coverage PAR 4 Photosynthetically active radiation RNA 10 Ribonucleic acid Rubisco 9 Ribulose - 1,5 - bisphosphate carboxylase/ oxygenase X V I 1 1 SI 33 Sensitivity index SLW 11 Specific leaf weight (W L /LA) SRR 32 Shoot:root ratio ( W S H / W R ) U V - A 10 Ultraviolet-A U V - B 1 Ultraviolet-B U V - B B E 26 Biologically effective ultraviolet-B W 79 Plant dry weight wL 79 Leaf dry weight wR 79 Root dry weight W s 33 Stem dry weight W S H 104 Shoot dry weight X 19 Species population density (plants per land area) X B 79 Population density (plants per land area) of broccoli X c 19 Population density (plants per land area) of crop denoted by subscript C xL 79 Population density (plants per land area) of lamb's-quarters Xw 19 Population density (plants per land area) of weed denoted by subscript W y 19 Mean yield per plant Y 19 Yield per unit land area z 21 Yield of a secondary measure in allometric relationships XIX Acknowledgements I would like to sincerely thank my supervisor Dr. Mahesh K. Upadhyaya for all his encouragement and support during these past years. I would also like to express appreciation to my committee members Dr. Peter Jolliffe, Dr. Doug Ormrod, and Dr. Chris Chanway for their valuable guidance and advice. I am grateful to Dr. Peter Jolliffe for many interesting and insightful conversations concerning plant competition. Many thanks to Dr. A . Kozak for advice regarding statistical analyses, Jerry Maedel for introducing me to image analysis, Dr. Elaine Humphrey for sharing her expertise in scanning electron microscopy, and David Kaplan for support in the greenhouse. I would also like to thank my lab-mates Mercy Mohan, Jennifer Cameron, and Dr. Quincy Dai for their help and encouragement. M y sincere appreciation also goes to Lisa Yu, Dr. Debbie Wheeler, and Lorri Moffatt for special friendships and for never letting me down. I am very grateful to Mike Moore and our daughter Katie Furness-Moore for their love, patience, and sense of humour during these past years, and to my mother Dorothy Furness for always believing in me. Without them this thesis would not be complete. 1 General Introduction Ultraviolet-B (UV-B) radiation (290-315 nm) is an environmental stress of current concern because enhanced levels of this radiation are reaching the earth's surface as the stratospheric ozone (O3) concentration declines (Madronich et al. 1995). Elevated U V - B radiation levels can alter competitive interactions among plants (Fox and Caldwell 1978, Bogenrieder and Klein 1982, Gold and Caldwell 1983, Barnes et al. 1988, Yuan et al. 1999). Research reported in this thesis explores the impact of elevated levels of U V - B radiation on crop-weed competitive interactions. Interference, the response of a plant to its neighbours (Harper 1961), is the net outcome of beneficial and/or deleterious influences on the target plant. Beneficial interactions between plants may occur through microclimate and soil modification, nutrient transfer, physical support, attraction of pollinators, and protection from herbivory (Hunter and Aarssen 1988, Hjalten and Price 1997). Allelopathic interactions (Rice 1984) and competition for limited resources (Connell 1983, Keddy and Shipley 1989) comprise some negative aspects of interference. The complexity of natural plant communities imposes logistic and analytical restrictions on studying plant interactions (Gibson et al. 1999). For example, presence of numerous species, spatial and temporal heterogeneity in both environmental factors and species abundance, and variation in size and age of plants often make it difficult to identify, or isolate and quantify the manner of interference contributing to plant responses in associations. Plants existing in close proximity however, generally make demands on the same resources (Hunter and Aarssen 1988), implying that competition for limited resources often comprises a key element of interference. Resources for which plants compete include mineral nutrients (Van Auken and Bush 1997), carbon dioxide (Reekie and Bazzaz 1991), oxygen (Sands etal. 2000), and light (Monteith 1981, Caldwell 1987, Keating and Carberry 1993). Begon et al. (1986) defined competition as "an interaction between individuals brought about by a shared requirement for a resource in limited supply, and leading to a reduction in the survivorship, growth, and/or reproduction of the species concerned". The abundance of literature documenting the resource-dependence of 2 high-density agricultural crops also supports the putative contribution of competition to interference (Radosevich and Roush 1990, Wanjau 1998). Weldon and Slauson (1986) considered restrictions in growth of a plant arising from its proximity to associated plants to be the result of competition. It is this meaning that will be imparted to the term "plant competition" throughout this thesis recognizing the fact that effects of the various components of interference will not be distinguished. Competitive relationships are often modified in response to environmental stresses (Mandak and Pysek 1999, Assaeed and Al-Doss 2001, Emery et al. 2001). U V - B radiation is an environmental stress of current relevance as natural and anthropogenic destruction of stratospheric ozone has been associated with elevated U V - B radiation levels at the Earth's surface (Russell et al. 1996). At elevated U V - B radiation levels predicted to reach the Earth's surface, UV-B-induced changes in plant morphology including reduced plant height (Barnes et al. 1993), leaf insertion height (Barnes et al. 1988), and leaf area (Greenberg et al. 1997) are considered to be of importance in determining competitive balances among species (Barnes et al. 1990b, Caldwell and Flint 1994, Allen et al. 1998). It has been suggested that even subtle differential changes in plant form in response to supplemental U V - B radiation may be sufficient to alter competition for sunlight due to the rapid extinction of light within plant canopies (Ryel et al. 1990). Sensitivity to U V - B radiation, defined as the relative change induced by this radiation on plant growth, morphology, or yield (Caldwell et al. 1980), varies considerably within and among plant species. Morphological responses to U V - B radiation, likely to affect competitive ability, in a variety of horticultural crop and weed species will be characterized in Chapter 2 of this thesis in order to select a suitable crop-weed species pair for use in studies detailing the influence of U V - B radiation on crop-weed competitive interactions. Current knowledge of the influence of U V - B radiation on competitive relationships (Fox and Caldwell 1978, Bogenrieder and Klein 1982, Gold and Caldwell 1983, Barnes et al. 1988, Yuan et al. 1999) is based on experiments carried out using the replacement series design (deWit 1960). In this design, plants compete directly for space and limited resources within that space. A constant overall plant density is maintained, 3 while proportions of the two species present are varied within a given area. Relative competitive ability of the target species is evaluated by comparing its performance in a mixture to its performance in a monoculture of the same density. Numerous criticisms, however, have cast serious doubt as to the appropriateness of the replacement series design as a method for studying plant competitive interactions (Jolliffe et al. 1984, Connolly 1986, Snaydon 1991, Snaydon 1994, Jolliffe 2000). Inverse yield-density relationships provide a more appropriate technique for a first approximation of competitive interactions (Wright 1981, Spitters 1983) and also allow separation of intra- and interspecific components of interference (Spitters 1983). Inverse yield-density models are utilized in Chapter 3 of this thesis in order to evaluate U V - B -induced shifts in crop-weed competitive relationships. These models also facilitated the testing of Barne's et al. (1988) hypothesis that the effects of U V - B radiation on intraspecific competition would be less consequential than those on interspecific competition. Competitive relationships are defined in more detail in Chapter 4 using plant growth indices and allometric analyses. Conventional plant growth analysis assesses plant responses to their neighbours using simple indices of growth and productivity, emphasizing physiological and morphological sources of variation in growth (Warren Wilson 1981, Hunt 1990). Some commonly used indices include specific leaf area, specific leaf weight, leaf weight ratio, leaf area ratio, and leaf area index (Hunt 1982). For example, specific leaf weight (leaf dry weight/leaf area) is a growth index commonly used to measure leaf thickness and/or density responses to U V - B radiation (Tevini and Teramura 1989, Bornman and Vogelmann 1990). Deckmyn and Impens (1995) used plant growth indices to detect shifts in biomass allocation and leaf morphology of bean plants (Phaseolus vulgaris L. cv. Label) grown at two levels of U V - B radiation. These growth indices can be linked to underlying functional processes and to plant structures. Allometric analysis explores quantitative relationships between different measures of an organism, commonly between one part of an organism and the whole, as growth advances (Jolliffe et al. 1988). It can be an effective technique for formulating testable hypotheses concerning the adaptive value of characteristics (Le Maitre and Midgley 1991). Allometric adjustments may occur in plants in response to environmental and/or 4 competitive influences. For example, above- or below-ground environmental stresses cause shifts in root: shoot partitioning in favour of the affected part of the plant (Hunt 1988). Population density has been shown to affect allometric relationships of carrot (Daucus carota L.), as well as orchardgrass (Dactylis glomerata L.) and timothy (Phleum pratense L.) (Stanhill 1977, Jolliffe et al. 1988). The expanded allometric equation developed by Jolliffe et al. (1988) enabled separation of the effects of U V - B radiation and population density on within-plant biomass partitioning in this research by evaluating sources of variation in allometric relationships. If interpretation of competitive responses relied entirely on final yield, dynamic aspects of species interactions would be overlooked (e.g. Connolly et al. 1990, Turkington and Jolliffe 1996). Non-destructive techniques used in Chapter 5 of this thesis enabled UV-B-induced shifts in species interactions to be tracked over time. Inverse yield-density models provided a method to study the influence of interception of photosynthetically active radiation (PAR) on competition for light at the canopy level over the course of the experiment by substituting the quantity of PAR intercepted for yield in the model (Dr. Jolliffe, Professor, Faculty of Agricultural Sciences, U.B.C., Pers. comm.). It was further possible to partition the contribution of each of the component species to P A R interception. Image analysis techniques were exploited to follow UV-B-induced shifts in overhead canopy coverage of the crop and weed species. Ryel et al. (1990) developed a model to quantitatively assess importance of UV-B-induced shifts in foliage positions of associated species within a canopy and determined that even slight changes in plant form altered competition for light. Barnes et al. (1988) reported that changes in plant morphology caused shifts in foliage height distribution in mixed canopies, resulting in wheat (Triticum aestivum L. cv. Bannock) overtopping wild oat (Avena fatua L.) to a greater degree under U V - B radiation. Wheat was more competitive in UV-B-enriched environments. Another dynamic pathway by which U V - B radiation might affect species' abilities to intercept P A R is through differential UV-B-induced modifications in leaf chlorophyll content (Lovelock et al. 1992, Middleton et al. 1996). Non-destructive measurements of leaf greenness taken over time intervals allowed the role of this pathway to be researched in Chapter 5. Other researchers have reported that chlorophyll content of various species, 5 measured at a single time increased (Teramura and Caldwell 1981), decreased (Garrard et al. 1976, Tevini et al. 1981, Vu et al. 1982), or remained unchanged (Smith et al. 2000) in response to U V - B radiation. Deckmyn and Impens (1995) reported a UV-B-induced reduction in chlorophyll content during the vegetative growth phase in beans (P. vulgaris L.). However, at the final harvest, plants grown at high U V - B radiation had a higher chlorophyll concentration than those grown at low U V - B radiation. They suggested that enhanced U V - B radiation delayed leaf senescence. These findings highlight the value of information gathered from more than a single time. Overall, the consequences of increased U V - B radiation on competitive interactions in agricultural systems may involve several complex pathways, rather than simply a reduction in overall productivity. While only field studies can provide realistic assessments of what will happen as the stratospheric ozone layer thins, greenhouse experiments provide valuable information on mechanisms and processes of U V - B action on competitive relationships (Caldwell et al. 1995). By manipulating spatial and temporal variability (Radosevich and Roush 1990) and reducing some components of interference, artificial greenhouse-grown plant communities provide an opportunity to explore intra-and interspecific competitive interactions under less complex conditions than in natural or field situations (Gibson et al. 1999). In addition, mechanisms of interaction (e.g. through root and shoot capture of resources) are more amenable to study under controlled conditions. Both quantity and quality of crop yield are influenced by competitive interactions (Chin 2001, Ngouajio et al. 2001). Therefore, the impact of U V - B radiation upon intra-and/or interspecific competitive interactions among crop and weed species could have serious implications for agricultural communities. Research Objectives The overall goal of this research was to expand our understanding of the impact of U V - B radiation stress on crop-weed competitive interactions. The research presented in this thesis focused on the influence of UV-B-induced morphological responses on components of interference in a horticulturally relevant crop-weed association. Competition 6 experiments were carried out in a greenhouse environment under simulated ambient and above-ambient levels of U V - B radiation. The specific objectives of this research were to: 1. characterize the influence of U V - B radiation on growth and morphology of selected weed, vegetable crop, and crop cultivar seedlings, and to compare relative sensitivity of plants within each of these groups to this radiation (Chapter 2), 2. evaluate intra- and interspecific components of interference in binary crop-weed associations by establishing inverse yield-density relationships for plant growth at ambient and above-ambient levels of U V - B radiation (Chapter 3), 3. elucidate the role of UV-B-induced morphological changes on components of interference in crop-weed associations using indices of plant growth and allometric analysis and (Chapter 4), 4. investigate the role of UV-B-induced shifts in leaf and canopy optical properties (PAR interception and leaf greenness) on components of interference in crop-weed associations (Chapter 5). 7 1.0 Literature Review 1.1 The changing UV-B radiation environment Release of anthropogenic pollutants such as chlorofluorocarbons (CFC's) and halocarbons into the earth's upper atmosphere has resulted in catalytic destruction of stratospheric ozone (0 3) (Rowland 1990). Subsequently, elevated levels of ultraviolet-B (UV-B; 290-315 nm) radiation are reaching the earth's surface. A 10% decrease in stratospheric ozone has been predicted to result in a 20% increase in UV-penetration at 305 nm and a 250% increase at 290 nm (Crutzen 1992). U V - B radiation below 300 nm comprises only 1% of the U V -radiation that reaches the earth's surface (D'Surney et al. 1993). However, the relatively large increase in this shorter wavelength region of the UV-B-spectrum is of concern because of its greater ability to disrupt physiological function and potentially induce D N A damage. In 1987, global concern over stratospheric ozone depletion resulted in ratification of 'The Montreal Protocol on Substances That Deplete the Ozone Layer' (1987; United Nations Environment Program Na.87-6106). This international treaty to protect the atmosphere from human impact by reducing or banning the future non-essential use of ozone-damaging substances was subsequently strengthened through amendments and adjustments in 1990 and 1992. Assuming full compliance to the Montreal Protocol by all nations, O 3 depletion was predicted to peak around 1998 with a maximum O 3 loss over the northern mid-latitudes (relative to 1960's levels) of 12-13% in the winter/spring and 6-7% in the summer/autumn (WMO 1995). A slow recovery (to pre-1960's ozone levels) was expected by about 2050 (Madronich et al. 1995). However, the validity of the assumptions upon which these predictions are based is currently unclear (Allen et al. 1998). Firstly, full compliance with the Montreal Protocol is uncertain (Greene 1995) and secondly, anthropogenic emission of greenhouse gases and other pollutants have the potential to change tropospheric O 3 concentrations, cloud cover and stratospheric air temperatures resulting in altered levels of U V - B radiation (Shindell et al. 1998). For example, a recent analysis using a global climate model predicted that rising levels of carbon dioxide and other greenhouse gases may cause stratospheric cooling and more stable polar vortices in winter, thereby delaying recovery of O 3 concentrations for a further 15 years following declines in CFC emissions (Shindell et al. 8 1998). Greenberg et al. (1997) also expressed concern that peak chlorofluorocarbon production was presently occurring and that it takes more than 20 years for the molecules to reach the stratosphere. Depletion of the stratospheric ozone layer, despite mitigation efforts is of particular concern to terrestrial plants (Caldwell and Flint 1994). Plants are especially vulnerable to potentially damaging increases in U V - B radiation levels due to their need to be exposed to sunlight during the process of photosynthesis (Koes et al. 1994). 1.2 Plant response to UV-B-enriched environments Several review articles concerning the effects of U V - B radiation on terrestrial plants are available (Bornman and Teramura 1993, Caldwell and Flint 1994, Teramura and Sullivan 1994, Caldwell et al. 1995, Greenberg et al. 1997). Exposure to U V - B radiation triggers numerous responses in plants including damage to the photosynthetic apparatus, membranes, proteins, and DNA, delayed maturation, growth reduction, upward curling and thickening of leaves, and activation of chemical stress and flavonoid synthesis (Greenberg et al. 1997). While some responses represent damage others, such as flavonoid synthesis, may represent acclimation. 1.2.1 Damage to the photosynthetic apparatus Decreased photosynthetic capacity and growth rate in plants have been linked to U V - B -induced structural damage to labile pigments and polypeptides, including chloroplasts and the D l and D2 proteins of the photosystem II core (Teramura 1996). Disruption of structural integrity of the chloroplast membrane partially damages components necessary for both light and dark reactions of photosynthesis (Taylor 1997). Studies involving numerous species have shown UV-B-induced declines in chlorophyll content (Garrard et al. 1976, Tevini et al. 1981, Vu et al. 1982). Such declines may be attributed to inhibition of chlorophyll biosynthesis or degradation of these pigments or their precursors. In contrast, no effect on total chlorophyll concentration was detected in soybean {Glycine max L. Merr.) grown under a range of photosynthetically active radiation (PAR) growth regimes in combination with low and moderate U V - B radiation, despite large 9 effects on net photosynthesis (Teramura 1980). Still other studies have detected stimulation in chlorophyll biosynthesis (Teramura and Caldwell 1981). Carbon fixation (Calvin cycle or reductive pentose phosphate cycle) is also affected by U V - B irradiation (Tevini and Teramura 1989, Ziska et al. 1993). Photomodification of aromatic residues in the central enzyme in the carbon fixation pathway, ribulose-1,5-bisphosphate carboxylase/ oxygenase (Rubisco), may render the protein inactive (Wilson et al. 1995). De novo synthesis of Rubisco is also diminished under U V - B irradiation (Jordan et al. 1994). 1.2.2 Damage to plant proteins and membranes The effects of U V - B radiation on photosystem II, Rubisco, and the plasma membrane ATPase indicate that proteins are hypersensitive to oxidation induced by this radiation (Greenberg et al. 1997). Absorbance of U V - B radiation, by aromatic amino acids and nucleotides, can lead to single electron oxidation of the residue and the reduction of oxygen to superoxide (Walrant and Santus 1974, Wilson et al. 1995), resulting in potentially disabling photochemical changes in the corresponding proteins and nucleic acids (Stapleton 1992, Britt 1995, Teramura 1996, Taylor 1997). UV-B-induced damage to plant membranes occurs when excited singlet-state oxygen, formed by the absorption of a photon by a molecule, attacks a bio-molecule to form an organic peroxide (Britt 1996). Lipid peroxides acutely inhibit membrane fluidity and function (Krinsky 1979, Thompson 1984, Girotti 1990, Kochevar 1990). For example, U V -B-induced changes to the plasma membrane result in dissipation of electrical potential, efflux of potassium and bicarbonate, and changes in cellular pH (Murphy 1983, Murphy et al. 1993, Greenberg et al. 1997). U V - B stress can be somewhat alleviated by scavenging of active oxygen and other radical species through either enzymatic or non-enzymatic systems (Jansen etal. 1996). 1.2.3 Damage to DNA Damage to the plant genetic code occurs when D N A absorbs U V - B photons (Greenberg et al. 1997). The most pervasive UV-induced damage is the photo-oxidation of D N A resulting 10 in formation of cyclobutane pyrimidine dimers (CPDs), and less commonly pyrimidine (6,4) photoproducts (Britt 1996). CPD's, resulting from the fusion of adjacent pyrimidine bases, eliminate information from the D N A code (Britt 1996). The inability of D N A polymerase II to read through unrepaired dimers leads to immediate defects in D N A replication, which can have long-term mutagenic ramifications. Equally important, however, are the toxic effects of CPDs and (6-4) photoproducts on transcription. R N A polymerase II stalls at fused pyrimidine dimers and remains bound to the damaged D N A (Donahue et al. 1994). The resulting decrease in available transcription complexes is likely to have serious consequences for the continued growth and development of affected cells (Britt 1996). Plants possess repair mechanisms to alleviate effects of UV-induced D N A damage on transcription and replication. The majority of UV-induced D N A damage can be ameliorated by a process known as photoreactivation by light of wavelengths between 300-600 nm given at the same time as or shortly after exposure to U V - B radiation in plants. Ultraviolet-A (UV-A) wavelengths (320-380 nm) appear to be the most effective (Pang and Hays 1991). The extent to which the D N A is repaired depends on the quality, timing and quantity of "photoreactivating" light. The process of photoreactivation elicits an electrochemical rearrangement, which restores the pyrimidine bases to their normal structures (Sancar 1996). UV-induced reduction in growth, caused by a reduction in rate of cell division, may enhance potential effectiveness of these repair mechanisms (Beggs et al. 1986). 1.2.4 Acclimation and protective mechanisms A l l organisms subjected to the U V - B radiation in sunlight have mechanisms to block that radiation from entering their cells (Taylor 1997). For example, U V - B radiation stimulates the general phenylpropanoid pathway, resulting in accumulation of flavonoids and sinapic esters, which afford protection to sensitive underlying targets by selectively filtering out much of the radiation between 280 and 340 nm (Jansen et al. 1998), while at the same time providing minimal attenuation in the 400-700 nm waveband which is effectively utilized in photosynthesis (Caldwell et al. 1983). At least 90% of the U V - B radiation that does reach the surface of plant leaves is attenuated before reaching the mesophyll tissues and very little 11 radiation in this waveband penetrates completely through the mesophyll tissue (Caldwell 1981). Trichomes (Skaltsa et al. 1994, Karabourniotis and Fasseas 1996) and epicuticular waxes of some species contain unbound flavonoids and flavones (Kolattukudy 1980, Wollenweber 1982). Removal of flavonoid-containing trichomes in UV-B-exposed holly oak (Quercus ilex L.) leaves resulted in reduction of photosystem II photochemical efficiency (Skaltsa et al. 1994). The removal of epicuticular waxes from some species (Clark and Lister 1975, Karabourniotis et al. 1992), but not others (Bornman and Vogelmann 1988, Day et al. 1992), has been shown to influence the amount of U V - B radiation penetrating into foliage. Carotenoids have also been shown to protect the photosynthetic apparatus against UV-radiation damage (Knox and Dodge 1985, Salin 1987). Tropical mangrove species were shown to differ significantly in their carotenoid/chlorophyll ratios (Lovelock et al. 1992). Species with relatively low carotenoid/chlorophyll ratios showed a decrease in chlorophyll content, while species with high carotenoid/ chlorophyll ratios showed no change in total chlorophyll content in response to U V - B radiation. Reduction in hypocotyl extension has been proposed as a mechanism to reduce exposure of seedlings to U V - B radiation while synthesis of UV-B-absorbing compounds occurs (Ballare et al. 1995). Reduction of leaf area in response to U V - B radiation is also believed to be a protective response, which reduces the amount of tissue exposed to this potentially harmful radiation (Bornman and Teramura 1993). Upward curling of leaves and cotyledons (Wilson and Greenberg 1993, Furness et al. 1999) provides another mechanism to reduce tissue exposed to U V - B radiation (Greenberg et al. 1997). The cost of these putative protective mechanisms however, is a decrease in leaf area to potentially intercept PAR (Dickson and Caldwell 1978). Many plant species acclimate to UV-B-enriched environments through increased specific leaf weight (SLW) [leaf dry weight/leaf area] (Biggs et al. 1981, V u et al. 1982, Cen and Bornman 1993). Although such an increase in leaf thickness and/or density may reduce penetration of U V - B radiation to sensitive targets (Teramura and Sullivan 1994), it alone provides insufficient protection against U V - B radiation (Teramura 1983). Greater SLW may not fully compensate for UV-B-induced losses in leaf area (Adamse and Britz 1992b). While distinctions among UV-B-damage, repair and acclimation mechanisms are not always clear, 12 the biological impact of this radiation on plants depends upon the balance attained among these responses (Jansen et al. 1998). 1.2.5 Growth and morphological responses Reduced growth and morphological changes occur more frequently than reduced photosynthetic competence in plants exposed to increased levels of U V - B radiation expected to reach the earth (Caldwell and Flint 1994, Allen et al. 1998). A number of studies have shown that total plant dry weight is often substantially reduced by exposure to U V - B radiation (Barnes et al. 1993, Furness et al. 1999). Total plant biomass accumulation is a convincing indicator of the influence of U V - B radiation on growth as it represents a long-term integration of all biochemical, physiological, and growth parameters (Teramura 1983). The cumulative impact of even subtle UV-B-induced effects on physiological processes could significantly alter biomass accumulation. Reductions in total biomass are often accompanied by substantial modifications in the partitioning of biomass into component plant organs (Teramura 1983). For example, despite an absolute reduction in leaf area in response to U V - B radiation, dicotyledons generally allocate a greater proportion of biomass into leaves and less into stems and roots. Gold and Caldwell (1983) suggested that changes in above-ground biomass under enhanced U V - B radiation may in part result from a shift in the allocation of plant resources between above-and below-ground biomass rather than a decrease in total biomass. In addition to altering dry matter partitioning into leaf tissue, within-leaf reallocation of dry matter can occur in response to U V - B radiation. For example, an increase in the leaf weight ratio (LWR) [leaf dry weight/ shoot dry weight] was reported to be due to an increase in specific leaf weight (SLW) [leaf dry weight/leaf area] in soybean, bean (Phaseolus vulgaris L.), pea (Pisum sativum L.), and cucumber (Cucumis sativus L.) exposed to U V - B radiation (Teramura 1983). Additionally, reduction in leaf area ratio (LAR) [leaf area/shoot dry weight] accompanied by increase in SLW was reported in soybean (cv. Essex) in response to U V - B radiation (Teramura and Sullivan 1987). UV-B-induced morphological changes include altered leaf shape (Wilson and Greenberg 1993), increased tillering (Barnes et al. 1988, Dai et al. 1994), and reduced plant height (Barnes et al. 1993), leaf insertion height (Barnes et al. 1988), and leaf area 13 (Greenberg et al. 1997). Relative effects of U V - B radiation on shoot morphology are usually greater than the effects on biomass production (Barnes et al. 1993). Morphological responses might actually represent allocation shifts (Barnes et al. 1995), as opposed to damage responses (Caldwell and Flint 1994). Some physiological responses, such as reduction in stem elongation, may reflect the cost of UV-B-induced re-allocation of resources to other functions or organs (Barnes et al. 1995). Such changes in the developmental agenda would entail reducing other activities, even without directly affecting photosynthesis and biomass accumulation. 1.2.6 Variation in inter-and intraspecific response to UV-B radiation Sensitivity to U V - B radiation, defined as the relative change induced by U V - B radiation on plant growth, morphology, or yield (Caldwell et al. 1980), varies considerably among plant species (Ambler et al. 1978, Becwar et al. 1982, Basiouny 1986, Barnes et al. 1990a). Intraspecific (cultivar) differences in U V - B response have been reported in soybean (Vu et al. 1978, Biggs et al. 1981), bean (Bennett 1981, Dumpert and Boscher 1982), collard (Brassica oleracea L. var. acephala) and cabbage (Brassica oleracea L. var. capitata) (Van et al. 1976, Garrard et al. 1976), wheat (Triticum aestivum L.), barley (Hordeum vulgare L.), corn (Zea mays L.), rice (Oryza sativa L.) (Biggs and Kossuth 1978), and spinach (Spinacia oleracea L.) (Dumpert and Boscher 1982). Considering all plant characteristics which are influenced by U V - B radiation including leaf area, height, and biomass partitioning pattern, there exists a huge array of U V -B responses expressed by different crop species (Teramura 1983). While the basis for differential response to U V - B radiation is largely unknown, differential response to elevated U V - B levels in soybean cultivars were suggested to be related to the ability to maintain high concentrations of UV-absorbing compounds in leaves (D'Surney et al. 1993). The combined effect of a multitude of morphological, anatomical and physiological processes, including differences in cuticle thickness, changes in specific leaf weight (SLW), leaf reflectivity, crop or leaf canopy development, as well presence of UV-absorbing pigments, likely result in differential species sensitivity to U V - B radiation. The issue of differential response to elevated U V - B radiation is further complicated by the dependence of UV-sensitivity upon experimental conditions (McCleod 1997) (section 1.5). 14 1.3 Mechanisms of UV-B-induced shifts in plant competitive interactions Several reviews of plant competition have been published (Milthorpe 1961, Harper 1977, Grime 1979, Tilman 1982, Silvertown 1987, Keddy 1989, Grace and Tilman 1990, Gibson et al. 1999). Resources for which plants compete include mineral nutrients (Van Auken and Bush 1997), carbon dioxide (Reekie and Bazzaz 1991), oxygen (Sands et al. 2000), and light (Monteith 1981, Caldwell 1987, Keating and Carberry 1993). Competition can occur for a single resource or several resources simultaneously. Light quickly becomes a limiting factor in crops where fertilizers and irrigation are applied (Keating and Carberry 1993). 1.3.1 UV-B radiation and interspecific competition Since plant species differ greatly in growth response to U V - B radiation, Caldwell (1977) hypothesized that reduced productivity of UV-sensitive species would result in increased productivity of associated UV-resistant species because of greater resource availability to the UV-resistant species. Bogenrieder and Klein (1982) suggested that an under-story species might gain a competitive advantage as a result of a relatively reduced UV-B-flux in the shelter of the taller species. Conversely, Barnes et al. (1988) found that taller species were more competitive in UV-B-enriched environments. UV-B-induced shifts in interspecific competitive balance have been reported (Fox and Caldwell 1978, Bogenrieder and Klein 1982, Gold and Caldwell 1983, Barnes et al. 1988) despite recent literature indicating that terrestrial plants do not generally experience appreciable damage to the photosynthetic apparatus upon exposure to elevated levels of U V -B radiation under field conditions (Caldwell and Flint 1994, Allen et al. 1998). At levels of predicted increase in ambient U V - B radiation, UV-B-induced changes in plant morphology, mediated by a putative U V - B photoreceptor (Wellmann 1983, Ballare et al. 1995), are considered to be a major determinant of the ability of a species to compete for light (Barnes et al. 1990b, Caldwell 1997). For example, in a six year study of mixtures of wheat (cv. Bannock) and wild oat (Avena fatua L.), supplemental U V - B radiation altered foliage distribution of the two species within the canopy (Barnes et al. 1988, Barnes et al. 1995). Although combined shoot biomass production of the species pair was unaffected, the competitive balance tended to shift in favour of wheat. It was proposed that the small daily carbon gain provided to the wheat plants by an upward shift in relative foliage position in the 15 canopy resulting from the U V - B treatment would accumulate through the season and be sufficient to change the competitive balance (Barnes et al. 1988). Barnes et al. (1993) further suggested that differential susceptibility of rice cultivars to UV-B-induced changes in competitive balance with associated weeds might occur as the result of intraspecific variation in morphological responses to U V - B radiation. Importance of even subtle UV-B-induced shifts in foliage positions of associated species within a canopy was quantitatively assessed through a model developed by Ryel et al. (1990). Significance of slight shifts in foliage position in the upper canopy can be magnified as absorption of sunlight within a canopy follows a logarithmic extinction pattern (Caldwell 1997). Leaves transmit less than 5% of PAR (Gates 1980), and therefore most of the sunlight is effectively absorbed in the uppermost portion of the canopy (Caldwell 1987). In addition, photosynthetic capacity of leaves in the upper foliage layers is usually greater than for leaves lower in the canopy (Caldwell 1997). PAR is an essential resource that must be intercepted and used immediately (Keating and Carberry 1993) and therefore a shift in foliage height that results in the overtopping of one species by another can quickly result in a shift in competition for light (Caldwell 1997). In plant canopies, U V - B radiation incident on foliage would be greatly attenuated due to shading by other foliage (Allen et al. 1975) as very little radiation within this waveband fully penetrates the leaf. Leaf inclination as an avoidance mechanism for solar U V - B radiation is less effective than shading by foliage, due to the large (40-75%) diffuse (scattered) component of radiation within this waveband (Caldwell 1981). Differential penetration of PAR and/or U V - B radiation as a result of accumulation of UV-B-absorbing compounds in the epidermal layer, quantitative or qualitative changes in carotenoids, (Middleton et al. 1996) chlorophylls (Lovelock et al. 1992, Middleton et al. 1996), trichomes (Skaltsa et al. 1994), or epicuticular waxes (Steinmuller and Tevini 1986, Barnes et al. 1996) may indirectly alter competitive interactions within plant communities. For example, following UV-B-exposure, quantity of wax on adaxial leaf surfaces has been shown to increase (Steinmuller and Tevini 1985), decrease (Barnes et al. 1996), or remain unchanged (Gordon et al. 1998). Removal of the surface waxes from some species (Clark and Lister 1975, Karabourniotis et al. 1992), but not others (Bornman and Vogelmann 1988, Day et al. 1992) has been shown to influence the amount of U V - B radiation penetrating into 16 foliage. It has been suggested that increased amounts of epicuticular waxes may attenuate UV-radiation through reflection or scattering (Steinmuller and Tevini 1986). However, epicuticular wax was not considered to be an important factor in acclimation of meadow salsify (Tragopogon pratensis L.) seedlings to U V - B radiation (Furness et al. 1999) as removal of epicuticular wax did not influence growth in response to U V - B radiation. Protection offered by leaf trichomes against UV-B-induced injury could be important, particularly until the underlying cells develop avoidance or repair mechanisms (Karabourniotis et al. 1995, Karabourniotis and Fasseas 1996). For the majority of species however, UV-B-reflectance from the leaf surface is less than 5% (Caldwell 1981), and most of the attenuation is the result of absorption and scattering within epidermal tissues (Caldwell 1971, Gausman et al. 1975). In the few species where leaf UV-reflectance is sizeable, the leaves do not selectively reflect U V radiation, but also reflect a large proportion of the visible radiation (Mulroy 1979). Root competition for soil moisture and nutrients is often more severe than above-ground competition for light (Caldwell 1987). Changes in shoot: root ratios might change competitive effectiveness of individual species for soil moisture and nutrients (Tilman 1988). Both positive and negative effects of enhanced U V - B radiation on shootroot ratios have been reported (Sullivan and Teramura 1989, Sullivan and Teramura 1992, Ziska and Teramura 1992, Barnes et al. 1993, Ziska et al. 1993, Sullivan et al. 1994). The consequences of UV-B-induced shifts in shoot:root ratios are currently a matter of speculation (Caldwell 1997). However, information regarding root interactions and responses is necessary in order to fully evaluate changes in competitive relationships in response to physicochemical stresses (Gold and Caldwell 1983). While some predictions concerning interspecific competitive ability can be made from the relative morphological responsiveness of individual species to elevated levels of U V - B radiation (Caldwell 1997), response of a single plant grown in isolation may not accurately reflect response of that plant when grown in association with other plants. Generalizations concerning the relative UV-B-induced advantages or disadvantages for crop plants in competition with weeds cannot be made based on the results of experiments to date (Caldwell 1997). 17 1.3.2 UV-B radiation and intraspecific competition The influence of U V - B radiation on intraspecific competition may be less consequential than the effects on interspecific competition since growth changes would likely be more uniform among individuals of the same species and no net change in competition would be expected (Barnes et al. 1988). If morphological changes consist of altered biomass allocation, such as reduced internode elongation, then monoculture productivity may be unaffected by increased exposure to U V - B radiation (Barnes et al. 1988, Barnes et al. 1990b, Barnes et al. 1995). Size inequality develops in dense monospecific stands as a result of asymmetric competition for light. Because light is largely a unidirectional resource, plants with a slight height advantage shade shorter individuals, while shorter plants have little influence on the taller ones (Weiner 1990). An initially small height advantage can therefore translate into considerable advantage over time (Caldwell 1997). The phytochrome system tends to counteract the tendency for size inequality to develop in dense populations (Ballare 1999). In plant associations, red light intercepted by phytochrome in the upper canopy tends to reduce internode length while far-red light resulting from shade deeper in the canopy tends to increase internode length. The U V - B photoreceptor causes some morphological changes similar to those elicited by phytochrome (Caldwell 1997). Therefore, elevated levels of U V - B radiation might also function to reduce size inequality in monocultures (Searles et al. (unpublished observations) cited from Caldwell 1997). 1.4 Evaluating competitive impacts on plant growth 1.4.1 Replacement series analysis Information concerning the influence of U V - B radiation on competitive relationships (Fox and Caldwell 1978, Bogenrieder and Klein 1982, Gold and Caldwell 1983, Barnes et al. 1988) is based primarily on experiments carried out using the replacement series design formalized by de Wit (1960). The replacement series is a model of plant competition in which plants compete directly for space and limited resources within that space. The experimental design maintains a constant overall plant density while varying the proportion of the two species present within a given area. The competitiveness of a species relative to 18 the other species in a binary association is evaluated by comparing its performance in a mixture to its performance in a monoculture of the same density. Numerous criticisms, however, have since cast serious doubt as to the appropriateness of the replacement series design as a method for studying plant competitive interactions (Jolliffe etal. 1984, Connolly 1986, Snaydon 1991, Snaydon 1994, Gibson etal. 1999, Jolliffe 2000). For example, Connolly (1988) expressed concern that when components in a binary mixture differ greatly in size, this approach may lead to consistent bias in favour of the component with larger individuals. Connolly and Nolan (1976) rejected the replacement series method in studies of mixed grazing of cattle and sheep due to the possible distorting or confounding of density effects occurring as a result of the replacement series consisting of points on a single line (one dimension) in a two-dimensional density plane. Mead (1979) claimed that the replacement method confounded the densities and spatial arrangements of the two component species by varying proportions of the two species while holding the overall crop density constant. Competitive outcome was found to be dependent upon the arbitrarily chosen experimental density (Inouye and Schaffer 1981). Firbank and Watkinson (1985) criticized the method on the basis that it confined attention to one density only. Jolliffe et al. (1984) noted that conventional replacement series experiments do not assess intra- and interspecific interference separately. 