UBC Theses and Dissertations

UBC Theses Logo

UBC Theses and Dissertations

Ecophysiology of the effect of red to far-red light ratio on selected weed and crop species Ma, Li 2017

Your browser doesn't seem to have a PDF viewer, please download the PDF to view this item.

Item Metadata

Download

Media
24-ubc_2017_november_ma_li.pdf [ 8.55MB ]
Metadata
JSON: 24-1.0355872.json
JSON-LD: 24-1.0355872-ld.json
RDF/XML (Pretty): 24-1.0355872-rdf.xml
RDF/JSON: 24-1.0355872-rdf.json
Turtle: 24-1.0355872-turtle.txt
N-Triples: 24-1.0355872-rdf-ntriples.txt
Original Record: 24-1.0355872-source.json
Full Text
24-1.0355872-fulltext.txt
Citation
24-1.0355872.ris

Full Text

 ECOPHYSIOLOGY OF THE EFFECTS OF RED TO FAR-RED LIGHT RATIO ON SELECTED WEED AND CROP SPECIES by  Li Ma  M.Sc., Northwest Agriculture and Forestry University, 2010  A THESIS SUBMITTED IN PARTIAL FULFILLMENT OF THE REQUIREMENTS FOR THE DEGREE OF  DOCTOR OF PHILOSOPHY in THE FACULTY OF GRADUATE AND POSTDOCTORAL STUDIES (Plant Science)  THE UNIVERSITY OF BRITISH COLUMBIA (Vancouver)   September 2017  © Li Ma, 2017 ii  Abstract Plants growing in canopies are exposed to reduced light intensity as well as low red/far-red light ratios, a signal of impending competition. In order to advance our understanding of the eco-physiological role of red/far-red ratio in agroecosystems, growth chamber and greenhouse studies were conducted to investigate 1. the growth and morphological responses of common lamb’s-quarters (Chenopodium album L.), redroot pigweed (Amaranthus retroflexus L.) and tomato (Solanum lycopersicum L.) to different red/far-red ratios, 2. how leaf optical properties at red (660 nm) and far-red (730 nm) wavelengths change with leaf position and plant development, and 3. if red/far-red ratio influences response of corn (Zea mays L.), lettuce (Lactuca sativa L.), and pigweed plants to UV-B radiation. Three red/far-red ratios were achieved by using supplemental far-red lamps in growth chambers, while maintaining the photosynthetically active radiation. Results showed that red/far-red ratio treatment influenced several growth and allometric parameters and tomato, lamb’s-quarters, and pigweed differed in this regard. This differential response among species suggests that fluctuations in the red/far-red ratio in canopies could impact inter-plant interactions among these species. Red/far-red ratio in a plant canopy is affected by leaf optical properties. A study of leaf optical properties of lamb’s-quarters, pigweed and tomato using a CI-710 Miniature Leaf Spectrometer showed that leaf optical properties changed with leaf position and plant development, which could modify the light environment in canopies comprising populations of these species. Interestingly, these species differed in this regard, suggesting differences in their ability to signal potential competition. Leaf optical properties, as well as leaf position and plant development effects, therefore, should be taken into consideration in assessing the eco-physiological functions of a vegetation cover. Effect of pre-exposure to low red/far-red ratio on plant susceptibility to UV-B radiation could impact plant interaction. However, my iii  research showed that red/far-red ratio pre-treatment and the associated change in anthocyanin concentration did not alter the response of corn, lettuce and pigweed seedlings to enhanced UV-B radiation. This improves our understanding of the eco-physiological role of these environmental stressors in agro-ecosystems, where both red/far-red ratio and UV-B levels fluctuate. iv  Lay Summary  This thesis aimed to study the ecophysiology of effects of red/ far-red light ratio, as an initial signal of potential competition, on selected weed and crop species. It was found that lamb’s-quarters, pigweed and tomato differed in their responses to changes in red/far-red ratio, suggesting that fluctuations in the red/far-red ratio in canopies of their mixed populations could impact inter-plant interactions among these species by affecting their growth and morphology. Leaf optical properties, which can affect the red/far-red ratio in plant canopies, changed with leaf position, plant development and species. These findings are important to our understanding of how leaf optical properties could influence inter-plant interactions. The observations that pre-exposure to different red/far-red ratios does not influence morphological responses of plants to UV-B radiation is significant to our understanding of the eco-physiology of plant-plant interaction in mixed-cropping systems where both of these environmental stressors fluctuate.      v  Preface  This research was designed, conducted, analyzed, and interpreted by me (Li MA). The dissertation was written by myself, and reviewed by my supervisory committee [Dr. M.K. Upadhyaya (Supervisor), Dr. C. Chanway, and Dr. D. Clements].  Chapters 2, 3, 4, and 5 have been written as free-standing chapters.  The research in Chapter 2 have been published in the Canadian Journal of Plant Science: L. Ma and M.K. Upadhyaya. 2017. Effects of red/far-red light ratio on common lamb’s-quarters, redroot pigweed, and tomato plants. Can. J. Plant Sci. 97: 1-7 dx.doi.org/10.1139/cjps-2016-0089. I carried out the experiments and data analysis, and drafted the manuscript. Prof. Upadhyaya supervised me and helped edit the draft.   The research in Chapter 3 has been accepted for publication by the Weed Research journal: L. Ma and M.K. Upadhyaya. 2017. Effects of leaf position on reflectance, transmittance and absorptance of red and far-red irradiance in tomato, Chenopodium album, and Amaranthus retroflexus leaves. I carried out the experiments and data analysis, and drafted the manuscript. Prof. Upadhyaya supervised me and helped edit it.  Contents of Chapter 4 and 5 will be submitted for publication shortly. I had some interaction with Prof. C.J. Swanton (University of Guelph) in planning of this study. He also provided corn vi  seeds for this study. I conducted all the research, analyzed data and wrote this chapter. Prof. Upadhyaya supervised me for this work.  vii  Table of Contents  Abstract .......................................................................................................................................... ii	Lay Summary ............................................................................................................................... iv	Preface .............................................................................................................................................v	Table of Contents ........................................................................................................................ vii	List of Tables ............................................................................................................................... xii	List of Figures ...............................................................................................................................xv	List of Abbreviations ................................................................................................................. xvi	Acknowledgements ................................................................................................................... xvii	Dedication ................................................................................................................................... xix	Chapter 1: Introduction ................................................................................................................1		 General Introduction ....................................................................................................... 1		 Literature Review ............................................................................................................ 4		 Red/far-red ratio and plant ecology ............................................................................ 4		 Leaf optical properties ................................................................................................ 8		 Ultraviolet-B radiation .............................................................................................. 10		 Plant species used in this study ................................................................................. 14	1.2.4.1	 Common lamb’s-quarters (Chenopodium album L.) ........................................ 14	1.2.4.2	 Redroot pigweed (Amaranthus retroflexus L.) ................................................. 16	1.2.4.3	 Tomato (Solanum lycopersicum L.) .................................................................. 18	Chapter 2: Effect of red/far-red light ratio on lamb’s-quarters, pigweed, and tomato plants .............................................................................................................................................21	viii  	 Summary ....................................................................................................................... 21