1.4.2 Inverse yield-density relationships Inverse yield-density relationships address many of the limitations of the replacement series design, and therefore provide a more suitable technique for a first approximation of competitive interactions (Wright 1981, Spitters 1983). These models also allow separation of intra- and interspecific components of interference (Spitters 1983). Influence of interception of photosynthetically active radiation (PAR) and/or U V - B radiation on competition for light at the canopy level may also be studied using a modified inverse yield-density model (Dr. P. Jolliffe, Professor, Faculty of Agricultural Sciences, U.B.C. , Pers. comm.). Monocrop yield increases (on a per unit land area basis) with increased population densities. The increases become progressively smaller at higher population densities until no further increase is evident (Harper 1977, Silvertown 1987, Shinozaki and Kira 1956 [cited from Firbank and Watkinson 1990]). The asymptotic nature of the relationship may reflect 19 both density-dependent mortality and plasticity in size of individuals within dense populations (Wiener 1988) and represents the limit on total biomass formation given the amount of resources available for crop growth. Yield-density models consistently provide good descriptions of such yield-density responses (Jolliffe 1997). Yield-density relationships are most often explored on a per plant basis (y) because population density (X) is not an independent variable in relation to yield per land area (Y): Y = yX (Willey and Heath 1969). Inverse mathematical forms have been commonly used to describe the resulting reduction in individual plant yield with increasing plant density (Kira etal. 1953, Wright 1981, Jolliffe 1988, Jolliffe 1997). The reciprocal yield equation introduced to describe the density-dependence of monoculture yield per plant (Shinozaki and Kira 1956) was expanded to include the effects of additional species (Wright 1981, Spitters 1983). For example, in a binary mixture of species denoted by subscripts c (crop) and w (weed), inverse yield-density functions link mean yield per plant (y) to species population density (X): First and second letters within a pair of subscripts designate the target species and the species that exerts an influence on the target species, respectively (Jolliffe and Wanjau 1999). Values of function parameters contain biologically relevant information: acc or aww represent intraspecific influences, acw or awe interspecific influences, and 1/aco or 1/awo growth potential per plant in the absence of competition. The balance of inter- to intraspecific competitive influences is indicated by the substitution rate (acw/acc o r awc/aww) (Spitters 1983). A population density interaction term (XcXw or XwXc) may be included in equation 1.1 or 1.2 if the intraspecific competitive influences of the target species are altered by the presence of the companion species (Jolliffe 1988, Shainsky and Radosevich 1991, Blackshaw and Schaalje 1993). In the inverse scale of measurement however, such interactions generally prove to be insignificant (Jolliffe 1997) and simple inverse yield-density models are usually applied without additional terms. In addition, slopes of the yield-density responses, as determined by the values of the regression coefficients (acc, acw, aww and awe), can be used as a measure of the intensity of competition (yield loss due to increased density or proximity of competitors) (Welden and yc _ aco + accXc + acwXw yw 1 = awo + awwXw + awcXc [1.1] [1.2] 20 Slauson 1986). The multiple coefficient of determination for a yield-density regression can be used to infer the importance of competition relative to other factors including microclimate, genetics, diseases, and site-specific variations (Shainsky and Radosevich 1991). Similarly, partial coefficients of determination for each density component can be used to establish the relative importance of each species' density. 1.4.3 Conventional plant growth analysis Competitive relationships can be further defined using methods of plant growth analysis. Conventional plant growth analysis transforms simple observations of the sizes of plants or plant parts into growth indices. It is, fundamentally, a time-based analytical model of growth (Jolliffe et al. 1982). Some plant growth indices such as crop growth rate, relative growth rate, and net assimilation rate require data from several harvests (Warren Wilson 1981). Other commonly used growth indices which can be constructed from observations taken during a single or several harvests include specific leaf area or specific leaf weight, leaf weight ratio, leaf area ratio, leaf area index, and the shoot: root ratio (Hunt 1982). These indices are then used to interpret the data. Several comprehensive reviews of conventional plant growth analysis are available (Evans 1972, Causton and Venus 1981, Hunt 1982). Conventional plant growth analysis facilitates assessment of plant responses using the simple indices of growth and productivity, emphasizing physiological and morphological sources of variation in growth (Warren Wilson 1981, Hunt 1990). For example, specific leaf weight (leaf dry weight/leaf area) is a growth index commonly used to measure leaf thickness and/or leaf density responses to U V - B radiation (Tevini and Teramura 1989, Bornman and Vogelmann 1990). UV-B-induced shifts in biomass allocation and leaf morphology of bean plants (cv. Label) were detected using plant growth analysis (Deckmyn and Impens 1995). Shifts in biomass partitioning to various plant organs may occur at differing population densities (Jolliffe and Gaye 1995), as well as in response to environmental conditions. While the use of plant growth indices compared with the primary measures reduces the information available in the primary data set, growth indices can in some instances conveniently summarize and express essential attributes of plant behaviour (Hunt 1982). Key features of growth addressed by conventional plant growth analysis encompass the efficiency and extent of assimilatory systems, and the duration and partitioning of growth (Jolliffe et al. 21 1982). The conceptual structure provided by conventional plant growth analysis provides a crucial link between primary measures of plant growth and crop performance (Jolliffe and Courtenay 1984). 1.4.4 Allometric relationships Allometric analysis explores quantitative relationships between different measures of an organism, and serves as a tool to facilitate insight into structural and functional relationships between different levels of organization (Jolliffe et al. 1988). The importance of allometry in biological organisms has been widely recognized following the works of Thompson (1917, 1942) and Huxley (1932). Allometric adjustments may occur in plants in response to environmental and/or competitive influences (Tilman 1988, Weiner 1990). For example, above- or below-ground environmental stresses can cause shifts in root: shoot partitioning in favour of the affected part of the plant (Hunt 1988). Allometric analysis showed that in increasingly competitive environments, lamb's-quarters (Chenopodium album L.) allocates relatively more biomass to stems than to leaves (Rohrig and Stutzel 2001). Regression models based on the simple, bivariate power function are commonly used to establish allometric relationships (Huxley 1932): y = az p [1.3] where y and z are two measures of an organism, or part of an organism, a is the allometric coefficient and p is the allometric exponent. It should be noted that a is strongly influenced by the arbitrary choice of the scale of measurement for z since it is the value of y when z equals (1). Parameter (3 is of greater biological interest, as it evaluates proportional changes and is not affected by the scale of measurement (Smith 1980). It can be used to evaluate intensity of differential variation between growth of two different plant parts (Caradus et al. 1995). Changes in biomass partitioning due to treatment effects will be manifested in p. Parameter P represents the slope of the double-logarithmic regression between ln(y) and ln(z). Therefore, for ease of interpretation, a ln-transformation is usually applied to equation 1.3 to produce a straight-line relationship with ln(a) as the y-intercept and p as the slope of the line. Statistical assumptions of normality and homogeneity of variance may also be fulfilled through ln-transformation of the biological data: 22 ln(y) = ln(a) + Bln(z) + ln(s) [1.4] where ln(s) accounts for residual variation in ln(y) not explained by l n ( o t ) , p or ln(z). A n expanded allometric equation developed by Jolliffe et al. (1988) enables responses of ln(y) to additional experimental factors to be evaluated. For example, the allometric equation can be expanded to account for the effects of crop (Xc) and weed ( X w ) population densities and level of U V - B radiation (UV-B) on ln(y): ln(y) = In (a") + p0ln(z) + PiUV-Bln(z) + p 2 X c ln(z) + p 3 X w ln(z) + [1.5] p 4 UV-BX c ln(z) + p 5 UV-BX w ln(z) + p 6 X c X w l n ( z ) + p 7 U V - B X c X w l n ( z ) + y,ln(UV-B) + y 2 ln(X c ) + Y 3 ln(X w ) + y 4 ln (UV-BX c ) + y 5 l n (UV-BX w ) + Y 6 l n ( X c X w ) + Y 7 l n ( U V - B X c X w ) + ln(s') The constant of the expanded allometric equation [ln(a')] is derived by grouping allometric and non-allometric sources of variation. This model allows separation of allometric responses (terms containing P), from non-allometric responses (terms containing y) due to treatment effects on ln(y). Through the use of the expanded allometric equation 1.5, population density was shown to affect allometric relationships of orchardgrass (Dactylis glomerata L.) and timothy (Phleum pratense L.) (Jolliffe et al. 1988). Allometric and non-allometric adjustments in response to population density and fertilizer level have been reported for mixtures of beans (cv. Contender) and beets (Beta vulgaris L. cv. Ruby Queen) (Wanjau 1998). 1.5 Studying plant response to UV-B radiation under greenhouse conditions Sensitivity to U V - B radiation is highly dependent upon experimental conditions (McLeod 1997). The diverse range of experimental protocols and methodologies used by different investigators therefore complicates the assessment of overall effects of elevated U V - B radiation on crops (Caldwell et al. 1995). For example, while response to U V - B radiation in greenhouse experiments has been found to compare well with relative performance in the field in some cases (Lydon et al. 1986, Barnes et al. 1988), plants grown in growth chambers or greenhouses are generally more sensitive to U V - B radiation than field-grown plants (Caldwell 1981, Dai et al. 1997). Contradictory results might be caused by methodological 23 differences, including levels of U V - B radiation, PAR, and interactions with other environmental factors (Jansen et al. 1998). Radiation emitted from artificial U V - B sources does not match the solar U V - B spectrum (Adamse and Britz 1992a) and must therefore be weighted using an appropriate action spectrum for comparison with expected effects of stratospheric ozone depletion (Caldwell et al. 1995). Due to the wavelength-dependency of biological responses to UV-B radiation, small differences in spectral irradiance may result in large differences in biologically effective irradiance. Biological action spectra, such as Caldwell's (1971) generalized plant damage action spectrum are assumed to be adequate weighting functions. Apart from action spectrum considerations, it is important in greenhouse experiments to maintain a realistic balance between different spectral regions since both U V - A radiation (315-400 nm) and P A R (400-700 nm) interact with U V - B radiation (Greenberg et al. 1997). When the P A R : U V - A : U V - B ratio is maintained at a level close to that found in solar radiation (approximately 100:10:1) many plants grow without signs of overt U V - B stress. In growth chamber and greenhouse experiments however, the visible and U V - A radiation is usually much less than in sunlight. Thus, even if realistic levels of U V - B radiation are used in simulating ozone reduction the plant response may be exaggerated relative to field conditions (Adamse and Britz 1992b). Another problem sometimes observed at low PAR levels is a lack of adequate acclimation to U V - B radiation, resulting in increased UV-B-sensitivity (Warner and Caldwell 1983, Mirecki and Teramura 1984, Tevini et al. 1989, Cen and Bornman 1990). Leaves expanding under high PAR are often thicker and contain more UV-absorbing pigments on a leaf area basis than those grown under low PAR (Caldwell et al. 1994). Low 9 1 PAR levels (<500 pmol m" s") may be inadequate to support flavonoid synthesis (Cen and Bornman 1990). Alternatively flavonoid synthesis may be impaired by U V - B radiation under low light conditions. Inhibition of photosystem II is exacerbated under a combination of low P A R and enhanced U V - B radiation (Cen and Bornman 1990). Leaf reflectivity measurements showed that the amount of PAR able to penetrate leaves of UV-B-treated plants under these conditions was reduced. It was suggested that a possible correlation may exist between the reduced PAR levels, loss of chlorophyll and lowered photosynthetic activity. 24 Solar radiation regimes in the field likely allow plants to undergo repair processes (Teramura et al. 1990) to a greater extent than in growth chambers and greenhouses. It has been demonstrated that higher amounts of U V - A radiation can ameliorate the effect induced by U V - B through the operation of photo-reactivation to restore pyrimidine bases to their normal structures, and other D N A repair mechanisms (Middleton and Teramura 1993). There are also indications that photosynthesis might be stimulated to a small degree by U V - A radiation since chlorophyll and accessory photosynthetic pigments absorb in the U V - A range (Caldwell 1981). If this radiation reaches mesophyll tissue it can drive the photosynthetic reaction. Plants growing under field conditions typically respond to several stress factors acting in concert (Caldwell et al. 1995). However, a significant portion of the stress complex is usually removed or altered in greenhouse-grown plant communities. It is important to'no.te that the overall effectiveness of U V - B radiation can be either greatly aggravated or ameliorated by factors present in the stress complex. For example Sullivan and Teramura (1990) demonstrated in a field study that UV-B-mediated reductions in photosynthesis and growth, observed in well-watered soybeans, were absent in water-stressed plants. A similar masking effect was found in studies of UV-B-irradiated, mineral deficient plants (Murali and Teramura 1985). Such interactions suggest that the impacts of U V - B radiation on plant growth might be greatest in well-watered, fertile environments. Despite inherent limitations, greenhouse studies can serve an important and necessary role in the initial and exploratory phases of U V - B research (Adamse and Britz 1992a) and provide insight into the potential responses to U V - B radiation (Dr. J. Sullivan, Pers. comm.). For example, greenhouse studies have proved useful in determining the relative sensitivities of different cultivars and species to U V - B radiation (Dai et al. 1997, Furness et al. 1999). Response of below-ground systems to U V - B radiation, difficult to investigate in field studies, is also facilitated in greenhouse experiments with container-grown plants. Attention has often focused on less complex greenhouse-grown communities (Gibson et al. 1999) due to difficulties in establishing the role of plant-plant interactions in determining community structure and dynamics in the field (Strong et al. 1984, Connell 1990). By diminishing some components of interference, greenhouse studies provide the opportunity to explore competitive interactions under more clearly defined conditions than 25 those found in field situations. Advantages of these studies include a high degree of experimental control, repeatability, precision, and amenability to rigorous statistical design (Harper 1983, Hairston 1989). Such controlled greenhouse studies facilitate mechanistic interpretation rather than simple phenomenological observation (Tilman 1987, Stiling 1992). In fact, unless plant interactions can be demonstrated in less complex greenhouse environments, they are unlikely to be of consequence in more complex natural situations (Gibson et al. 1999). While interpretation of enhanced U V - B radiation in an ecological context requires consideration of other components in the stress complex, greenhouse experiments can provide valuable information on underlying mechanisms and processes of U V - B action on competitive relationships (Caldwell et al. 1995). 26 2.0 Influence of UV-B radiation on growth and morphology of weed, vegetable, and broccoli cultivar seedlings. 2.1 A B S T R A C T U V - B radiation effects on seedling growth and morphology, and relative UV-B-sensitivity of common agricultural weeds, vegetable crops, and broccoli cultivars were investigated to identify a suitable crop:weed species pair for use in subsequent competition experiments. Seedlings were grown at 0, 7, and 11 kJ m"2 d"1 of biologically effective ultraviolet-B ( U V - B B E ) radiation (290-315 nm) in a greenhouse for 6 weeks. Weed, vegetable crop, and broccoli cultivars had differential morphological and growth responses to UV-B-enhanced environments. Relative sensitivity of plants was ranked using a formula based on percent changes in stalk dry weight and leaf area in response to U V - B radiation treatments. Among the weeds, common chickweed (Stellaria media (L.) Vill .) was the most sensitive to U V - B radiation, and redroot pigweed (Amaranthus retroflexus L.) and lady's-thumb (Polygonum persicaria L.) the least. Lamb's-quarters (Chenopodium album L . ) was intermediate in sensitivity to U V - B radiation. Beet (Beta vulgaris L . cv. Early Wonder) was the most, and cabbage (Brassica oleracea var. capitata L . cv. Christmas Drumhead) the least sensitive vegetable species to U V - B radiation. Broccoli (Brassica oleracea L . var. italica) was intermediate in U V - B sensitivity for the vegetable crops studied, and the cultivar Purple Sprouting among the most sensitive of the broccoli cultivars. Differential morphological sensitivity to U V - B radiation may alter ability to intercept photosynthetically active radiation, thereby shifting competitive relationships among weeds and associated crop species. Broccoli cv. Purple Sprouting and lamb's-quarters were chosen as the crop and weed species for use in subsequent competition experiments based on growth and morphological sensitivity to U V - B radiation, agricultural relevance of these species, and uniform seedling emergence and similar seedling growth. 2.2 INTRODUCTION During the past two decades, release of anthropogenic pollutants into the upper atmosphere has 27 resulted in significant reductions in the stratospheric ozone layer over much of the earth (WMO 1995). This decline is expected to increase in magnitude into the 21 s t century (Caldwell and Flint 1994, W M O 1995). Increased ultraviolet-B (UV-B 290-315 nm) radiation reaching the Earth's surface as a result of the decline in stratospheric ozone levels could have potentially adverse effects on agricultural productivity (Corriea et al. 1999). Elevated levels of U V - B radiation have been reported to damage the photosynthetic apparatus in a number of plant species (Greenberg et al. 1997). However, the elevated levels of U V - B radiation predicted to reach the Earth's surface are more likely to influence plant growth and morphology than to directly reduce photosynthetic activity (Barnes et al. 1990b, Allen et al. 1998). UV-B-induced morphological changes include change in leaf shape (Wilson and Greenberg 1993), and reduction in plant height (Barnes et al. 1993), leaf insertion height (Barnes et al. 1988), and leaf area (Allen et al. 1998). Sensitivity to U V - B radiation varies greatly among plant species (Tevini et al. 1981, Teramura 1983), as well as among cultivars of the same species. For example, intraspecific differences in growth and yield in response to U V - B radiation have been reported among cultivars of soybean (Glycine max L. Merr.) (Teramura and Murali 1986, Reed et al. 1992), rice (Oryza sativa L.) (Dai et al. 1994), and maize (Zea mays L.) (Correia et al. 1999). Inherent ability to rapidly accumulate and maintain high levels of UV-B-absorbing compounds has been suggested to contribute to differential inter- and intraspecific sensitivity to U V - B radiation in some studies (Gonzalez et al. 1996), but not in others (Dumpert and Boscher 1982, Smith et al. 2000). Differential sensitivity to U V - B radiation has also been attributed to numerous morphological and anatomical features, such as inherent differences in leaf surfaces, leaf thickness and canopy architecture (Murali et al. 1988). Additionally, differential UV-sensitivity may be partly due to the dependence of the response upon the experimental environment (McCleod 1997). Differential UV-B-induced shifts in plant morphology at the seedling stage could play a pivotal role in shifting long-term relationships among plant species by altering their ability to compete for resources. For example, Gold and Caldwell (1983) suggested that changes in above-ground biomass under enhanced U V - B radiation may in part result from a shift in the 28 allocation of plant resources between above- and below-ground biomass rather than a decrease in total biomass. A shift in biomass allocation to the roots might change competitive effectiveness of individual species for soil moisture and nutrients. Barnes et al. (1988) showed that shifts in the competitive balance between wheat (Triticum aestivum L.) and wild oat (Avena fatua L.) were associated with differential effects of U V - B radiation on shoot morphology, which altered the relative positioning of leaf area for the two species in the mixed canopies. In dense canopies, even subtle effects on internode elongation and/or leaf size can change light interception and canopy photosynthesis of competing species (Barnes et al. 1990a, Ryel et al. 1990). A change in the competitive balance between crop and weed species, as a result of decreased ability of one of the species to intercept light, could have far-reaching consequences for agricultural systems. The objectives of this study were to: 1) characterize the influence of U V - B radiation on seedling growth and morphological features that may influence competitive ability in a variety of weeds, vegetable crops, and cultivars of a selected crop, 2) compare their sensitivities to U V - B radiation, and 3) identify a weed and vegetable crop species (including a specific cultivar) for use in subsequent competition experiments. 2.3 M A T E R I A L S A N D M E T H O D S 2.3.1 Seed source and plant culture Redroot pigweed (Amaranthus retrojlexus L.), shepherd's-purse (Capsella bursa-pastoris L.), lamb's-quarters (Chenopodium album L.), lady's-thumb (Polygonumpersicaria L.), and common chickweed (Stellaria media (L.) Vill .) seeds were collected from field populations in southern British Columbia. Green foxtail (Setaria viridis L.) seeds were collected from Lethbridge, Alberta. Vegetable seeds were obtained from the following sources: Johnsons Seeds (Boston, Lines, UK) , McKenzie Seeds (Brandon, MN), West Coast Seeds Ltd. (Delta, BC), Pacific Northwest Seeds (Vernon, BC), and Stokes Seeds Ltd. (St. Catharines, ON). Seed sources for individual vegetable species and cultivars are listed in Table 2.1. Seeds were sown in 6.4 cm diameter pots containing 90 % peat moss-perlite mixture, 10 % mineral soil, and a slow-release fertilizer (Osmocote Plus fertilizer, Scotts-Sierra Table 2.1. Common names, species, cultivars, and seed sources for vegetable crops. Common name Species Cultivar Seed source Beet Beta vulgaris var. esculenta L. Cylindra Early Wonder Johnsons Seeds3 McKenzie Seeds Broad bean Vicia faba L. Aquadulce Johnsons Seeds Broccoli Brassica oleracea var. botrytis Brassica oleracea var. italica Arcadia Emperor Everest Legend Patriot Purple Sprouting White Sprouting Stokes Seeds Ltd. Stokes Seeds Ltd. Stokes Seeds Ltd. Stokes Seeds Ltd. Stokes Seeds Ltd. West Coast Seeds Ltd. Johnsons Seeds Bush Bean Phaseolus vulgaris L. Contender Pacific Northwest Seeds Cabbage Brassica oleracea var. capitata Christmas Drumhead Johnsons Seeds Corn Zea mays subsp. mays Northern Super Sweet Pacific Northwest Seeds Cucumber Cucumis sativus L. Straight Eight McKenzie Seeds Kohlrabi Brassica oleracea var. gongylodes Purple Vienna Johnsons Seeds Leaf beet Beta vulgaris L. var. cicla Perpetual Spinach Johnsons Seeds Lettuce Lactuca sativa L. Little Gem Cos Lojoits Green Cos Saladin Johnsons Seeds Johnsons Seeds Johnsons Seeds Pac Choi Brassica rapa var. chinensis Pueblo Johnsons Seeds Radish Raphanus sativus L Mooli Mino Early Johnsons Seeds Spinach Spinacia oleracea L. Long Standing Bloomsdale McKenzie Seeds Tomato Lycopersicon esculentum M i l l . Cuor Di Bue McKenzie Seeds aSeed source locations are given in Section 2.3.1. 30 Horticultural Products Co., Marysville, OH, US). Weeds were grown from seed for six weeks under U V - B radiation treatments described below. High-pressure sodium lamps (Philips Lighting Co., Somerset, NJ, US) (16 h photoperiod), provided approximately 15 umol m"2 s"1 supplemental photosynthetically active radiation (PAR). Weekly PAR measurements were taken at noon with a LI-COR LI- 185B portable light meter (Li-Cor Inc., Lincoln, N B , US) (Table 2.2). Minimum and maximum temperatures were recorded weekly for each experiment (Table 2.2). 2.3.2 UV-B radiation treatments U V - B treatments were provided by ten UVB-313 40W fluorescent tubes, installed 1.10m above the greenhouse bench, in a 1.20 m long x 1.20 m wide x 1.25 m high frame enclosed with Mylar film (Type D, 0.127 mm thick) (Cadillac Plastics Ltd., Burnaby, BC, CN). Within this frame, smaller frames (60.0 cm x 32.5 cm x 32.0 cm high) were covered with either a layer of Mylar film, or one or two layers of cellulose acetate film (diacetate type, 0.127 mm) (McMaster-Carr, New Brunswick, NJ, US). Mylar film absorbs UV-radiation below 320 nm and cellulose acetate below 290 nm (Barnes et al. 1988). The UVB-313 fluorescent tubes were pre-burnt for a minimum of 72 h prior to experiments to stabilize U V - B radiation output; total spectral irradiance of a single unfiltered UVB-313 lamp has been shown to decline by 20 % during the first 70 h of burning (Adamse and Britz, 1992a). Cellulose acetate filters were exposed to U V - B radiation for at least 10 h prior to experiments. Absorbance by cellulose acetate of wavelengths above 285 nm increases by approximately 75 % of the initial value during the first 10 h of U V - B exposure. (Adamse and Britz, 1992a). U V - B radiation (290 - 315 nm) was measured in single nanometer increments using an International Light IL1700 Radiometer interfaced with an IL782A double-slit monochromator (Harvard Apparatus, St. Lauren, PQ, CN). Single nanometer readings were taken in an experimental chamber covered with opaque black plastic to exclude interference from visible light. Biologically effective U V - B (UV-BBE) radiation was estimated from these readings using o CJ 00 CJ O fi < 00 cj > < fi ^ .0. CO CS o tH CJ > o cj Q c fi CJ CL X W in r o 00 c n CN m CN 00 CN c n CN c n c n c n in c n •4 (N 00 CN t> CN CN CN CN CN CN CN 00 00 00 CN CN CN CN CN CN OO 00 00 O r--O O CN 0 CN ON 0 0 r--O in O in CN 0 m c n O U0 in 0 00 c n 0 m O O in 0 CN fi 1—s l-l CL < •a CJ CJ C/3 00 < I 3 3 CJ fi 3 +-» CJ 00 cj > 1 CJ VH X> CJ fi-s CJ * H -S 0 CL (-) CJ ^ 1 CJ 3 fi 00 CJ HH <, CJ 00 3 t SH 3 32 Caldwell's (1971) generalized plant damage action spectrum normalized to 300 nm. Integrated estimates of U V - B between 290 and 315 nm were obtained under light conditions with an International Light SED240 Solar Blind Bulk Sensor (Harvard Apparatus, St. Lauren, PQ, CN). Relationships between bulk sensor readings and UV-BBE estimates obtained using the monochromator were determined through regression analyses. Daily UV-BsE-exposure for 8 h, centered around solar noon, provided approximately 0 (control), 7 (low), and 11 (high) kJ m 2 d"1 of UV-BBE radiation in frames covered with a layer of Mylar film, and two, or one layer(s) of cellulose acetate film, respectively. According to the model of Bjorn and Murphy (1985), 7 and 11 kJ m" d" UV-BBE radiation represent ozone depletion scenarios of 18 and 30%, respectively, for Vancouver BC at summer solstice. Since greenhouse glass attenuates almost all solar U V - B radiation, 0 kJ m" d" UV-BBE radiation was used as the control in these experiments. 2.3.3 Growth and morphological response to UV-B radiation Plant (main shoot) height (H) and leaf number were recorded at harvest. Roots were gently washed and blotted dry on paper towels. Leaf (lamina) area (LA) was measured using a LI-COR LI-3000 portable leaf area meter (Li-Cor Inc., Lincoln, N B , US). Roots, stalks (stems and petioles), and leaves were dried at 70 C for 72 h and weighed. Dry weights of individual plants were used to construct the following growth indices: specific leaf weight (SLW) [lamina dry weight/leaf area (g m")], leaf weight ratio (LWR) [lamina dry weight/shoot dry weight], leaf area ratio (LAR) [leaf area/shoot dry weight (cm2 g"1); L A R = LWR x SLW" 1], and shoot.root dry weight ratio (SRR) (Hunt 1990). A completely randomized design with 8 replicates (one plant per replicate) was used in the weed study. Due to the larger size of the vegetable study, a randomized block design was used with two blocks and two replicates per block (one plant per replicate). Primary growth data generated by these experiments met the assumptions of normality and homogeneity of variance. A l l experiments were repeated in time. 'Experiment x treatment' interactions were absent; therefore data from two experiments were pooled. 33 A completely randomized experimental design with six replicates (one plant per replicate) was used for the broccoli cultivar experiment. The experiment was repeated in time. However, data from the two experiments were analyzed separately due to significant interactions between experiments for some of the treatment effects. Primary data were transformed to natural logarithms prior to analysis of variance to achieve homogeneity of variance. Growth indices were constructed from natural logarithm-transformed primary data. Data presented are untransformed means. Values in weed, vegetable, and broccoli cultivar graphs are expressed as percent of control to facilitate comparisons among treatments. Statistical analyses, however, were performed on the measured, quantitative data. The following orthogonal contrasts (P = 0.01, and P = 0.05) were carried out using SYSTAT 9.0 (SYSTAT Inc. Evanston, IL, US) software. Contrast 1: Control versus U V - B treatments (0 versus (7+11 kJ m"2 d"1 UV-B B E )/2)) Contrast 2: Low versus high U V - B treatment (7 versus 11 kJ m" d" UV-BBE) 2.3.4 Relative seedling sensitivity to UV-B radiation Based on a ranking model advanced by Lydon et al. (1986), a measure of relative sensitivity to U V - B radiation was developed. Average percent changes in stalk dry weight and leaf area per plant were summed to construct sensitivity indices (SI) for control versus U V - B and for low versus high U V - B comparisons as follows: Slcont vereiiv UV-B-treated - ((((WS(|ow UV-B) + Ws(|,igh UV-B))/2)- Ws(cont))/ WS(Cont)) x 100) + P-1A] ((((LA(i„w uv-B) + LA(high uv-B ) ) /2) -LA ( c o i l t ) ) /LA ( c o n t ) x 100) SIlow remix high UV-B treated = ((Ws(high UV-B) " W S(| 0 W UV-B)V W*s(|0w UV-B) X 1 00) + [2. IB] ((LA(higi, uv-B) - LA(| 0 W uv-B))/LA (| 0 W uv-B) X 100) Here, Ws = stalk dry weight, L A = leaf area, and subscripts cont, low U V - B , and high U V - B are 0, 7, and 11 kJ m"2 d"1 U V - B B E radiation, respectively. 34 Using the sensitivity indices, weed, vegetable, and broccoli cultivar seedlings were classified as relatively insensitive (SI > -50), intermediately sensitive (SI between -51 and -100), or sensitive (SI < -101) to U V - B radiation. These values were chosen to facilitate identification of the most and least sensitive species by visual inspection. Sensitivity indices for weed and vegetable species were calculated using data pooled from two experiments. Separate sensitivity indices were calculated for the two broccoli cultivar experiments due to significant 'experiment x treatment' interactions. 2.3.5 Scanning electron microscopy Samples (0.5 cm each) excised from the necrotic regions of UV-B-treated redroot pigweed leaves were fixed for 6 h in 2 % glutaraldehyde (J. B. E M Services Inc., Pointe-Claire, Dorval, PQ, CN) in 0.05 M sodium cacodylate (J. B. E M Services Inc., Pointe-Claire, Dorval, PQ, CN) buffer and rinsed (3x) with 0.05 M sodium cacodylate buffer. The samples were fixed in 1 % osmium tetroxide (Ted Pella Inc., Redding, C A , US) in 0.05 M sodium cacodylate buffer for 1 h, quickly rinsed with distilled water, and dehydrated for 20 min each in 30, 50, 70, 85, 95, and 100 % ethanol series. Samples were embedded with liquid CO2 (at 15 C) in a Balzers 020 Critical Point Dryer (Balzers Union, Balzer, LI). Chamber temperature was increased to 40 C to evaporate the CO2. Samples were mounted on aluminum stubs using double-sided adhesive tabs, coated with gold (about 20-30 nm thickness) using a Nanotech Semprep sputtering device (Nanotech Ltd., Prestwich, Manchester, UK) and the adaxial leaf surface was examined using a Cambridge 250T scanning electron microscope (LEO Electron Microscopy Ltd., Cambridge UK) at 20 kV. 2.3.6 Leaf greenness measurements Greenness of the second true-leaf of broccoli cultivar seedlings was measured with a Minolta SPAD-502 meter (Minolta Camera Co., Osaka, JP). The meter readings correlate highly with total chlorophyll concentration (Tenga et al. 1989). 35 2.4 R E S U L T S 2.4.1 Response of weed seedlings to UV-B radiation 2.4.1.1 Visual effects Leaf surfaces of several weed species were visibly affected by U V - B radiation. Effects included: upward cupping of leaves (at high U V - B only) and leaf bronzing in shepherd's-purse, thickening and upward cupping of cotyledons and leaves and darker green leaf colouration in common chickweed, and occasional necrotic lesions (at high U V - B only) on redroot pigweed leaves. Scanning electron micrographs of the adaxial surface of redroot pigweed leaves showed collapsed epidermal cells in the necrotic regions (Fig. 2.1 A) . No sign of UV-B-induced leaf injury was evident in other weed species. 2.4.1.2 Effects on growth parameters Weed species differed significantly in their growth response to U V - B radiation. U V - B -exposure reduced the height of green foxtail and redroot pigweed seedlings (Fig. 2. IB and Table 2.3) in the control versus U V - B contrast. In lamb's-quarters both control versus U V - B and the low versus high U V - B contrasts detected significant reductions in seedling height. Lady's-thumb seedling height was not affected by U V - B radiation. Height was not measured due to sprawling and rosette growth habits in common chickweed and shepherd's-purse seedlings, respectively. Leaf area (Table 2.3), and leaf, stalk, and root biomass (Fig. 2.2A, D, and F) of U V - B -treated common chickweed, lamb's-quarters, and shepherd's-purse seedlings were significantly lower compared with respective controls. These parameters were also lower for common chickweed and lamb's-quarters seedlings treated with the high, compared with the low, dose of U V - B radiation. Leaf area (Table 2.3) and leaf biomass for lady's-thumb and redroot pigweed seedlings, and stalk and root biomass for lady's-thumb (Fig 2.2C and E) were not significantly Fig. 2.1. Effects of U V - B radiation treatments on redroot pigweed and green foxtail seedlings. (A) a collapsed epidermal cell (arrow) on the adaxial leaf surface of a redroot pigweed leaf developed under 11 kJ m"2 d"1 of UV-BBE radiation (bar = 20pm), and (B) green foxtail seedlings developed under 0,7, and 11 kJ m"2 d'1 of UV-BBE radiation (left to right). Table 2.3. Effect of U V - B radiation on height and leaf area of weed species. Weed species U V - B dose Height Leaf area (kJ m' 2 d"1) (cm) (cm2) Common chickweed 0 _a 6 5 . 6 ± 5 . 2 b 7 - 44.4 ± 4.7 11 - 17.0 ±3 .9 Control versus U V - B Low versus high U V B ** Green foxtail 0 29.6 ±2.0 95.0 ± 14.4 7 18.5 ± 1.4 55.7 ± 7.0 11 17.4+1.9 56.5 ± 9.6 Control versus U V - B ** ** Low versus high U V B NS NS Lady's-thumb 0 16.2 ±0 .9 31.0 ± 3.8 7 14.8 ±0.8 30.0 ±2 .9 11 14.1 ± 1.0 30.5 ±2 .9 Control versus U V - B NS NS Low versus high U V B NS NS Lamb's-quarters 0 24.5 ± 0.8 49.7 ± 5.9 7 20.9 ± 0.9 46.5 ± 6.9 11 17.6 ±0 .9 26.6 ± 4.2 Control versus U V - B ** Low versus high U V B * ** Redroot pigweed 0 25.8 ± 1.1 113.1 ± 8.9 7 19.8 ±0 .7 112.3 ± 11.7 11 17.1 ±0 .8 100.2 ± 6.9 Control versus U V - B * NS Low versus high U V B NS NS Shepherd' s-purse 0 _a 88.2 ± 10.1 7 - 51.8 ± 8.4 11 - 36.5 ± 4.8 Control versus U V - B ** Low versus high U V B NS aData not recorded due to sprawling and rosette growth habits of common chickweed and shepherd's-purse, respectively. Values are means + SE of 16 replicates pooled from two experiments. °*Significant at P - 0.05, ** Significant at P = 0.01, and NS = not significant. 38 Leaf Stalk Root Leaf Stalk Root Fig. 2.2. Effect of U V - B radiation on leaf, stalk, and root dry weights of common chickweed, green foxtail, lady's-thumb, lamb's-quarters, redroot pigweed, and shepherd's-purse seedlings. Average biomass values (mg plant'1) for control seedlings are given in parentheses above respective control bars. U V - B B E treatment doses were 0 (closed bars), 7 (open bars), and 11 (shaded bars) kJ m"2 d"1 39 affected (P < 0.05) by U V - B treatments. U V - B radiation reduced leaf area and leaf biomass in green foxtail seedlings (Table 2.3 and Fig. 2.2B). Stalk and root biomass of green foxtail and redroot pigweed seedlings were reduced in U V - B treatments compared with controls (Fig 2.2. B and E). Leaf number was not affected by U V - B treatment in redroot pigweed, shepherd's-purse, lamb's-quarters, lady's-thumb, and green foxtail (data not shown). Common chickweed seedlings exposed to the high dose of UV-B-radiation had many small leaves in rosette-type clusters and it was not possible to gain an accurate leaf count for this weed. Tillering increased in UV-B-exposed green foxtail plants compared with controls (P < 0.05) (Fig. 2.1B). The tiller numbers per plant were 2.1 ± 0.3, 3.5 ± 0.4, and 3.9 ± 0.3 (means + SE of 16 replicates pooled from two experiments) for UV-BBE radiation levels of 0, 7, and 11 kJ m"2 d"1, respectively. 2.4.1.3 Effects on growth indices The control versus U V - B contrast showed that UV-B-exposed common chickweed and shepherd's-purse had greater specific leaf weight (SLW; a measure of leaf thickness and/or density) than controls (Table 2.4). In the case of common chickweed, SLW increased at the high compared with low U V - B level. SLW of redroot pigweed, lamb's-quarters, lady's-thumb and green foxtail was not affected. The leaf weight ratio (LWR; leafiness of the plant on a dry weight basis) of common chickweed increased in response to U V - B radiation in the control versus UV-B-treated contrast (Table 2.4). LWR of other weeds was not affected. The leaf area ratio (LAR; a morphological index of leafiness) decreased for common chickweed grown at the high contrasted to the low dose of U V - B radiation (Table 2.4). In shepherd's-purse, the L A R decreased in response to U V - B radiation in the control versus UV-B-treated contrast. Exposure to U V - B radiation did not alter the L A R in other weed species. The shoot: root ratio (SRR; a measure of above- versus below-ground biomass allocation) of redroot pigweed increased in the UV-B-treated compared with control contrast, as well as in the high, compared with low radiation dose (Table 2.4). The SRR increased for common chickweed grown at the high, compared with low level of U V - B radiation. The SRR of the other species was not affected. 40 Table 2.4. Effect of U V - B radiation on growth indices of weed species. Weed species UV-B dose SLW a LWR LAR SRR (kJ m"2 d"1) (g m"2) (cm2 g 1) Common chickweed 0 16.1 ± 0 . 9 b 0.48 ±0.01 314.6 ± 19.5 7.6 ± 0.4 7 15.5 ±0 .8 0.53 ± 0.02 356.2 ±20.9 5.9 ±0 .4 11 29.1 ±4.1 0.55 ±0 .02 241.8 ±28.8 8.4 ± 1.3 Control versus UV-B * c ** NS NS Low versus high UVB ** NS ** * Green foxtail 0 16.9 ±0 .6 0.49 ± 0.04 304.2 ± 34.2 8.2 ± 1.7 7 16.0 ±0 .4 0.48 ± 0.02 307.4 ±21.2 9.4 ± 1.3 11 18.4 ±0 .7 0.50 ±0.03 273.7 ± 13.8 5.9 ±0 .4 Control versus UV-B NS NS NS NS Low versus high UVB NS NS NS NS Lady's-thumb 0 23.5 ±3 .9 0.54 ± 0.02 269.1 ± 19.1 9.0 ± 0.6 7 20.0 ± 1.8 0.56 ± 0.02 308.2 ±25 .9 10.1 ±0 .9 11 19.1 ± 1.2 0.54 ± 0.02 295.0 ± 18.9 8.4 ±0.8 Control versus UV-B NS NS NS NS Low versus high UVB NS NS NS NS Lamb's-quarters 0 24.2 ± 0.9 0.51 ±0 .02 206.8 ± 13.8 10.7 + 0.9 7 20.9 ± 1.5 0.44 ± 0.03 201.2 ± 15.1 11.9± 1.2 11 24.0 ±2 .6 0.42 ± 0.03 181.2 ± 19.0 12.7 ± 1.1 Control versus UV-B NS NS NS NS Low versus high UVB NS NS NS NS Redroot pigweed 0 19.6 ±0 .7 0.54 + 0.01 279.5 ± 9.6 8.2 ± 0.6 7 19.0 ±1.1 0.61 ±0.02 335.6 ±21 .9 8.5 ±0.5 11 21.5 ± 1.6 0.61 ±0.03 302.2+ 11.8 10.0 ± 1.2 Control versus UV-B NS NS NS * Low versus high UVB NS NS NS * Shepherd's-purse 0 14.9 ±0.5 0.64 ± 0.02 434.2 ± 16.0 5.0 ±0.3 7 17.8 ±0 .9 0.63 ± 0.02 366.3 ± 17.4 5.8 ±0 .6 11 17.7 ±0 .8 0.61 ±0.03 353.1 ±24.0 5.4 ±0 .8 Control versus UV-B NS ** NS Low versus high UVB NS NS NS NS a S L W = specific leaf weight, L W R = leaf weight ratio, L A R = leaf area ratio, SRR = shoot: root ratio b Values are means ± SE of 16 replicates pooled from two experiments. c*Significant at P = 0.05, ** Significant at P = 0.01, and NS = not significant. 41 2.4.1.4 Relative sensitivity to UV-B radiation Relative sensitivity of weeds to U V - B radiation, based on cumulative percent changes in stalk dry weight and leaf area for the control versus U V - B contrast (Eqn. 2.1 A), ranked from the most sensitive to the most tolerant as follows: common chickweed, green foxtail, shepherd's-purse, lamb's-quarters, redroot pigweed, and lady's-thumb (Fig. 2.3A). Weed ranking in order of decreasing sensitivity based on comparison of the high versus low U V - B radiation effects (Eqn. 2.IB) was: common chickweed, lamb's-quarters, shepherd's-purse, redroot pigweed, green foxtail, and lady's-thumb (Fig. 2.3B). 2.4.2 Response of vegetable seedlings to UV-B radiation 2.4.2.1 Visual effects Upward cupping of cotyledons and leaves occurred in broccoli cvs. Purple Sprouting (Fig. 2.4) 2 1 and White Sprouting, pac choi and spinach in response to 7 and 11 kJ m" d" UV-BBE radiation. Bronzing of the leaf surface, necrotic leaf margins, and darker green leaf colouration in response to 7 and 11 kJ m"2 d"1 UV-BBE radiation were apparent in cucumber, bush bean, and broccoli cultivars, respectively. 2.4.2.2 Effects on growth parameters Height of several vegetable species declined in response to U V - B radiation. For example, reduction in shoot height in the UV-B-exposed seedlings in the control versus UV-B-treated contrast was detected in beet cv. Early Wonder, broad bean, broccoli cvs. Purple Sprouting and White Sprouting, bush bean, corn, cucumber, and leaf beet (Table 2.5). Exposure to high versus low U V - B radiation reduced shoot height in broccoli cv. Purple Sprouting, corn, and tomato seedlings. Height was not measured, due to a rosette habit of growth, for the lettuce cultivars. No effect of U V - B radiation on seedling height was detected in the other vegetable species. Tiller number in corn was not influenced by U V - B radiation (data not shown). Number of leaves produced declined in broccoli cv. White Sprouting, and lettuce cvs. Little Gem Cos and Lobjoits Green Cos in response to U V - B treatments (data not shown). Lettuce cvs. Lobjoits Green Cos produced less, and Saladin more leaves in seedlings grown at 42 125 -100 -75 -50 -25 0 Sensitivity Index 25 Common chickweed Green foxtail Shepherd's-purse Lamb's-quarters Redroot pigweed Lady's-thumb Common chickweed Lamb's-quarters Shepherd's-purse Redroot pigweed Green foxtail Lady's-thumb Fig. 2.3. Relative sensitivity of common chickweed, green foxtail, lady's-thumb, lamb's-quarters, redroot pigweed, and shepherd's-purse seedlings to U V - B radiation based on cumulative percent changes in stalk biomass and leaf area for the following contrasts: (A) Control versus U V - B radiation, and (B) Low versus high U V - B radiation dose. . 2.4. Upward cupping of leaves in seedlings of broccoli cv. Purple Sprouting grown at 11 kJ m"2 d"1 of UV - B B E radiation for 6 weeks. TT CO c o UH •*-» c o U CO 0 0 G T J CJ CJ CO CJ DO CJ > s w3 .2? ' S - 3 CJ J=, -*-» a o a o • i-H T J CO fc. 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OJ B 3 o 3 > & 3 OH 1 o o . 3 c o a o co cj 3 s & o CJ -CJ U X) C3 u ^ 3 1—1 c/3 O u 3 CJ CJ UH o , 0 T J O CO >-) CO CJ CJ o o 3 3 CJ CJ hJ 1—1 T3 s O O CQ S3 P Q o 3 - ° 1 3 § O ^ I 1 i CO CO OH 0< 0 0 u§ « I ^  CO Q op »_ 3 O O 3 J O CO CO .S B D . O co E-H mei noi UH I CJ DH CO X CJ T J two an B © o . U H © <+H I T J oole atP hab OH - 3 co CO CJ o "CO O UH o '2 0 0 "OH op CJ •4—* CJ UH CO H—» CJ 0 0 * CO O <+* UH o i n o W © » CJ 3 CO © 4-1 I T J CO 3 OH ed cO •4—» CO T 3 CJ UH s H—» 3 CJ ica tre alues; o alues; 3 3 alues; 0 0 cO alues; CO +-> CO * x> Q o the high, compared with low, U V - B radiation level. Leaf number was not affected by U V - B radiation in any of the other vegetable species (data not shown). U V - B radiation caused a decline in leaf area accumulation per plant in a number of vegetable species. Leaf area accumulation per plant declined in response to U V - B radiation in the control versus U V - B -treated contrast for beet cvs. Cylindra and Early Wonder, broccoli cvs. Purple Sprouting and White Sprouting, bush bean, corn, cucumber kohlrabi, leaf beet, lettuce cv. Little Gem Cos, radish, spinach, and tomato (Table 2.6). A further decrease in leaf area was detected at high versus low U V - B radiation for beet cv. Early Wonder, bush bean, corn, cucumber, leaf beet, and spinach. Leaf area of broad bean, cabbage, lettuce cvs. Lobjoits Green Cos and Saladin, and pac choi were not affected by exposure to U V - B radiation. Leaf, stalk, and/or root biomass accumulation declined in response to U V - B radiation in several vegetable species (Figures 2.5A and 2.5B). While Figures 2.5A and 2.5B represent data from the same experiments, the number of graphs presented precludes all graphs being displayed on a single page. Statistical significance of control versus UV-B-treated, and low versus high UV-B-treated contrasts were carried out at P < 0.05 for biomass data displayed in graphs. Leaf biomass declined in response to U V - B radiation in the control versus U V - B -treated contrast for beet cv. Cylindra and Early Wonder, broccoli cvs. Purple Sprouting and White Sprouting, bush bean, cucumber (Fig. 2.5A; A , B, D, E, F, and I), kohlrabi, lettuce cv. Little Gem Cos, spinach, and tomato (Fig.2.5B; A , C, H, and I). A decline in leaf biomass occurred at high versus low U V - B radiation for beet cv. Cylindra and Early Wonder, bush bean, cucumber (Fig. 2.5 A ; A , B, F, and I), leaf beet, and spinach (Fig. 2.5B; B and H). Exposure to U V - B radiation did not affect leaf biomass accumulation of broad bean, cabbage (Fig 2.5A; C and G), lettuce cvs. Lobjoits Green Cos and Saladin, and pac choi (Fig 2.5B; D, E, and F). Declines in stalk biomass in UV-B-treated versus control contrasts were detected for beet cvs. Cylindra and Early Wonder, broccoli cvs. Purple Sprouting and White Sprouting, com, cucumber (Fig. 2.5 A ; A , B, D, E, H , and I), kohlrabi, leaf beet, lettuce cvs. Little Gem Cos, Lobjoits Green Cos, and Saladin, radish, spinach and tomato (Fig. 2.5B; A , B, C, D, E, G, H , and I). A reduction in stalk biomass occurred in beet cvs. Cylindra and Early Wonder, corn, CO cS HI o U CO .3 CJ CJ CO CJ I -+-» CJ 00 CJ > C*H o s— s S3 CJ t H cs <HH u C o fi o 'I '•B a I-I m i > <HH o CJ o c CJ c H H V O CN cj CJ CO O PQ I > pa i > ID DO S S» CQ ? 5 o U X ! 60 * CO * * 2 o H J fi o U co CJ o, co CJ 1 H-» CJ 60 CJ > O0 t 1/1 W 73 2 * 2 £ 2 00 2 * 00 2 M CO W 71 W CO £ 2 2 2 2 * 2 * * * 2 * * „ O0 00 00 „ „ * 2 2 2 * * 0 0 CO Os CO 0 0 Os CO in r» o o o CN vd in f-- •4 •4 in od od Os © •4 +1 -H -H -H -H -H -H -H -H -H 41 41 41 41 41 41 41 41 © CO r - CN Os — : CO VO oo oo CO CO H H CO H H in CN CN CN vd 0 0 Tl- in CN m r - vd oo r - in CO CO r - v© o so CO in r -CN CN oo CN in CN Os oo CN CO r - r - SO o •"fr CO i> Os *fr so so CO •4 1—H CO CO © Os in in CO CO -H -H -H -H -H -H -H -H -H -H 41 41 41 41 41 41 41 41 sq i> v© CO CN Os CN CN o CN v© oo r - Os oo oo oo CN CO od CO oo in ^d in Os SO o CN CN m Os SO SO Os oo CN uo in r» SO r-~-od CO CO o 01 © CN Os © CN CN Os 0 0 CN so CO 0 0 in CN in CO < j *© Os Os CO Os m l - H -H -H -H -H -H -H -H -H -H 41 41 41 41 41 41 41 41 41 m O v© CO — i 0 0 oo v© so "fr in Os CO Os «™H os co © m •4 o •4 r-« in 0 0 4 in m CN o SO Os SO Os CO in Os in m r-»' O od o od cS HI e CJ o CJ ~0 ^ < .3 « CJ CJ OH ^ CJ •H* C o U H3 —I 3 IT C CJ CJ CJ O HI o o o o o o o o pa pa PQ X l pa pa pa •3 „ CJ P ^ C 00 Q § 2 •i e CJ ° oo 2 £ I ccj O u u X l ccj °P 2 CJ • I—I > XI •a H i CH "H* LH oo 3 xi CJ CS .3 -co CL O 00 CJ ~c3 3 3 & ° Cr cj cj X 3 3 cj 3 •8 t H XI o CJ PH CJ -CJ CJ CO O u Cl CJ CJ t H o 3 ,o xj ' j ? cO O cS H-5 00 CJ CO 3 o o pa S oo w .3 o e T3 C CS CJ 3 pa X C+H cs CJ CJ » 14^  VL/ U ^ H J H J CJ o 3 *S cj H J CJ o 3 t-CJ H J X CJ 3 &H o X o o CS o o 00 Q 60 H, 3 O O 3 J U X. co •3 XI O cs g 'SH 00 O 3 o e CJ 3 c cj O H X CJ o 15 o '3 60 O c II oo 2 O O II PH ts • c CS o '3 2 o r > H H - H T3 "o O CL CO CJ H-» CS o 0 0 H - H o 00 41 CO fi CS CJ fi JH CS CO CJ 3 * * m o PH ts -H* c CS o H H '3 00 Is oo r-TT o m o o O O 00 <+H cd CD .-I o i n o o m o o o i n o o m o o o i n § o UH "—' • ° UH" , — v cu 1) 3 -3 "2 ° u § U % >, % .2? 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PH CJ CJ H-H O .fi o CO CJ) CJ y s CJ Cd CO —- CH -T3 "cd _. ••r ca *> co « u o £ t- £ PH CJ O H H H—» •§ @ s 8 o « - -< g « , CJ ^3 H-H £ CO o o fi CO (H) - t n J2 .2? o 5 § -g1 oo > 9 d C? - ^ •rt CJ C g 5 co 2 _CJ cu "3 CO " .fi c o o cda • CH CO G O c o T3 cd in m i > o U fi CJ o CJ "2 $ Cd CJ W u ffi o -H> -H> CJ HH> fi cj cd CJ u c CQ I > P CO t H cd fi o cj CJ _> *H—» o CJ CH CO CJ tH CJ > O cd CO CJ CO CJ HH1 +-» fi CJ tH cd CH .c fi CJ > '5b cj CO co .fi "3 CJ CJ CO fi o o tH fi >? O fi P <« O C J SHH O O fi .23 fi JH "HH CJ) cd > CO CO cd - H H * .2 W co g jo o un o o O I T ) o o in o o o in o o m o o o in CQ in CN ' £ CO a X> T 3 CJ i d cd -fi in T3 fi cd cd X> C cj CH O cd HO T3 CJ CO O CJ CO CJ CO O ( J O J T U O O jo lusoidd) sseuioig cucumber (Fig. 2.5 A ; A , B, H, and I), leaf beet, pac choi, spinach, and tomato (Fig. 2.5B; B, F, H , and I) exposed to the high compared with low dose of U V - B radiation. No change in stalk biomass accumulation in response to U V - B radiation was detected for broad bean, bush bean, or cabbage (Fig. 2.5A; C, F, and G). Root biomass declined in response to U V - B radiation in the control versus U V - B contrast for beet cv. Early Wonder, broccoli cv. White Sprouting, cucumber (Fig. 2.5A; B, E, and I), kohlrabi, and spinach (Fig. 2.5B; A and H). In cabbage, cucumber (Fig.2.5A; G and I), and spinach (Fig. 2.5B; H) seedlings exposed to the high compared with low U V - B dose had lower root biomass. No effect of U V - B radiation on root biomass accumulation was detected in other vegetable seedlings. 2.4.2.3 Effects on growth indices Growth indices of vegetable species differed in response to U V - B radiation. For example, specific leaf weight (SLW; a measure of leaf thickness and/or density) decreased in response to U V - B radiation in control versus UV-B-exposed broccoli cv. White Sprouting and bush bean, and increased in broccoli cv. Purple Sprouting, cucumber, lettuce cvs. Lobjoits Green Cos and Saladin, pac choi, and tomato (Table 2.7). The SLW of beet cv. Cylindra decreased, and broccoli cv. Purple Sprouting, bush bean, lettuce cv. Saladin, and spinach increased in seedlings exposed to high, compared with low U V - B radiation. The SLW of other vegetable species was unaffected by U V - B radiation. Leaf weight ratio (LWR; leafiness of the plant on a dry weight basis) increased in U V -B-exposed seedlings of broccoli cvs. Purple Sprouting and White Sprouting, corn, kohlrabi, leaf beet, lettuce cv. Saladin, pac choi, spinach, and tomato, and decreased in beet cv. Early Wonder and broad bean in the control versus UV-B-treated contrast (Table 2.8). The LWR decreased in lettuce cv. Lobjoits Green Cos seedlings exposed to a high compared with low level of U V - B radiation. The LWR of other vegetables did not respond to U V - B radiation. Response of the leaf area ratio (LAR; a morphological index of leafiness) to UV-B radiation varied among the vegetable species in both magnitude and direction. 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Early Wonder, broccoli cv. White Sprouting, kohlrabi, leaf beet, lettuce cv. Saladin, and spinach, and a decrease in the L A R of pac choi in response to U V - B radiation (Table 2.9). The L A R increased in leaf beet and pac choi seedlings, and declined in broccoli cv. White Sprouting, lettuce cv. Saladin, and spinach grown at the high compared with low U V - B radiation dose. The L A R of the other species was not affected by U V - B radiation. The shoot: root ratio (SRR; a measure of above- versus below-ground biomass partitioning) declined in response to U V - B radiation for bush bean and lettuce cv. Saladin, and increased for kohlrabi, lettuce cv. Little Gem Cos, and spinach in the control versus U V - B -treated contrast (Table 2.10). SRR decreased for corn, and increased for lettuce cv. Lobjoits Green Cos, and spinach in seedlings exposed to a high, compared with low level of U V - B radiation. U V - B radiation did not affect SRR of other vegetable species. 2.4.2.4 Relative sensitivity to U V - B radiation Ranking of vegetable seedlings from most to least sensitive to U V - B radiation, based on cumulative percent changes in leaf area and stalk biomass for the control versus U V - B contrast (Eqn. 2.1 A) was as follows: beet cv. Early Wonder, spinach, broccoli cv. White Sprouting, kohlrabi, lettuce cv. Little Gem Cos, broccoli cv. Purple Sprouting, leaf beet, cucumber, beet cv. Cylindra, lettuce cv. Saladin, radish, pac choi, lettuce cv. Lobjoits Green Cos, tomato, corn, bush bean, broad bean, and cabbage (Fig. 2.6A). Vegetable seedlings ranked in order of decreasing sensitivity to high versus low U V - B radiation (Eqn. 2. IB) as follows: leaf beet, beet cv. Early Wonder, beet cv. Cylindra, cucumber, spinach, corn, bush bean, pac choi, lettuce cv. Little Gem Cos, tomato, cabbage, kohlrabi, broccoli cv. Purple Sprouting, broad bean, lettuce cv. Lobjoits Green Cos, broccoli cv. White Sprouting, lettuce cv. Saladin, and radish (Fig. 2.6B). 2.4.3 Response of broccoli cultivar seedlings to U V - B radiation Broccoli was identified as a suitable crop for use in competition experiments. Justification for this selection is provided in Section 2.5.3. Further experiments were conducted using a range CO cO UH o U CO 60 .g T J CJ CD co CJ u 60 CJ > 601 2 o o7 < o UH CO CJ <+H CO JD CJ X -*-» 3 o s o 1 UH PQ > <+H o cj CJ 3 cj 3 3 I—H O N CN CJ ? 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O fl 'P? ca o ca H H 00 13 "O to s O O X H H ea C J 3 U H H CJ CJ H H H H CJ o 3 ts CJ H J CJC J 3 ts CJ H H *i w o c X H H ^ !s ^ t3 S3 ca rt & H C H X? CQ oo c cj 3 CQ O0 Q 00 c o H H X o CCS fi ' C H oo t H o 3 u o ta a o o ' f i 00 fi CJ a c CJ C L X cj O a o r>H H H T 3 H H o CL co CJ ta o 00 C H O W 00 +1 co CJ CJ IH ca o fl 00 z -a © © ' ll cu-ts C J c 'c E .5? «J 00 in © Cu ta i o H H c 00 co CJ 3 la oo I > * 55 A B mm J I I L Beet 'Early Wonder' Spinach 'Long Standing Bloomsdale' Broccoli 'White Sprouting' Kohlrabi 'Purple Vienna' Lettuce 'Little Gem Cos' Broccoli 'Purple Sprouting' Leaf beet 'Perpetual Spinach' Cucumber 'Straight Eight' Beet 'Cylindra' Lettuce 'Saladin' Radish 'Mooli Mino Early' Pac choi 'Pueblo' Lettuce 'Lobjoits Green Cos' Tomato 'Cuor Di Bue' Corn 'Northern Super Sweet' Bush bean 'Contender' Broad bean 'Aquadulce' Cabbage 'Christmas Drumhead' Leaf beet 'Perpetual Spinach' Beet 'Early Wonder' Beet 'Cylindra' Cucumber 'Straight Eight' Spinach 'Long Standing Bloomsdale' Corn 'Northern Super Sweet' Bush bean 'Contender' Pac choi 'Pueblo' Lettuce 'Little Gem Cos' Tomato 'Cuor Di Bue' Cabbage 'Christmas Drumhead' Kohlrabi 'Purple Vienna' Broccoli 'Purple Sprouting' Broad bean 'Aquadulce' Lettuce 'Lobjoits Green Cos' Broccoli 'White Sprouting' Lettuce 'Saladin' Radish 'Mooli Mino Early' •125 -100 -75 -50 -25 0 25 Sensitivity Index 50 Fig. 2.6. Relative sensitivity of vegetable seedlings to U V - B radiation based on cumulative percent changes in stalk biomass and leaf area for the following contrasts: (A) Control versus U V - B radiation, and (B) Low versus high U V - B radiation dose. 56 of broccoli cultivars in order to identify the most appropriate cultivar for use in competition studies. 2.4.3.1 Visual effects U V - B radiation caused visible effects on broccoli cultivar seedlings. For example, cotyledons of UV-B-exposed broccoli cultivar seedlings were smaller and thicker compared with those of control plants. Cotyledons and leaves of UV-B-treated seedlings often curled upwards and appeared to be darker green in both experiments. Leaf margins of cv. Patriot seedlings grown at high U V - B radiation were chlorotic. Visual differences among cultivars not due to U V - B radiation also existed. For example, cultivar Purple Sprouting had a more upright growth habit compared with the other cultivars. The basal portion of stalks of cultivars Arcadia, Everest, Emperor, Legend, and Patriot tended to be horizontally oriented. 2.4.3.2 Effects on growth parameters Statistical significance of control versus UV-B-treated, and low versus high UV-B-treated contrasts for height and leaf area data of broccoli cultivars were carried out at P < 0.05. Although UV-B-exposure generally reduced the height of broccoli seedlings (Fig. 2.7A and B), cultivar response differed between two experiments. In the first experiment, the control versus U V - B contrast detected height reductions in UV-B-exposed, compared with control plants, with the exception of cv. Everest (Fig. 2.7A). Exposure to a high, compared with low, U V - B dose resulted in shorter plants for cvs. Emperor and Legend. In the second experiment decreases in height in response to U V - B radiation were detected for the control versus U V - B -treated contrast in all cultivars except Legend (Fig. 2.7B). Height declined for all cultivars grown at high compared with low U V - B radiation. Leaf area accumulation generally declined in broccoli cultivars exposed to U V - B radiation, compared with controls in the first experiment (Fig. 2.8A). In this experiment, leaf area accumulation declined in all UV-B-exposed cultivars compared with controls, with the 57 Arcadia Emperor Everest Legend Patriot Purple Sprouting Fig. 2.7. Effect of U V - B radiation on the height (cm) of seedlings of broccoli cvs. Arcadia, Emperor, Everest, Legend, Patriot, and Purple Sprouting. Each value is the mean ± SE of 6 replicates. U V - B B E treatment doses were 0 2 1 (closed bars), 7 (open bars), and 11 (shaded bars) kJ m" d* . Fig. A and B represent results of two different experiments. 58 150 100 50 0 o +-» c o o o c § 150 cd CD ^ 100 50 0 Arcadia Emperor Everest Legend Patriot Purple Sprouting 2 Fig. 2.8. Effect of U V - B radiation on leaf area (cm ) of seedlings of broccoli cvs. Arcadia, Emperor, Everest, Legend, Patriot, and Purple Sprouting. Average leaf area for control seedlings are given in parentheses above respective control bars. U V - B B E treatment doses were 0 (closed bars), 7 (open bars), and 2 1 11 (shaded bars) kJ m" d" . Fig. A and B represent results of two different experiments. 59 exception of Everest. Leaf area of cultivars Emperor and Legend was reduced in seedlings exposed to the high compared with low U V - B dose. In the second experiment, leaf area decreased in all cultivars exposed to U V - B radiation compared with controls, except cvs. Arcadia and Emperor (Fig. 2.8B). Leaf area accumulation was reduced in all the cultivars exposed to high compared with low U V - B radiation. Leaf number was unaffected by U V - B treatments (data not shown). In all cases, leaf area reduction was due to smaller, rather than fewer leaves (data not shown). Leaf, stalk, and root biomass accumulation of broccoli cultivars and response of growth indices to U V - B radiation also differed between experiments. As identification of the most suitable broccoli cultivar for use in competition experiments was based mainly on height, leaf area, and relative sensitivity to U V - B radiation, detailed results of biomass accumulation and growth indices are not described here, but are presented in Appendices 2.1, 2.2, 2.3, and 2.4. 2.4.3.3 Effects on leaf greenness Leaves of broccoli cultivars appeared darker green compared with controls, in seedlings treated with 7 and 11 kJ m"2 d"1 UV-BBE radiation (Section 2.4.2.1). Leaf greenness was therefore measured using a Minolta SPAD meter in the broccoli cultivar experiments. In the first experiment, SPAD readings (a measure of leaf greenness) were significantly higher for all UV-B-treated broccoli cultivars in the control versus UV-B-treated contrast, with the exception of Emperor (Table 2.11). No difference in leaf greenness for any cultivar was observed in the low versus high U V - B contrast in this experiment. In the second experiment SPAD readings were higher for UV-B-treated 'Purple Sprouting' leaves compared with controls (Table 2.11). No difference in SPAD readings for the control versus U V - B contrast was detected for any of the other cultivars. SPAD readings increased, however, in cvs. Arcadia, Emperor, and Everest grown at the high compared with low U V - B level. 60 Table 2.11. Influence of U V - B radiation on leaf greenness (SPAD units) of the second true leaf of broccoli cultivars Arcadia, Emperor, Everest, Legend, Patriot, and Purple Sprouting. Cultivar U V - B dose (kJ i n 2 d"1) Experiment I Experiment II Arcadia 0 41.4 ± l . l a 41.2 ±0.9 7 45.1 ± 1.3 38.1 ±0.7 11 44.2 ± 0.7 42.8 ± 1.1 Control versus U V - B *b NS Low versus high U V - B NS ** Emperor 0 44.3 ± 0.7 40.3 ± 0.9 7 45.9 ±0 .7 37.0 ±0.8 11 45.8+1.4 42.4 ± 1.1 Control versus U V - B NS NS Low versus high U V - B NS ** Everest 0 41.8 ± 1.2 42.2 ± 0.6 7 44.6 ± 1.0 38.5 ±0.4 11 46.0 ± 1.2 42.4 ± 1.4 Control versus U V - B * NS Low versus high U V - B NS ** Legend 0 43.8 + 1.1 41.5 ±0.5 7 48.8 ±0.7 43.5 ±2.3 11 48.4 + 0.7 42.9 ± 1.3 Control versus U V - B ** NS Low versus high U V - B NS NS Patriot 0 45.1 ± 1.1 44.0 ± 1.3 7 50.1 + 1.0 43.5 ±0.6 11 50.1 ±0 .8 44.6 ± 0.7 Control versus U V - B ** NS Low versus high U V - B NS NS Purple Sprouting 0 39.3 ± 1.3 35.6 ± 1.2 7 42.6 ± 1.6 40.1 ± 1.6 11 44.7 ± 1.1 40.9 ± 1.0 Control versus U V - B * ** Low versus high U V - B NS NS Values are means ± SE of six replicates. b * Significant at P = 0.05, ** Significant at P = 0.01, and NS = not significant. 61 2.4.3.4 Relative sensitivity to UV-B radiation Broccoli cultivars ranked from most to least sensitive to U V - B radiation, compared with controls (Eqn. 2.1 A) in the first experiment as follows: Purple Sprouting, Patriot, Legend, Emperor, Arcadia, and Everest (Figure 2.9A). Ranking in order of decreasing sensitivity based on comparison of the high versus low U V - B radiation effects (Eqn. 2. IB) was: Legend, Emperor, Purple Sprouting, Everest, Patriot, and Arcadia (Fig. 2.9B). In the second experiment, relative cultivar sensitivity to U V - B radiation, compared with controls (Eqn. 2.1 A) ranked from most to least sensitive as follows: Patriot, Purple Sprouting, Everest, Legend, Arcadia, and Emperor (Figure 2.9C). Ranking in order of decreasing sensitivity based on comparison of the high versus low U V - B radiation effects (Eqn. 2.IB) was as follows: Purple Sprouting, Emperor, Arcadia, Patriot, Everest, and Legend (Fig. 2.9D). In the first experiment, cultivars generally responded more to the control versus U V - B contrast and less the to low versus high U V - B contrast, compared with the second experiment. Although cultivars differed in relative sensitivity to U V - B radiation between experiments and contrasts, cv. Purple Sprouting ranked among the three most UV-B-sensitive cultivars in both contrasts in both experiments. 2.5 DISCUSSION Success in a mixed population depends on the ability of a particular crop or weed to compete with neighbouring species. Any stress that differentially influences seedling growth and morphology of an individual species within an association could alter competitive interactions by influencing the relative abilities of these species to intercept light and absorb nutrients. This study compares UV-B-induced growth and morphological responses expected to influence competitive ability of some important agricultural weeds, vegetable crops, and cultivars of a selected vegetable crop. Discussion regarding potential shifts in competitive ability as a result of UV-B-induced morphological responses in individual species will be restricted to a greenhouse environment. The results demonstrate a significant interspecific and, in the case of lettuce and broccoli cultivars, intraspecific diversity in response to U V - B radiation. CN N O D O c 60 C 3 o CH m CJ 3 OH CJ CM £ 2 •3 cd o CS C M CO CU In CJ > c CJ D O CU H J UH X D O c .1 CD X W D O c o LH CH m ' & 3 OH cd OH co HI <D > T 3 3 cj D O CU 2 -3 o cj CL £ IT) CN C N o in i in • o o in CN -a a i—i > i n CN in CN o i n in i o o i n CN CO D O cu cu CO 00 m cu & 3 OH T 3 C cS 2 H0 2 •§> 8 ° a oo «n 3 o <2 i—' H 3 Q •4—» ^ ' c2 3. cS O CS SH P. CS OH M H ^ CS c CJ CS -o fl >^  CS CJ H J co cu CO CS _> *H-» o CJ CM CO CJ CU ~-\ W 1 <N tn- C rt cu CS* o T3 cS o t H CO > c CU O HI CU C L cu > 3 cu 4) CJ CJ 3 CH .2 u •-H _3 o o o o t H JO M H o 5^ CJ H CO > * ~ s 'co .2 g 3 CO fl CJ cs > *-'•5 w CS I 13 > oi H0 ON CN D O 'Cu fl M CS T 3 S PQ > "3 3 H J <U to g 9> OH H H CJ 8 cu £ o -I S | "2 'B a rt CS t H 63 2.5.1 Response of weed species to UV-B radiation 2.5.1.1 Effects on plant morphology, biomass, and leaf area Due to the rapid extinction of light within plant canopies, even subtle reductions in plant height could alter competitive balance among associated species through reduced PAR interception (Ryel et al. 1990). U V - B radiation reduced the height of green foxtail, lamb's-quarters, and redroot pigweed, but not of lady's-thumb seedlings (Table 2.3). Competitive advantage of affected species may diminish due to their differential height reduction in a U V -B-enriched environment. While seedling height may be a good indicator of the ability to intercept P A R in species with a single, upright stem, stalk biomass may be a better indicator for species with creeping, tillering, or rosette forms. In this study, stalk biomass was reduced in weed species with sprawling (common chickweed) and tillering (green foxtail) growth habits, as well in species with a single main stem (lamb's-quarters and redroot pigweed) (Fig. 2.2). Due to rosette growth habits, petioles constituted the main component of stalk biomass in shepherd's-purse. However, this component was unchanged in response to exposure to U V - B radiation. Stalk biomass of lady's-thumb was not influenced by U V - B radiation. Biomass reductions in UV-B-exposed plants are often accompanied by substantial modifications in biomass partitioning into component plant organs (Teramura 1983). Morphological responses, including reduced stem elongation, may represent a shift in biomass allocation as opposed to a damage response (Caldwell and Flint 1994, Barnes et al. 1995). U V -B radiation reduced plant height, but significantly increased tiller number in green foxtail (Fig. 2. IB) suggesting a possible UV-B-induced loss of apical dominance. Barnes et al. (1990b) also reported a reduction of internode elongation and an augmentation of axillary shoot production in response to U V - B radiation in monocots. The leaf surface is the primary target of all radiation incident on the plant. The influence of U V - B radiation on leaf morphology and appearance varied significantly among 64 the species employed in this study. U V - B radiation reduced leaf area and/or leaf biomass of common chickweed, green foxtail, lamb's-quarters, and shepherd's-purse (Table 2.3 and Fig. 2.2). Since number of leaves per seedling was not affected in green foxtail, lamb's-quarters, and shepherd's-purse (data not shown) reduction in leaf area in these species was due to smaller, rather than fewer, leaves. Because of a large number of very small leaves in U V -irradiated common chickweed plants, leaf number was not counted in this species. Reduction of leaf area is believed to be a protective response, which reduces the amount of tissue exposed to U V - B radiation (Bornman and Teramura 1993). The cost of this putative protective mechanism is a decrease in leaf area to intercept PAR, which in turn would reduce photosynthesis and possibly the ability to compete for sunlight (Dickson and Caldwell 1978). In contrast, the ineffectiveness of U V - B radiation in reducing leaf area accumulation and leaf biomass of lady's-thumb and redroot pigweed (Table 2.3 and Fig. 2.2) suggests that these weed species may have a relative competitive advantage in UV-B-enriched environments. U V - B induced upward cupping of cotyledons and leaves in common chickweed, and shepherd's-purse. A similar response has been reported for the rangeland weed houndstongue (Cynoglossum officinale L.) (Furness et al. 1999) and canola {Brassica napus L. var. Topas) (Wilson and Greenberg 1993). This response offers yet another mechanism to reduce the leaf area exposed to U V - B radiation (Greenberg et al. 1997). While the mesophyll can be protected from U V - B radiation by accumulation of screening pigments in epidermal cells, the epidermis itself remains susceptible to U V - B radiation damage. For example, bronzing of cotyledon and leaf surfaces was observed in shepherd's-purse in response to U V - B radiation. Similar bronzing has also been reported in soybean (cv. Hardee) and houndstongue (Teramura 1983, Furness et al. 1999) and has been attributed to the presence of oxidized, polymerized, phenolic compounds (Cline and Salisbury 1966). Necrotic leaf margins were observed in bush bean, and necrotic lesions containing collapsed epidermal cells were occasionally noted on the adaxial surface of redroot pigweed leaves grown at the high U V - B radiation dose (Fig. 2.1 A). In contrast, leaves of common chickweed exposed to U V - B radiation appeared to be darker green compared with controls 65 suggesting an increase in chlorophyll content, which may compensate in part for U V - B -induced damage, as has been implied for mouse-ear cress (Arabidopsis thaliana (L.) Heynh genotype LER) (Ormrod et al. 1997). 2.5.1.2 Effects on growth indices Measurement of response of growth indices to U V - B radiation showed that not all weed species in this study responded similarly. For example specific leaf weight (SLW; the ratio of leaf mass to area) increased in common chickweed and shepherd's-purse, but did not change in green foxtail, lady's-thumb, lamb's-quarters, and redroot pigweed (Table 2.4) in response to U V - B exposure. While increased SLW may increase the path length for photons, thereby reducing transmission of U V - B radiation to sensitive underlying targets (Teramura and Sullivan 1994), this anatomical manifestation alone has been deemed insufficient to protect plants from U V - B radiation (Teramura 1983). It is unknown whether weeds in which U V - B -induced reduction of leaf area is not associated with increased SLW rely on some other strategy (e.g. accumulation of U V - B absorbing compounds in epidermal cells) for protection from U V - B damage. Leaf weight ratio (LWR) represents leafiness of the plant on a dry weight basis. Increased L W R in common chickweed (Table 2.4) seedlings exposed to U V - B radiation represents relatively greater expenditure by the UV-B-exposed plants on potentially photosynthesizing organs. However, the increase in the proportion of dry weight found in the leaves may have been due in part to an increase in SLW, as has been reported in soybean, bean, pea (Pisum sativum L.), and cucumber (Teramura 1983). Leaf area ratio (LAR) is a morphological index of leafiness and reflects leaf area production per unit above-ground biomass. The fact that the control versus U V - B contrast was not significant for common chickweed seedlings suggests that SLW and L W R responses to U V - B radiation compensated each other resulting in no change in L A R (LAR = L W R x SLW" 1) (Table 2.4). A decline in the L A R due to an increase in SLW was detected in common chickweed as the U V - B level increased from low to high, and in shepherd's-purse in response to U V - B treatments (control versus U V - B contrast). The decrease in the L A R of these weed 66 species in response to U V - B radiation indicates a shift between the rates of dry-weight gain and leaf area growth. Reduction in L A R accompanied by increase in SLW has also been reported in soybean (cv. Essex) (Teramura and Sullivan 1987). A change in L A R may represent within leaf reallocation of dry matter without significantly altering partitioning of dry matter into the leaf tissue. No effect of U V - B radiation on L A R of green foxtail, lady's-thumb, lamb's-quarters, or redroot pigweed seedlings was observed (Table 2.4). Stability in L A R implies some balance between rates of dry-weight gain and leaf area growth (Warren Wilson 1981). Differential sensitivity of L A R to U V - B radiation detected for these agricultural weeds may contribute to potential UV-B-induced shifts in competitive ability as L A R has been reported to be a major contributor to aggressiveness of some species (Roush and Radosevich 1985). Changes in above-ground biomass under enhanced U V - B radiation may in part result from a shift in the allocation of plant resources between above- and below-ground biomass rather than a decrease in total biomass (Gold and Caldwell 1983). A shift in biomass allocation to the roots might enhance competitive effectiveness of individual species for soil moisture and nutrients. A UV-B-induced shift in biomass compartmentation, with a relatively greater investment in the shoot (increased SRR) however, was detected in common chickweed and redroot pigweed (Table 2.4). U V - B radiation decreased root biomass in all the weeds, except lady's-thumb (Fig. 2). 2.5.1.3 Relative sensitivity of weed seedlings to UV-B radiation Ranking of sensitivity of the common agricultural weed species to U V - B radiation was based on cumulative percent changes in stalk biomass and leaf area. Stalk biomass, as opposed to plant height, was included in the sensitivity indices as it may more accurately reflect the ability of plants with sprawling, tillering, or rosette habits to intercept PAR. Leaf area and angle are important parameters in determining PAR interception. However, since it is difficult to measure individual angles of large numbers of leaves within a canopy, only leaf area was included in the sensitivity indices. Leaf biomass was omitted because UV-B-induced increases in SLW, reflected in the leaf biomass values, may not fully compensate for losses in leaf area 67 (Adamse and Britz 1992b), resulting in under-estimation of UV-B-sensitivity in affected weeds. Common chickweed was classified sensitive, shepherd's-purse, lamb's-quarters, and green foxtail intermediately sensitive, and redroot pigweed and lady's-thumb tolerant to U V - B radiation (Fig. 2.3A). When sensitivity between the low and high U V - B dose was compared, common chickweed was deemed sensitive, shepherd's-purse and lamb's-quarters intermediately sensitive, and redroot pigweed, lady's-thumb, and green foxtail relatively tolerant to the increased U V - B radiation dose (Fig. 2.3B). In an enhanced U V - B environment, competitive ability of relatively UV-B-tolerant weeds such as lady's-thumb and redroot pigweed may increase, while that of sensitive species such as common chickweed may decline. 2.5.2 Response of vegetable seedlings to UV-B radiation 2.5.2.1 Effects on plant morphology, biomass, and leaf area Plant height and the overtopping of potential competitors may be especially important during the rapid canopy development, which occurs in agricultural situations. Beet (cv. Early Wonder), broad bean, broccoli (cvs. Purple Sprouting and White Sprouting), bush bean, corn, cucumber, leaf beet, and tomato heights were decreased in response to U V - B radiation (Table 2.5). Differential height reduction may reduce competitive advantage of these crop species under conditions of enhanced U V - B radiation. Stalk biomass of all vegetables was reduced in response to U V - B radiation with the exceptions of broad bean, bush bean, and cabbage (Fig. 2.4A; C, F, and G). Stalk biomass of the lettuce cultivars, comprised mainly of petiole tissue, was reduced in response to U V - B radiation. U V - B radiation reduced leaf area and/or leaf biomass of vegetable species including beet cultivars, broccoli cultivars, bush bean, corn, cucumber, kohlrabi, leaf beet lettuce (cv. Little Gem Cos), radish, spinach, and tomato (Table 2.6 and Fig. 2.4A; A , B, D, E, F, H , and I). Since the number of leaves per seedling was usually not affected (data not shown), the reduction in leaf area was generally due to smaller, rather than fewer, leaves. A n increased number of smaller leaves produced in lettuce (cv. Saladin) at high, compared with low U V - B radiation, however, resulted in unchanged leaf area (Table 2.6). Conversely, a decreased 68 number of leaves were produced in broccoli (cv. White Sprouting) and lettuce (cvs. Little Gem Cos and Lobjoits Green Cos) in response to U V - B radiation. UV-B-induced upward curling of cotyledons and leaves in broccoli (Fig. 2.4), pac choi, and spinach further reduced leaf area exposed to U V - B radiation. Possible consequences of reduced leaf area were discussed in Section 2.5.1. In contrast, the ineffectiveness of U V - B radiation in reducing leaf area accumulation and leaf biomass of broad bean, cabbage (Table 2.6 and Fig. 2.4A; C and G), lettuce (cvs. Lobjoits Green Cos and Saladin), and pac choi (Table 2.6 and Fig. 2.4B; D, E, and F) may translate into a relative competitive advantage for these vegetable species in UV-B-enriched environments. Signs of UV-B-induced changes in leaves were visible in several vegetable species. For example, bronzing of cotyledon and leaf surfaces in cucumber, and a darker green colouration in broccoli cultivars in response to U V - B radiation was noted. These effects were also observed in some of the weed species and were discussed in Section 2.5.1.1. Occasionally, necrotic leaf margins were observed in bush bean, grown at the high U V - B radiation dose. 2.5.2.2 Effects on growth indices Response of growth indices to U V - B radiation showed that not all vegetable species in this study reacted in a similar way. For example specific leaf weight (SLW; the ratio of leaf mass to area) increased in broccoli (cv. Purple Sprouting), cucumber, lettuce (cvs. Lobjoits Green Cos and Saladin), pac choi, and tomato, and decreased in broccoli (cv. White Sprouting) and bush bean in response to U V - B exposure (Table 2.7). SLW declined in beet (cv. Cylindra), and increased in broccoli (cv. Purple Sprouting), bush bean, lettuce (cv. Saladin), and spinach grown at a high, compared with low, U V - B dose. UV-induced reduction in leaf area was not always associated with a corresponding increase in SLW. Similarly, in a screening experiment of 82 economically important species Biggs and Kossuth (1978) reported that SLW changes do not always correspond with U V - B radiation resistance. In dicotyledons, a greater proportion of biomass is commonly allocated to leaves, despite an absolute reduction in leaf area in plants exposed to a high level of U V - B radiation (Teramura 1983). Increased leaf weight ratio (LWR; leafiness of the plant on a dry weight 69 basis) in broccoli cultivars, corn (a monocotyledon), kohlrabi, leaf beet, lettuce (cv. Saladin), pac choi, spinach, and tomato seedlings exposed to U V - B radiation (Table 2.8) represents relatively greater expenditure by the UV-B-exposed plants on potentially photosynthesizing organs. The increase in the proportion of dry weight found in the leaves may have been due in part to an increase in SLW in some of these species, as has been reported in numerous other crop species (Teramura 1983). Leaf area ratio (LAR; a morphological index of leafiness) of broccoli (cv. Purple Sprouting) (Table 2.9) did not change in response to U V - B radiation, suggesting that SLW and L W R responses to U V - B radiation balanced each other resulting in no change in L A R (LAR -L W R x SLW" 1). A decline in the L A R for lettuce (cv. Saladin) and spinach grown at high compared with low U V - B radiation, and for UV-B-exposed compared with control seedlings of pac choi (Table 2.9) may also be due, in part, to an increase in SLW (Table 2.7). Reduction in L A R accompanied by increase in SLW has also been reported in soybean (cv. Essex) (Teramura and Sullivan 1987). In contrast, L A R for the control versus UV-B-treated seedlings of broccoli (cv. White Sprouting), kohlrabi, leaf beet, and lettuce (cv. Saladin) increased in response to U V - B radiation (Table 2.9). This may have been the result of increased L W R in these species (Table 2.8). Such changes in the L A R may represent reallocation of dry matter within the leaf. L A R declined in broccoli (cv. White Sprouting), and increased in leaf beet and pac choi seedlings grown at high compared with low U V - B radiation (Table 2.9) despite lack of UV-B-induced changes in SLW (Table 2.7) or L W R (Table 2.8). The L A R has been reported to be a major contributor to aggressiveness of some species (Roush and Radosevich 1985). Therefore the range of UV-B-induced responses of L A R exhibited by these vegetable crops may contribute to potential shifts in competitive ability. The L A R of the majority of vegetable seedlings used in this research, however, was unaffected by U V - B radiation (Table 2.9). Such stability suggests equilibrium between rates of dry-weight gain and leaf area growth (Warren Wilson 1981). Shoot:root ratio (SRR) declined in response to U V - B radiation for bush bean and lettuce (cv. Saladin), and in response to high compared with low U V - B radiation for corn (Table 2.10). Conversely, a UV-B-induced shift in biomass compartmentation, with a 70 relatively greater investment in the shoot (increased SRR), was detected in response to U V - B radiation for kohlrabi and lettuce (cv. Little Gem Cos), and at high compared with low U V - B radiation dose for lettuce (cv. Lobjoits Green Cos) and spinach. Both positive and negative effects of enhanced U V - B radiation on shootroot ratios have been reported (Sullivan and Teramura 1992, Barnes et al. 1993, Sullivan et al. 1994, Ziska and Teramura 1992, Ziska et al. 1993). A shift in shoot:root biomass allocation might alter competitive effectiveness of individual species for soil moisture and nutrients (Tilman 1988). 2.5.2.3 Relative sensitivity of vegetable seedlings to UV-B radiation While the goal of these experiments was to identify suitable plant material for use in greenhouse competition experiments, the reduction in growth of UV-B-exposed vegetable seedlings, compared with controls may have implications in cases where vegetable crops are seeded in a UV-B-free environment, such as a glasshouse and transplanted as seedlings into the field. While only beet (cv. Early Wonder) was classified as UV-B-sensitive, intermediately sensitive vegetables including spinach, broccoli, kohlrabi, lettuce leaf beet, cucumber, radish, and pac choi (Fig. 2.6A) may also be affected. When sensitivity to a high compared with low U V - B dose was quantified, only leaf beet was UV-B-sensitive, while the beet cultivars, cucumber, spinach, corn and bush bean were intermediately sensitive, and the remaining vegetable crops were UV-B-insensitive (Fig. 2.6B). 2.5.3 Selection of weed and vegetable species for use in competition experiments Broccoli (crop) and lamb's-quarters (weed) were selected as the crop-weed species pair for use in subsequent competition experiments for the following reasons: (1) morphological responsiveness to U V - B radiation, (2) economic importance of each species, (3) agricultural relevance of the crop:weed pair, and (4) similar emergence time and seedling growth (Bitterlich and Upadhyaya 1990). Height and leaf area, factors expected to influence the ability of plants to compete for light, declined in broccoli and lamb's-quarters in response to U V - B radiation. Broccoli is a commercially important agricultural crop in British Columbia (BCMAF 1985). Lamb's-quarters is a deleterious weed of cultivated crops (Weaver 2001), and is a 71 serious problem in the Lower Fraser Valley of BC. The effect of lamb's-quarters interference on broccoli growth has been shown to be severe (Bitterlich and Upadhyaya 1990). For example, under field conditions, 15 lamb's-quarters plants m" reduced total broccoli yield by 50 - 78%. 2.5.4 Selection of broccoli cultivar for use in competition experiments Brassicaceae is regarded as one of the most UV-B-sensitive crop families studied (Teramura 1990). Separate experiments were conducted to explore the range of sensitivity among a number of broccoli cultivars in order to identify the most appropriate one for use in the competition studies. U V - B radiation reduced height (Fig. 2.7A and 2.7B), leaf area (Fig. 2.8A and 2.8B) and increased leaf greenness (Table 2.11) in all broccoli cultivars used in this study. Increased leaf greenness implies an increase in chlorophyll content (Tenga et al. 1989). High levels of chlorophyll have been correlated with U V - B tolerance in some studies (Bornman and Vogelmann 1990, Greenberg et al. 1997), but not in others (Smith et al. 2000). No clear relationship was observed in this study between leaf greenness and tolerance to U V - B radiation. For example, in the first experiment leaf greenness increased in response to U V - B radiation in UV-B-sensitive (Purple Sprouting) as well as UV-B-insensitive (Everest) cultivars. While variations in sensitivity within species tend to be more subtle than between species (Murali and Teramura 1986), this research showed qualitative and quantitative intraspecific differences in UV-B-sensitivity among broccoli cultivars. Cultivar response to U V - B radiation differed between experiments. Some speculations can be made concerning the reasons underlying these differences. Firstly, with the exception of the Fi hybrid cv. Emperor (Garden Seed Research Committee and American Seed Trade Association 1980) broccoli cultivars used in this study are open-pollinated. Therefore, they may be expected to exhibit a greater degree of variability in growth and U V - B responses than more genetically uniform cultivars. Breeders of broccoli cultivars (with the exception of cv. Emperor) have not applied for Plant Breeders' Rights and therefore additional information regarding parentage of the cultivars is unavailable. Secondly, the average midday (11:00-72 13:00h) P A R levels in the greenhouse ranged between 620 (overcast sky) and 1250 (clear sky) 2 1 2 1 umol m s in the first experiment and only 380 (overcast sky) to 920 (clear sky) pmol m" s" during the second experiment (Table 2.2). This may have resulted in significantly greater leaf area accumulation in control plants in the first, compared with the second experiment (Fig. 2.8A and 2.8B). Interestingly, response of all cultivars to U V - B radiation was greater in the first, compared with the second experiment. This was unexpected as exaggerated response to U V - B radiation is commonly reported as a result of low PAR:UV-B ratios (Caldwell and Flint 1994) partly due to inhibited accumulation of U V - B absorbing compounds below a certain critical threshold of P A R (Tevini and Teramura 1989, Cen and Bornman 1990). In both experiments, 2 1 however, the average levels of PAR provided were within the range (>500 pmol m" s") necessary to facilitate UV-acclimation processes (Cen and Bornman 1990). Smith et al. (2000) suggested that rapidly growing plants are more susceptible to U V - B damage and reduction in growth rate may afford opportunity for other protective mechanisms to be fully implemented. Results of this study implied that broccoli may be more sensitive to U V - B radiation when plants are grown under conditions that enhance rapid growth. Broccoli (cv. Purple Sprouting) was selected for use in subsequent competition experiments, based on relative sensitivity to U V - B radiation. It consistently ranked amongst the most sensitive of the cultivars to U V - B radiation (Fig. 2.9), suggesting that Brassica oleracea var. italica may be more sensitive to U V - B radiation than var. botrytis. Other cultivars were either less sensitive or had inconsistent responses to U V - B radiation. Furthermore, cv. Purple Sprouting had a more upright habit of growth, compared with the other cultivars, making it more suitable for a space-constrained competition study in association with the weed species lamb's-quarters. 73 3.0 Competitive interactions in broccoli (Brassica oleracea L. var. italica cv. Purple Sprouting) and lamb's-quarters (Chenopodium album L.) associations grown at two levels of UV-B radiation. 3.1 ABSTRACT Effects of ultraviolet-B radiation on intra- and interspecific competition in broccoli (Brassica oleracea L. var. italica cv. Purple Sprouting) and lamb's-quarters (Chenopodium album L.) were studied in a greenhouse. A bivariate factorial experiment with a randomized block design was performed with three monoculture densities for each species (144 (low), 256 (medium), 2 2 and 400 (high) plants m") and all binary combinations grown at simulated ambient (4 kJ m" d"1) and above-ambient (7 kJ m"2 d"1) UV-BBE radiation for 4 weeks in 1999 and 5 weeks in 2000. Per plant biomass of each species generally decreased with increasing species population density. Inverse yield-density relationships using biomass per plant were more discernible at 4, than at 7 kJ m"2 d"1 UV-BBE radiation. Substitution rates for broccoli were smaller at 7, than at 4 kJ mf2 d"1 UV-BBE radiation, largely due to reduced interspecific competitive influences at the elevated U V - B level. Conversely, substitution rates increased for lamb's-quarters grown at 7 kJ m"2 d"1 UV-BBE radiation, as a result of both a decrease in intraspecific competition and an increase in interspecific competition. Lamb's-quarters biomass per plant was less affected by broccoli, and broccoli was more affected by lamb's-quarters than by members of their own species, except at 7 kJ m"2 d"1 UV-BBE radiation in 2000. These results show that lamb's-9 1 quarters was a stronger competitor than broccoli at 4 kJ m" d" UV-BBE radiation in both years and at 7 kJ m"2 d"1 UV-BBE radiation in 1999. In 2000, broccoli was the stronger competitor at 7 kJ m"2 d"1 UV-BBE radiation. Overall, interspecific competition was influenced by U V - B radiation to a greater extent than intraspecific competition. Competitiveness of broccoli was 2 1 enhanced, while that of lamb's-quarters was diminished at 7 compared with 4 kJ m" d" UV-BBE radiation. 3.2 INTRODUCTION The structure and dynamics of plant communities are strongly influenced by intra- and interspecific interactions (Grime 1979, Tilman 1988, Keddy 1989, Grace and Tilman 1990). The net outcome, whether positive or negative, of such interactions is termed 74 interference. Competition is the component of interference that is arbitrated via the utilization of limited, communal resources (Bazzaz 1990). Competition occurs when two or more individuals compete for limited resources. Intraspecific (within-species) competition becomes increasingly important in communities that are largely monospecific, e.g. agricultural systems (Gold and Caldwell 1983). Nevertheless, the existence and importance of interspecific (between-species) competition due to the presence of weed species, is also well documented in agricultural systems (Radosevich and Holt 1984, Blackshaw et al. 1999). The competitive balance of a species within a population is under the control of the entire environmental complex. Even subtle changes in environmental stress may alter the competitive ability of individual species within associations (Fox and Caldwell 1978). Although U V - B radiation has been recognized as an environmental stress for more than a century (Caldwell 1981) the recent decline in stratospheric ozone, the primary attenuator of solar U V - B , has enhanced the status of this radiation as an environmental stress. Enhanced levels of U V - B radiation have been shown to cause shifts in competitive balance (Fox and Caldwell 1978, Gold and Caldwell 1983, Barnes et al. 1988, Yuan et al. 1999). Even ambient compared with below ambient levels of U V - B radiation are sufficient to alter competitive relationships (Bogenrieder and Klein 1982). Such UV-B-induced shifts in intra- and/or interspecific competitive interactions could ultimately influence the structure and dynamics of plant communities. In agricultural systems, UV-B-induced shifts in competitive relationships could impact weed management strategies. Differential sensitivity of associated species to U V - B radiation might partially explain altered competitive interactions in species mixtures in response to enhanced U V - B radiation (Caldwell 1977). Reduced productivity of a relatively UV-sensitive species could increase productivity of an associated less UV-sensitive species as a result of increased resource availability to the tolerant species. Shifts in competitive ability have been associated with differential morphological changes in associated species under U V - B enhancement (Barnes et al. 1990b, Caldwell and Flint 1994, Allen et al. 1998). For example, UV-B-induced shifts in competitive balance between wheat (Triticum aestivum L. cv. Bannock) and wild oat (Avena fatua L.) were associated with shifts in relative positioning of leaf area for the two species within the 75 mixed canopies (Barnes et al. 1988). Changes in canopy structure were computed to be sufficient to alter light interception and canopy photosynthesis for these species (Ryel et al. 1990). In the previous chapter it was shown that broccoli (Brassica oleracea L. var. italica cv. Purple Sprouting) and a common weedy competitor, lamb's-quarters (Chenopodium album L.), were morphologically responsive to U V - B radiation when grown individually in pots within a greenhouse environment. Response of an individual species grown in isolation under enhanced U V - B radiation, however, may have little bearing on its response in a competitive environment (Fox and Caldwell 1978, Barnes et al. 1988). Since competitive interactions are ubiquitous within plant communities, effects of U V - B stress should be evaluated in a competitive environment (Yuan et al. 1999). Competition studies under greenhouse conditions permit more precise examination of intra- and-interspecific interactions compared with environmentally more complex field situations by providing relatively uniform and controlled environmental conditions (Gibson et al. 1999). However, plants are generally less sensitive to increased U V - B radiation in the field than in the greenhouse (Caldwell 1981, Dai et al. 1994) where a significant portion of the stress complex has been removed or altered. Decreased plant sensitivity in field compared with greenhouse UV-B-studies, might be due to dissimilarities between the two growth environments, including levels of U V - A (320-400 nm) and U V - B radiation, photosynthetically active radiation (PAR), and interactions with other environmental factors (Jansen et al. 1998). Such inherent limitations restrict the usefulness of greenhouse studies in predicting the response to a change in U V - B levels under natural conditions. Nonetheless, greenhouse experiments allow underlying mechanisms, as opposed to simple phenomenology, to be explored (Tilman 1987, Stiling 1992). In addition, unless plant-plant interactions can be detected in a less complex greenhouse environment, they are unlikely to be of consequence in natural communities (Gibson et al. 1999). Inverse yield-density models have been used to describe the reduction in mean size per plant with increasing plant population density in monocultures (Kira et al. 1953). Such models have been expanded to evaluate competition in binary mixtures. Quantitative measures of intra- and interspecific competitive influences can be derived from the parameters of inverse yield-density models (Wright 1981, Spitters 1983). The objective of 76 this research was to use inverse yield-density models of biomass per plant to study the effect of U V - B radiation on intra- and interspecific competition in broccoli and lamb's-quarters associations in a greenhouse. 3.3 M A T E R I A L S A N D M E T H O D S 3.3.1 Seed source and plant culture Broccoli (Brassica oleracea L. var. italica cv. Purple Sprouting) seeds were obtained from West Coast Seeds Ltd. (Delta, BC). Lamb's-quarters (Chenopodium album L.) seeds were collected from field populations in southern coastal British Columbia. Broccoli and lamb's-quarters seeds were germinated separately in 25 x 50 x 5 cm (depth) plastic flats in Redi-earth media (Eddi's Wholesale Garden Supplies, Surrey BC) in a greenhouse. Lamb's-quarters seeds were sown five to seven days prior to broccoli seeds in order to ensure that both species had similar above-ground per plant biomass at the time of transplanting. Seedlings of each species were transplanted to 25 cm x 25 cm x 12 cm (depth) plastic flats filled with Redi-earth media. This media was chosen to facilitate washing of roots during harvesting and to maximize leaf-soil colour contrast for image analysis (Chapter 5). Crop and weed monocultures [0, 144 (low), 256 (medium), and 400 (high) plants m" ] and all binary combinations were established (Table 3.1 A) . These densities were chosen to ensure that interference among plants would occur. Actual numbers of observations used in statistical analyses of the competition experiment are shown in Table 3.IB. An alternative "offset-square" planting pattern was used with equal within- and between-row distances (Dr. P. Jolliffe, Professor, Faculty of Agricultural Sciences, U.B.C., Pers. comm.), and included an unsampled border consisting of a single row of plants. Templates were used to ensure precision of seedling placement in each flat. A bivariate factorial experiment with a randomized block design and four blocks was conducted with 2 1 all the monocultures and mixtures grown at ambient (4 kJ m" d") and above-ambient (7 kJ m"2 d"1) UV-BBE radiation. Some components of resource dependence were diminished by providing plants with sufficient water and nutrients. Flats were fertigated with Peters E X C E L 15-5-15 Cal-Mag (The Scotts Company, Marysville, OH, US) twice a week for the first two weeks 77 Table 3.1 A. Population densities of broccoli and lamb's-quarters monocultures and binary associations. Broccoli density Lamb's-quarters density (plants m") (plants m") 0 144 256 400 0 0:0 (0)a 0:144 (144) 0:256 (256) 0:400 (400) 144 144:0 (144) 144:144 (288) 144:256 (400) 144:400 (544) 256 256:0 (256) 256:144 (400) 256:256 (512) 256:400 (656) 400 400:0 (400) 400:144 (544) 400:256 (656) 400:400 (800) „, . ,„„.,— aNumbers in parentheses are total plant densities (plants m ' ) of both species. Table3.1B. Numbers of broccoli and lamb's-quarters plants from monocultures and binary associations used in statistical analyses. Broccoli density Lamb's-quarters density (plants m") (plants m"2) 0 144 256 400 0 0:0 (0)a 0:8b (8) 0:15 (15) 0:24 (24) 144 8:0 (8) 8:8 (16) 8:15 (23) 8:24 (32) 256 15:0 (15) 15:8 (23) 15:15 (30) 15:24 (39) 400 24:0 (24) 24:8 (32) 24:15 (39) 24:24 (48) lumbers in parentheses are total plants used in statistical analyses per experimental unit. Numbers do not include one randomly chosen sample for each species from each experimental unit frozen for possible future analysis. 78 following transplanting and subsequently every second day for the remainder of the experiment. Daylight was supplemented with high-pressure sodium lamps (Philips Lighting Co., Somerset, NJ, US), which provided 15-20 umol m"2 s"1 supplemental P A R (16 h photoperiod). Ambient PAR was measured weekly at noon with a LI-COR LI-185B portable light meter (Li-Cor Inc., Lincoln, N B , US). Daily minimum and maximum temperatures were recorded. In the first experiment, three flats were sown with lamb's-quarters seeds each day from June 28 to July 12, 1999, and three flats with broccoli seeds each day from July 5 to July 18, 1999. Seedlings of both species were transplanted (one block every second day) from July 16 to July 22, 1999 and exposed to U V - B treatments for 4 wk during the first experiment. Average weekly PAR and U V - A levels are given in Appendix 3.1. Greenhouse 2 1 P A R levels ranged between an average of 520 (overcast sky) and 1240 umol m" s" (clear day), while temperatures ranged between 33 and 44 C (day) and 18 and 22 C (night). In the second experiment, three flats were sown with lamb's-quarters seeds each day from October 13 to October 27, 2000, and three flats with broccoli seeds each day from October 20 to November 2, 2000. In the second experiment, seedlings were transplanted (one block every second day) from October 31 to November 6, 2000 and exposed to U V - B treatments for 5 wk. In this experiment P A R levels ranged between an average of 140 2 1 (overcast sky) and 645 umol m' s' (clear day), and ambient temperatures ranged between 21 and 27 C (day) and 18 and 22 (night). 3.3.2 UV-B radiation treatments U V - B treatments were provided within frames as described in Chapter 2 (Section 2.3.2). Within these frames, flats were placed inside smaller frames (32.5 cm x 32.5 cm x 60.0 cm high) covered with either three or two layers of cellulose acetate film (diacetate type, 0.127 mm) (McMaster-Carr, New Brunswick, NJ, US). Daily UV-B-exposure for 8 h centred around solar noon, provided approximately 4 and 7 kJ m" d" of UV-BBE radiation in frames covered with three or two layers of cellulose acetate film, respectively. According to the model of Bjorn and Murphy (1985) 4 kJ m"2 d"1 UV-BBE radiation represents ambient 79 U V - B radiation for Vancouver BC at summer solstice, while 7 kJ m"2 d"1 U V - B B E radiation represents an ozone depletion scenario of 18%. 3.3.3 Harvesting and data collection A l l plants, excluding border rows, were harvested. Flats were submerged in water and roots were gently washed. Actual number of plants harvested from 0, 144 (low), 256 (medium), and 400 (high) plants m"2 density treatments were 0, 9, 16, and 25, respectively. One randomly selected plant from each flat was placed in a plastic bag and frozen at -85 C for possible future analysis. Numbers of observations used in statistical analyses are shown in Table 3. IB. Leaf area (LA) of individual plants was measured as described in Chapter 2 (Section 2.3.3). Leaf, stem, and root material were placed in a drying oven at 70 C for 72 h prior to recording leaf (WL), stem (Ws), root (WR) and total plant (W) biomass of individual plants. 3.3.4 Statistical analysis For each flat, mean WR, WS, WL, and W, as well as L A per plant were calculated. Variance typically declines with increasing population density (Spitters 1983). Hence, an F-max test (Dr. A . Kozak, Professor, Faculty of Forestry U.B.C. , Pers. comm.) was carried out on all the data sets. Heterogeneity of variance was detected. Mean values were therefore weighted by the inverse of the variance, thus affording greater weight to means with lesser variance. Analyses of variance were carried out on the weighted means of all primary plant measures. In order to quantify intra- and interspecific influences, inverse yield-density models were developed using multiple non-linear regressions of the inverse of yield on component species population densities (X): YB'] = aBo + aBB^B + a B L A l [3.0] yL ' = aL0 + aLL^L + aLB^B [3.1] Subscript B of the inverse yield-density models designates broccoli and subscript L designates lamb's-quarters. Parameters of the inverse yield-density models were used to 80 evaluate growth potential of the target species in the absence of competition (l/aeo and 1/aLo), intraspecific competitive effects (aeB and am), and interspecific competitive effects (aBL and atB)- The ratios SLBL/^BB and ate/an, known as substitution rates (Spitters 1983), were used to indicate the balance of intra- to interspecific effects. Multiple non-linear regressions of reciprocal yield per plant (y"1) were weighted by the inverse of the variance of y"1. Measures of yield included W R , Ws, WL, W, and L A . Data from blocks were pooled because 'block x treatment' interactions were rarely significant. Data from each year required independent models, as significant effects of time were observed. Similarly, separate models were developed for the two levels of U V - B radiation, due to significant effects of U V - B treatments. Regression parameters were evaluated individually using T-tests. A density interaction term (XBXL) was not included in the regressions as interaction terms were not significant. T-tests were also used to estimate whether regression parameters of models developed for 4 kJ m"2 d"1 UV-BBE radiation differed from corresponding regression parameters of models developed for 7 kJ m"2 d"1 UV-BBE radiation. 3.4 R E S U L T S 3.4.1 Visual observations In the first experiment, upward cupping of cotyledons and leaves occurred in broccoli 2 1 seedlings during the first week of exposure to 7 kJ m" d' UV-BBE radiation. Lamb's-quarters seedlings grown at the two U V - B levels were indistinguishable. During the second week of U V - B exposure, lamb's-quarters seedlings were at the six-leaf stage. Broccoli seedlings were generally taller (at the one- to two-leaf stage), and therefore had a height advantage over lamb's-quarters seedlings. Canopy closure (i.e. foliage overlap among neighbouring shoots) occurred in all but the low density flats. Upward cupping of broccoli leaves and occasional bleaching of broccoli leaf margins was observed in both U V - B treatments. Plant density was high enough to impede air circulation, which may result in ethylene build up and cause upward leaf cupping (Dr. B. Holl, Professor, Faculty of Agricultural Sciences, U.B.C. , Pers. comm.). Little ethylene was detected, however, in four air samples (1 ml/ sample) taken randomly from within plant canopies of each chamber and examined using a 5830A Gas chromatograph (Hewlett Packard Chicago, IL, US). 81 Chambers were elevated by placing blocks (10 cm high) under each corner, in order to ensure adequate air circulation. Mylar flaps on two sides of each chamber were also pulled back and secured daily from 3 p.m. to 8 a.m. while the U V - B lamps were turned off. Broccoli plants appeared to be much more vigorous during the third week of 2 1 treatments. Leaves of broccoli monocultures grown at 7 kJ m" d" UV-BBE radiation 2 1 appeared darker green compared with those grown at 4 kJ m" d" UV-BBE radiation. At this time, lamb's-quarters plants generally had gained a height advantage over broccoli plants. 2 1 In the final week of growth, lamb's-quarters leaves exposed to 7 kJ m" d" UV-BBE radiation appeared lighter green and upper leaves were occasionally chlorotic. In the second experiment, upward cupping of leaves and' cotyledons occurred in broccoli seedlings grown at both U V - B radiation levels, while no UV-B-effects were visible in lamb's-quarters seedlings during the first week. Canopy closure began in the high-density flats during the second week of UV-B-treatments. Necrotic leaf margins were occasionally observed in lamb's-quarters plants grown at both U V - B levels. Lamb's-quarters plants grew less vigorously in the second compared with the first experiment. Senescence of lower leaves in the lamb's-quarters canopy was observed during the final week of UV-B-exposure. Due to less vigorous growth during the second experiment, harvesting was carried out following 5 weeks of UV-B-treatments compared with 4 weeks in the first experiment. 3.4.2 Analysis of variance These results focus on biomass accumulation per plant as it provides a good indication of treatment effects on growth. Means and standard errors of all data are given in Appendix 3.2. Detailed results of components of per plant biomass accumulation and growth (WR, Ws, WL, and L A ) are not described here, but are presented in Appendices 3.3 and 3.4. Differential changes in these components due to treatment effects are dealt with in Chapter 4 through the use of plant growth indices and allometric analyses. In 1999, analysis of variance (ANOVA) indicated that broccoli biomass per plant (W) declined in response to increasing population densities of broccoli (X B) and lamb's-quarters (XL), but was unaffected by U V - B radiation (Table 3.2). Block effects were Table 3.2. A N O V A results (variance ratios) for the effect of U V - B radiation, and broccoli (XB), and lambs-quarters (XL) densities on broccoli and lamb's-quarters weighted mean biomass per plant (W) in 1999. Source of variation df Broccoli Lamb's-quarters Block 3 12.617b* 3.643 * U V B 1 0.283 NS 56.576 * X B 2 13.400 * 33.232 * X L 3 55.715 * 68.841 * Block x U V B 3 0.541 NS 1.868 NS Block x XB 6 1.051 NS 1.450 NS U V B x X B 2 0.392 NS 8.662 * Block x X L 9 1.806 NS 0.463 NS U V B x X L 3 1.595 NS 17.225 * X B X X L 6 4.886 * 8.919 * U V B x X B x X L 6 0.308 NS 6.016 * Error 51 * Significant at P < 0.05; NS: not significant. bMeans and standard errors of data are given in Appendix 3.2 83 detected, however, block x treatment interactions were absent. The population density interaction (X B x X L ) was significant. Lamb's-quarters biomass per plant in 1999 was significantly reduced by U V - B radiation, and broccoli and lamb's-quarters population densities (Table 3.2). Block effects were significant and block x treatment interactions absent. Other interactions including U V - B radiation x broccoli population density (UV-B x XB), U V - B radiation x lamb's-quarters population density (UV-B x X L ) , broccoli x lamb's-quarters population densities (X B x XL) and the three-way interaction (UV-B x X B x X L ) were significant for lamb's-quarters per plant biomass. In 2000, broccoli biomass per plant increased in response to enhanced U V - B radiation and decreased in response to broccoli and lamb's-quarters population densities (Table 3.3). Block effects and block x treatment interactions were absent. There was a significant U V - B radiation x lamb's-quarters population density interaction (UV-B x XL) and population density interaction (X B x XL). Lamb's-quarters biomass per plant declined in response to 7 kJ m" d" U V - B B E radiation, and in response to increasing broccoli and lamb's-quarters population densities (Table 3.3) in 2000. Block effects and treatment interactions were absent, however all other interactions including U V - B radiation x broccoli population density (UV-B x X B ) , U V - B radiation x lamb's-quarters population density (UV-B x XL), broccoli x lamb's-quarters population densities (X B x XL), and the three-way interaction (UV-B x X B x XL) were significant for lamb's-quarters biomass per plant. 3.4.3 Inverse yield-density relationships Both inverse yield-density regressions and A N O V A s were carried out to test for effects of population density and U V - B radiation. These methods of analysis differ however, in the scale of the treatment means used to evaluate treatment effects. Treatment means (y) weighted by the inverse of the variance were used in A N O V A s , while the means of the inverse of biomass per plant (y"1) weighted by the inverse of the variance of y"1 were used to evaluate treatment effects in inverse yield-density regressions. Significant inverse yield-density relationships were obtained for average reciprocal biomass per plant (W"1) of Table 3.3. A N O V A results (variance ratios) for the effect of U V - B radiation, and broccoli (XB), and lambs-quarters (XL) densities on broccoli and lamb's-quarters weighted mean biomass per plant in 2000. Source of variation df Broccoli Lamb's-quarters Block 3 0.613" NS 1.430 NS U V B 1 77.713 * 366.526 * X B 2 84.256 * 151.816 * X L 3 45.059 * 206.422 * Block x U V B 3 0.555 NS 1.765 NS Block x XB 6 0.485 NS 1.064 NS U V B x X B 2 0.665 NS 8.008 * Block x X L 9 1.05 NS 1.299 NS U V B x X L 3 3.501 * 110.303 * X B x X L 6 10.484 * 21.736 * U V B x X B x X L 6 1.485 NS 12.435 * Error 51 * Significant at P < 0.05; NS: not significant. bMeans and standard errors of data are given in Appendix 3.2. 85 broccoli a n d lamb's-quarters, at both levels of U V - B radiation and in both years (Tables 3.4 and 3.5). The inverse of the regression intercepts (i.e. l/aso and 1/aLo) (Eqns. 3.0 and 3.1) quantify growth potential of the target species in isolation. Significance was detected for half of the regression intercepts (Tables 3.4 and 3.5). Actual growth of a target plant in a mixture deviates from potential growth in isolation due to intraspecific competition (quantified by coefficients aeB (broccoli) and aLL (lamb's-quarters)) and interspecific competition (quantified by coefficients a B L (broccoli in competition with lamb's-quarters) and aLB (lamb's-quarters in competition with broccoli)). Regression coefficients, evaluated using T-tests, differed significantly from zero (Tables 3.4 and 3.5) with the exception of the intraspecific coefficient for lamb's-quarters ( a ix ) grown at 7 kJ m"2 d"1 U V - B B E radiation in 2000. T-tests were used to estimate whether corresponding regression intercepts and coefficients from inverse yield-density models for 4 versus 7 kJ m" d" U V - B B E radiation differed significantly. Corresponding regression intercepts and coefficients from inverse yield-density models for 4 versus 7 kJ m"2 d"1 UV-BBE radiation did not differ significantly with the following exceptions: regression intercepts for lamb's-quarters (auO in both years and the coefficient aBL (broccoli in competition with lamb's-quarters) in 2000 (Tables 3.4 and 3.5). In both years, the slope of the regression plane assessing sensitivity of broccoli to lamb's-quarters density was less steep at 7, than at 4 kJ m"2 d"1 U V - B B E radiation (Figs. 3.1 and 3.2), indicating that broccoli biomass per plant was less affected by increasing density of lamb's-quarters at the elevated compared with ambient level of U V - B radiation. In 2000, the slope of the regression plane for lamb's-quarters shifted in response U V - B treatments, becoming steeper (more sensitive) to increasing broccoli density and less steep (less 2 i sensitive) to increasing lamb's-quarters density at 7 compared with 4 kJ m" d" UV-BBE radiation. In 1999, with broccoli as the target species, values of regression coefficients aBL were greater than aeB at both levels of U V - B radiation (Table 3.4). Consequently, the substitution rates asi/aBB were greater than 1.0. However, the coefficient representing • 2 1 intraspecific competitive influences ( a B B ) increased in magnitude in response to 7 kJ m" d" UV-BBE radiation, while the coefficient representing interspecific competitive influences 86 Table 3.4. Regression parameters and statistics for inverse yield-density models (Eqns. 3.1 and 3.2) describing the response of reciprocal biomass per plant ( W 1 ) of (a) broccoli and (b) densities in 1999. lamb's-quarters to broccoli and lamb's-quarters population (a) Broccoli UV-BBE (kJ m' 2 d"1) aBo aBB aBL aBL/a BB R 2 P 4 1.11 NS (0.82) 0.0120* (0.0043) 0.0266 * (0.0035) 2.22 0.71 O.0001 7 1.34 NS (1.04) 0.0138 * (0.0048) 0.0185 * (0.0030) 1.34 0.55 O.0001 4 vs. f NS NS NS (b) Lamb's-quarters aLo aix aLB aiVaix R 2 P 4 -0.69 * (0.29) 0.0088 * (0.0017) 0.0040 * (0.0007) • 0.45 0.81 <0.0001 7 0.33 NS (0.35) 0.0062 * (0.0019) 0.0057 * (0.0012) 0.92 0.72 O.0001 4 vs. 1 * NS NS * Regression intercepts and coefficients differ significantly from zero according to a T-test (P < 0.05) or do not differ (NS). Values in brackets below the regression parameters are standard errors of estimate. •Corresponding regression intercepts and coefficients from inverse yield-density models developed at 4 versus 7 kJ m"2 d"1 U V - B B E according to a T-test; differ significantly (P < 0.05, *) or do not differ (NS). 87 Table 3.5. Regression parameters and statistics for inverse yield-density models (Eqns. 3.1 and 3.2) describing the response of reciprocal biomass per plant (W 1 ) of (a) broccoli and (b) lamb's-quarters to broccoli and lamb's-quarters population densities in 2000. (a) Broccoli UV-BBE (kJ m"2 d"1) aBO aBB aBL aBi/aBB R 2 P 4 3.57 * (1.27) 0.021 * (0.0054) 0.0217 * (0.0018) 1.0 0.94 O.0001 7 3.01 * (0.88) 0.020 * (0.0045) 0.0079 * (0.0022) 0.4 0.76 O.0001 4 vs. f NS NS * (b) Lamb's-quarters a w aLL aLB aLB/aLL R2 p 4 0.018 NS (0.41) 0.012 * (0.0021) 0.0047 * (0.001) 0.4 0.80 O.0001 7 2.96* (0.87) 0.0063 NS (0.0033) 0.0075* (0.0021) 1.2 0.77 O.0001 4 vs. 1 * NS NS *Regression intercepts and coefficients differ significantly from zero according to a T-test (P < 0.05) or do not differ (NS). Values in brackets below the regression parameters are standard errors of estimate. Corresponding regression intercepts and coefficients from inverse yield-density models developed at 4 versus 7 kJ m"2 d"1 U V - B B E according to a T-test; differ significantly (P < 0.05, *) or do not differ (NS). 88 1999 Broccoli Lamb's-quarters Broccoli Lamb's-quarters 7 kJ m"2 di1 U V - B B E radiation 3.1. Inverse yield-density relationships (planar surfaces) for mean inverse total biomass per plant (W"1) of broccoli (left panels) and lamb's-quarters (right panels) grown at 4 kJ m"2 d"1 UV-BBE radiation (top panels) and 7 kJ m"2 d"1 U V - B B E radiation (bottom panels) in 1999. 89 2000 Broccoli Lamb' s-quarters Broccoli Lamb's-quarters 7 kJ m"2 d"1 U V - B B E radiation Fig. 3.2. Inverse yield-density relationships (planar surfaces) for mean inverse total biomass per plant ( W 1 ) of broccoli (left panels) and lamb's-quarters (right panels) grown at 4 kJ m"2 d"1 UV-BBE radiation (top panels) and 7 kJ m' 2 d"1 U V - B B E radiation (bottom panels) in 2000. 90 (aeO declined. This resulted in a decline in the magnitude of the substitution rate (aBi/aBB) at 7 compared with at 4 kJ m"2 d"1 U V - B B E radiation. With lamb's-quarters as the target species in 1999 (Table 3.4), coefficients representing intraspecific competitive influences (aix) were greater than those representing interspecific competitive influences (3LB) at both levels of UV - B radiation, resulting in substitution rates (aLe/aix) less than 1.0. The coefficient a ^ decreased and the coefficient 3LB increased in response to the elevated UV - B level, resulting in a n increase in the magnitude of the substitution rate at 7 compared with 4 kJ m"2 d"1 UV-BBE radiation. In 2000, broccoli plants were equally sensitive to both species (substitution rate = 1.0) at 4 kJ m"2 d"1 U V - B B E radiation (Table 3.5). However, at 7 kJ m"2 d"1 U V - B B E radiation, intraspecific influences (aBB) were greater than interspecific influences (asO indicating that broccoli biomass per plant was more affected by presence of broccoli than by members of the companion species. Both coefficients &BB and &BL declined at 7 kJ m" d" UV-BBE radiation, however the extent of the decline was greater for interspecific influences (aBL) resulting in a decline in the magnitude of the substitution rate in response to 7 compared with 4 kJ m"2 d"1 U V - B B E radiation. With lamb's-quarters as the target species in 2000, the coefficient representing intraspecific competitive influences (aix) was greater than that representing interspecific competitive influences (ay}) at 4 kJ m"2 d"1 UV-BBE radiation (Table 3.5). The substitution rate was therefore less than 1.0 indicating the lamb's-quarters was more sensitive to other lamb's-quarters plants than to broccoli. At 7 kJ m"2 d"1 UV-BBE radiation, the coefficient aLB increased, while the coefficient a i x decreased. In fact, a i x (the regression coefficient expressing intraspecific effects) was not significant at 7 kJ m" d" UV-BBE, indicating that lamb's-quarters density did not make a significant contribution to the inverse yield-density relationship at this level of UV - B radiation. The substitution rate at 7 kJ m" d" UV-BBE radiation was greater than 1.0. To facilitate discussion of results, a summary of Tables 3.4 and 3.5 is presented as direction and magnitude of percent changes in regression coefficients and substitution rates at 7 compared with 4 kJ m"2 d"1 U V - B B E radiation (Table 3.6). Table 3.6 Percent change in regression coefficients and substitution rates for inverse yield-density models describing the response of reciprocal biomass per plant (W"1) of (a) broccoli and (b) lamb's-quarters to broccoli and lamb's-quarters population densities established at 7 compared with 4 kJ iri d ' ' U V - B B E radiation in 1999 and 2000. (a) Broccoli Year a B B* a B L a B L / a B B 1999 +15 -31 -40 2000 -7 -64 -60 (b) Lamb's-quarters Year a L L a L B a L B / a L L 1999 -30 +43 +104 2000 -47 +59 +200 * a B B a n d a B L = i n t r a - a n d i n t e r s p e c i f i c c o m p e t i t i v e e f fects , r e s p e c t i v e l y w i t h b r o c c o l i as the target s p e c i e s . aLL a n d aLB = i n t r a - a n d i n t e r s p e c i f i c c o m p e t i t i v e e f fec ts , r e s p e c t i v e l y w i t h l a m b ' s - q u a r t e r s as the target s p e c i e s . aBi7aBB a n d a L B / a L L ~ s u b s t i t u t i o n rate i n d i c a t i n g b a l a n c e o f i n t e r - to i n t r a s p e c i f i c c o m p e t i t i v e e f fec ts (Sp i t t er s 1983). 92 The multiple coefficient of determination (R 2) for the inverse yield-density model measures the importance of competition as a source of overall yield variation (Weldon and Slauson 1986). These values varied widely (0.55 to 0.94) with respect to •y species and level of U V - B radiation. However, R values were consistently higher, indicating a greater importance of competition, for relationships derived at 4 compared with 7 kJ m"2 d" 'UV-B B E radiation (Tables 3.4 and 3.5). 3.5 DISCUSSION Under non-stressful, or weakly stressful conditions, plant growth exhibits plasticity in response to resource availability. In the present study, total biomass accumulation per plant of both broccoli and lamb's-quarters declined in response to increasing population density of members of their own species, as well as that of the associated species (Tables 3.2 and 3.3). These findings are in agreement with other studies. For example, increasing lamb's-quarters density has been shown to severely reduce growth of associated species including broccoli (Brassica oleracea L. var. botrytis cv. Emperor) (Bitterlich and Upadhyaya 1990), and tomato (Lycopersicon esculentum Mill. cv. Ailsa Craig) (Qasem and Hi l l 1994). Lamb's-quarters is also sensitive to intraspecific interference (Qasem and Hil l 1994, Nicotra and Rodenhouse 1995). In addition to decreasing biomass per plant in response to increasing plant density, lamb's-quarters biomass per plant also declined in response to elevated U V - B radiation in both years (Tables 3.2 and 3.3). Conversely, broccoli biomass per plant was unaffected by U V - B radiation in 1999 (Table 3.2) and increased at 7 kJ m"2 d' 1 U V - B B E in 2000 (Table 3.3). Caldwell (1977) hypothesized that that since plants differ in growth response to U V - B radiation, reduced productivity of a relatively more UV-sensitive species could increase productivity of an associated less UV-sensitive species as a result of increased resource availability to the tolerant species. Resources that broccoli and lamb's-quarters may have competed for include above-ground (light and carbon dioxide), and below-ground (nutrients and oxygen) resources. Some components of resource dependence were diminished in this study by supplying plants with ample water and nutrients. In crops where fertilizers and irrigation are applied, P A R often becomes a limiting factor (Caldwell 1987). Whether broccoli had 93 increased access to P A R at 7 compared with 4 kJ m"2 d"1 U V - B B E radiation as a result of UV-B-induced biomass reduction or morphological changes in the companion species will be explored in Chapter 5. Most studies on effects of elevated U V - B radiation on competition have been limited to influences on above-ground biomass (Fox and Caldwell 1978, Bogenrieder and Klein 1982, Gold and Caldwell 1983, Barnes et al. 1988). Total, as opposed to above-ground, biomass per plant may better express overall competitive outcome at differing levels of U V - B radiation (Yuan et al. 1999) because dry-matter distribution within the plant can vary with U V - B exposure (Teramura 1983) and competitive stress (Rohrig and Stutzel 2001). For example, enhanced U V - B radiation may alter the shootroot ratio in some species (Teramura 1980, Sullivan and Teramura 1989, Furness et al. 1999). Changes in above-ground biomass under enhanced U V - B radiation may result from a shift in the allocation of plant resources between above- and below-ground biomass rather than a decrease in total plant biomass (Gold and Caldwell 1983). Similarly, a limiting resource often stimulates a shift in biomass allocation, resulting in a proportional increase favouring that component of the plant that draws most upon the limiting environmental factor (Hunt 1988). For example, nutrient- and/or water-limited plants may increase relative biomass allocation to roots, whereas shaded plants may increase relative biomass allocation to shoots. Additionally, total plant biomass accumulation represents a cumulative integration of all biochemical, physiological, and growth parameters (Teramura 1983) and therefore provides a good indication of treatment effects on growth. Total biomass accumulation per plant also reflects resource acquisition, and hence competition. Total biomass ( W 1 ) per plant was therefore used to evaluate overall competitive outcome in this study. Shifts in components of biomass per plant, including root, stem, and leaf biomass, as well as leaf area will be dealt with through the use of plant growth indices and allometric analyses in Chapter 4. The relationship between decreasing individual plant size and increasing density is termed the "reciprocal yield law" (Shinozaki and Kira 1956). The decline in per plant yield with increasing plant population density has been effectively expressed in a large number of studies using inverse yield-density relationships (Jolliffe 1997). Such relationships 94 provide a useful way to condense copious yield data into biologically relevant information. In this study, inverse yield-density models described the behaviour of broccoli and lamb's-2 1 quarters in response to increasing plant population densities at 4 and 7 kJ m" d" UV-BBE radiation in both years (Tables 3.4 and 3.5). Declining biomass per plant, in response to increasing plant density, implied that competition for resources was likely a fundamental determinant of biomass per plant at both levels of UV-B radiation. However, as noted by Wanjau (1998) and Turkington and Jolliffe (1996), other mechanisms of interference cannot be discounted. Such mechanisms might include allelopathy, or sheltering from potentially adverse conditions including exposure to elevated U V - B radiation. The inverse yield-density models developed in this study (Eqns. 3.0 and 3.1) were always significant (P < 0.0001) (Tables 3.4 and 3.5) and usually explained a substantial portion of the variation in total biomass (W"1) per plant. The importance of competition in relation to all other factors affecting plant growth is indicated by the coefficient of determination (R 2) (Weldon and Slauson 1986), which ranged from 0.71 to 0.94 at 4 kJ m"2 d"1 UV-BBE , and 0.55 and 0.77 at 7 kJ m"2 d"1 U V - B B E radiation in this study. The lower R 2 values obtained at 7 kJ m"2 d"1 U V - B B E may be a result a more stressful environment due to enhanced U V - B radiation. When plant yield is restricted by non-competitive factors, inverse yield-density models may not be applicable (Turkington and Jolliffe 1996). In inverse yield-density relationships, the growth potential of the target species in the absence of competition is estimated by the reciprocal of the regression intercepts (i.e. 1/aso and 1/aLo for broccoli and lamb's-quarters, respectively) (Eqns. 3.0 and 3.1). However, the regression intercepts were significant in only half of the cases (Tables 3.4 and 3.5). Estimation of the regression intercept requires extrapolation well beyond the range of experimental data described by inverse yield-density equations (Spitters 1983). Extrapolation is problematic because at very low densities, the nature of the relationship may change. For example, plants growing at low densities may be fully exposed to the elevated level of potentially harmful U V - B radiation thereby decreasing their growth potential. Alternatively, facilitation (beneficial influences) may occur through protection from the UV-B-enriched environment. At a limited range of very low densities, there may be no effect of increasing density on resource allocation, including P A R interception. In addition, variance typically declines with increasing plant population density (Spitters 95 1983). Greater weight is given to data with lesser variance (i.e. data generated by the highest density) and this may further influence the estimated value of the intercept. Because only half of the regression intercepts were significant in this study, a comparison of growth potential in the absence of competition at the two levels of U V - B radiation would not be appropriate. Interpretations of the relative competitive abilities of broccoli and lamb's-quarters in associations however, remain valid as they do not rely on these parameter estimates. Actual growth of the target species in a mixture deviates from potential growth in response to intra- and interspecific competitive influences. Regression coefficients asB (broccoli) and a^L (lamb's-quarters) measure intraspecific competitive influences. Interspecific competitive influences are measured by aBL (broccoli as the target species) and aLB (lamb's-quarters as the target species). Simple T-tests were used to determine whether corresponding regression coefficients from inverse yield-density models 9 1 developed at 4 versus 7 kJ m" d" U V - B B E radiation differed significantly. Significance was not detected between coefficients that measured either intra- or interspecific competitive influences with the exception of interspecific competitive influences for broccoli (ast) in 2000 (Tables 3.4 and 3.5). These findings do not preclude the usefulness of the regression coefficients as the T-tests provide only an approximation of treatment effects (Dr. A . Kozak, Professor, Faculty of Forestry, U .B .C., Pers. comm.). Additionally, population density effects were detected in analyses of variance carried out using the relatively more sensitive F-test. The slopes of the plane for each inverse yield-density model describe the relationship between reciprocal per plant biomass (W*1) and population density of broccoli (X B) and lamb's-quarters (XL) (Figs. 3.1 and 3.2). A shift in either or both slope(s) in response to treatment effects would indicate a change in the sensitivity of W"1 to variation in broccoli and/or lamb's-quarters population density. In 1999, broccoli sensitivity to lamb's-quarters diminished in response to UV - B radiation as indicated by the less steep slope of the regression plane in response to lamb's-quarters density (Fig. 3.1). However, at both levels of UV - B radiation broccoli was more sensitive to lamb's-quarters than to members of its own species as indicated by substitution rates (aBi/-BB) of >1.0 (Table 3.4). At 4 kJ m' 2d"' UV-BBE radiation, with broccoli as the 96 target species interspecific competitive influences (asO were greater than intraspecific influences (aeB)- At 7 kJ m' 2 d"1 UV-BBE radiation, interspecific competitive influences still dominated, however the intensity of intraspecific influences increased slightly while interspecific influences declined (Table 3.6). Conversely with lamb's-quarters as the target species in 1999, intraspecific competitive (aix) influences were greater than interspecific influences ( a L B ) at both levels of U V - B radiation (Table 3.4). However, intraspecific competitive influences declined and interspecific influences increased in intensity at 7 compared with 4 kJ m"2 d"1 U V - B B E radiation (Table 3.6). Lamb's-quarters responded more to itself than to broccoli at both levels of U V - B radiation (Fig. 3.1). In 2000, broccoli responded less to lamb's-quarters presence compared with the year 1999 (Figs. 3.1 and 3.2). For example, broccoli responded similarly to itself and to lamb's-quarters presence at 4 kJ m"2 d"1 U V - B B E radiation (Table 3.5). At 7 kJ m"2 d"1 UV-BBE radiation however, interspecific competition declined to a much greater extent than intraspecific competition (Table 3.6). It has been suggested that for lamb's-quarters, intensity of intraspecific competition is greatest under optimal resource conditions (Nicotra and Rodenhouse 1995). Above-ambient U V - B levels may represent a stressful environment for lamb's-quarters. As a result of UV-B-induced reductions in per plant biomass, lamb's-quarters might have been unable to fully exploit available resources. This is supported by the decreased intensity of intraspecific competitive influences of lamb's-quarters grown at the above-ambient compared with ambient U V - B level (Tables 3.5 and 3.6). In 2000, the regression coefficient representing intraspecific competitive influences (aa) at the above-ambient U V - B level did not differ significantly from zero, revealing that lamb's-quarters population density contributed to the inverse yield-density relationship at the ambient, but not the above-ambient U V - B radiation level (Table 3.5). Lower PAR levels in 2000 (140 (overcast 9 I sky) - 645 pmol m" s" (clear day)) compared with the year 1999 (520 (overcast sky) -9 1 1240 pmol m" s" (clear day)) may also have contributed in part to the less vigorous growth and reduced competitive ability of lamb's-quarters in 2000. Additionally, 9 1 interspecific competition increased at 7 kJ m" d" UV-BBE radiation (Tables 3.5 and 3.6). 97 Barnes et al. (1988) hypothesized that intraspecific competition would be less influenced than interspecific competition by U V - B radiation since growth changes among individuals of the same species would probably be more uniform resulting in no net change in intraspecific competition. Enhanced UV - B radiation did not affect intraspecific competitive responses of wheat (cv. Bannock), wild oat, or jointed goatgrass (Aegilops cylindrica Host.) grown at a range of monoculture densities (Gold and Caldwell 1983). Conversely, Fox and Caldwell (1978) found, in numerous species mixtures, that relative proportion of intra- versus interspecific competition had inconsistent effects on the suppression of biomass of individual species by UV - B radiation. These findings however, were based on experiments carried out using a replacement series experimental design. Such designs have been criticized for confounding species density treatments and as a result they are not conducive to separation of the individual contributions to interference by each species in the mixture (Jolliffe 2000). Through the regression coefficients of the inverse yield-density models, however, intra- and interspecific components can be separated mathematically (Spitters 1983). In agreement with the hypothesis of Barnes et al. (1988), interspecific competition (a sL and aLB; broccoli and lamb's-quarters respectively) was influenced by U V - B radiation to a greater extent than intraspecific competition (aBB and aLL; broccoli and lamb's-quarters, respectively) in both years (Table 3.6). These findings imply that U V - B effects might be exacerbated in mixed plant associations, and of less importance in pure stands. Substitution rates (Connolly 1987), the ratio between the inter- and intraspecific effects ( a B i 7aB B , broccoli as the target species and aLB/aLL, lamb's-quarters as the target species), express how many individuals of the companion species would be needed to substitute for a single individual of the target species in order to maintain constant yield per individual of the target species in mixture (Turkington and Jolliffe 1996). The substitution rates indicate the balance of inter- to intraspecific competitive influences (Spitters 1983) and therefore conveniently summarize overall competitive influences. Substitution rates for broccoli as the target species (a B i /aB B ) declined in both years at 7 compared with 4 kJ m" d" U V - B B E radiation indicating that broccoli increased in competitiveness at the above-ambient compared with the ambient level of U V - B radiation (Tables 3.4, 3.5, and 3.6). Conversely, the substitution rate for lamb's-quarters ( aLB/aLL) 98 increased in both years at 7 compared with 4 kJ m" d" UV-BBE radiation suggesting decreased competitiveness of lamb's-quarters at the above-ambient compared with the ambient level of U V - B radiation. The magnitude of UV-B-induced change in substitution rate was notably greater for lamb's-quarters than for broccoli (Table 3.6). The magnitude of the substitution rate was used as a measure of the relative competitive strength of broccoli (aai/aBB) versus lamb's-quarters (3LB/&LL)- For example, at 4 kJ m"2 d"1 UV - B BE radiation, lamb's-quarters was the stronger competitor in both years as indicated by the smaller value of the substitution rate compared with broccoli (Tables 3.4 and 3.5). However, responsiveness to interference differed between experiments at 7 kJ m"2 d"1 UV-BBE radiation, with lamb's-quarters being a stronger competitor than broccoli in 1999, and broccoli being the stronger competitor in 2000. In field studies, inconsistent results in competition experiments repeated over time or space often are attributed to environmental variation (Zimdahl 1980, Coble et al. 1981, Hagood et al. 1981). For example, shifts in competitive balance with wheat (cv. Bannock) gaining a competitive advantage over wild oat under UV - B enhancement in the field were significant in two out of six years when precipitation was relatively high, but not in years of drought (Barnes et al. 1988). Although a potentially more uniform and controlled environment is attainable under greenhouse conditions where some components of resource dependence can be reduced, variation in PAR levels as noted earlier, occurred between experiments carried out in 1999 and 2000. Interactions between UV - B radiation and PAR levels have been documented with the effects of U V - B radiation being more pronounced at relatively low P A R :UV - B ratios (Cen and Bornman 1990). While PAR levels differed between experiments, UV-B-levels remained constant resulting in a difference in the PAR :UV -B ratios in the two experiments. It is conceivable that lamb's-quarters is relatively more sensitive than broccoli to low PAR :UV -B ratios. This may have contributed to the inconsistent competitive response as growth of lamb's-quarters plants was less vigorous at 7 kJ m"2 d"1 U V - B B E radiation under the lower P A R levels in the experiment carried out in 2000. Shifts in competitive balance have been shown to occur in response to U V - B radiation at ambient compared with below ambient UV-B-levels (Bogenrieder and Klein 1982), as well as at enhanced UV-B-levels (Fox and Caldwell 1978, Gold and Caldwell 99 1983, Barnes et al. 1988). Such shifts observed in the field and glasshouse experiments were associated with UV-B-induced morphological changes, such as differential effects on height and leaf area accumulation for two species within a mixed canopy. While an under-story species might be expected to gain a competitive advantage as a result of a relatively reduced UV-B-flux in the shelter of the taller species (Bogenrieder and Klein 1982), taller species generally increase in competitive status even under UV-B-enhancement as a result of increased ability to intercept PAR (Barnes et al. 1988). Whether shifts in biomass allocation or morphology of broccoli or lamb's-quarters plants can be associated with ability to compete for light will be explored in Chapters 4 and 5. In conclusion, inverse yield-density models successfully described per plant biomass accumulation of broccoli and lamb's-quarters in response to increasing plant densities (Tables 3.4 and 3.5). Inverse yield-density relationships were often stronger (higher R 2 values) at 4 (simulated ambient UV - B radiation) compared with 7 kJ m"2 d"1 UV-BBE radiation (above-ambient UV - B radiation), implying that biomass per plant may have been somewhat restricted by non-competitive factors under enhanced U V - B radiation. For example, plant responsiveness to density treatments may have been depressed by UV-B-induced suppression of lamb's-quarters growth. At the ambient level of U V - B radiation, interspecific competitive influences prevailed for broccoli, while intraspecific competitive influences were dominant for lamb's-quarters (Tables 3.4 and 3.5). At above-ambient UV-B radiation, the competitive relationship between broccoli and lamb's-quarters differed between the two years of study. Lamb's-quarters was a stronger competitor than broccoli at the ambient level of U V - B radiation in both years and at the above-ambient level in 1999. In 2000, broccoli was the stronger competitor at the above-ambient level of U V - B radiation. In both years and for both species however, intraspecific competition was less influenced than interspecific competition by UV - B radiation (Table 3.6). Overall, broccoli gained in competitiveness relative to lamb's-quarters in response to above-ambient UV - B radiation. Although it is inappropriate to extrapolate results from UV-B-greenhouse studies to field conditions, these findings indicate that above-ambient U V - B radiation has the potential to alter the population structure of plant communities through its influence on intra- and interspecific competitive interactions. 100 4.0 Growth indices and allometric analyses of broccoli (Brassica oleracea var. italica cv. Purple Sprouting) and lamb's-quarters (Chenopodium album L.) associations grown at two levels of UV-B radiation. 4.1 A B S T R A C T Broccoli (Brassica oleracea L. var. italica cv. Purple Sprouting) and lamb's-quarters (Chenopodium album L.) were grown in a greenhouse in monocultures (144, 256, and 400 plant m"2) and all binary mixtures during the summer of 1999 and the fall of 2000. Plants were exposed to 4 or 7 kJ m"2 d"1 U V - B B E radiation in a randomized block design with four replications. Per plant leaf area, shoot height and biomass components (root, stem, leaf, and shoot) were obtained at harvest and used to construct plant growth indices and expanded allometric relationships. Growth indices indicated that morphology and biomass partitioning were influenced by U V - B radiation and species density treatments. Response to U V - B radiation over the two years differed for broccoli, and was consistent for lamb's-quarters. Leaf area index of broccoli increased, while that of lamb's-quarters declined at high U V - B radiation in both years. Both species responded to increasing plant densities by a decrease in specific leaf weight (thinner and/or less dense leaves). Leaf area index of both species generally increased with increasing density of their own species and declined with increasing density of the companion species. For other indices including leaf weight ratio, leaf area ratio, and shootroot ratio, lamb's-quarters was generally more sensitive to presence of its own species, while broccoli was more sensitive to lamb's-quarters in 1999 and to itself in 2000. Simple allometric models showed total plant growth of broccoli and lamb's-quarters was proportional to growth of components of biomass and leaf area. Simple allometric models relating total biomass per plant to height explained less variation in total biomass per plant. Allometric adjustments accounted for further variation in total biomass per plant, although direct, non-allometric responses to U V - B radiation and species densities also occurred. Overall, plant densities had greater influence compared with U V - B treatments on allometric adjustments. Direct, non-allometric treatment effects were more often detected in broccoli than in lamb's-quarters. The range of responses of broccoli and lamb's-quarters to U V - B radiation and 101 competitive influences is reflected in the complex behaviour of the plant growth indices, as well as in allometric and non-allometric relationships. 4.2 I N T R O D U C T I O N Ultraviolet-B radiation has the potential to alter the population structure of plant communities through its influence on competitive interactions (Chapter 3). Inverse yield-density relationships using total biomass per plant as a measure of yield were used to gain a first approximation of plant performance. Total plant biomass accumulation is a convincing indicator of treatment effects on growth as it represents a long-term integration of all biochemical, physiological, and growth parameters (Teramura 1983). However, reductions in total biomass in response to both U V - B radiation and competitive influences are often accompanied by substantial modifications in the partitioning of biomass into component plant organs. Effects of environmental factors (Rohrig and Stutzel 2001) and interference (Jolliffe et al. 1990) on the physiological plant variables controlling dry matter distribution are complicated, and often poorly understood. The complex nature of competitive interactions among closely associated plants may be further described using a variety of measures of plant performance and indices of growth (Wanjau 1998). Methods of plant growth and allometric analyses can expand our understanding of plant responses to environmental influences including U V - B radiation and competitive interactions. Conventional plant growth analysis emphasizes physiological and morphological sources of variation in growth rates (Warren Wilson 1981, Hunt 1990), and deals with plant proportions, as expressed through ratios of morphological measures. An even more profound interpretation of plant growth indices can be achieved using allometric techniques, which typically assess proportionality between pairs of growth measures (Wanjau 1998). Conventional plant growth and allometric analyses are often used to evaluate trends in growth and yield variation over time. However, data from a single time of observation can be used to construct some growth indices including the following: specific leaf weight (SLW) [leaf dry weight/leaf area], leaf weight ratio (LWR) [leaf dry weight/ shoot dry weight], leaf area ratio (LAR) [leaf area/shoot dry 102 weight], leaf area index (LAI) [leaf area x plant density], and shoot:root ratio (SRR) [shoot dry weight/root dry weight). Plant growth indices have been used to investigate plant responses to environmental influences, including U V - B radiation and competition. For example, despite an absolute reduction in leaf area in response to U V - B radiation, dicotyledonous species generally allocate a greater proportion of biomass into leaves and less into stems (Teramura 1983) resulting in an increased LWR. Wanjau (1998) reported that L W R of beans (Phaseolus vulgaris L.) decreased in response to increasing bean population density. A limiting resource (eg. light) can stimulate shifts in biomass allocation, resulting in a proportional increase favouring that component of the plant (eg. shoot) that draws most upon the limiting environmental factor (Hunt 1988). Reallocation of dry matter can occur within a leaf without significantly altering partitioning of dry matter into the leaf tissue. For example, an increase in the L W R was reported to be due to an increase in SLW in soybean (Glycine max (L.) Merr.), bean, pea (Pisum sativum L.), and cucumber (Cucumis sativus L.) exposed to U V - B radiation (Teramura 1983). Additionally, reduction in L A R accompanied by increase in SLW was reported in soybean cv. Essex in response to U V - B radiation (Teramura and Sullivan 1987). Roush and Radosevich (1985) reported that increased L A R contributed to aggressiveness of some weeds. Jolliffe et al. (1990) used conventional plant growth analysis to interpret the complex physiological and morphological adjustments in yield due to effects of population density on forage maize (Zea mays L.). Bakker and Wilson (2001) used plant growth indices to study competition between C 3 and C 4 grasses at differing levels of water availability. They found that competition resulted in increased biomass allocation to below-ground structures. The interpretation of plant growth indices can be furthered using allometric techniques, which typically assess the proportionality between pairs of growth measures. Allometric relationships in plants reveal size-correlated variation in form and development and characterize the relative growth of a part of a plant in comparison with the whole (Reddy et al. 1998). In such cases the evaluation of allometry can help us to understand structural and functional relationships between different levels of organization (Jolliffe et al. 1988). 103 Plant allometry has been investigated for a variety of purposes, including tests of the adaptive value of traits (Le Maitre and Midgley 1991). Some studies suggest that the ability of plants to adapt to the environment can affect allometric relationships (Reekie and Bazzaz 1987, Marvel et al. 1992, Weiner and Thomas 1992). Allometric relations have been reported in a broad variety of plant species, and are viewed as the consequence of natural selection and adaptive evolutionary changes (Niklas 1994). Allometric relations have often been treated as genetically fixed characteristics of plant species (Weller 1987), or as features of a group of species (Niklas 1995), and used as the basis for comparison of different taxonomic groups (Niklas 1994). Geiger et al. (1996) use allometric analysis to assess structure-function relationships. They reported that under diverse environmental conditions regulation of dry matter partitioning acted to maintain allometric growth among plant parts, and to maintain the functional balance between the supply and-use of carbon. Many experimental treatments are known to affect plant form, and some studies have specifically examined allometric responses to treatments (Stanhill 1977, L i et al. 1996). Teramura and Sullivan (1987) found that the effects of U V - B radiation on the allometry of soybeans varied with plant growth stage. Modifications of the root:shoot ratio were reported by Caradus et al. (1995), where white clover (Trifolium repens L.) cultivars responded differently to phosphorus. Gedroc et al. (1996) presented a significant linear allometric relationship for lamb's-quarters plants in a high nutrient regime showing only minor deviations from linearity when the nutrient supply was reduced. Effects of population density-related stress on stem mass versus stem height scaling was observed for annual plants (Weiner and Thomas 1992), and for trees grown in different density stands (Niklas 1994). Using allometric relationships McLachlan et al. (1995) determined that competition altered reproductive effort in redroot pigweed (Amaranthus retroflexus L.), while Weiner and Fishman (1994) recorded significant influences of plant competition on allometry of Russian thistle (Kochia scoparia L.). Stutzel and Aufhammer (1991) showed that planting density and ontogenetic processes significantly influenced dry matter partitioning between leaves and stems of lamb's-quarters. With increasing competition, lamb's-quarters allocated relatively more biomass 104 to stems than to leaves. However, such effects may not be evident i f differences in plant density do not cause variations in stress level (West et al. 1989). The discovery of allometric relationships could help quantify and explain the effect of plant population density and U V - B radiation on growth and development of associated species. The interpretation of allometric relationships can be furthered by the formulation of models, such as the commonly used simple power function, which effectively describe those relationships. Huxley's (1932) suggestion that such a function has wide biological applicability has been supported by subsequent work. An expanded equation introduced by Jolliffe et al. (1988) enables evaluation of several experimental factors on bivariate allometric relationships. Intensity of plant competitive interactions was varied in this thesis by growing broccoli and lamb's-quarters at different population densities as mixtures and monocultures, at two levels of U V - B radiation. As reported in Chapter 3, observations in the competition study permitted intra- and interspecific components of interference to be separated and quantified. Observations from Chapter 3 were used in the current chapter as inputs for plant growth and allometric analyses to explore the influence of experimental treatments on several aspects of growth per plant. The main objective of this work was to expand the understanding of plant response to U V - B radiation at increasing population densities. Specifically, this work was intended to identify the aspects of growth, including dry matter partitioning which were most strongly affected by the experimental treatments, and to explore the stability of growth indices and allometric relationships in response to population density and U V - B radiation treatments. 4.3 M A T E R I A L S A N D M E T H O D S As reported in Chapter 3, broccoli and lamb's-quarters were grown at different population densities in monocultures (144, 256, and 400 plants m") and all binary combinations at 4 and 7 kJ m" d" UV-BBE radiation. Analyses reported here were based on observations from both the 1999 and 2000 experiments on the following primary variables (mean values per plant): biomass of plant (W), root (WR), stem (Ws), leaf (WL), and shoot (WSH); leaf area (LA); and shoot height (H). Actual numbers of observations 105 for each experimental unit used in statistical analyses of data are shown in Chapter 3; Table 3.IB. 4.3.1 Plant growth indices Plant growth indices were used to describe the influence of UV-B-induced morphological changes on interactions in broccoli and lamb's-quarters associations described in Chapter 3. The following growth indices were constructed from data obtained during the final harvest: specific leaf weight (SLW) [lamina dry weight/leaf area (g m")], leaf weight ratio (LWR) [lamina dry weight/shoot dry weight], leaf area ratio (LAR) [leaf area/shoot dry weight (cm2 g 1 ) ; L A R = L W R x SLW" 1], leaf area index (LAI) [leaf area x plant density], and shoot:root ratio (SRR) [shoot dry weight/ root dry weight] (Hunt 1978). Growth indices were constructed for individual plants and means of indices from each experimental unit were calculated. The ratios generated lacked homogeneity of variance and were therefore weighted by the inverse of the variance of the ratio. Analyses of variance were carried out separately for data from the two experiments due to significant time x treatment interactions detected in A N O V A s of the primary growth data in Chapter 3. 4.3.2 Allometric analysis Simple bivariate allometric relationships between biomass per plant (y = W) and various other primary measures per plant including root (z = WR), stem (z = Ws), and leaf (z = WL) biomass, leaf area (z = LA) , and shoot height (z = H) were expressed using the In-transformed allometric power function (Huxley 1932): ln(y) = ln(cc) + pin(z) + In (s) [Eqn. 4.1 ] In this equation, allometric adjustments are characterized through changes in the allometric coefficient [ln(a)] that specifies ln(y) at unit values of z, and the allometric exponent P that reflects the ratio of relative growth rates of y and z (Whitehead and Myerscough 1962). The term ln(e) expresses residual, non-allometric variation in ln(y) not accounted for through allometry with ln(z). Experimental treatment factors may 106 influence ln(y) and its relationship with ln(z) via allometric influences assessed by ln(ot), and P, or via non-allometric influences assessed by ln(s). The expanded allometric equation developed by Jolliffe et al. (1988) was used to detail potential allometric responses to experimental treatments: ln(W) = ln(a') + p0ln(z) + (3,UVBln(z) + p 2X Bln(z) + p 3X Lln(z) + [Eqn. 4.2] p 4 UV-BX B ln(z) + p 5 UV-BX L ln(z) + p 6 X B X L ln (z ) + p 7 U V - B X B X L l n ( z ) + y i ln(UV-B) + y 2 ln(X B) + y 3ln(X L) + y 4 ln (UV-BX B ) + y 5 ln(UV-BX L ) + y 6 l n (X B X L ) + y 7 l n ( U V - B X B X L ) + ln(s') Independent variables of Eqn. 4.2 were generated from ln-transformed primary values [e.g. ln(z) = ln(WR), ln(Ws), III(WL), ln(LA), or ln(H)] and the experimental treatments of U V - B B E radiation (4 or 7 kJ m*2 d"1), broccoli population density ( X B , plants m"2), and lamb's-quarters population density (XL, plants m"). Terms containing p express proportionality between ln(W) and ln(z) and explain allometric adjustments to treatments. Terms involving y account for non-allometric effects of treatments on ln(W). The term ln(a') is the result of grouping of terms and includes both allometric and non-allometric components. Equation 4.2 was developed from experimental data using multiple linear regression analysis. For each measure of ln(z), a best subset multiple regression procedure (SigmaStat 2.0; SPSS Inc., Chicago IL, US) was used to select the subset of terms which best expressed variation in ln(W). Candidate best subset models were rejected if variance inflation factors (a measure of multicollinearity) were greater than 4.0 for any model term. Mallow's Cp (process capability index), calculated for each subset, measured the sum of the squared biases plus the squared random errors in the dependent variable at all data points. The best subset among all the multiple regressions constructed was the subset that gave a relatively low Cp value and high R value. Parameters Pk and yi< (k < 0) in each model differ in units of measurement, depending on the identity of the independent variable with which they are associated. To facilitate comparisons, standard partial regression coefficients were calculated for Pk and Yk values, allowing the contributions of different independent variables to be evaluated in the same scale of measurement. The size and sign of the standard partial regression 107 coefficients indicate relative magnitude and direction of relationship between ln(W) and a specific Z j . 4.4 R E S U L T S Preliminary analysis of the data using the analysis of variance indicated that in both years broccoli biomass per plant (W) declined significantly in response to increasing population densities of broccoli (XB) and lamb's-quarters (XL) (Chapter 3; Table 3.2 and 3.3). Population density interactions were also significant. In 1999, U V - B treatments did not influence broccoli biomass per plant. However, in 2000, exposure to above-ambient (7 kJ m"2 d"1) compared with ambient (4 kJ m"2 d"1) U V - B B E radiation resulted in increased broccoli biomass per plant, and a small, but significant U V - B x XL interaction. In both years lamb's-quarters biomass per plant (W) declined in response to elevated U V - B radiation as well as to increasing population densities of both broccoli and lamb's-quarters (Chapter 3; Tables 3.2 and 3.3). Treatment interaction effects (UV -B x XB, UV - B x XL, XB X XL, and UV - B x XB x XL) were also significant. 4.4.1 Growth Indices 4.4.1.1 Broccoli A N O V A results for growth indices of broccoli are presented in Tables 4.1 and 4.2. Means and standard errors of growth indices are presented in Appendix 4.1. In 1999, SLW decreased with exposure to elevated UV - B radiation, and was also influenced by lamb's-quarters density (Table 4.1 and Fig. 4.1). L W R decreased with increasing lamb's-quarters density (Table 4.1 and Fig. 4.3). L A R was significantly greater for broccoli grown at the elevated UV - B level (Table 4.1 and Fig. 4.4). There were no independent effects of species density on broccoli L A R in 1999. However, treatment interactions between U V - B radiation and species densities were significant. L A I generally increased at elevated U V - B radiation (Table 4.1 and Fig. 4.6). Broccoli LAI increased with increasing broccoli density, and declined with increasing lamb's-quarters density. The SRR was unaffected by treatments in 1999 (Table 4.1). In 2000, SLW increased at elevated UV - B radiation and was also affected by 108 Table 4.1. A N O V A results (variance ratios) for the effect of U V - B radiation, broccoli density (XB), and lamb's-quarters (X L) densities on broccoli plant growth indices weighted by the inverse of the variance of the ratios in 1999. Source of y Variables variation vcuiauuii df S L W 3 LWR L A R LAI SRR Block 3 5.951 b* c 5.310 * 1.682 NS 15.233 * 2.618 NS U V - B 1 46.475 * 3.275 NS 48.917 * 8.540 * 0.948 NS X B 2 0.870 NS 1.859 NS 0.384 NS 67.869 * 1.584 NS X L 3 5.363 * 21.100 * 0.697 NS 44.965 * 1.371 NS Block x U V - B 3 1.752 NS 0.475 NS 0.886 NS 2.301 NS 2.159 NS Block x XB 6 20.83 NS 1.516NS 0.961 NS 1.899 NS 1.444 NS U V - B x X B 2 2.263 NS 0.036 NS 3.868 * 0.788 NS 1.903 NS Block x X L 9 1.591 NS 1.777 NS 1.210NS 3.210 * 0.451 NS U V - B x X L 3 0.337 NS 2.023 NS 6.378 * 0.365 NS 0.230 NS X B X X L 6 0.419 NS 1.600 NS 1.340 NS 0.932 NS 1.409 NS U V - B x X B x X L 6 1.415 NS 1.667 NS 2.147 NS 1.110NS 0.382 NS Error 51 a S L W = specific leaf weight, L W R = leaf weight ratio, L A R = leaf area ratio, LAI = leaf area index, and SRR = shootroot ratio. Means and standard errors of data are given in Appendix 4.1. c*significant at P < 0.05; NS: not significant. 109 Table 4.2. A N O V A results (variance ratios) for the effect of U V - B radiation, broccoli density (XB), and lambs-quarters (XL) densities on broccoli plant growth indices weighted by the inverse of the variance of the ratios in 2000. Source of variation y Variables df S L W a L W R L A R LAI SRR Block 3 1.397^8 1.294 NS 6.115 * 0.457 NS 2.469 NS U V - B 1 87.781 * c 0.970 NS 48.879 * 20.178 * 0.969 NS X B 2 6.275 * 1.861 NS 6.625 * 158.511 * 3.638 * X L 3 3.140 * 2.524 NS 1.039 NS 27.083 * 0.339 NS Block x U V - B 3 0.597 NS 1.343 NS 0.032 NS 0.432 NS 0.507 NS Block x X B 6 1.140 NS 1.535 NS 1.121 NS 1.092 NS 0.740 NS U V - B x X B 2 1.600 NS 2.439 NS 0.060 NS 1.671 NS 3.657 * Block x X L 9 1.643 NS 1.224 NS 0.730 NS 0.722 NS 0.723 NS U V - B x X L 3 2.035 NS 0.409 NS 1.950 NS 6.413 * 1.684 NS X B x X L 6 2.086 NS 1.156 NS 4.011 * 6.345 * 2.164 NS U V - B x X B x X L 6 1.437 NS 0.539 NS 1.507 NS 0.459 NS 2.189 NS Error 51 a S L W = specific leaf weight, L W R = leaf weight ratio, L A R = leaf area ratio, LAI = leaf area index, and SRR = shootroot ratio. Means and standard errors of data are given in Appendix 4.1. c*significant at P < 0.05; NS: not significant. 110 30 25 20 15 h 10 h 5 h Lamb's-quarters density (plants m" ) Fig. 4.1. Effect of lamb's-quarters density (plants m"2) on specific leaf weight (SLW) [lamina dry weight/leaf area (g m"2)], (mean ± SE) of broccoli plants grown at 4 (open bars) and 7 (shaded bars) kJ m"2 d"1 UV-BBE radiation for 4 weeks in 1999. I l l Fig. 4.2. Effect of broccoli (X B ) and lamb's-quarters (X L ) densities (plants m"2) on specific leaf weight (SLW) [lamina dry weight/leaf area (g m"2)] of broccoli plants grown at low (4 kJ m"2 d"1) and high (7 kJ m"2 d"1) UVBBE radiation for 5 weeks in 2000. 112 144 256 400 Lamb's-quarters density (plants m" ) Fig. 4.3. Effect of lamb's-quarters density (plants m"2) on leaf weight ratio (LWR) [lamina dry weight/shoot dry weight] (mean ± SE) of broccoli plants exposed to treatments for 4 weeks in 1999. 113 4 kJ m"2 d"1 U V B D C radiation . 4.4. Effect of broccoli (X B) and lamb's-quarters (X L) densities (plants m"2) on leaf area ratio (LAR) [leaf area/shoot dry weight (cm2 g"1)] of broccoli plants grown at low (4 kJ m"2 d"1) and high (7 kJ m" d"1) U V B B E radiation for 4 weeks in 1999. 114 4.5. Effect of broccoli (X B ) and lamb's-quarters (X L ) densities (plants m"2) on leaf area ratio (LAR) [leaf area/shoot dry weight (cm2 g"1)] of broccoli plants grown at low (4 kJ m"2 d"1) and high (7 kJ m"2 d"1) U V B B E radiation for 5 weeks in 2000. 4 kJ m"2 d"1 U V - B R F radiation 115 7 kJ m"2 d"1 U V - B R F radiation i - l Fig. 4.6. Effect of broccoli (X B ) and lamb's-quarters (X L ) densities (plants m"2) on leaf area index (LAI) [leaf area x plant density (cm 2 x X)] of broccoli plants grown at low (4 kJ m"2 d"1) and high (7 kJ m"2 d"1) UVBBE radiation for 4 weeks in 1999. 116 broccoli and lamb's- quarters density (Table 4.2 and Fig. 4.2). L W R did not respond to treatments (Table 4.2). L A R generally declined in response to increased U V - B radiation and increased slightly in response to broccoli density (Table 4.2 and Fig. 4.5). The density interaction term was also significant. LAI generally increased at elevated U V - B radiation and with increasing broccoli density, and declined with increasing lamb's-quarters density in 2000 (Table 4.2 and Fig.4.7). Interaction terms (UV-B x X L and X B x X L ) were also significant. SRR declined in response to increasing broccoli density at low U V - B radiation and increased in response to increasing broccoli density at high U V - B radiation (Table 4.2 and Fig. 4.8). Variance ratios for interaction effects were generally small in comparison with independent treatment effects (Tables 4.1 and 4.2). 4:4.1.2 Lamb's-quarters A N O V A results for growth indices of lamb's-quarters are presented in Tables 4.3 and 4.4. The SLW of lamb's-quarters was not affected by U V - B radiation, or broccoli density, but decreased in response to lamb's-quarters density in both years (Table 4.3 and Fig. 4.9A). The LWR of was unaffected by treatments in 1999 (Table 4.3). L A R was not affected by treatments (Table 4.3). LAI declined at elevated U V - B radiation and with increasing broccoli density, and increased with increasing lamb's-quarters density in 1999 (Table 4.3 and Fig. 4.12). Interaction terms (UV-B x X B , U V - B x X L , and U V - B x X B x X L ) were significant. Enhanced U V - B radiation, and increasing lamb's-quarters density resulted in an overall increase in the SRR (Table 4.3 and Fig. 4.14). In 2000, similarly to 1999, SLW of lamb's-quarters was not affected by U V - B radiation, or broccoli density, but decreased in response to lamb's-quarters density (Table 4.4 and Fig. 4.9B). L W R increased slightly with increasing lamb's-quarters density in 2000 (Table 4.4 and Fig. 4.10). L A R generally increased slightly with increasing lamb's-quarters density, and the density interaction term (X B x XL) was significant (Table 4.4 and Fig. 4.11). LAI declined at elevated U V - B radiation and with increasing broccoli density, and increased with increasing lamb's-quarters density (Table 4.4 and Fig. 4.13). U V - B radiation, and species densities influenced SRR (Table 4.4 and Fig. 4.15). 117 4.7. Effect of broccoli (X B ) and lamb's-quarters (X L ) densities (plants m"2) on leaf area index (LAI) [leaf area x plant density (cm2 x X)] of broccoli plants grown at low (4 kJ m"2 d"1) and high (7 kJ m"2 d"1) U V B B E radiation for 5 weeks in 2000. 118 40 03 S-H O o -*-> o o GO 30 20 H 10 h 144 256 400 -2N Broccoli density (plants m") Fig. 4.8. Effect of broccoli density (plants m"2) on shoot:root ratio (SRR) [shoot dry weight: root dry weight] (mean ± SE) of broccoli plants grown at 4 (open bars) and 7 (shaded bars) kJ m"2 d"1 UV-BBE radiation for 5 weeks in 2000. 119 Table 4.3. A N O V A results (variance ratios) for the effect of U V - B radiation, and broccoli (XB), and lamb's-quarters (XL) densities on lamb's-quarters plant growth indices weighted by the inverse of the variance of the ratios in 1999. Source of y Variables variation Df S L W a LWR L A R LAI SRR Block 3 1.019b NS 1.237 NS 0.692 NS 3.103 * 2.386 NS U V - B 1 2.383 NS 2.789 NS 2.178 NS 54.817 * 4.200 * X B 2 0.547 NS 0.759 NS 0.884 NS 14.238 * 1.461 NS X L 3 6.913 * c 1.394 NS 2.831 NS 11.800 * 5.918 * Block x U V - B 3 0.447 NS 0.724 NS 0.714 NS 2.524 NS 0.172 NS Block x XB 6 0.463 NS 0.760 NS 1.100 NS 1.676 NS 1.301 NS U V - B x X B 2 0.264 NS 2.333 NS 0.907 NS 2.787 * 1.913 NS Block x X L 9 0.458 NS 0.822 NS 0.989 NS 1.364 NS 1.161 NS U V - B x X L 3 1.119NS 0.388 NS 1.234 NS 1.401 NS 0.012 NS X B X X L 6 1.687 NS 0.430 NS 1.448 NS 2.950 * 1.732 NS U V - B x X B x X L 6 1.441 NS 1.433 NS 1.856 NS 3.410 * 0.856 NS Error 51 SLW = specific leaf weight, L W R = leaf weight ratio, L A R = leaf area ratio, LAI = leaf area index, and SRR = shootroot ratio. Means and standard errors of data are given in Appendix 4.1. c*significant at P < 0.05; NS: not significant. 120 Table 4.4. A N O V A results (variance ratios) for the effect of U V - B radiation, and broccoli (XB) and lamb's-quarters (XL) densities on lamb's-quarters plant growth indices weighted by the inverse of the variance of the ratios in 2000. Source of y Variables variation df S L W a L W R L A R LAI SRR Block 3 1.150b NS 0.114 NS 2.804 * 0.186 NS 3.179 * U V - B 1 0.591 NS 0.149 NS 0.080 NS 55.263 * 9.589 * X B 2 1.126 NS 0.269 NS 1.600 NS 23.488 * 4.857 * X L 3 5.082 *c 5.428 * 6.920 * 85.272 * 6.721 * Block x U V - B 3 0.266 NS 2.608 NS 1.390 NS 3.617 * 1.507 NS Block x XB 6 1.839 NS 0.426 NS 1.864 NS 0.949 NS 1.533 NS U V - B x X B 2 1.149 NS 1.038 NS 2.081 NS 1.370 NS 3.975 * Block x X L 9 0.190 NS 1.713 NS 0.166 NS 0.809 NS 0.440 NS U V - B x X L 3 1.291 NS 0.327 NS 0.142 NS 11.238 * 1.413 NS X B X X L 6 1.195 NS 1.125 NS 3.108 * 3.580 * 1.049 NS U V - B x X B x X L 6 1.204 NS 0.907 NS 1.850 NS 1.029 NS 1.537 NS Error ,51 a S L W = specific leaf weight, L W R = leaf weight ratio, L A R = leaf area ratio, LAI = leaf area index, and SRR = shoot:root ratio. bMeans and standard errors of data are given in Appendix 4.1. °*significant at P < 0.05; NS: not significant. 121 w O o CD on 30 20 r-10 0 30 20 h 10 0 144 256 400 2 Lamb's-quarters density (plants m") Fig. 4.9. Effect of lamb's-quarters density (plants m"2) on specific leaf weight (SLW) [lamina dry weight/leaf area (g m"2)] (mean ± SE) of lamb's-quarters plants exposed to treatments for (A) 4 weeks in 1999 and (B) 5 weeks in 2000. 122 144 256 400 2 Lamb's-quarters density (plants m") Fig. 4.10. Effect of lamb's-quarters density (plants m"2) on leaf weight ratio (LWR) [lamina dry weight/shoot dry weight] (mean ± SE) of lamb's-quarters plants exposed to treatments for 5 weeks in 2000. 123 . 4.11. Effect of broccoli (XB) and lamb's-quarters (X L) densities (plants m"2) on leaf area ratio (LAR) [leaf area/shoot dry weight (cm2 g"1)] of lamb's-quarters plants exposed to treatments for 5 weeks in 2000. 124 4 kJ m"2 d"1 U V - B R F radiation cd § 7 kJ m"2 d"1 UV-BBE radiation a CD 1-1 Fig. 4.12. Effect of broccoli (X B) and lamb's-quarters (XL) densities (plants m"2) on leaf area index (LAI) [leaf area x plant density (cm2 x X)] of lamb's-quarters plants grown at low (4 kJ m"2 d"1) and high (7 kJ m"2 d"1) UVBBE radiation for 4 weeks in 1999. 125 Fig. 4.13. Effect of broccoli (X B ) and lamb's-quarters (X L ) densities (plants m"2) on leaf area index (LAI) [leaf area x plant density (cm2 x X)] of lamb's- quarters plants grown at low (4 kJ m"2 d"1) and high (7 kJ m"2 d"1) UVBBE radiation for 5 weeks in 2000. 126 18 Q 12 O S-c +-> O O r: 6 > o o 00 144 2 5 6 4 0 0 Lamb's-quarters density (plants m" ) Fig. 4 .14. Effect of lamb's-quarters density (plants m"2) on shootroot ratio (SRR) [shoot dry weight: root dry weight] (mean ± SE) of lamb's-quarters plants grown at 4 (open bars) and 7 (shaded bars) kJ m"2 d"1 UV-BBE radiation for 4 weeks in 1999. 127 Fig. 4.15. Effect of broccoli (X B ) and lamb's-quarters (X L ) densities (plants m"2) on shoot:root ratio (SRR) [shoot dry:root dry weight] of lamb's-quarters plants grown at low (4 kJ m"2 d"1) and high (7 kJ m"2 d"1) U V B B E radiation for 5 weeks in 2000. 128 Interaction effects (UV-B x XB) were also significant. As with broccoli, interactions when significant, tended to be small in comparison with main treatment effects as indicated by magnitudes of variance ratios (Tables 4.3 and 4.4). 4.4.2 Allometric analysis 4.4.2.1 Broccoli Simple allometric models for broccoli, relating ln(W) to other ln-transformed measures per plant (WR, WS, WL, L A and H) are given in Table 4.5. These models were all significant, involved positive values of p, and for WR, W S , WL, and L A explained between 67% and 99% of the variation in ln(W). Hence, growth in ln(W) of broccoli tended to be proportional to growth in these plant measures. Simple allometric models relating ln(W) to ln(H) were also significant, involved positive values of P, but explained much less of the variation (8% to 61%) in ln(W). Expanded allometric models for broccoli were based on parameter values from the best subset regressions (Tables 4.6, 4.7, 4.8, 4.9, and 4.10). The expanded allometric equations improved the fraction of variation accounted for in ln(W), as indicated by high R values (0.73 to 0.99). Allometric exponents (terms containing P) were significant (P < 0.05) in all of the best subset regressions. Parameters a' and Po were significant in each model indicating direct allometric relationships between W and other measures of growth, independent of experimental treatments. Significant terms containing Pk (k > 0) indicated that allometry was also affected by the experimental treatments. U V - B treatments (signified by Pi) did not influence the allometric relationships between y and z\ for broccoli in either year. However, increasing independent species population densities often significantly lowered the ratios of y/zi, as indicated by the negative signs on terms involving p2and/or p 3. Two-way interaction terms, occasionally included in best subset regressions, were significant and positive. Significant terms containing yk express relationships that were not due to allometry between W and the other primary measures. In 1999, species density treatments (XB and/or XL) reduced ln(W). This was indicated by significant values of y 2 and/or y 3 and 129 Table 4.5. Parameters and statistics for simple bivariate allometric models (Eqn 4.1) between ln(W) and other ln-transformed measures (z) of broccoli grown at two levels of U V - B radiation. Independent a P Residual mean square R2 variable (z) 1999 (4 kJ m"2 d"1 UV-BBE radiation) ln(WR) 2.999* 0.828* 0.0766 0.67 ln(Ws) 0.852* 1.028* 0.0130 0.94 ln(WL) 0.991* 0.912* 0.00427 0.98 ln(LA) -2.798* 4.858* 0.000782 0.99 ln(H) 1.563* 1.403* 0.135 0.43 1999 (7 kJ m"2 d"1 UV-BBE radiation) ln(WR) 3.088* 0.775* 0.0574 0.77 ln(Ws) 1.036* 0.975* 0.0193 0.92 ln(WL) 0.844* 0.952* 0.00655 0.97 ln(LA) -2.924* 4.940* 0.000616 0.99 ln(H) 0.864* 1.659* 0.0992 0.61 2000 (4 kJ i n 2 d' 1 UV-BBE radiation) ln(WR) -2.246* 4.494* 0.0000174 0.99 ln(Ws) 1.331* 0.966* 0.0170 0.85 ln(WL) 0.652* 0.947* 0.00389 0.96 ln(LA) 1.118* 1.025* 0.00868 0.92 ln(H) 1.688* 1.404* 0.0812 0.32 2000 (7 kJ m"2 d"1 UV-BBE radiation) ln(WR) -2.648* 4.752* 0.0000474 0.99 ln(Ws) 1.591* 0.898* 0.00580 0.91 ln(WL) 0.540* 0.971* 0.00187 0.97 ln(LA) 1.354* 0.985* 0.00792 0.88 ln(H) 3.309* 0.716* 0.0613 0.08 * Significant at P < 0.05; NS: not significant according to a T-test. A l l regressions were significant at P < 0.05. 130 Table 4.6. Parameters and statistics for best subset multiple regression models (Eqn. 4.2) of the allometric relationship between total biomass (W) and root biomass (W R) per plant in broccoli. Potential independent Parameter Parameter estimates Standard partial variable regression coefficient 1999 2000 1999 2000 Intercept ln(a') 4.143 * -2.359 * (0.327) (0.0285) ln(WR) Po 0.704 * 4.580 * 0.753 1.003 (0.0441) (0.0207) (UVB)ln(W R) P, - a - - -X B ln (W R ) p2 - . . . X L ln(W R ) p3 -0.000459 * - -0.296 (0.0000724) (UVB)X B ln(W R ) p4 - . . . (UVB)X L ln(W R ) p5 - . . . X B X L l n ( W R ) p6 - . . . (UVB)X B X L l n (W R ) p7 - - - -ln(UVB) Y l - -0.0108 * - -0.00906 (0.00542) ln(X B) Y2 -0.127* - -0.108 (0.0528) ln(X L) Y 3 - . . . ln((UVB)X B) Y4 " . . . ln((UVB)X L) y 5 - . . . l n (X B X L ) y 6 - . . . ln( (UVB)X B X L ) y 7 - . . . Statistics Residual mean square 0.0454 0.00175 R 2 0.82 0.99 d.f. 3^92 2^93 *Regression parameters differ significantly from zero according to a T-test (P < 0.05). Values in brackets are the standard errors of the parameter estimates given above. a Dash indicates term not included in best subset regression. 131 Table 4.7. Parameters and statistics for best subset multiple regression models (Eqn. 4.2) of the allometric relationship between total biomass (W) and stem biomass (W s) per plant in broccoli. Potential independent variable Parameter Parameter estimates Standard partial regression coefficient 1999 2000 1999 2000 Intercept ln(Ws) (UVB)ln(Ws) ln(oc') Po P. 1.708 * (0.133) 0.872 * (0.0291) a 1.834 * (0.143) 0.806 * (0.0438) 0.844 0.799 X B ln (W s ) X L ln (W s ) (UVB)X B ln(W s ) P2 P3 P4 -0.0000947 * (0.0000256) -0.000172 * (0.0000367) -0.0000891 * (0.0000267) -0.0798 -0.162 -0.124 (UVB)X L ln(W s ) Ps - - -X B X L l n ( W s ) P6 - - -( U V B ) X B X L l n ( W s ) P7 - - -ln(UVB) ln(X B) YI Y2 _ 0.154 * (0.0447) 0.129 ln(X L) ln((UVB)X B ) Y3 Y4 -0.0404 * (0.00572) _ -0.201 ln((UVB)X L) Y5 - - -l n (X B X L ) Y6 - - -ln ( (UVB)X B X L ) Y7 - - -Statistics Residual mean square R 2 d.f. 0.00919 0.92 4,91 0.0101 0.96 . 3,92 *Regression parameters differ significantly from zero according to a T-test (P < 0.05). Values in brackets are the standard errors of the parameter estimates given above. a Dash indicates term not included in best subset regression. 132 Table 4.8. Parameters and statistics for best subset multiple regression models (Eqn. 4.2) of the allometric relationship between total biomass ( W ) and leaf biomass (WL) per plant in broccoli. Potential independent Parameter variable Parameter estimates Standard partial regression coefficient Intercept ln(a') l n ( W L ) Po (UVB)ln(W L ) Pi XBIII(WL) .. -P 2 X L l n ( W L ) P3 ( U V B ) X B l n ( W L ) p 4 ( U V - B ) X L l n ( W L ) p 5 X B X L I I I ( W L ) P6 ( U V - B ) X B X L l n ( W L ) p 7 ln(U-VB) Yi l n ( X B ) Y2 l n ( X L ) Y3 l n ( (UVB)X B ) Y4 ln( (UVB)X L ) Ys l n ( X B X L ) Y6 l n ( ( U V - B ) X B X L ) Y 7 Statistics Residual mean square R 2 d.f. 1999 0.626 * (0.113) 0.979 * (0.0222) 0.00000688 * (0.00000248) 0.0135 * (0.00482) 0.00496 0.98 3,92 2000 0.927 * (0.159) 0.918 * (0.0208) -0.0000213 * (0.0000107) -0.0255 NS (0.0159) 0.00276 0.98 3,92 1999 2000 1.038 0.956 -0.0370 0.0423 -0.0317 0.0669 *Regression parameters differ significantly from zero according to a T-test (P < 0.05). Values in brackets are the standard errors of the parameter estimates given above. a Dash indicates term not included in best subset regression. 133 Table 4.9. Parameters and statistics for best subset multiple regression models (Eqn. 4.2) of the allometric relationship between total biomass (W) and leaf area (LA) per plant in broccoli. Potential independent Parameter Parameter estimates Standard partial variable regression coefficient 1999 2000 1999 2000 Intercept ln(a') -2.536 * 1.972 * (0.0822) (0.251) ln(LA) Po 4.778 * 0.854 * 0.974 0.763 (0.0384) (0.0434) (UVB)ln(LA) Pi a - -X B ln (LA) P2 - - -X L ln(LA) P3 - -0.0000954 * -0.132 (0.0000220) (UVB)X B ln(LA) P 4 - - -(UVB)X L ln(LA) Ps - - -X B X L l n ( L A ) P6 - - -(UVB)X B X L l n (LA) P7 - - -ln(UVB) Yi - 0.246 * 0.206 (0.0341) ln(X B) Y2 -0.0201 * -0.105 * -0.0172 -0.131 (0.00622) (0.0263) ln(X L) Y3 -0.00587 * - -0.0291 (0.00154) ln((UVB)X B ) Y4 - - -ln((UVB)X L) Ys - - -l n (X B X L ) Y6 - - -ln( (UVB)X B X L ) Y7 - - -Statistics Residual mean square 0.000586 0.00663 R 2 0.99 0.94 d.f. 3,92 4,91 * Regression parameters differ significantly from zero according to a T-test (P < 0.05). Values brackets are the standard errors of the parameter estimates given above. a Dash indicates term not included in best subset regression. 134 Table 4.10. Parameters and statistics for best subset multiple regression models (Eqn. 4.2) of the Potential independent Parameter Parameter estimates Standard partial variable regression coefficient 1999 2000 1999 2000 Intercept ln(a') 4.911* 2.644 * (0.602) (0.290) ln(H) Po 0.888 * 0.875 * 0.421 0.318 (0.137) (0.142) (UVB)ln(H) Pi a - - -X B ln(H) P2 - -0.000829 * - -0.508 (0.0000791) X L ln(H) Ps - -0.000501 * - -0.423 (0.0000602) (UVB)X B ln(H) P4 - - - -(UVB)X L ln(H) Ps - - - -X B X L l n ( H ) P6 0.000000942 * - 0.204 -(0.000000477) (UVB)X B X L ln (H) P7 - - - -ln(UVB) Yi - 0.509 * 0.426 (0.0588) ln(X B) Y2 -0.316 * - -0.271 -(0.0826) ln(X L) Y3 -0.133 * - -0.663 -(0.0201) ln((UVB)X B ) Y4 - - - -ln((UVB)X L) Y5 - - - -l n (X B X L ) Y6 - - - - • ln( (UVB)X B X L ) Y7 - - - -Statistics Residual mean square 0.0251 0.0664 R 2 0.79 0.73 d.f. 4,91 4,91 *Regression parameters differ significantly from zero according to a T-test (P < 0.05). Values in brackets are the standard errors of the parameter estimates given above. a Dash indicates term not included in best subset regression. 135 negative values of the standard partial regression coefficients associated with these terms in the relationships between ln(W) and ln(WR), ln(W s), ln(LA) and ln(H) (Tables 4.6, 4.7, 4.8 and 4.9). In 1999 lamb's-quarters population density (XL) significantly increased ln(W) as shown by significant values of 73 and positive values of the standard partial coefficient associated with this term in the relationship between ln(W) and M(WL) (Table 4.8). In 2000, U V - B radiation had small, negative direct (non-allometric) effects on ln(W) in the relationship between ln(W) and ln(WR) and ln(WL) (Tables 4.6 and 4.8), and positive effects on ln(W) in the relationship between ln(W) and ln(Ws), ln(LA), and ln(H) (Tables 4.7, 4.9 and 4.10). Broccoli population density (X B ) had a direct, negative effect on ln(W) in the relationships between ln(W) and ln(WY) and ln(LA) (Tables 4.8 and 4.9). Lamb's-quarters population density had no direct effects on any of the relationships. Interaction terms were not significant for non-allometric treatment effects in either year. 4.4.2.2 Lamb's-quarters Simple allometric relationships between ln(W) and other ln-transformed measures per plant (WR, WS, WL, and LA) are given in Table 4.11. These relationships were significant, had positive values of P, and explained between 63% and 98% of the variation in ln(W). Therefore, as with broccoli, growth in ln(W) of lamb's-quarters tended to be proportional to growth in these plant measures. In 1999, simple allometric models relating ln(W) to ln(H) were significant, and involved positive values of p. However, much less of the variation in ln(W) was explained by these relationships (24% to 33%), compared with variation explained by relationships with other plant measures. In 2000, the model relating ln(W) to ln(H) was significant (R = 0.49) and involved a positive value of p at 4 9 I 9 1 kJ m" d" UV-BBE radiation. At 7 kJ m" d* UV -B B E radiation however, a simple bivariate, allometric relationship did not exist between ln(W) and ln(H). Parameter values specifying the best subset regressions for the expanded allometric models and standard partial regression coefficients for lamb's-quarters are presented in Tables 4.12,4.13,4.14, 4.15, and 4.16. Allometric exponents (terms containing Po) were significant (P< 0.05) in all the best subset regressions. Together with 136 Table 4.11. Parameters and statistics for simple bivariate allometric models (Eqn. 4.1) between ln(W) and other ln-transformed measures (z) of lamb's-quarters grown at two levels of U V - B radiation. . _ Independent a P Residual mean square R2 variable (z) 1999 (4 kJ r n 2 d"1 UV-BBE radiation) ln(WR) 3.297* 0.772* 0.0278 0.89 ln(Ws) 0.796* 0.984* 0.00399 0.98 ln(WL) 0.845* 1.008* 0.00718 0.97 ln(LA) 1.431* 1.090* 0.0167 0.93 ln(H) 1.293* 1.505* 0.184 0.33 1999 (7 kJ i n 2 d*1 U V - B B E radiation) ln(WR) 3.464* 0.731* 0.0223 0.84 ln(Ws) 0.596* 1.013* 0.00919 0.93 ln(WL) 0.845* 1.008* 0.00718 0.97 ln(LA) 1.431* 1.090* 0.0167 0.93 ln(H) 3.285* 0.825* 0.106 0.24 2000 (4 kJ m"2 d"1 UV-BBE radiation) ln(WR) 3.048* 0.829* 0.0119 0.92 ln(Ws) 1.068* 0.923* 0.00599 0.96 ln(WL) 0.812* 1.016* 0.0102 0.93 ln(LA) 1.780* 1.010* 0.0287 0.82 ln(H) -1.490 NS 2.353* 0.0804 0.49 2000 (7 kJ m' 2 d _ 1 UV-BBE radiation) ln(WR) 2.960* 0.863* 0.0130 0.78 ln(Ws) 1.009* 0.927* 0.00254 0.95 ln(WL) 1.066* 0.957* 0.00411 0.93 ln(LA) 2.866* 0.695* 0.0225 0.63 ln(H) 3.903* 0.470 NS 0.0582 0.04 NS * Significant at P < 0.05; NS: not significant according to a T-test. A l l regressions were significant at PO.001. 137 Table 4.12. Parameters and statistics for best subset multiple regression models (Eqn 4.2) of the allometric relationship between total biomass (W) and root biomass (WR) per plant in Potential independent variable Parameter3 Parameter estimates Standard partial regression coefficient 1999 2000 1999 2000 Intercept ln(WR) (UVB)ln(W R) ln(a') Po Pi 3.815 * (0.123) 0.710 * (0.0263) a 3.494 * (0.108) 0.751 * (0.0284) 0.891 0.862 X B ln (W R ) P2 - - -X L ln(W R ) (UVB)X B ln(W R ) (UVB)X L ln(W R ) Ps P4 Ps -0.000215 * (0.0000417) -0.0000186 * (0.00000557) -0.000203 * (0.0000416) -0.0000244 * (0.00000563) -0.159 -0.144 -0.114 -0.146 X B X L l n ( W R ) P6 - - -(UVB)X B X L l n (W R ) P7 - - -ln(UVB) Yi - - -ln(X B) Y2 - - -ln(X L) Y3 - - -ln((UVB)X B ) Y4 - - -ln((UVB)X L) Ys - - -l n (X B X L ) Y6 - - -ln ( (UVB)X B X L ) Y? - - -Statistics Residual mean square R 2 d.f. 0.0189 0.92 3,92 0.00940 0.94 3,92 *Regression parameters differ significantly from zero according to a T-test (P < 0.05). Values in brackets are the standard errors of the parameter estimates given above. a Dash indicates term not included in best subset regression. 138 Table 4.13. Parameters and statistics for best subset multiple regression models (Eqn. 4.2) of the allometric relationship between total biomass (W) and stem biomass (Ws) per plant in Potential independent Parameter Parameter estimates Standard partial variable regression coefficient 1999 2000 1999 2000 Intercept ln(a') 1.354 * 1.462 * (0.230) (0.154) ln(Ws) Po 0.965 * 0.882 * 0.943 0.916 (0.0213) (0.0237) (UVB)ln(Ws) P. a -0.00498 * -0.0860 (0.00119) X B ln (W s ) P2 - -0.0000437 * -0.0776 (0.0000127) X L ln(W s ) P3 - -0.0000407 * -0.0493 (0.0000182) (UVB)X B ln(W s ) P4 - - -(UVB)X L ln(W s ) Ps - - -X B X L l n ( W s ) P6 - - -(UVB)X B X L l n (W s ) P7 - - -ln(UVB) Yi -0.0919 * - -0.0543 (0.0306) ln(X B) Y2 - - -ln(X L) Y3 -0.0583 * - -0.0513 (0.0226) ln((UVB)X B) Y4 - - -ln((UVB)X L) Ys - - -l n (X B X L ) Y6 - - -ln ( (UVB)X B X L ) Y7 - - -Statistics Residual mean square 0.00618 0.00380 R 2 0.97 0.97 d.f. 3,92 4,91 *Regression parameters differ significantly from zero according to a T-test (P < 0.05). Values in brackets are the standard errors of the parameter estimates given above. a Dash indicates term not included in best subset regression. 139 Table 4.14. Parameters and statistics for best subset multiple regression models (Eqn. 4.2) of the allometric relationship between total biomass (W) and leaf biomass (WL) per plant in lamb's-quarters. Potential independent variable Parameter Parameter estimates Standard partial regression coefficient 1999 2000 1999 2000 Intercept ln(WL) (UVB)ln(W L) X B ln(W L ) XLIII(WL) (UVB)X B ln(W L ) ln(a') Po P. P2 Ps P4 1.324 * (0.114) 0.927 * (0.0206) a -0.0000400 * (0.0000137) 1.465 * (0.207) 0.923 * (0.0334) -0.00387 * (0.00167) -0.0000387 * (0.0000177) -0.0000875 * (0.0000236) 0.957 0.896 -0.0633 -0.0623 -0.0660 -0.103 (UVB)X L ln(W L) Ps - - -X B X L l n (W L ) P6 - - -(UVB )X B X L ln (W L ) P 7 - - -ln(UVB) Yi - - -ln(X B) Y2 - - -ln(XL) 73 - - -ln((UVB)X B) Y4 - - -ln((UVB)X L) Y5 - - -ln(X B X L ) 76 - - -ln((UVB)X B X L ) Y7 - - -Statistics Residual mean square R 2 d.f. 0.00844 0.96 2, 93 0.00634 0.96 4,91 *Regression parameters differ significantly from zero according to a T-test (P < 0.05). Values in brackets are the standard errors of the parameter estimates given above. Dash indicates term not included in best subset regression. 140 Table 4.15. Parameters and statistics for best subset multiple regression models (Eqn. 4.2) of the allometric relationship between total biomass (W) and leaf area (LA) per plant in lamb's-quarters. Potential independent variable Parameter Parameter estimates Standard partial regression coefficient 1999 2000 1999 2000 Intercept ln(LA) (UVB)ln(LA) X B ln (LA) X L ln (LA) ln(a') Po P. P2 PB 3.196 * (0.368) 0.852 * (0.0397) a -0.000107 * (0.0000270) 4.799 * (0.350) 0.681 * (0.0452) -0.0144* (0.00305) -0.000182 * (0.0000302) 0.851 0.668 -0.183 -0.136 -0.246 (UVB)X B ln(LA) P4 - - -(UVB)X L ln(LA) Ps - - -X B X L l n ( L A ) P6 - - -(UVB)X B X L l n (LA) P7 - - -ln(UVB) Yi - - -ln(X B) Y2 - - -ln(X L) ln((UVB)X B ) 73 Y4 -0.112 * (0.0428) -0.258 * (0.0361) -0.0987 -0.290 ln((UVB)X L) Ys - - -l n (X B X L ) Y6 - - -ln ( (UVB)X B X L ) Y7 - - -Statistics Residual mean square R 2 d.f. 0.0211 0.91 3,92 0.0161 0.90 4,91 *Regression parameters differ significantly from zero according to a T-test (P < 0.05). Values in brackets are the standard errors of the parameter estimates given above. a Dash indicates term not included in best subset regression. 141 Table 4.16. Parameters and statistics for best subset multiple regression models (Eqn. 4.2) of the allometric relationship between total biomass (W) and height (H) per plant in lamb's-quarters. Potential independent Parameter Parameter estimates Standard partial variable regression coefficient 1999 2000 1999 2000 Intercept ln(a') 6.586 * 3.828 * (0.732) (0.558) ln(H) po 0.887 * 0.936 * 0.393 0.341 (0.142) (0.164) (UVB)ln(H) p, -0.0285 * -0.0315 * -0.293 -0.343 (0.00570) (0.00520) X B ln(H) p 2 -0.000438 * -0.000423 * -0.447 -0.488 (0.0000572) (0.0000420) X L ln(H) p 3 - a -0.000510 * - -0.403 (0.0000685) (UVB)X B ln(H) p 4 - . . . (UVB)X L ln(H) p 5 - . . . X B X L l n ( H ) p 6 - . . . (UVB)X B X L ln (H) p 7 - . . . ln(UVB) Y l - . . . ln(X B) y 2 - . . . ln(X L) Y 3 -0.465 * - -0.409 (0.0719) ln((UVB)X B ) Y 4 " . . . ln((UVB)X L) Y 5 " . . . l n (X B X L ) Y 6 " . . . ln ( (UVB)X B X L ) Y? " . . . Statistics Residual mean square 0.0733 0.0306 R 2 0.69 0.79 d.f. 4_9_ 4_91 •Regression parameters differ significantly from zero according to a T-test (P < 0.05). Values in brackets are the standard errors of the parameter estimates given above. a Dash indicates term not included in best subset regression. 142 the significant allometric coefficients (a') this indicates the common occurrence of direct allometric relationships, independent of the experimental treatments. In 1999, significant interactions between treatments and allometry occurred for lamb's-quarters. For example, elevated U V - B radiation and increasing broccoli density significantly lowered the ratio of ln(W)/ln(H), as indicated by the negative sign on the standard partial coefficients for Piand P2 (Table 4.16). Increasing broccoli density lowered the ratios of ln(W)/ln(WL) and ln(W)/ln(LA) (Tables 4.14 and 4.15), while increasing lamb's-quarters density lowered the ratio of ln(W)/ln(WR) (Table 4.12). A single two-way interaction term [(UV-B)XB] in the relationship between ln(W) and ln(WR) was significant and negative. Direct treatment effects on ln(W), independent of allometry, were detected. For example, ln(UVB) reduced ln(W) in the relationship between ln(W) and ln(Ws) (Table 4.13). Contributions from ln(XiJ reduced ln(W) as indicated by significant values of 73 and negative values of the standard partial coefficient associated with this term in the relationship between ln(W) and ln(Ws), ln(LA) or ln(H) (Tables 4.13,4.15 and 4.16). In 2000, direct allometric relationships, independent of the experimental treatments occurred for lamb's-quarters. Except in the relationship between ln(W) and ln(WR), U V - B and broccoli density (XB) significantly reduced the ratios of ln(W) to all other ZJS (Tables 4.13, 4.14, 4.15, and 4.16). Lamb's-quarters density (XL) reduced the ratio of ln(W) to all Z j ' s except ln(LA) (Tables 4.12, 4.13, 4.14, and 4.16). The two-way interaction term [(UV-B)X B ] in the relationship between ln(W) and ln(WR) was significant and negative (Table 4.12). Direct treatment effects on ln(W) were absent, with the exception of a negative contribution of ln(XL) in the relationship between ln(W) and ln(LA) (Table 4.15). 4.5 DISCUSSION Plants can exhibit considerable plasticity in response to environmental influences. For example, in Chapter 3 it was shown using inverse yield-density relationships with total biomass per plant as a measure of yield, that U V - B radiation and plant density altered the population structure of broccoli and lamb's-quarters associations by influencing competitive interactions. While total biomass accumulation per plant is a convincing indicator of treatment effects on growth, reductions in biomass in response to both UV-B 143 radiation and competitive influences are often accompanied by substantial modifications in the partitioning of biomass into component plant organs. Effects of environmental factors (Rohrig and Stutzel 2001) and interference (Jolliffe et al. 1990) on the physiological plant variables controlling dry matter distribution are complicated, and often poorly understood. This study attempted to further detail the plastic response of plants within broccoli and lamb's-quarters associations to U V - B radiation and population densities. Attention was focused on evaluating morphological sources of variation in growth using plant growth indices and bivariate allometric relationships. 4.5.1 Growth indices While the use of growth indices reduces the information available in the primary data set (Hunt 1982), it can summarize and express key aspects of plant behaviour more clearly than the primary measures. Also, the conceptual framework provided by conventional plant growth analysis provides a context for relating primary measures of plant growth to plant performance (Jolliffe and Courtney 1984). Relative effects of U V - B radiation on shoot morphology are usually greater than the effects on biomass production (Barnes et al. 1993). Therefore, while total biomass may best express competitive outcome at differing levels of U V - B radiation (Yuan et al. 1999), partitioning biomass into various components may provide insights into the process of competition. The growth indices used here were ratios of the primary measures whose treatment responses were presented in Appendices 3.3 and 3.4. Absence of treatment effects on growth indices, could be due to either lack of treatment effects, or proportional effects, on the primary variables from which the indices were constructed. Conversely, where indices were significantly affected by treatments, it is apparent that treatments had differential effects on the primary variables used to derive the indices, implying morphological plasticity in treatment response. Growth indices were not always uniformly affected by the experimental treatments in this study; differences between species and years of study sometimes occurred. For example, specific leaf weight (SLW) of broccoli decreased in 1999 (Table 4.1 and Fig. 4.1), but increased in 2000 in response to U V - B radiation (Table 4.2 and Fig. 4.2). SLW, a measure of leaf thickness and/or density, is commonly reported to increase in response to elevated U V - B levels (Greenberg et al. 1997, Furness and Upadhyaya 144 2002). It is considered to be a protective mechanism as it increases the path length for photons, thereby reducing transmission of U V - B radiation to sensitive underlying targets (Teramura and Sullivan 1994). Additionally increasing lamb's-quarters density in both years, and increasing broccoli density in 2000 (when broccoli growth was the most vigorous) reduced SLW in broccoli (Tables 4.1 and 4.2 and Figs. 4.1 and 4.2). SLW of broccoli was more responsive to U V - B radiation than to species densities (Figs. 4.1 and 4.2). Decreased SLW in response to increasing plant density may have been a response (thinner leaves) to increased shading. Similarly, Wanjau (1998) reported that specific leaf area (SLW" 1) of beets and beans increased with increase in species population densities. SLW of lamb's-quarters grown in associations was not affected by U V - B radiation (Tables 4.3 and 4.4). Lamb's-quarters SLW declined slightly at increased lamb's-quarters densities (Fig. 4.9). This response may represent a characteristic morphological adjustment to allow lamb's-quarters to exploit environmental resources and tolerate crowding, but it is not a significant protective mechanism against U V - B -induced damage. Despite an absolute reduction in leaf area in response to U V - B radiation, dicotyledons generally allocate a greater proportion of biomass into leaves and less into stems (Teramura 1983) when grown in the absence of competing plants. The leaf weight ratio (LWR) provides a measure of treatment effects on such "leafiness". In this study however, U V - B radiation did not influence L W R of broccoli or lamb's-quarters (Tables 4.1, 4.2, 4.3, and 4.4). Broccoli LWR decreased in 1999 in response to presence of lamb's-quarters (Table 4.1 and Fig. 4.3), indicating reduced expenditure on photosynthesizing organs, in favour of support tissue. The decrease in the proportion of dry weight found in broccoli leaves in response to lamb's-quarters was a combined result of reduced leaf area (Appendices 3.2 and 3.3) and decreased SLW (Table 4.1, Fig. 4.1) (i.e. smaller, thinner leaves). In 1999, lamb's-quarters growth was vigorous and an increased investment in stem tissue by broccoli might have resulted in taller plants, thereby facilitating interception of PAR by broccoli. Stoller and Myers (1989) reported that under reduced irradiance resulting from increased plant density, the ratio of support tissue to leaves of lamb's-quarters plants was unaffected. In agreement with these findings, L W R of lamb's-quarters was unaffected by increasing population densities in 1999 (Table 4.3). 145 Conversely in 2000, L W R of broccoli was unaffected by species densities (Table 4.2), while L W R of lamb's-quarters generally increased slightly with increasing lamb's-quarters density (Table 4.4 and Fig. 4.10). Broccoli growth was notably more, and lamb's-quarters less, vigorous in 2000 than in 1999. Such inconsistent responses make it challenging to interpret L W R responses as characteristic adjustments to allow each species to exploit environmental resources and tolerate crowding. The leaf area ratio (LAR) is a morphological index of leafiness and reflects leaf area production per unit above-ground biomass. At high U V - B radiation, the L A R of broccoli increased in 1999 (Table 4.1 and Fig. 4.4), and decreased in 2000 (Table 4.2 and Fig. 4.5) as a result of corresponding adjustments in SLW (LAR = L W R x SLW" 1). The change in L A R thus represented within leaf reallocation of dry matter without significantly altering partitioning of dry matter into the leaf tissue. Reduction in L A R accompanied by an increase in SLW has also been reported in soybean (Glycine max cv. Essex) (Teramura and Sullivan 1987). In contrast, U V - B radiation did not influence the L A R of lamb's-quarters indicating that this radiation was ineffective in altering the balance between rates of dry weight gain and leaf area growth in this weed. L A R has been reported to be a major contributor to aggressiveness of some species (Roush and Radosevich 1985). Differential sensitivity of L A R to U V - B radiation was not a major contributor to UV-B-induced shifts in competitive ability in broccoli and lamb's-quarters associations in this study. As reported in Chapter 3 of this thesis, broccoli gained and lamb's-quarters declined in competitiveness at high U V - B radiation in both years, regardless of differential changes in L A R . Jolliffe et al. (1990) reported that L A R of forage maize (Zea mays L.) was not influenced by plant density. Small, but significant responses of L A R to increasing plant densities have been observed in beets and beans (Wanjau 1998). There were no independent effects of plant density on L A R of broccoli or lamb's-quarters in 1999. However, small, but significant interactions between U V - B radiation and plant population densities occurred in broccoli indicating subtle linkages between effects of U V - B level and plant population densities (Table 4.1 and Fig. 4.4). In 2000, L A R of broccoli and lamb's-quarters both increased in response to increasing density of members of their own species (Table 4.2 , Fig. 4.5 and Table 4.4, Fig. 4.11, respectively). These 146 responses however, were modified by presence of the companion species, again highlighting the complexity of the underlying morphological adjustments. For each species, the extent of the leaf array available for photosynthetic carbon assimilation is represented by the leaf area index (LAI) (Wanjau 1998). Enhanced U V - B radiation caused LAI to increase in broccoli (Table 4.1, Fig. 4.6 and Table 4.2, Fig. 4.7), and decline in lamb's-quarters (Table 4.3, Figs. 4.12 and Table 4.4 Fig. 4.13). These treatment effects correspond to leaf area responses (Appendices 3.2 and 3.3). Increased investment in leaf area might have enhanced competitive ability, as competitiveness of broccoli was shown to increase at high U V - B radiation (Chapter 3). In order to improve PAR interception, the plant can either invest in more leaf area or in height. Investing in leaf area would not necessarily enable the plant to gain a competitive advantage over taller plants. Whether broccoli relies on differential UV-B-induced modifications in height to gain a further advantage over lamb's-quarters is explored in Chapter 5. LAI of both species generally increased with increasing density of their own species and declined with increasing density of the companion species (Tables 4.1, Fig. 4.6; Table 4.2, Fig. 4.7; Table 4.3, Fig. 4.12; and Table 4.4, Fig. 4.13). In 1999 when lamb's-quarters growth was vigorous, LAI for broccoli was notably greater in absence compared with presence of lamb's-quarters (Fig. 4.6). Treatment effects on LAI of broccoli and lamb's-quarters were large in comparison with effects on other growth indices. Jolliffe and Gaye (1995) also observed an increase in LAI with increase in plant population of bell peppers (Capsicum annuum L.). It should also be noted, that LAI is not independent of population density [LAI = L A x plant density (X)]. Therefore, LAI for a species would be expected to increase as its population density increases. The decrease in LAI corresponds to the decline in L A per plant (Appendices 3.2 and 3.3) in response to increasing population density of the companion species. Enhanced U V - B radiation has been reported to alter the shoot:root ratio (SRR) in numerous species (Sullivan and Teramura 1989, Furness et al. 1999, Furness and Upadhyaya 2002). It has been suggested that changes in above-ground biomass under enhanced U V - B radiation may be due in part to a shift in the allocation between above-and below-ground systems, as opposed to a decrease in overall biomass (Gold and Caldwell 1983). Yuan et al. (1999) reported that a decrease in SRR in wheat (Triticum aestivum L.) and wild oat (Avena fatua L.) by U V - B treatments was insufficient to alter 147 the effects of U V - B enhancement on competitive balance, likely due to similarity of biomass shifts for the two species. In this study, U V - B radiation affected SRR of the two species differently. It did not have an independent effect on SRR of broccoli (Tables 4.1 and 4.2, Fig. 4.8), while it generally increased SRR of lamb's-quarters (Tables 4.3 and 4.4, and Fig 4.14 and 4.15). Correia et al. (1998) suggested that suppressed photosynthetic activity of lower leaves in response to enhanced U V - B radiation, might limit carbohydrates supplied to the root, thereby reducing root activity in sensitive species. Whether this mechanism was responsible for UV-B-induced increases in SRR found in this study is unknown. Plant population density influenced SRR of both broccoli and lamb's-quarters. SRR of broccoli declined with increasing broccoli density under ambient U V - B radiation and increased with increasing broccoli density under above-ambient U V - B radiation in 2000 (Table 4.2 and Fig. 4.8). These responses were similar in magnitude and indicate complex underlying shifts in biomass allocation in response to treatment effects. SRR of lamb's-quarters increased with increasing lamb's-quarters density in both years (Tables 4.3 and 4.4, and Figs. 4.14 and 4.15). In 2000 SRR of lamb's-quarters generally increased with increasing broccoli density at ambient U V - B radiation (Fig. 4.15). At above-ambient U V - B radiation, SRR of lamb's-quarters appeared to be less responsive to broccoli density. Rohrig and Stutzel (2000) have also reported shifts in biomass allocation in lamb's-quarters in response to contrasting competitive environments. As a limiting resource can stimulate shifts in biomass allocation, resulting in a proportional increase favouring that component of the plant which draws most upon the limiting environmental factor (Hunt 1988), these results suggest that competition for light was a factor in treatment responses. In summary, the range of responses of broccoli and lamb's-quarters to U V - B radiation and competitive influences is reflected in the complex behaviour of the plant growth indices. Both broccoli and lamb's-quarters had morphological plasticity in response to U V - B radiation as indicated by treatment effects on plant growth indices Tables 4.1, 4.2, 4.3, and 4.4). However, it is interesting to note that UV-B-induced morphological response of broccoli differed between the two years of study (Tables 4.1 and 4.2), while the response of lamb's-quarters was consistent (Tables 4.3 and 4.4). Despite differential responses of several of the plant growth indices, the total leaf array 148 potentially available for photosynthesis (LAI), as well as the competitive ability (Chapter 3) of broccoli increased (Tables 4.1 and 4.2, and Figs. 4.6 and 4.7), while that of lamb's-quarters declined at high U V - B radiation in both years (Tables 4.3 and 4.4, and Figs. 4.12 and 4.13). Both species responded to increasing plant densities by a decrease in SLW (thinner and/or less dense leaves). The LAI of both species tended to increase with increasing density of their own species and decline with decreasing density of the companion species. Lamb's-quarters tended to be more sensitive to presence of its own species, than to presence of broccoli. Broccoli, on the other hand, tended to be more sensitive to lamb's-quarters in 1999 and to itself in 2000. While significant interaction terms reflected the underlying complexity of relationships, these effects tended to be small in comparison with the magnitude of the main effects for both species. 4.5.2 Allometric analysis Broccoli and lamb's-quarters exhibited considerable morphological plasticity in response to U V - B radiation and competitive influences, as revealed by examination of plant growth indices. The expanded allometric equation (Eqn 4.2) (Jolliffe et al. 1988) was implemented to further detail this plasticity by evaluating sources of variation in bivariate allometric relationships. In this equation, the term ln(oc') is the result of grouping of terms and includes both allometric and non-allometric components. Terms containing p express proportionality between ln(W) and ln(z) and explain allometric adjustments to treatments. Parameter P is called the allometric exponent and measures the intensity of the differential variation between the growth of the plant parts (Caradus et al. 1995). Hence, treatment effects on partitioning will become evident through changes in p. Unlike intercept a, parameter P is independent of the unit of measurement (Smith 1980); emphasis in this study therefore focused on p. Terms involving y account for direct, non-allometric effects of treatments on W. In both years, simple allometric relationships existed between W and other primary growth parameters in broccoli and lamb's-quarters (Tables 4.5 and 4.11). Inclusion of significant Po terms, with relatively large, positive values of the standard partial regression coefficients, in best subset regressions indicated that strong allometric relationships (ie. proportional growth) existed between W and WR, WS, WL, or L A . 149 2 While simple allometric relationships explained much of the variation in W, (R = 0.63 - 0.99), the expanded model facilitated interpretation by clarifying allometric and non-allometric influences of U V - B radiation and competitive interactions (Tables 4.6, 4.7, 4.8, 4.9, 4.10, 4.12, 4.13, 4.14, 4.15, and 4.16). To facilitate this discussion results from tables for the expanded allometric model have been summarized in Tables 4.17 and 4.18). Allometric coefficients (terms containing Pk, k>0) were significant in many of the best subset regressions, indicating that treatment factors often changed the allometry between W and other measures of plant growth. Here, species population densities were more influential than U V - B radiation, indicating that similar proportions were generally maintained at both U V - B levels, with the exceptions of lamb's-quarters Ws, WL, and L A in 2000. Where plant density effects were significant, standard partial regression coefficients were negative, indicating decreasing proportionality of the different measures with W. This implies that competitive interactions lowered the measures of the components of biomass per plant in relation to W, as has been reported in other species (Weiner and Fishman 1994, McLachlan et al. 1995, Wanjau 1998). Interestingly, in earlier analyses (Chapter 3) it was determined that lamb's-quarters was more competitive than broccoli in all cases, except at the high level of U V - B radiation in 2000. In these instances, the effects of elevated U V - B radiation on allometric relationships between W and Ws, WL or L A were similar in direction and magnitude to that of increasing broccoli density (Tables 4.13, 4.14, 4.15, and 4.18). This .suggests that U V - B radiation may have exerted influence over lamb's-quarters' competitive ability in 2000 by altering above-ground biomass partitioning. With the exception of the term (UV-B)Xeln(z) treatment interaction terms (terms containing Pk, k>3) were not significant (Tables 4.17 and 4.18). The standard partial regression coefficients for this term were negative in the allometric relationship between W and W R (lamb's-quarters, both years) and positive between W and W L (broccoli, 1999) indicating differential allometric responses. In both cases the magnitude of the interaction was small in comparison with the po term. There were no obvious patterns as to which terms involving Pk, (k>0) were significant between the two years. 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N N xf £ co "TJ X X S ' N " 0-T ffl G CD T J X X > > N TJ X CQ X N ^—' TJ X oa PQ I > > ef CQ _1 X X ef ef CO -I X X PQ m o > > x !!& ef ef ef X CQ X i > 152 Direct, non-allometric effects of population densities had greater influence on broccoli in 1999, while U V - B radiation had greater effect in 2000 as indicated by inclusion of more terms containing y in best subset regressions (Table 4.17). Non-allometric effects were less commonly observed in lamb's-quarters (Table 4.18). In accord with much previous research, the direct effects, via y of increasing population densities on W were always negative (Jolliffe 1988). Conversely, the influence of U V - B radiation was simply to increase W in broccoli, and decrease it in lamb's-quarters, without changing the proportionality of W to other plant measures. Non-allometric effects in any case did not reveal complex underlying behaviour in response to experimental treatments, as they did not involve any interacting relationships between population densities and U V - B radiation. In both years, simple allometric relationships between W and H were much weaker [R 2 = 0.08 - 0.61 (broccoli) and 0.04 - 0.49 (lamb's-quarters)] compared with other measures of growth (Tables 4.10 and 4.16). Much more variation could be accounted for [R = 0.73 - 0.79 (broccoli) and 0.69 -0.79 (lamb's-quarters)] by using the expanded allometric model to characterize allometric and non-allometric influences of U V - B radiation and competitive interactions (Tables 4.17 and 4.18). Magnitudes of standard partial regression coefficients indicated that expanded allometric and/or direct treatment effects were often stronger than simple allometric relationships between W and H. This is in contrast to responses of components of biomass and leaf area. Inclusion of significant, positive values of Po in best subset regressions however, indicated that proportionality between W and H existed (ie. more massive plants were taller). In agreement with results for other parameters, significant population density effects were always negative. For broccoli, mechanisms affecting the relationship between W and H differed between the two years. For example, in 1999 direct, non-allometric effects of population densities dominated. Therefore, increasing plant densities resulted in decreased W without changing the proportionality of W in relation to H. In 2000 however, population effects were manifested allometrically; increasing plant density lowered the ratio of ln(W)/ln(H). Additionally, an independent positive effect of U V - B radiation indicated that under high U V - B broccoli plants had greater biomass per plant and were taller while proportionality was maintained. In lamb's-quarters, allometric 153 influences of U V - B radiation and plant density both decreased the ratio of ln(W)/ln(H) and were of greater consequence than direct treatment effects (Table 4.18). In 1999, lamb's-quarters density had a negative non-allometric influence on lamb's-quarters biomass per plant. In summary, the range of responses of broccoli and lamb's-quarters to U V - B radiation and competitive influences is reflected in the complex behaviour of the plant growth indices as well as in allometric and non-allometric responses. Significant treatment effects on plant growth indices showed that both broccoli and lamb's-quarters had morphological plasticity in response to U V - B radiation and population densities. U V -B-induced morphological response differed between the two years of study for broccoli, but was consistent for lamb's-quarters. Despite differential responses of several of the plant growth indices leaf area index (LAI) representing extent of the leaf array available for photosynthesis, appeared to be a key factor in determining competitive ability in both broccoli and lamb's-quarters. In both years at elevated U V - B radiation, LAI and relative competitiveness (Chapter 3) of broccoli increased, while that of lamb's-quarters declined. SLW of both species declined, signifying production of thinner and/or less dense leaves at increasing plant densities. The LAI of both species generally increased with increasing density of their own species and declined with decreasing density of the companion species. Lamb's-quarters generally responded more to presence of its own species, than to broccoli. Conversely, broccoli responded more to lamb's-quarters in 1999 and to itself in 2000. While significant interaction terms reflected the underlying complexity of relationships, these effects tended to be small in comparison with the magnitude of the main effects for both species. Much variation in combined dry mass per plant was related to adjustments in allometry, although direct, non-allometric experimental influences on per plant biomass also occurred. Overall, plant densities had greater influence compared with U V - B treatments on allometric adjustments, indicating that broccoli and lamb's-quarters generally maintained similar proportions at both levels of U V - B radiation. Direct treatment effects were more often detected in broccoli compared with lamb's-quarters. Since measures were taken only at the final harvest, it was not possible to characterize dynamics of dry matter partitioning using of plant growth analysis or 154 allometry. Teramura and Sullivan (1987) reported that effectiveness of enhanced U V - B radiation on biomass partitioning in soybean was dependent upon the growth stage of the plant. They suggested that this variation might be due in part to changes in micro-environmental conditions within the plant canopy. The current study provided a static description of the effects of U V - B radiation on competition, which is of course a time-dependent process. An attempt to illustrate the dynamic aspect of UV-B-induced shifts in competitive interactions, including changes in the canopy characteristics is made in Chapter 5 through a series of non-destructive measures. 155 5.0 Influence of UV-B-induced shifts in leaf and canopy optical properties on broccoli (Brassica oleracea L . var. italica cv. Purple Sprouting) and lamb's-quarters (Chenopodium album L.) competitive interactions. 5.1 A B S T R A C T Broccoli (Brassica oleracea L. var. italica cv. Purple Sprouting) and lamb's-quarters (Chenopodium album L.) were grown at three monocultures densities (144, 256 and 400 2 2 1 plants m") and all binary mixtures at 4 and 7 kJ m" d" UV-BBE radiation in a greenhouse during the summer of 1999 and the fall of 2000. Shoot height and leaf number were recorded at final harvests. Non-destructive measures of above- and below-canopy photosynthetically active radiation (PAR) levels, overhead canopy coverage (OCC), and leaf greenness were periodically recorded. Over time, the contribution of broccoli to PAR interception decreased at 4 kJ m"2 d"1 UV-BBE radiation and increased at 7 kJ m"2 d"1 UV-BBE radiation. Lamb's-quarters, accordingly, became relatively more successful over time at intercepting PAR at the low UV - B level and less successful at the elevated U V - B level. Height and leaf number of lamb's-quarters tended to decline at the high U V - B level. Conversely, broccoli height increased while leaf number was unaffected by exposure to elevated UV - B radiation. Lamb's-quarters was generally the stronger competitor based on lesser magnitude of substitution rates compared with broccoli. Differential U V - B -induced effects on height, leaf area and/or leaf number however, ultimately translated into a gain in competitiveness of broccoli relative to lamb's-quarters at 7 kJ m"2d"' UV-BBE radiation when OCC was used as a measure of resource acquisition. UV-B-induced changes in leaf greenness implied that chlorophyll content of broccoli leaves increased at the elevated U V - B level while that of lamb's-quarters declined. These results suggest that adjustments in both canopy architecture and physiology of broccoli and lamb's-quarters may contribute to increased competitiveness of broccoli relative to lamb's-quarters at elevated U V - B radiation reported in Chapter 3. 5.2 INTRODUCTION While the importance of competitive interactions among plants is well appreciated and the focus of much study (Harper 1961, Grace and Tilman 1990, Wanjau 1998), relatively 156 little progress has been made toward understanding how plants compete with one another for resources and which traits are of particular advantage in this competition. Direct investigation of the process of competition is difficult and only a few studies have dealt with the process or mechanisms of competition, as opposed to outcome (Tilman 1982, Tilman 1988). Evidence suggests that competitive balances are not fixed, but are modified in response to conditions under which the associated species are grown (Turkington and Jolliffe 1996, Jolliffe 1997). Competitive relationships may shift in response to resource availability, as plant species react in different ways to varying resource levels (King and Purcell 1997). In vegetable production systems in developed nations however, irrigation and fertilizers are commonly applied, so that insufficient supply of either factor is not likely to occur (Rohrig and Stutzel 2000). Light is usually the resource for which weed-crop competition arises (Tremmel and Bazzaz 1993). Plant canopy architecture influences light attenuation within canopies, and morphological attributes of plants affect competition for light (Tremmel and Bazzaz 1993). Several studies have investigated the role of canopy structure in the direct competition for light within plant associations (Ryel et al. 1990, Campbell and Norman 1998, Green et al. 2001). Competitive species should be capable of rapidly establishing a canopy through both height and lateral spread (Caldwell 1987). Direct competitive advantage of a species ensues from the ability to capture PAR and by quickly shading competitors. Physiological traits, including elevated photosynthetic rates per unit foliage would also contribute to a competitive advantage. Successful acquisition of limited resources depends on placement of plant components (the consequence of biomass allocation and morphology) and the physiological ability to efficiently exploit available resources. Mechanisms of competitive interactions involve ways in which plants acquire and utilize resources and subsequently make these resources unavailable to associated plants. Response of plants to resource pre-emption (i.e. asymmetric competition) would also play a role in competitive mechanisms (Bazzaz 1990). Asymmetric competition occurs when there is an uneven sharing of resources amongst competing plants as a consequence of individual plants or species removing a disproportionately large amount of a resource (Freckleton and 157 Watkinson 2001). In plant associations asymmetric competition for light may occur due to initial size advantage or over-topping by individuals or species. Over time, successful species are the ones that are able to acclimate to modified environmental conditions, including climate shifts. For example, plant species respond differentially to elevated levels of U V - B radiation currently reaching the earth's surface (Barnes et al. 1990a, Smith et al. 2000, Furness and Upadhyaya 2002). Elevated U V - B radiation levels can change plant morphology, most notably height (Barnes et al. 1990a, Greenberg et al. 1997, Furness and Upadhyaya 2002). UV-B-induced morphological changes, resulting in changes in canopy architecture can alter ability of associated species to compete for light (Caldwell 1987, Barnes et al. 1990b, Ryel et al. 1990, Barnes et al. 1995). Enhanced U V - B radiation environments also influence plant biochemistry, including levels of UV-B-absorbing compounds (Grammatikopoulos et al. 1999, Krause et al. 2001) and chlorophyll (Smith et al. 2000). Chlorophyll levels of various species decrease, increase, or remain unchanged in response to enhanced U V - B radiation (Smith et al. 2000). Deckmyn and Impens (1995) reported that UV-B-induced adjustment in chlorophyll level in bean plants (Phaseolus vulgaris L.) varied with time of observation. Such findings highlight the importance of obtaining information from more than a single time. At final harvest, higher chlorophyll concentration of plants exposed to a high compared with low level of U V - B radiation was attributed to a delayed reduction in chlorophyll content by leaf ageing in plants exposed to the high UV-B-level. Previous chapters of this thesis explored UV-B-induced shifts in competitive interactions between broccoli and lamb's-quarters in terms of biomass accumulation (Chapter 3) and biomass partitioning and allometry (Chapter 4). This chapter focuses on dynamics of UV-B-induced changes among broccoli - lamb's-quarters associations in chlorophyll content and canopy architecture in competition for light. The specific objectives were to: 1) evaluate the influence of U V - B radiation on PAR interception in broccoli and lamb's-quarters associations, and 2) quantify UV-B-induced shifts in overhead shoot canopy coverage of broccoli and lamb's-quarters grown in association. 158 5.3 M A T E R I A L S A N D M E T H O D S As reported in Chapter 3, broccoli and lamb's-quarters were grown in 25 cm x 25 cm x 12 cm (depth) plastic flats filled with Redi-earth media at three monoculture densities (144, 256 and 400 plants m"2) and all binary mixtures (Chapter 3; Table 3.1a) at 4 and 7 kJ m" d* UV-BBE radiation. Actual numbers of plants per experimental unit used in analyses are presented in Chapter 3; Table 3.1b. Shoot height (H) and leaf number (LN) data from the final harvests of 1999 and 2000 experiments were used. Non-destructive measures of leaf greenness, above- and below-canopy PAR levels, and overhead canopy coverage were also recorded over time. 5.3.1 Canopy interception of P A R PAR levels were measured weekly at noon above and below the plant canopy in each flat using a LI-COR LI-185B Light Meter (LI-COR Inc., Lincoln, Nebraska). Measurements started at canopy closure (following 2 and 3 weeks of U V - B exposure in 1999 and 2000, respectively). Because green leaves scatter little radiation in the visible band (Walker et al. 1988), radiation data were collected with a quantum sensor (LI-190SB Quantum Sensor, LI-COR Inc., Lincoln, Nebraska) responding only in the 400- 700 nm range. The sensor, fastened to a 25 cm long dowel was inserted approximately 2 cm above the soil surface in order to minimize canopy disturbance. Three random above- and below-canopy measurements were taken per flat. Based on the concept that per plant interception of P A R ought to decline with increasing population density, data were recorded as fraction of incident PAR. Then, using multiple non-linear regression analysis inverse yield-density relationships were established using the following equation: ra - ioVioi D "' = a X B + a x L [Eqn. 5.1] I 0 and I signify above- and below-canopy PAR levels, respectively, and D indicates total density of the association. Subscripts B and L of the inverse yield-density model designate broccoli and lamb's-quarters, respectively, while X indicates species population density. Parameters of the inverse yield-density models (axB and axO were 159 used to evaluate competitive effects. The ratio axi/axB was used to indicate the balance of 2 1 interspecific competitive effects. Separate models were developed for 4 and 7 kJ m" d" UV-BBE radiation. However, since the dependent variable [reciprocal P A R interception per plant (y"1)] is a property of the entire mixture, constants of the models ( a a o and a L o ) were omitted and separate models were not established for broccoli and lamb's-quarters. 5.3.2 Height, leaf number, and leaf greenness For each plant, height from soil level to shoot tip and total leaf number were recorded. Mean shoot height and leaf number per plant were calculated for each species. Greenness of the second leaf from the apex of three randomly selected broccoli and/or lamb's-quarters plants per flat was measured with a Minolta SPAD-502 meter (Minolta Corp., Osaka, Japan). SPAD readings were recorded following two and four weeks of U V - B treatments in 1999, and three and five weeks of U V -B treatments in 2000. SPAD readings have been shown to correlate with total chlorophyll concentration (Tenga et al. 1989). 5.3.3 Overhead canopy coverage Overhead photographs of monoculture and mixture canopies in the competition experiment were taken weekly using a 35 mm Pentax K1000 camera on a tripod. The portion of the overhead canopy in mixtures comprised of broccoli leaves was manually coloured on each photograph with an indelible red marking pen to facilitate differentiation between the two species. Photographs were scanned using a flat-bed scanner and saved as .tif files (150 dpi) in Adobe Photoshop 5.0 software. Using Arc View GIS 3.2 software, images were manually digitized by creating a series of polygons to represent overhead canopy coverage (OCC) for each species. Total area of polygons for each species was calculated and data for each week were downloaded into an Excel 7.0 spreadsheet. Inverse yield-density models were utilized to evaluate competitive responses (aa and a^ using the following equations: y"1 = [(BOCC/AFVXB]-1 = a B 0 + a B B X B + a B L X L [Eqn. 5.2] y"1 = [(LOCC/AFVXL]-1 = a L 0 + a L L X L + a L B X B [Eqn. 5.3] 160 Boccand Locc represent overhead canopy coverage (i.e. total area of polygons) for broccoli and lamb's-quarters, respectively. AF is proportion of the flat captured in the image. Subscripts B and L of the inverse yield-density model signify broccoli and lamb's-quarters, respectively, while X indicates species population density. Inverse model constants (l/a Bo and 1/aLo) evaluate potential OCC of the target species in the absence of competition. Parameters of the inverse yield-density models were used to evaluate intra- (aeB and aLL) and interspecific (a B L and 3LB) competitive responses, while the ratios SLQI/SLBB and aLB/aLL were used to indicate the balance of inter- to intraspecific competitive effects. Separate models were developed for 4 and 7 kJ m" d" UV-BBE radiation. T-tests (P < 0.05) were used to determine whether individual regression parameters differed significantly from zero. 5.4 R E S U L T S 5.4.1 P A R interception PAR interception results for broccoli - lamb's-quarters associations in 1999 and 2000 are summarized in Tables 5.1 and 5.2, respectively. Regressions for PAR interception in broccoli - lamb's-quarters associations at both levels of U V - B radiation were significant • • * • 2 in both years. In 1999 multiple coefficients of determination (R values) relating PAR interception and species associations were highest in weeks 2 and 3 (R = 0.91 - 0.96) (Table 5.1). At week 4, R 2 values declined (0.65 and 0.82 at 4 and 7 kJ m"2 d"1 U V - B B E radiation, respectively). At 4 kJ m" d" U V - B B E radiation the proportion of P A R intercepted by lamb's-quarters increased each week as indicated by the increased magnitude of the substitution rate (axL/axB-) Conversely at 7 kJ m" d" U V - B B E radiation the substitution rate decreased each week. In 2000 the multiple coefficients of determination tended to increase with time at both U V - B levels (Table 5.2). However, at each corresponding time the R value was 2 1 greater for the inverse yield-density model developed at 4 compared with 7 kJ m" d" U V - B B E radiation. Substitution rates ranged between 1.00 and 1.13 for P A R interception at 4 kJ m"2 d"1 UV-BBE radiation. At 7 kJ m"2 d"1 UV-BBE radiation substitution rates were relatively stable (0.93 - 0.97) for weeks two through four. During the fifth week the substitution rate declined to 0.74. 161 Table 5.1. Regression parameters and statistics for inverse yield-density models (Eqn 5.1) describing the effect of broccoli (axB) and lamb's-quarters (axO densities on P A R interception by plants grown at (a) 4, and (b) 7 kJ m"2 d"1 UV-BBE radiation in 1999. 2 Week aXB a XL a XL/axB R P (a) 4 kJ m"2 d"1 UV -B B E radiation 2 1.37* 1.20 * 0.88 0.92 O.0001 (0.04) (0.04) 3 1.23 * 1.13 * 0.92 0.96 O.0001 (0.03) (0.03) 4 1.06 * 1.33 * 1.26 0.65 O.0001 (0.07) (0.07) (b) 7 kJ m"2 d"1 UV-BBE radiation 2 1.25 * 1.11 * 0.89 0.91 <0.0001 (0.04) (0.04) 3 1.42 * 1.12 * 0.79 0.92 <0.0001 (0.04) (0.04) 4 1.38 * 1.07* 0.77 0.82 O.OOOl (0.06) (0.06) * Regression parameters differ significantly from zero according to a T-test (P < 0.05). Values in brackets are standard errors of the above regression parameters. 162 Table 5.2. Regression parameters and statistics for inverse yield-density models (Eqn 5.1) describing the effect of broccoli (axB) and lamb's-quarters (axO densities on PAR interception for plants grown at (a) 4, and (b) 7 kJ m"2 d"1 UV-BBE radiation in 2000. Week axB axL axi/axB R 2 P (a) 4 kJ i r f 2 d"1 UV-BBE radiation 2 2.10 * (0.17) 2.12 * (0.17) 1.00 0.54 O.0001 3 1.43 * (0.08) 1.62 * (0.08) 1.13 0.81 O.0001 4 1.30 * (0.05) 1.37 * (0.05) 1.06 0.88 O.0001 5 1.30* (0.05) 1.46* (0.05) (b) 7 kJ m"2 d" 1.13 U V - B B E radiation 0.87 O.0001 2 2.05 * (0.19) 1.91 * (0.19) 0.93 0.21 0.003 3 1.57 * (0.10) 1.47* (0.10) 0.94 0.68 <0.0001 4 1.36* (0.06) 1.33 * (0.06) 0.97 0.79 O.0001 5 1.57 * (0.06) 1.17 * (0.06) 0.74 0.81 O.0001 * Regression parameters differ significantly from zero according to a T-test (P < 0.05). Values in brackets are standard errors of the above regression parameters. 163 5.4.2 Height and leaf number Broccoli seedlings grown at 7 kJ m' 2 d'1 UV-BBE radiation were taller than those grown at 4 kJ m"2 d"1 UV-BBE radiation (Table 5.3 and Fig. 5.1). Broccoli height diminished in the presence of lamb's-quarters in 1999. In 2000, U V - B radiation and lamb'-quarters density effects, as well as UV - B x XB and XB X XL interaction terms on broccoli height were significant (Table 5.3 and Fig. 5.2). No clear pattern of response emerged from these interactions. Height of lamb's-quarters plants grown at 7 kJ m"2 d"1 UV-BBE radiation was less than those grown at 4 kJ m"2 d"1 UV-BBE radiation in 1999 (Table 5.3 and Fig. 5.3). Lamb's-quarters height decreased with increasing lamb's-quarters density in 1999 (Table 5.3 and Fig. 5.3). In 2000, lamb's-quarters height decreased at the elevated U V - B radiation level (Table 5.3 and Fig. 5.4). Independent density effects (XB and XL), and 2 1 interaction terms (UV-B x XL and XB X XL ) were also significant. At 4 kJ m" d" UV-BBE radiation, lamb's-quarters height was greatest at low lamb's-quarters density (Fig. 5.4). At 7 kJ m"2 d"1 UV-BBE radiation lamb's-quarters height was generally less at low lamb's-quarters density. Species density interactions were complex and no pattern of response was distinguishable. Number of broccoli leaves declined with increasing lamb's-quarters density in 1999 (Table 5.4 and Fig. 5.5). Broccoli density also had a small, but significant effect on leaf number. Number of broccoli leaves was unaffected by treatments in 2000 (Table 5.4). Number of lamb's-quarters leaves generally declined at elevated UV - B radiation and with increasing broccoli and lamb's-quarters densities in both years (Table 5.4 and Figs. 5.6 and 5.7). In 2000, interaction terms U V - B x XB and XB X XL were also significant. Leaf number generally 7 1 declined more with increasing broccoli density at 7 kJ m" d" UV-BBE radiation compared with at 4 kJ m"2 d"1 U V - B B E radiation (Fig. 5.7). 5.4.3 Overhead canopy coverage Overhead canopy coverage (OCC) results for broccoli and lamb's-quarters in 1999 and 2000 are summarized in Tables 5.5 through 5.8. Significant inverse yield-density relationships were obtained for OCC" 1 of broccoli and lamb's-quarters, at both levels of U V - B radiation and in both years, following one week of exposure to UV - B radiation treatments. Potential OCC of the target species in isolation is estimated by the reciprocal of the regression intercept (i.e. l/aso and l/aLo; Eqns. 5.2 and 5.3, respectively). Regression 164 Table 5.3. Variance ratios for the effect of U V - B radiation, broccoli (X B ) , and lamb's-quarters (X L ) densities on mean shoot height (H) weighted by the inverse of the variance. Broccoli Lamb's-quarters Source of variation df 1999 2000 1999 2000 Block 3 11.47*a 1.30 NS 29.96* 1.28 NS U V - B 1 6.67* 4.97* 15.95* 74.99 * X B 2 0.41NS 2.80 NS 1.06 NS 6.55 * x L 3 22.86* 6.04* 26.64* 13.27 * Block x U V - B 3 1.25NS 1.38 NS 1.49 NS 0.44 NS Block x X B 6 0.47NS 1.18NS 1.67 NS 1.27 NS U V - B x X B 2 1.45NS 11.29** 1.33 NS 2.58 NS Block x X L 9 1.94NS 1.32 NS 2.1 NS 0.96 NS U V - B x X L 3 0.77NS 0.59 NS 1.34 NS 3.90 * X B X X L 6 0.86NS 10.91 * 1.27 NS 3.70 * U V - B x X B x X L 6 1.95NS 0.73 NS 0.92 NS 1.47 NS Error 51 a*Significant at P < 0.05; NS: not significant 165 16 12 144 256 400 2 Lamb's-quarters density (plants m") Fig. 5.1. Effect of U V - B radiation and lamb's-quarters density (plants m"2) on shoot height (mean ± SE) of broccoli plants grown at 4 (open bars) and 7 (shaded bars) kJ m"2 d"1 U V - B B E radiation for 4 weeks in 1999. 166 Fig. 5.2. Effect of broccoli (X B) and lamb's-quarters (X L) densities (plants m"2) on shoot height of broccoli plants grown at low (4 kJ m"2 d"1) and high (7 kJ m"2 d"1) UVBBE radiation for 5 weeks in 2000. 167 40 30 r o -<-> '53 20 h 10 1 1 144 256 400 Lamb's-quarters (plant m" ) Fig. 5.3. Effect of U V - B radiation and lamb's-quarters densities (plants m"2) on shoot height (mean ± SE) of lamb's-quarters plants grown at 4 (open bars) and 7 (shaded bars) kJ m"2 d"1 UV-BBE radiation for 4 weeks in 1999. 4 kJ m"2 d"1 U V - B R F radiation 168 Fig. 5.4. Effect of U V - B radiation and broccoli (X B ) and lamb's-quarters (X L ) densities (plants m"2) on shoot height of lamb's-quarters plants grown at low (4 kJ m"2 d"1) and high (7 kJ m"2 d"1) U V B B E radiation for 5 weeks in 2000. 169 Table 5.4. Variance ratios for the effect of U V - B radiation, broccoli (X B ) , and lamb's-quarters (XL) densities on mean leaf number weighted by the inverse of the variance. Broccoli Lamb's-quarters Source of variation df 1999 2000 1999 2000 Block 3 4.08* 2.43 NS 8.32* 2.40 NS UV - B 1 0.0001NS 0.79 NS 24.28* 17.06* X B 2 5.63* 0.61 NS 2.82* 10.98 * X L 3 47.50* 0.39 NS 10.44* 19.37 * Block x U V - B 3 0.93NS 0.13 NS 0.99 NS 0.28 NS Block x XB 6 0.13NS 0.55 NS 1.93 NS 0.41 NS UV - B x X B 2 0.0008NS 0.60 NS 0.71 NS 10.18 * Block x X L 9 1.02NS 0.66 NS 0.38 NS 1.42 NS UV - B x X L 3 0.56NS 0.45 NS 2.17NS 0.94 NS X B X X L 6 1.63NS 0.90 NS 1.32 NS 5.73 * U V - B X X B X X L 6 2.24NS 0.76 NS 1.81 NS 0.44 NS Error 51 •Significant at P < 0.05; NS: not significant 170 5.5. Effect of broccoli (XB) and lamb's-quarters (XL) densities (plants m"2) on inverse leaf number of broccoli plants exposed to treatments for 4 weeks in 1999. Data are shown as inverse values in order to reverse slopes, thereby facilitating presentation. 4 kJ m"2 d"1 U V - B R F radiation 171 7 kJ m"2 d"1 U V - B R F radiation 5.6. Effect of broccoli (X B ) and lamb's-quarters (X L ) densities (plants m"2) on inverse leaf number of lamb's-quarters plants grown at low (4 kJ m"2 d"1) and high (7 kJ m"2 d"1) UVBBE radiation for 4 weeks in 1999. Data are shown as inverse values in order to reverse slopes, thereby facilitating presentation. 4 kJ m"2 d"1 U V - B R F radiation | 7 kJ m"2 d"1 U V - B B E radiation =3 i_3 CO 5.7. Effect of broccoli (XB) and lamb's-quarters (X L) densities (plants m~2) on inverse leaf number of lamb's-quarters plants grown at low (4 kJ m"2 d"1) and high (7 kJ m"2 d'1) U V B B E radiation for 5 weeks in 2000. Data are shown as inverse values in order to reverse slopes, thereby facilitating presentation. 173 Table 5.5. Regression parameters and statistics for inverse yield-density models (Eqn. 5.2) describing the response of reciprocal per plant overhead canopy coverage (OCC) of broccoli plants grown for 4 weeks at (a) 4, and (b) 7 kJ m"2 d"1 U V -BBE radiation to broccoli and lamb's-quarters densities in 1999. Week aBoa aBB aBL a B L /a B B R 2 P (a) 4 kJ r n 2 d"1 U V - B B E radiation 1 4.86* 0.019 * 0.014* 0.74 0.33 0.001 (1.85) (0.0068) (0.0041) 2 -0.34 NS 0.027 * 0.039 * 1.42 0.60 O.0001 (2.35) (0.0073) (0.0052) 3 3.05 NS 0.026 NS 0.065 * 2.48 0.31 0.0003 (6.75) (0.021) (0.015) 4 10.69 NS 0.0050 NS 0.033 * 6.66 0.13 0.04 (5.76) (0.018) (0.013) (b) 7 kJ r n 2 d"1 U V - B B E radiation 1 0.28 * 0.0013 * 0.0008 * 0.62 0.34 <0.0001 (0.11) (0.0003) (0.0002) 2 0.22 NS 0.0012 * 0.0016 * 1.33 0.33 0.0001 (0.18) (0.0005) (0.0004) 3 0.18 NS 0.0020 NS 0.0028 * 1.40 0.28 0.006 (0.34) (0.0011) (0.0008) 4 -0.27 NS 0.0035 * 0.0050 * 1.43 0.33 0.0001 (0.52) (0.0016) (0.0012) •Regression parameters differ significantly from zero according to a T-test (P < 0.05). Values in brackets are standard errors of the above regression parameters. aaBo = reciprocal of the growth potential of broccoli in the absence of competition, a B B = intraspecific competitive responses with broccoli as the target species, aBL = interspecific competitive responses with broccoli as the target species, a B i7a B B = substitution rate (Spitters 1983) indicating balance of inter- to intraspecific competitive effects. 174 Table 5.6. Regression parameters and statistics for inverse yield-density models (Eqn. 5.3) describing the response of reciprocal per plant overhead canopy coverage (OCC) of lamb's-quarters plants grown for 4 weeks at (a) 4, and (b) 7 kJ m"2 d"1 UV-BBE radiation to broccoli and lamb's-quarters densities in 1999. Week aLo aLL a L B aLB/aLL R P (a) 4 kJ m"2 d"1 UV-BBE radiation 1 1.35 NS 0.018 * 0.010 * 0.56 0.49 O.0001 (1.13) (0.004) (0.003) 2 -0.89 NS 0.021 * 0.006 * 0.29 0.77 O.0001 (0.60) (0.002) (0.001) 3 -0.21 NS 0.015 * 0.001 * 0.08 0.88 O.0001 (0.26) (0.0008) (0.0006) 4 -0.013 NS 0.012 * 0.001 * 0.10 0.94 O.0001 (0.15) (0.0005) (0.0003) (b) 7 kJ m' 2 d _ 1 UV-BBE radiation 1 3.37 NS 0.022 * 0.002 NS 0.09 0.27 0.001 (1.76) (0.006) (0.0039) 2 0.70 NS 0.012 * 0.003 * 0.24 0.71 O.0001 (0.39) (0.001) (0.0009) 3 0.22 NS 0.012 * 0.001 * 0.10 0.88 <0.0001 (0.22) (0.0007) (0.0005) 4 -0.18 NS 0.013 * 0.002 * 0.13 0.84 O.0001 (0.28) (0.0009) (0.0006) * Regression parameters differ significantly from zero according to a T-test (P < 0.05). Values in brackets are standard errors of the above regression parameters. aau) = reciprocal of growth potential of lamb's-quarters in absence of competition, aLL -intraspecific competitive responses with lamb's-quarters as the target species, a L B = interspecific competitive responses with lamb's-quarters as the target species, 3LB/&LL = substitution rate (Spitters 1983) indicating balance of inter- to intraspecific competitive effects. 175 Table 5.7. Regression parameters and statistics for inverse yield-density models (Eqn. 5.2) describing the response of reciprocal per plant overhead canopy coverage (OCC) of broccoli plants grown for 5 weeks at (a) 4, and (b) 7 kJ m"2 d"1 U V -BBE radiation to broccoli and lamb's-quarters population densities in 2000. Week aBo aBB aBL aBL/aBB R 2 P (a) 4 kJ m"2 d"1 U V - B B E radiation 1 2.04 NS (1.22) 0.019* (0.004) 0.009 * (0.003) 0.44 0.44 <0.0001 2 0.31 NS (1.37) 0.020 * (0.004) 0.017* (0.003) 0.86 0.53 O.0001 3 1.50 NS (1.87) 0.012 * (0.006) 0.026 * (0.004) 2.18 0.49 <0.0001 4 0.061 NS (2.01) 0.020 * (0.006) 0.025 * (0.005) 1.24 0.48 O.0001 5 3.16 * (1.30) 0.007 NS (0.004) 0.031 * (0.003) 4.38 0.72 O.0001 1 2.54 * (1.25) (b) 0.017* (0.004) 7 kJ rn 2 d - 1 0.009 * (0.003) UV-BBE radiation 0.52 0.38 <0.0001 2 1.30 NS (0.92) 0.016 * (0.003) 0.013 * (0.002) 0.80 0.62 <0.0001 3 0.76 NS (1.47) 0.013 * (0.005) 0.023 * (0.003) 1.72 0.56 <0.0001 4 -0.67 NS (1.33) 0.019* (0.004) 0.023 * (0.003) 1.22 0.64 O.0001 5 -0.32 NS (1.50) 0.022 * (0.005) 0.023 * (0.003) 1.01 0.60 O.0001 * Regression parameters differ significantly from zero according to a T-test (P < 0.05). Values in brackets are standard errors of the above regression parameters. aaBo = reciprocal of the growth potential of broccoli in the absence of competition, &BB = intraspecific competitive responses with broccoli as the target species, aBL = interspecific competitive responses with broccoli as the target species, aBi/a BB = substitution rate (Spitters 1983) indicating balance of inter- to intraspecific competitive effects. 176 Table 5.8. Regression parameters and statistics for inverse yield-density models (Eqn. 5.3) describing the response of reciprocal per plant overhead canopy coverage (OCC) of lamb's-quarters plants grown for 5 weeks at (a) 4, and (b) 7 kJ m"2 d"1 UV-BBE radiation to broccoli and lamb's-quarters population densities in 2000. Week aLo a L L a L B aLB/aLL R P (a) 4 kJ m"2 d"1 U V - B B E radiation 1 3.00 * (1.38) 0.016* (0.004) 0.010* (0.003) 0.65 0.35 O.0001 2 1.59 * (0.69) 0.014 * (0.002) 0.006 * (0.002) 0.44 0.57 O.0001 3 0.71 NS (0.41) 0.013 * (0.001) 0.004 * (0.0009) 0.33 0.74 O.0001 4 0.38 NS (0.38) 0.013 * (0.001) 0.005 * (0.0009) 0.40 0.76 O.0001 5 -0.62 NS (0.35) 0.016* (0.001) 0.006 * (0.0008) 0.37 0.86 O.0001 (b) 7 kJ m"2 d"1 UV-BBE radiation 1 2.59* (1.20) 0.023 * (0.004) 0.006 * (0.003) 0.27 0.48 <0.0001 2 1.64* (0.82) 0.018 * (0.003) 0.006 * (0.002) 0.32 0.57 O.0001 3 1.91 * (0.43) 0.012* (0.001) 0.002 * • (0.0009) 0.19 0.65 <0.0001 4 1.75 * (0.68) 0.011 * (0.002) 0.004 * (0.002) 0.35 0.43 O.0001 5 0.41 NS (0.49) 0.015 * (0.002) 0.006 * (0.001) 0.39 0.74 O.0001 * Regression parameters differ significantly from zero according to a T-test (P < 0.05). Values in brackets are standard errors of the above regression parameters. aaLo = reciprocal of growth potential of lamb's-quarters in absence of competition, aix = intraspecific competitive responses with lamb's-quarters as the target species, a E B = interspecific competitive responses with lamb's-quarters as the target species, aLB/aLL = substitution rate (Spitters 1983) indicating balance of inter- to intraspecific effects. 177 intercepts (aBo and aLo) often did not differ significantly from zero, with the exception of lamb's-quarters in 2000 (Table 5.8). Intraspecific competition was quantified by coefficients am (broccoli) and an (lamb's quarters) and interspecific competition was quantified by coefficients a BL (broccoli in competition with lamb's-quarters) and aLB (lamb's-quarters in competition with broccoli) (Eqns. 5.2 and 5.3). Regression coefficients, evaluated using T-tests, differed significantly from zero (Tables 5.5 through 5.8) with several exceptions: the intraspecific coefficient for broccoli (am) grown at 4 kJ m' 2 d"1 UV-BBE radiation for weeks 3 and 4, and at 7 kJ m"2 d"1 U V - B B E radiation for week 3 in 1999 (Table 5.5), the interspecific coefficient for lamb's-quarters (aLB) grown at 7 kJ m' 2 d"1 U V - B B E radiation for week 1 in 1999 (Table 5.6), and the intraspecific coefficient for broccoli (aBB) grown at 4 kJ m"2 d"1 U V - B B E radiation for week 5 2000 (Table 5.7). In 1999, with broccoli as the target species, values of regression coefficients aBL were less than am at both levels of U V - B radiation at week 1 (Table 5.5). At weeks 2, 3, and 4 however, values of regression coefficients aBL were greater than aBB at both levels of UV-B radiation. Consequently, the substitution rates aBi7aBB were less than 1.0 for week 1 and greater than 1.0 thereafter. At 4 kJ m*2 d"1 UV-BBE radiation, during weeks 2 to 4, coefficient aBB declined, while coefficient aBL increased during weeks 1 to 3 and declined at week 4: At low U V - B radiation, the substitution rate increased over time. At elevated U V - B radiation, during weeks 2 to 4, both coefficients aBB and aBL increased, resulting in relatively stable substitution rates. At weeks 3 and 4, the substitution rates at low U V - B radiation exceeded those at the elevated U V - B level. With lamb's-quarters as the target species in 1999, intraspecific ( a L L ) always exceeded interspecific (aLB) competitive effects, resulting in substitution rates ranging from 0.08 to 0.56 (Table 5.6). Magnitude of substitution rates did not differ consistently between U V - B levels or over time. In 2000, broccoli responded more to itself (substitution rates <1.0) at weeks 1 and 2, and more to lamb's-quarters (substitution rates >1.0) at weeks 3, 4, and 5 at both levels of U V - B radiation (Table 5.7). Corresponding substitution rates were similar at both UV-B-levels, with the exception of week 5. In this case, at 4 kJ m"2 d"1 U V - B radiation, the substitution rate increased as an overall result of decreased intraspecific (am) and 178 9 1 increased interspecific competitive effects (asO- At 7 kJ m" d" U V - B radiation, the substitution rate declined due to an increase in aBB and an unchanged aBL-Results obtained with lamb's-quarters as the target species in 2000 were similar to those in 1999; intraspecific (aLL) always exceeded interspecific (3LB) competitive effects for lamb's-quarters in 2000, resulting in substitution rates less than 1.0 (Table 5.8). Again, magnitude of substitution rates did not differ consistently between U V - B levels or over time. Importance of species population densities as a source of overall variation in OCC was measured by the multiple coefficient of determination (R ) for the inverse yield density model. These values varied widely (0.13 to 0.94) with respect to time, species, and level of U V - B radiation (Tables 5.5, 5.6, 5.7 and 5.8). R 2 values for broccoli OCC were generally lower than corresponding R 2 values for lamb's-quarters. For broccoli no trends were apparent with regards to changing R 2 values in response to U V - B radiation or time (Tables 5.5 and 5.7). For lamb's-quarters R 2 values generally increased with time at both U V - B levels (Tables 5.6 and 5.8) indicating increasing importance of species densities with time. 5.4.4 Leaf greenness A N O V A results for leaf greenness measurements are presented in Tables 5.9 and 5.10 for broccoli and lamb's-quarters, respectively. For broccoli, leaf greenness increased at 7, 9 I compared with 4 kJ m' d" UV-BBE radiation for both the 2 and 4 weeks measurement in 1999 (Table 5.9 and Fig. 5.8). Small, but significant XL, U V - B x X B , and block x XL effects were also detected (Table 5.9). In 2000, leaf greenness tended to increase at elevated U V - B radiation for measurements at weeks three and five (Table 5.9 and Fig. 5.9). At week three, XB, and the density (XB X XL), and three-way (UV-B x X B x XL) interactions were significant. At the five weeks measurement XL, and interaction terms U V - B x XL, XB X XL, and U V - B x X B x XL were significant. 9 1 Lamb's-quarters leaf greenness tended to decline at 7, compared 4 kJ m" d" UV-BBE radiation for both the two and four weeks measurement in 1999 (Table 5.10 and Fig. 5.10). Independent species densities (X B and XL) were significant at week two, while broccoli density (XB) and the density interaction term (X B x XL) were significant at week four. In 2000, only the 179 Table 5.9. Variance ratios for the effect of U V - B radiation, and broccoli (XB) and lamb's-quarters (XQ densities on mean SPAD values for broccoli. 1999 2000 Source of variation df Week 2 Week 4 Week 3 Week 5 Block 3 37.85 * 6.54 * 11.83 * 2.55 NS U V - B 1 59.19 * 77.37 * 71.77 * 88.19* X B 2 0.37 NS 0.89 NS 3.24* 1.37 NS X L 3 3.02 * 15.49 * 1.51 NS 3.01 * Block x U V - B 3 0.57 NS 2.02 NS 1.97 NS 1.08 NS Block x XB 6 0.85 NS 1.11 NS 2.08 NS 1.86 NS U V - B x X B 2 1.42 NS 1.62 NS 1.47 NS 0.63 NS Block x X L 9 2.24 * 1.46 NS 1.49 NS 1.59 NS U V - B x X L 3 1.88 NS 1.50 NS 2.38 NS 3.30 * X B X X L 6 1.68 NS 2.14 NS 6.97 * 4.14* U V - B X X B X X L 6 2.07 NS 1.18NS 5.75 * 2.31 * Error 243 * Significant at P < 0.05; NS: not significant 180 Table 5.10. Variance ratios for the effect of U V - B radiation, and broccoli (X B) and lamb's-quarters (X L) densities on mean SPAD values for lamb's-quarters. 1999 2000 Source of variation df Week 2 Week 4 Week 3 Week 5 Block 3 35.89 * 21.72 * 42.86 * 16.07 * U V - B 1 36.01 * 41.37 * 3.12NS 9.24 * X B 2 5.69* 8.88 * 2.08 NS 10.62 * X L 3 7.41 * 2.57 NS 1.93 NS 2.17NS Block x U V - B 3 2.19 NS 1.58 NS 1.05 NS 2.01 NS Block x XB 6 1.68 NS 1.06 NS 1.41 NS 0.69 NS U V - B x X B 2 1.25 NS 0.11 NS 3.61 * 6.82 * Block x X L 9 1.64 NS 1.25 NS 1.84 NS 1.83 NS U V - B x X L 3 0.42 NS 1.89 NS 2.82 NS 0.48 NS X B X X L 6 1.48 NS 4.22 * 1.47 NS 1.35 NS U V - B x X B x X L 6 2.08 NS 0.69 NS 2.07 NS 0.61 NS Error 243 * Significant at P < 0.05; NS: not significant 181 co -<—> a Q < c o 0) c a a> &> 20 10 0 50 40 30 20 10 0 0 144 256 400 2 Lamb's-quarters density (plants m") Fig. 5.8. Effect of U V - B radiation and lamb's-quarters density (plants m"2) on leaf greenness (SPAD units) (mean ± SE) of broccoli plants grown for (A) two weeks and (B) four weeks in 1999. U V - B B E treatment doses were 4 (open bars) and 7 (shaded bars) kJ m"2 d"1. 182 Fig. 5.9. Effect of U V - B radiation and broccoli (X B ) and lamb's-quarters (X L ) densities (plants m"2) on leaf greenness (SPAD units) of broccoli plants grown for (A) and (C) two weeks, and (B) and (D) four weeks in 2000. 183 Fig. 5.10. Effect of U V - B radiation and broccoli (X B ) and lamb's-quarters (X L ) density (plants m"2) on leaf greenness (SPAD units) of lamb's-quarters plants grown for (A) and C) two weeks, and (B) and (D) four weeks in 1999. 184 UV - B x XB interaction term had a small, but significant effect on leaf greenness (Table 5.10 and Fig. 5.11). At week five, independent UV - B and broccoli density (XB) effects, in addition to the U V - B x X B interaction term, were significant. Significant interaction terms were generally small in magnitude based on visual assessment of data (Figs. 5.8, 5.9, 5.10, and 5.11). 5.5 DISCUSSION Initial analysis of competitive interactions using inverse-yield density relationships indicated that broccoli gained in competitiveness relative to lamb's-quarters at 7 compared with 4 kJ m' d" UV-BBE radiation based on biomass at final harvest (Chapter 3). In chapter 4 a more detailed investigation into UV-B-induced shifts in competitive response was performed using plant growth indices and allometry to assess biomass partitioning and growth. These studies provided a static description of effects of UV - B radiation on competition which is essentially a time-dependent process. Research on weed-crop competition has traditionally focused heavily on competitive outcomes as opposed to processes of competition (Radosevich 1987). Evidence indicates that competitive relationships shift in response to conditions to which associated species are exposed (Turkington and Jolliffe 1996, Jolliffe 1997). The dynamic aspect of UV - B effects has been largely ignored (Barnes et al. 1995). One of the limitations of previous studies in assessing the significance of morphological responses to UV - B radiation is that data have generally been gathered from a single point at the end of an arbitrarily defined time period (Barnes et al. 1995). Teramura and Sullivan (1987) reported that the stage of plant growth influenced the effectiveness of enhanced UV - B radiation on biomass partitioning in soybean (Glycine max. (L.) Merr.). They suggested that changing micro-environmental conditions within the plant canopy might have contributed to this variation. Deckmyn and Impens (1995) reported that UV-B-induced adjustment in chlorophyll level in bean plants (Phaseolus vulgaris L.) varied with time of observation. This chapter attempts to address these concerns and provide deeper insight into the chronology of U V - B radiation effects on direct competition for light. The present study extends the application of inverse yield-density relationships to evaluate canopy interception of PAR in broccoli and lamb's-quarters associations. As 185 144 256 400 Broccoli density (plants m"2) Fig 5.11. Effect of U V - B radiation and broccoli density (plants m") (mean ± SE) on leaf greenness (SPAD units) of lamb's-quarters plants grown for (A) three weeks and (B) five weeks in 2000. U V - B B E treatment doses were 4 2 1 (open bars) and 7 (shaded bars) kJ m" d" . 186 implied by physiological concepts of limiting factors (Huston and DeAngelis 1994), competition for light should be comparatively important in irrigated, nutrient-rich sites (Wanjau 1998). Under such conditions, competitiveness of a weed is primarily governed by the ability to overgrow and shade crop foliage (Qasem 1992, Rohrig and Stutzel 2000), thus stressing importance of stem elongation and relative canopy height (Kiniry et al. 1992). Although it is not possible to rule out contributions from other components of interference, such as allelopathy or sheltering from U V - B radiation, results obtained here suggest that competition for light was occurring. For example, significant inverse yield-density relationships were developed for PAR at all times and both levels of U V - B radiation (Tables 5.1 and 5.2). In 1999, declining R 2 values indicated that the importance of plant population densities to variation in PAR interception declined as growth progressed from the third to fourth week of UV - B treatments (Table 5.1). Interestingly, ratios representing competitive interactions (axi7axB) revealed that, during the fourth week the contribution of broccoli to PAR interception decreased at 4 kJ m"2 d"1 UV-BBE radiation and increased at 7 kJ m"2 d"1 UV-BBE radiation (Table 5.1). Lamb's-quarters, on the other hand, became more successful over time at intercepting PAR at the low U V - B level and less successful at the elevated U V - B level. This may reflect the dynamic nature of the mixed canopy and emphasizes the importance of time-course studies. For instance, it was noted in mixtures that broccoli plants were generally taller than lamb's-quarters plants at two weeks (Chapter 3). Despite upward cupping and a more vertical orientation of broccoli leaves, compared with relatively horizontal lamb's-quarters leaves, broccoli was more successful at intercepting P A R early in the study. By the third week of the study, lamb's-quarters plants were generally taller than broccoli plants. In 2000, increasing R 2 values indicated that species population densities increased their contributions to variation in P A R interception as time progressed at both levels of UV - B radiation (Table 5.2). This may reflect the less vigorous growth of plants during 2000; as plant canopy closure progressed however, plant interactions increased. In this case, species tended to share equally in PAR interception during the second, third, and fourth week of U V - B treatments. As in 1999, during the final week of growth lamb's-187 quarters intercepted more P A R than broccoli at low, and less at high U V - B radiation ((axi/axB) =1.13 and 0.74, respectively). At the final time-point, results obtained using inverse yield-density models to determine proportion of incident PAR intercepted by constituent species were in accord with results obtained when biomass was used as measures of yield (Chapter 3) in both years; i.e. broccoli gained a competitive advantage relative to lamb's-quarters at the elevated U V - B level. Highly significant regressions (P < 0.01) when both biomass and PAR were used as measures of resource acquisition suggest that competition for light was indeed occurring. Competition for light within mixed associations is a complex process and profoundly impacts growth and morphology of constituent plants. While light intensity influences biomass production, the quality, or spectral distribution, of light induces specific morphogenic responses to neighbouring plants (Ballare 1999). The phytochrome system tends to counteract the tendency for size inequality to develop in dense populations (Ballare 1999). In plant associations, red light intercepted by phytochrome in the upper canopy tends to reduce internode length. Crowding signals (such as reflected far-red radiation) from shade deeper in the canopy elicit shade-avoidance responses (Ballare et al. 1995, Smith and Whitelam 1997), one of which is stem elongation. Increased height relative to associated plants is of utmost importance in order to place foliage in a position to intercept light. In both years broccoli and lamb's-quarters plants revealed morphological plasticity in response to crowding by reducing height in response to increasing lamb's-quarters population density (Table 5.3 and Figs. 5.1, 5.2, 5.3 and 5.4). McConnaughay and Bazzaz (1992) also reported that increasing neighbour shoot density reduced lamb's-quarters main shoot height and leaf production. Rohrig and Stutzel (2000) found that under reduced light levels due to crowding, lamb's-quarters' dry matter production was reduced so significantly that its height potential could not be realized. The U V - B photoreceptor causes some morphological changes similar to those elicited by phytochrome (Caldwell 1997). Exposure to elevated U V - B radiation commonly results in reduced plant height (Barnes et al. 1993, Furness and Upadhyaya 2002), thereby adding to the complexity of competitive interactions. When grown in 2 1 mixtures however, height of broccoli plants increased at 7, compared with 4 kJ m" d" 188 UV-BBE radiation, while the opposite was true for lamb's-quarters plants (Table 5.3 and Figs. 5.1, 5.2, 5.3, and 5.4). Barnes et al. (1990b) showed that relative shifts in foliage placement in response to elevated U V - B radiation resulted in wheat overtopping wild oats, and this was deemed sufficient to alter competition for light between the two species (Ryel et al. 1990). It should also be noted that broccoli leaf area increased, while lamb's-quarters declined in response to elevated U V - B radiation (Appendix 3.2). Simple over-topping of one species by another due to a height differential would lead to a predictable outcome in competition for light (Caldwell 1987). Walker et al. (1988) determined that the sunlit foliage area was an appropriate measure of the ability of each species in a mixture to compete for available light and could be used to quantify light competition between species. The situation may be much more complicated however, i f height differences are subtle and foliage of competing plants is distributed vertically throughout the stand. At different layers, leaves of a plant are being shaded and are shading leaves of the same individual as well as those of neighbouring plants. Lamb's-quarters has a rigidly erect growth form which may limit its flexibility in positioning leaves in response to crowding (McConnaughay and Bazzaz 1992). Conversely broccoli has a flexible, decumbent stem and relatively long petioles. Less rigid structure within the canopy may have enabled greater flexibility in leaf deployment by broccoli as has been suggested for giant foxtail (Setaria faberii (Herm.)) (McConnaughay and Bazzaz 1992). Such flexibility may cause relationships to change quickly through time. By using the area of overhead canopy coverage (OCC) of associated species as the measure of resource acquisition (y) in inverse yield-density models (Eqns.5.2 and 5.3), it was possible to track the influence of U V - B radiation on the complex process of competition for light over time. Unlike total plant biomass, OCC encompasses architectural aspects (including leaf cupping, orientation, area, and height) of plant growth. Some studies have shown that architecture of neighbouring plants is at least as important in determining effects on light availability within a canopy as are the amount of leaf area or biomass produced (Caldwell 1987, Barnes et al. 1990b). Because data were available for each species, in addition to partitioning total sunlit foliage into contributions 189 by the constituent species, intra- and interspecific effects on OCC could also be separated. Significant inverse yield-density relationships were established for broccoli and lamb's-quarters at both levels of U V - B radiation and in both years using OCC as a measure of resource acquisition (Tables 5.5, 5.6, 5.7, and 5.8). Overall, R values varied widely with respect to species, time, and U V - B level (0.13 - 0.94). Additionally, R values for OCC tended to be somewhat lower compared with values achieved when biomass (Chapter 3) or PAR interception (Tables 5.1 and 5.2) were used as measures of resource acquisition. This implies that competition, in relation to all other factors affecting OCC, played a less important role in determining OCC than other measures of resource acquisition, such as biomass or PAR. These results suggest that a situation existed which was more complex than simple over-topping of one species by another. For example, relative differences in height and leaf area may have been subtly changing throughout the study. In addition, foliage of competing plants may have been present throughout several strata of the associations. Potential OCC of the target plant grown in isolation is represented by the reciprocal of the regression intercept (aeo or ai.o) (Eqns. 5.2 and 5.3). As intercepts were often not significant (Tables 5.5, 5.6, 5.7, and 5.8), comparison of potential OCC in the absence of competition at the two levels of U V - B radiation was not justified. As noted in Chapter 3 however, interpretation of the relative competitive abilities of broccoli and lamb's-quarters in associations remain legitimate, as they do not depend on these parameter estimates. Actual OCC of a target plant in a mixture diverges from potential OCC in isolation due to intraspecific [quantified by coefficients aBB (broccoli) and 3LL (lamb's-quarters)] and interspecific competition [quantified by coefficients a B L (broccoli in competition with lamb's-quarters) and aLB (lamb's-quarters in competition with broccoli)] (Eqns. 5.2 and 5.3). In 1999, with broccoli as the target species at 4 kJ m" d" UV-BBE radiation, intraspecific competitive effects (aBB) strengthened between weeks one and two, and declined thereafter (Table 5.5). Interspecific competitive effects (aBL) intensified during the first three weeks and declined at week four. The substitution rate (aaL/aBB) however, increased each week, indicating the increasing competitiveness of lamb's-190 quarters with time at the low U V - B level. At 7 kJ m' 2 d"1 U V - B B E radiation, both intra-and interspecific components of interference increased with time. However, the magnitude of these parameters was lower than corresponding parameters at the low UV-B level. Elevated U V - B radiation may have constituted a more stressful environment for lamb's-quarters plants, resulting in less intense competition when broccoli was the target species. Overall, the substitution rate increased with time. At both U V - B levels, broccoli responded more to members of its own species at week one and thereafter more to lamb's-quarters (Table 5.5). Substitution rates were similar at both U V - B levels at weeks one and two. At weeks three and four however, substitution rates at the elevated U V - B level were substantially less than those at the low UV-B level, indicating that when OCC is used as a measure of competitiveness, broccoli gained an advantage at the elevated U V - B level following three weeks of U V - B exposure. When lamb's-quarters was the target species in 1999, no trends in competitiveness with respect to time or U V - B level were detected (Table 5.6). While both species contributed to the inverse yield-density relationship, lamb's-quarters always responded more to itself than to broccoli. A slightly different pattern emerged with broccoli as the target species in 2000 (Table 5.7). At both U V - B levels, broccoli responded more to itself than to lamb's-quarters during weeks one and two, and more to lamb's-quarters thereafter. It is interesting to note that if the final harvest had taken place at week four as in 1999, it might have been concluded that U V - B had no influence on relative competitive abilities of broccoli and lamb's-quarters when OCC was used as a measure of resource acquisition. These results again emphasize the significance of the time-course study. However, at week five, the substitution rate was four-fold greater at 4, compared with 7 kJ m" d" UV-BBE radiation. Results at final harvest therefore, indicated that broccoli gained a competitive advantage relative to lamb's-quarters in accord with results from 1999. With lamb's-quarters as the target species in 2000, and OCC as a measure of resource acquisition, there were no evident trends in competitiveness with respect to time or U V - B level (Table 5.8). While broccoli and lamb's-quarters both contributed to the 191 inverse yield-density relationship, lamb's-quarters always responded more to itself than to broccoli. These results for lamb's-quarters were similar to 1999. Ability to intercept light and effectively use it in photosynthesis determines success of species within mixed canopies to compete for light (Barnes et al. 1990b). The photosynthetic apparatus has been shown to be a target of U V - B radiation (Renger et al. 1989, Teramura and Sullivan 1994, Smith et al. 2000). Since photosynthesis is dependent on the light-harvesting properties of the chlorophylls, UV-B-induced changes in chlorophyll levels (Smith et al. 2000) might be expected to influence biomass accumulation. Many plants display reduced photosynthesis and chlorosis as symptoms of U V - B stress (Renger et al. 1989, Strid et al. 1994). In the present study leaf greenness measurements, shown to be correlated with chlorophyll content (Tenga et al. 1989), suggested that chlorophyll content of lamb's-quarters declined in plants grown at 7, compared with 4 kJ m~2 d"1 U V - B B E radiation (Table 5.10 and Figs. 5.10, and 5.11). Conversely, leaf greenness measurements of broccoli leaves increased at 7, compared with 4 kJ m' 2 d"1 U V - B B E radiation (Table 5.9 and Figs. 5.8 and 5.9). Results are in accord with research indicating that plants which maintain or enhance chlorophyll levels during U V - B exposure were often less negatively affected by this radiation (Bornman and Vogelmann 1990, Greenberg et al. 1997). Other researchers reported that changes in chlorophyll content were useful indicators of plant response to U V - B radiation, but could not be used to predict sensitivity of plant growth to U V - B radiation (Smith et al. 2000). In summary, this study attempted to address the effect of U V - B radiation on the extent of shoot canopies exposed to light and the capacity of those surfaces to intercept light for broccoli and lamb's-quarters plants grown in associations. By using inverse yield density models on non-destructive measures of resource acquisition [i.e. canopy PAR interception and overhead canopy coverage (OCC)] it was possible to track the process of competition through time. Over time broccoli became more and lamb's-quarters less successful in intercepting PAR (Tables 5.1 and 5.2). When OCC was used as a measure of resource acquisition broccoli gained in competitiveness relative to lamb's-quarters at the elevated U V - B level (Tables 5.5, 5.6, 5.7, and 5.8). These findings are in agreement with results obtained using inverse yield-density models on biomass, 192 and suggest that competition for light was occurring. UV-B-induced changes in leaf greenness implied that chlorophyll content of broccoli leaves increased at the elevated U V - B level, while that of lamb's-quarters declined (Table 5.9 and 5.10 and Figs. 5.8, 5.9, 5.10, and 5.11). Results from this study suggest that both canopy architecture and physiological plasticity might have contributed to increased competitiveness of broccoli relative to lamb's-quarters at elevated U V - B radiation. 193 General discussion Anthropogenic and natural destruction of stratospheric ozone have been linked to increased U V - B radiation at the earth's surface (Russell et al. 1996). Although levels of ozone-destroying pollutants are expected to peak within the next decade, their long atmospheric lifetime and future natural atmospheric perturbations ensure that high levels of U V - B radiation will persist at the earth's surface well into the 21 s t century (Madronich et al. 1995). This thesis investigates consequences of exposure to U V - B radiation (290-315 nm) on plant morphology and assesses effects of these morphological modifications on intra- and interspecific competition. This work focuses on shifts in ability to compete for light in broccoli (Brassica oleracea L. var. italica cv. Purple Sprouting) and lamb's-quarters (Chenopodium album L.) associations grown at 4 (ambient) versus 7 (above-ambient) kJ m"2 d"1 UV-BBE radiation in a greenhouse environment. Although this study embraced a range of issues, the main themes were as follows: (1) Characterization of the influence of U V - B radiation on growth and morphology of selected weed, vegetable crop, and crop cultivar seedlings, and comparison of relative susceptibility of plants within each of these groups to this radiation (Chapter 2), (2) Evaluation of intra- and interspecific components of interference in broccoli-lamb' s-quarters associations grown at ambient and above-ambient U V - B radiation levels (Chapter 3), (3) Elucidation of the role of UV-B-induced morphological changes on competitive interactions in broccoli-lamb's-quarters associations using plant growth indices and allometry (Chapter 4), and (4) Investigation of the role of UV-B-induced shifts in leaf and canopy optical properties (PAR interception and leaf greenness) on components of interference in broccoli-lamb's-quarters associations (Chapter 5). This study shows that agricultural weeds, crops, and crop cultivars vary in their susceptibility to elevated U V - B radiation levels (Chapter 2). Relative sensitivity of plants was ranked using a formula based on percent changes in stalk (stem and petiole) weight and leaf area in response to U V - B radiation. These results have significance in 194 agricultural situations because differential sensitivity to elevated U V - B radiation may affect competitive interactions in crop and weed associations. Based on growth and morphological sensitivity to U V - B radiation, agricultural relevance of species, uniform seedling emergence and similarity in seedling growth, broccoli cv. Purple Sprouting and lamb's-quarters were chosen as the crop and weed pair for subsequent experiments to study effects of U V - B radiation on their competitive interactions. To the best of my knowledge, this work uses inverse yield-density relationships for the first time to explore the influence of U V - B radiation on plant competitive interactions (Chapter 3). Inverse yield-density models describe the decline in yield per plant as population densities increase (Jolliffe 1997). Interference was considered to be due to competition for limited resources, although other forms of interference cannot be discounted. Parameters of inverse yield-density models showed that competitive balances in greenhouse-grown broccoli and lamb's-quarters associations differed between the two years of study and between UV - B radiation levels. Competition was a more important source of overall yield variation at 4 compared with 7 kJ m"2 d"' UV-BBE radiation as indicated by higher R 2 values (Weldon and Slauson 1986) for inverse yield-density models established at the elevated U V - B radiation level. Responsiveness of both species to density treatments at the elevated U V - B level may have been depressed by UV-B-induced suppression of lamb's-quarters growth. Lamb's-quarters was the stronger competitor at 4 kJ m"2 d"1 UV-BBE radiation in both years and at 7 kJ m"2 d"1 UV-BBE radiation in 1999. In 2000, when light intensity during the experiment was much lower compared with the 1999 growing season, broccoli was the 2 1 stronger competitor at 7 kJ m" d" UV-BBE radiation. Thus, variation in competitive strength of the two species over the two years could be due in part to modification of competitive effects by climatic variation (i.e. light and/or temperature) or by the interaction between climatic variation and U V - B radiation treatments. UV - B radiation influenced interspecific competition more than intraspecific competition in agreement with Barne's et al. (1988) hypothesis. These findings are important as they imply that UV - B effects might be exacerbated in mixed plant associations, and less consequential in pure stands. This could have far reaching implications for weed management strategies in agricultural situations as either crop or 195 weed could benefit from elevated U V - B radiation depending on the manner in which interspecific competition is affected. While total biomass effectively expressed competitive outcome (Chapter 3), insights into the influence of U V - B radiation on the process of competition were gained by partitioning biomass into various components (Chapter 4). The plastic nature of competitive interactions among closely associated broccoli and lamb's-quarters plants grown at ambient and above-ambient levels of U V - B radiation was characterized using indices of growth and allometry. Analysis of variance of plant growth indices indicated that morphology and biomass partitioning were often influenced by U V - B radiation but that differences also occurred between broccoli and lamb's-quarters. Broccoli responded differently to elevated U V - B radiation between the two years of study. For example, leaf thickness and/or density decreased in response to elevated U V - B radiation in 1999 and increased in 2000. U V - B radiation did not influence biomass partitioning to leaves, or above- versus below-ground biomass partitioning in broccoli. Leafiness as measured by L A R , is a major contributor to aggressiveness in some species (Roush and Radosevich 1985). It did not, however emerge as a key factor in determining competitiveness in this study. Leaf area index (LAI), which represents the extent of leaf array available for photosynthetic carbon assimilation increased at elevated U V - B radiation in broccoli in both years. LAI response to U V - B radiation corresponded to competitive outcome using inverse yield-density relationships on plant biomass (i.e. both LAI and competitiveness of broccoli increased at the elevated U V - B level). U V - B radiation did not affect leaf thickness and/or density, partitioning of biomass to leaves, or leafiness of lamb's-quarters grown in associations. LAI of this weed declined at the elevated U V - B level in both years, corresponding with its decreased competitiveness at elevated U V - B radiation. Elevated U V - B radiation also influenced above- versus below-ground biomass partitioning manifested as increased biomass allocation to the shoot. These results show that although elevated U V - B radiation influences biomass partitioning differently in broccoli and lamb's-quarters grown in associations, UV-B-induced change in LAI was a major contributor to shifts in competitive ability of both species. Additionally, some U V - B effects on broccoli growth indices differed between the two years while those of lamb's-quarters did not. 196 This research revealed some common characteristic responses of broccoli and lamb's-quarters to crowding. Both species produced thinner and/or less dense leaves at increasing plant densities. LAI of both species tended to increase with increasing density of their own species and decline with decreasing density of the companion species. Other growth indices differed between species and years making it problematic to identify responses as typical adjustments to permit each species to exploit environmental resources and tolerate crowding. While significant U V - B and species interaction terms reflected the underlying complexity of relationships, these effects tended to be small in comparison with the magnitude of main treatment effects. Broccoli and lamb's-quarters biomass growth tended to be proportional to growth of biomass components and leaf area (Chapter 4). While variation in biomass per plant was strongly related to allometric adjustments, direct (non-allometric) U V - B radiation and species density effects were also observed. Simple bivariate, allometric relationships between total biomass and height explained less variation in total biomass. Overall, U V -B-effects on allometric adjustments were less than density-effects. Direct (non-allometric) treatment effects were more often detected in broccoli compared with lamb's-quarters. The diversity of responses of broccoli and lamb's-quarters to U V - B radiation and competition were reflected in the complex behaviour of plant growth indices as well as in allometric and non-allometric relationships. Dynamics of UV-B-induced shifts in competitive interactions including changes in canopy characteristics were illustrated in this thesis using non-destructive measures (Chapter 5). For example, inverse yield-density models showed that over time broccoli contributed more at the elevated U V - B level, and lamb's-quarters less to overall interception of PAR. These findings correspond to results obtained when relative competitive abilities of broccoli and lamb's-quarters were assessed using inverse yield-density models on total biomass. Further investigation of canopy structure supported earlier evidence, including results from inverse yield-density models on biomass (Chapter 3) and PAR interception (Chapter 5), indicating that broccoli gained in competitiveness relative to lamb's-quarters at elevated U V - B radiation, and that competition for light was a major factor. Broccoli height and leaf area increased, while number of leaves was unaffected by exposure to elevated U V - B radiation. Conversely, height, leaf area, and number of leaves in lamb's-197 quarters tended to decline at the high U V - B level, again highlighting differential species response to elevated U V - B radiation. Inverse yield-density models with overhead canopy coverage (OCC) as the measure of resource acquisition showed that lamb's-quarters was the stronger competitor at both U V - B levels. Over time, differential UV-B-induced effects on height, leaf area and/or number of leaves translated into a gain in competitiveness of broccoli relative to lamb's-quarters at above-ambient compared with ambient U V - B radiation. These results show that differential growth and morphological changes (i.e. height, leaf number, and leaf area) induced by elevated U V - B radiation were sufficient to cause significant shifts in OCC of lamb's-quarters and broccoli grown in associations. Competition for PAR within a mixed association depends the ability of an individual species to intercept light and to efficiently use it in photosynthesis. U V - B -induced changes in leaf greenness reported in Chapter 5 implied that elevated U V - B radiation increased chlorophyll content of broccoli leaves and decreased that of lamb's-quarters leaves. Differential UV-B-effects on chlorophyll content could contribute to increased competitive ability of broccoli at the elevated U V - B level indicating that competition for light may be influenced by physiological as well as morphological factors. Competition for light can be asymmetric in that larger individuals remove disproportionately more of the resource compared with smaller ones (Freckleton and Watkinson 2001). This may result from height differences between species enabling one species to overtop another, thereby pre-empting access to light. Due to its asymmetrical character (Freckleton and Watkinson 2001), competition for light can potentially magnify UV-B-effects on shoot morphology and may therefore be central to UV-B-induced alteration of the composition of plant associations (Barnes et al. 1996). The vast range of U V - B responses expressed by different species reflects the diversity of plant characteristics that are influenced by U V - B radiation including leaf area, height, biomass partitioning pattern, and chlorophyll content. While the basis for differential response to U V - B radiation is beyond the scope of this thesis, results suggest that at elevated U V - B radiation both differential architectural and physiological plasticity contributed to increased ability of broccoli relative to lamb's-quarters to compete for PAR. 198 It is necessary to acknowledge several limitations in studying UV-B-effects on plant competitive relationships in greenhouse environments. Firstly, in greenhouse studies all U V - B radiation originates from lamps whose spectral composition differs from sunlight. Potential exists for errors associated with assumptions of action spectra used to weight artificial U V - B sources in order to attain a measure of biologically effective U V -B radiation (Dr. P. Barnes, Pers. comm.). Secondly, unrealistic P A R : U V - A : U V - B ratios in greenhouse studies may reduce the natural ability of plants to undergo acclimation or repair processes resulting in exaggerated UV-B-sensitivity (Adamse and Britz 1992b, Caldwell et al. 1994, Greenberg et al. 1997, McCleod 1997, Allen et al. 1998). In greenhouse environments, PAR and U V - A radiation are usually less than in field conditions. Thirdly, field-grown plants commonly encounter several stress factors concurrently. Several studies have shown that overall effectiveness of U V - B radiation might be ameliorated by additional factors present in the stress complex (Murali and Teramura 1985, Sullivan and Teramura 1990). However, by removing some components of interference such as competition for nutrients and/or water, greenhouse studies provide the opportunity to explore competitive interactions under more controlled and clearly-defined conditions than those found in field situations. While an ecologically relevant interpretation of elevated U V - B radiation necessitates further studies under field conditions, greenhouse experiments provide valid information regarding fundamental mechanisms and processes of UV-B-induced influences on competitive relationships (Caldwell etal. 1995, Dr. J. Sullivan, Pers. comm.). In summary, research reported in this thesis has advanced our understanding of the influence of U V - B radiation on plant competitive interactions by using inverse yield-density models for the first time to demonstrate that elevated levels of U V - B radiation have the potential to alter population structure of plant communities. When grown in association, broccoli gained in competitiveness relative to lamb's-quarters at above-ambient compared with ambient U V - B radiation in both years of study. U V - B radiation effects were greater on inter- compared with intraspecific competition. Shifts in competitive abilities were the result of differential morphological and physiological responses to elevated U V - B radiation which altered species' abilities to compete for PAR. 199 Literature Cited Adamse, P. and S.J. Britz. 1992a. Spectral quality of two fluorescent U V sources during long-term use. Photochem. Photobiol. 56: 641-644. Adamse, P. and S.J. Britz. 1992b. 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(21) D Legend (209) (93) (27) F Purple Sprouting (119) (59) (19) Leaf Stalk Root Leaf Stalk Root Appendix 2.1 Effect of U V - B radiation on leaf, stalk, and root dry weights of broccoli cultivars: A) Arcadia, B) Emperor, C) Everest, D) Legend, E) Patriot, and F) Purple Sprouting seedlings in experiment 1. Average biomass values (mg plant*1) for control seedlings are given in parentheses above respective control bars. U V - B B E treatment doses were 0 (closed bars), 7 (open bars), 2 1 and 11 (shaded bars) kJ m" d" . 220 E Patriot (84) (42) F Purple Sprouting (76) (42) (11) Leaf Stalk Root Leaf Stalk Root Appendix 2.2. Effect of U V - B radiation on leaf, stalk, and root dry weights of broccoli cultivars: A) Arcadia, B) Emperor, C) Everest, D) Legend, E) Patriot, and F) Purple Sprouting seedlings in experiment 2. Average biomass values (mg plant*1) for control seedlings are given in parentheses above respective control bars. U V - B B E treatment doses were 0 (closed bars), 7 (open bars), 2 1 and 11 (shaded bars) kJ m" d" . c 2 •3 CQ I > o *-» CO L H as 3 o g H-» W , L H -cd 'ofi| ed E H-H O KJ ' U —1 o o o o L H x> o co cu o •3 C o L H , 00| <H-H o CD co e to CD Pi CN * c <u . < .SP '53 <H-S 3 ' o ,°o| S o u a. on CD /-to — •8 ^ § 3 £ LU UJ c a> E 0 J u E *c + 1 +l +l ^ NO OO ON Z — ro O ro ro ro r- NO + 1 + 1 O N O r~- oo ro CN CN m + 1 o on Z CN — — o o o °" "=> °" + 1 + 1 + 1 g ON — ro ^" NO r~- r-~ o d d — — CN o o p o d d + 1 + 1 + 1 NO -fl" CN r~- r~ r-; o d d C/3 p CN + 1 m CN CN NO d + 1 NO r f CN CN r> o d + 1 + 1 £ ON 0 0 O ro CN CN N O O + 1 + I x > c y 3 NO oo NO vi CN CN o o <J o u OQ o r-CQ I > i o O -a CO CJ CQ I > •Si — J3 ro oo o + 1 + 1 + 1 o in o ^ Z — CN O ro ro ro + 1 + 1 + 1 O O N O N 0 0 oo CN CN tN CN CN CN O O p o d d + 1 + 1 + 1 ON r- ON NO NO NO o d d c/3 c n Z Z CN '—1 '—1 p o p o d d 4 4 4.1 4 4 ^ 1 / 5 + 1 +l +l z Z — ( N O r- r~- r-d d d NO d + 1 o +l on ro Z Z CN O CN CN CN CN in in in o d —'• + 1 + 1 + 1 <5 % ~t ON —; Z Z •fl-' fl1 in CN CN CN <=> r- ~ CQ CQ > > o o > <-> I CD a. 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Influence of U V - B radiation on the shoot: root ratio (SRR) of broccoli cultivars. Cultivar U V - B dose (kJm" 2 d-1) Experiment I Experiment II Arcadia 0 11.8 ± 1.0a 11.4 ± 1.0 "7 14.2 ± 1.8 13.3 ±0.6 11 15.7 ± 1.3 18.1 ±2.1 Control v*. U V - B N S b * Low vs. high U V - B NS * Emperor 0 9.0 ± 1.0 12.3 ± 1.2 7 11.1 ± 0.8 13.3 ±0 .6 11 14.8 ± 1.4 10.7 ±0 .9 Control vs. U V - B ** NS Low vs. high U V - B * NS Everest 0 11.3 ±0 .8 11.9 ± 1.2 7 14.4 ± 0.9 13.5 ± 1.4 11 17.9 ± 1.0 16.8 ±0 .7 Control vs. U V - B ** * Low vs. high U V - B * NS Legend 0 11.4±0.8 10.9 ±0.5 7 14.3 ± 1.3 12.7 ±0 .6 11 17.0 ± 1.6 17.9 ± 1.6 Control vs. U V - B * ** Low vs. high U V - B NS ** Patriot 0 11.3 ± 1.0 11.8 ±0 .4 7 14.4 ± 1.2 14.4 ±0 .9 11 19.3 ±2.3 14.2 ±0 .9 Control vs. U V - B ** * Low vs. high U V - B * NS Purple Sprouting 0 9.6 ±0 .4 10.8 ±0 .4 7 13.9 ± 1.1 13.9 ±0 .4 11 19.8 ± 1.5 14.5 ±0 .9 Control vs. U V - B ** ** Low vs. high U V - B ** NS "Values are means ± SE of six replicates. b * Significant at P = 0.05, ** Significant at P = 0.01, and NS = not significant. Appendix 3.1. Ambient greenhouse PAR and U V - A levels during the competition experiments. Date Weather conditions PAR U V - A (umol m"2 s"1) (uW cm'2) 1999 July 16 Clear 1320a 430 July 23 Clear 1170 410 July 30 Lightly overcast 620 290 Aug 6 Overcast 420 100 Aug 13 Clear 1220 400 2000 Oct. 31 Clear 725 280 Nov. 7 Lightly overcast 190 80 Nov. 14 Clear 675 250 Nov. 21 Overcast 120 60 Nov. 28 Overcast 110 50 Dec. 5 Clear 540 230 aValues are means of three measurements. Measurements were taken weekly at noon. TT C N C N O N O N O N C/3 "3 .2 c s > 5-o o C J o o fc c j T J c 03 co CJ > c CS u ccj CN c C J < CCS -o O N O N O N O c j O o UH CQ -J Ofl > .6. 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A N O V A results (variance ratios) for the effects of U V - B radiation, broccoli population density (X B), and lamb's-quarters density (X L) on broccoli mean primary variables weighted by the inverse of the variance in 1999. Source of Yield variables variation df w R a W s W L L A Block 3 10.491" * 12.401 * 9.144 * 14.938 * U V B 1 2.035 NS 1.214 NS 0.067 NS 4.160 * X B 2 4.392 * 4.853 * 21.631 * 17.383 * X L 3 23.857 * 34.823 * 63.607 * 56.822 * Block x U V B 3 0.191 NS 0.483 NS 0.583 NS 1.511 NS Block x XB 6 0.774 NS 0.853 NS 1.482 NS 0.672 NS U V B x X B 2 0.471 NS 0.377 NS 0.844 NS 2.193 NS Block x X L 9 1.440 NS 2.640 * 1.616 NS 1.757 NS U V B x X L 3 2.382 NS 2.621 NS 0.627 NS 0.067 NS X B X X L 6 2.419 * 1.758 NS 6.835 * 6.421 * U V B x X B x X L 6 0.563 NS 0.987 NS 0.307 NS 0.521 NS Error 51 3WR = root biomass, Ws = stem biomass, WL = leaf biomass, and L A = leaf area. Means and standard errors of data are given in Appendix 3.2a. * Significant at P = 0.05; NS: not significant. 229 Appendix 3.3b. A N O V A results (variance ratios) for the effects of U V - B radiation, broccoli population density (X B), and lamb's-quarters density (X L) on lamb's-quarters mean primary variables weighted by the inverse of the variance in 1999. Source of Yield variables variation df w R a W s W L L A Block 3 5.833 b* 2.693 NS 2.434 NS 2.607 NS U V B 1 54.009 * 40.114 * 60.812 * 70.448 * X B 2 20.712 * 27.728 * 29.728 * 25.628 * X L 3 47.885 * 54.738 * 76.057 * 67.599 * Block x U V B 3 1.465 NS 1.277 NS 1.792 NS 1.147 NS Block x XB 6 1.687 NS 1.255 NS 1.475 NS 2.263 NS U V B x X B 2 9.191 * 8.514* 6.311 * 6.091 * Block x X L 9 0.547 NS 0.219 NS 0.964 NS 1.274 NS U V B x X L 3 14.787 * 17.724 * 13.138 * 8.479 * X B X X L 6 5.419 * 7.811 * 8.390 * 7.619* U V B x X B X X L 6 5.279 * 6.212 * 4r874 * 5.534 * Error 51 WR = root biomass, Ws = stem biomass, WL = leaf biomass, and L A = leaf area. * Significant at P = 0.05; NS: not significant Means and standard errors of data are given in Appendix 3.2b. 230 Appendix 3.3c. A N O V A results (variance ratios) for the effect of U V - B radiation, broccoli population density (XB), and lambs-quarters density (XL) on broccoli mean primary variables weighted by the inverse of the variance in 2000. Source of Yield variables variation D f w R a W s W L L A Block 3 1.252b NS 2.568 NS 0.591 NS 1.460 NS U V B 1 20.270 * 50.697 * 97.052 * 21.745 * X B 2 21.102 * 43.361 * 98.765 * 75.065 * X L 3 17.730 * 30.573 * 50.172 * 40.789 * Block x U V B 3 0.880 NS 0.357 NS 0.378 NS 0.770 NS Block x XB 6 0.547 NS 0.412 NS 2.029 NS 2.114NS U V B x X B 2 2.902 NS 2.102 NS 0.335 NS 0.162 NS Block x X L 9 0.950 NS 1.872 NS 0.513 NS 0.465 NS U V B x X L 3 3.595 * 1.488 NS 3.359 * 9.878 * X B X X L 6 6.183 * 10.928 * 11.799 * 9.623 * U V B x X B x X L 6 4.532 * 1.539 NS 1.553 NS 1.925 NS Error 51 3 WR = root biomass, Ws = stem biomass, WL = leaf biomass, and L A = leaf area. Means and standard errors of data are given in Appendix 3.2c. * Significant at P = 0.05; NS: not significant. 231 Appendix 3.3d. A N O V A results (variance ratios) for the effect of U V - B radiation, broccoli population density (X B), lambs-quarters density (X L) and on lamb's-quarters mean primary variables weighted by the inverse of the variance in 2000. Source of Yield variables variation df w R a W s W L L A Block 3 2.949b * 0.444 NS 1.288 NS 0.583 NS U V B 1 261.966 * 263.299 * 148.160* 90.201 * X B 2 96.281 * 114.773 * 61.706 * 28.920 * X L 3 118.265 * 163.915 * 80.192 * 44.710* Block x U V B 3 3.756 * 1.207 NS 0.944 NS 1.749 NS Block x XB 6 1.182 NS 0.613 NS 0.775 NS 0.850 NS U V B x X B 2 14.807 * 6.985 * 2.792 * 0.854 NS Block x X L 9 0.793 NS 1.000 NS 0.622 NS 1.012 NS U V B x X L 3 81.396 * 81.466 * 41.234* 31.888 * X B X X L 6 13.394 * 17.631 * 10.086 * 4.815 * U V B x X B x X L 6 12.824 * 9.328 * 4.498 * 1.181 NS Error 51 a W R = root biomass, Ws = stem biomass, WL = leaf biomass, and L A = leaf area. bMeans and standard errors of data are given in Appendix 3.2d. *Significant at P = 0.05; NS: not significant. 232 Appendix. 3.4a. Regression parameters and statistics for inverse yield-density models (Eqns. 3.1 and 3.2) describing the response of reciprocal root biomass (WR"1) of (a) broccoli and (b) lamb's-quarters to broccoli and lamb's-quarters population densities in 1999. (a) Broccoli UV-BBE (kJ m' 2 d"1) aBO aBB aBL aBi/aBB R2 P 4 23.5120 NS (14.3612) 0.1389 * (0.0636) 0.2367 * (0.0462) 1.70 0.47 O.0001 7 7.3658 NS (13.7261) 0.2136 * (0.0698) 0.2417 * (0.0458) 1.13 0.51 O.0001 4 vs. 1 NS NS NS (b) Lamb's-quarters aLO aLL aLB aLB/aLL R2 P 4 -12.2448 * (4.3680) 0.1279 * (0.0280) 0.0419 * (0.0105) 0.33 0.68 O.0001 7 5.7371 NS (5.7852) 0.0922 * (0.0333) 0.0607 * (0.0186) 0.66 0.56 O.0001 4 vs. 7 * NS NS *Regression intercepts and coefficients differ significantly from zero according to a T-test (P = 0.05). Values in brackets below the regression parameters are standard errors of estimate. Corresponding regression intercepts and coefficients from inverse yield-density models developed at 4 versus 7 kJ m"2 d"1 U V - B B E according to a T-test; differ significantly (P = 0.05, *) or do not differ (NS). 233 Appendix. 3.4b. Regression parameters and statistics for inverse yield-density models (Eqns. 3.1 and 3.2) describing the response of reciprocal stem biomass (W s"') of (a) broccoli and (b) lamb's-quarters to broccoli and lamb's-quarters population densities in 1999. (a) Broccoli UV-BBE (kJ m"2 d"1) aBO aBB aBL aBiTaBB R2 P 4 5.5943 * (2.3878) 0.0223 NS (0.0114) 0.0589 * (0.0101) 2.64 0.61 O.0001 7 3.9815 NS (2.9862) 0.0329 * (0.0132) 0.0352 * (0.0080) 1.07 0.37 O.0001 4 vs. 1 * NS * (b) Lamb's-quarters ano a L L aLB a L B / a L L R2 P 4 -1.2644* (0.6149) 0.0169 * (0.0038) 0.0085 * (0.0015) 0.50 0.85 O.0001 7 0.9704 NS (0.6683) 0.0101 * (0.0037) 0.0116 * (0.0024) 1.15 0.67 O.0001 4 vs. 1 NS NS NS *Regression intercepts and coefficients differ significantly from zero according to a T-test (P = 0.05). Values in brackets below the regression parameters are standard errors of estimate. Corresponding regression intercepts and coefficients from inverse yield-density models developed at 4 versus 7 kJ m"2 d"1 U V - B B E according to a T-test; differ significantly (P = 0.05, *) or do not differ (NS). 234 Appendix. 3.4c. Regression parameters and statistics for inverse yield-density models (Eqns. 3.1 and 3.2) describing the response of reciprocal leaf biomass (WL"1) of (a) broccoli and (b) lamb's-quarters to broccoli and lamb's-quarters population densities in 1999. (a) Broccoli UV-BBE aBO aBB aBL a B L /a B B R 2 P (kJ m"2 d"1) 4 0.1220 NS 0.0318 * 0.0487 * 1.53 0.73 O.0001 (1.6624) (0.0092) (0.0069) i 3.0615 NS 0.0219* 0.0455 * 2.08 0.73 <0.0001 (2.0524) (0.0098) (0.0064) 4 vs. 7 NS . NS NS (b) Lamb's-quarters aLO a LL aLB a L B /a L L R 2 P 4 -2.144* 0.0243 * 0.0075 * 0.31 0.76 O.0001 (0.7042) (0.0042) (0.0022) 7 0.4555 NS 0.0171 * 0.0133 * .78 0.72 O.0001 (1.0534) (0.0056) (0.0033) 4 vs. 1 * NS NS * Regression intercepts and coefficients differ significantly from zero according to a T-test (P = 0.05). Values in brackets below the regression parameters are standard errors of estimate. Corresponding regression intercepts and coefficients from inverse yield-density models developed at 4 versus 7 kJ m"2 d"1 U V - B B E according to a T-test; differ significantly (P = 0.05, *) or do not differ (NS). 235 Appendix. 3.4d. Regression parameters and statistics for inverse yield-density models (Eqns. 3.1 and 3.2) describing the response of reciprocal leaf area (LA" 1) of (a) broccoli and (b) lamb's-quarters to broccoli and lamb's-quarters population densities in 1999. (a) Broccoli UV-BBE (kJ m"2 d"1) aBO aBB aBL aBiVaBB R2 P 4 0.001471 NS (0.002856) 0.00005904 * (0.00001702) 0.0009425 * (0.00001506) 15.96 0.71 <0.0001 7 0.006181 NS (0.004118) 0.00004687 * (0.00001948) 0.00007025 * (0.00001279) 1.50 0.58 <0.0001 4 vs. 1 NS NS * (b) Lamb's-quarters aLo a L L aLB a L B / a L L R2 P 4 0.002093 NS (0.001600) 0.00004555 * (0.00000889) 0.00002037 * (0.000005097) 0.45 0.76 <0.0001 7 0.003654 NS (0.002655) 0.00003605 * (0.00001313) 0.00002966 * (0.000007923) 0.82 0.65 O.0001 4 vs. 1 NS NS NS * Regression intercepts and coefficients differ significantly from zero according to a T-test (P = 0.05). Values in brackets below the regression parameters are standard errors of estimate. Corresponding regression intercepts and coefficients from inverse yield-density models developed at 4 versus 7 kJ m"2 d"1 U V - B B E according to a T-test; differ significantly (P = 0.05, *) or do not differ (NS). 236 Appendix. 3.4e. Regression parameters and statistics for inverse yield-density models (Eqns. 3.1 and 3.2) describing the response of reciprocal root biomass (WR"1) of (a) broccoli and (b) lamb's-quarters to broccoli and lamb's-quarters population densities in 2000. (a) Broccoli UV-BBE (kJ m"2 d"1) aBo aBB aBL aBi/aBB R2 P 4 93.5992 * (33.3813) 0.8486 * (0.0855) 0.4335 * (0.0556) 0.51 0.80 O.0001 7 94.9859 * (31.9273) 0.3959 * (0.1448) 0.817 NS (0.0706) 2.06 0.39 O.0001 4 vs. 7 NS * * (b) Lamb's-quarters au) aLL aLB aLB/aLL R2 p 4 -14.9839 NS (6.2947) 0.2142 * (0.0348) 0.0679 * (0.0166) 0.32 0.73 O.0001 7 44.0708 * (13.5983) 0.0813 NS (0.0605) 0.1070 * (0.0371) 1.32 0.67 O.0001 4 vs. 1 * NS NS * Regression intercepts and coefficients differ significantly from zero according to a T-test (P = 0.05). Values in brackets below the regression parameters are standard errors of estimate. Corresponding regression intercepts and coefficients from inverse yield-density models developed at 4 versus 7 kJ m"2 d"1 U V - B B E according to a T-test; differ significantly (P = 0.05, *) or do not differ (NS). 237 Appendix. 3.4f. Regression parameters and statistics for inverse yield-density models (Eqns. 3.1 and 3.2) describing the response of reciprocal stem biomass (Ws"1) of (a) broccoli and (b) lamb's-quarters to broccoli and lamb's-quarters population densities in 2000. (a) Broccoli UV-BBE aBO a B B aBL aBi /a B B R 2 P (kJ m"2 d"1) 4 5.8275 NS 0.1068 * 0.0861 * 0.81 0.89 O.0001 (4.3739) (0.0183) (0.0063) 7 9.6876 * 0.0674 * 0.0324 * 0.48 0.64 O.0001 (3.4434) (0.0192) (0.0088) 4 vs. 7 NS NS * (b) Lamb's-quarters aw a L L aLB a L B/a L L R 2 P 4 0.1737 NS 0.0245 * 0.0083 * 0.34 0.75 <0.0001 (0.7810) • (0.0040) (0.0021) 7 5.1223 * 0.0149 * 0.0135 * 0.93 0.72 O.0001 (1.6312) (0.0061) (0.0038) 4 vs. 1 * NS NS * Regression intercepts and coefficients differ significantly from zero according to a T-test (P = 0.05). Values in brackets below the regression parameters are standard errors of estimate. Corresponding regression intercepts and coefficients from inverse yield-density models developed at 4 versus 7 kJ m"2 d"1 UV-BBE according to a T-test; differ significantly (P = 0.05, *) or do not differ (NS). 238 Appendix. 3.4g. Regression parameters and statistics for inverse yield-density models (Eqns. 3.1 and 3.2) describing the response of reciprocal leaf biomass (WL"1) of (a) broccoli and (b) lamb's-quarters to broccoli and lamb's-quarters population densities in 2000. (a) Broccoli UV-BBE aBO aBB aBL aBL/aBB R 2 P (kJ m"2 d"1) 4 4.3450 * 0.0395 * 0.0320 * 0.84 0.92 O.0001 (2.0565) (0.0088) (0.0028) 7 4.4017 * 0.0300 * 0.0140 * 0.47 0.78 O.0001 (1.6002) (0.0074) (0.0041) 4 vs. 7 NS NS * (b) Lamb's-quarters ay) a L L a L B a L B / a L L R 2 P 4 0.6593 NS 0.0253 * 0.0125 * 0.49 0.78 O.0001 (1.0453) (0.0055) (0.0027) 7 7.6933 * 0.0006 NS 0.0165 * 27.50 0.31 O.0001 (2.4348) (0.0092) (0.0056) 4 vs. 1 * * NS * Regression intercepts and coefficients differ significantly from zero according to a T-test (P = 0.05). Values in brackets below the regression parameters are standard errors of estimate. Corresponding regression intercepts and coefficients from inverse yield-density models developed at 4 versus 7 kJ m"2 d"1 U V - B B E according to a T-test; differ significantly (P = 0.05, *) or do not differ (NS). 239 Appendix. 3.4h. Regression parameters and statistics for inverse yield-density models (Eqns. 3.1 and 3.2) describing the response of reciprocal leaf area (LA" 1) of (a) broccoli and (b) lamb's-quarters to broccoli and lamb's-quarters population densities in 2000. (a) Broccoli UV-BBE (kJ m"2 d"1) aBo aBB aBL asi/as B R 2 P 4 0.01260 * (0.004640) 0.00005313 * (0.00001975) 0.00005947 * (0.000006069) 1.12 0.89 O.0001 7 0.0170* (0.004153) 0.00005226 * (0.00001812) 0.00002309 * (0.000009941) 0.35 0.57 <0.0001 4 vs. 1 NS NS * (b) Lamb's-quarters aw aLL aLB aLB/aLL R2 p 4 0.001858 NS (0.002578) 0.00006688 * (0.00001295) 0.00002785 * (0.000005905) 0.42 0.78 O.0001 7 0.2125 * (0.005432) 0.00001399 NS (0.00002104) 0.00004416 * (0.00001226) 3.16 0.43 O.0001 4 vs. 1 * * NS * Regression intercepts and coefficients differ significantly from zero according to a T-test (P = 0.05). Values in brackets below the regression parameters are standard errors of estimate. Corresponding regression intercepts and coefficients from inverse yield-density models developed at 4 versus 7 kJ m"2 d"1 U V - B B E according to a T-test; differ significantly (P = 0.05, *) or do not differ (NS). 

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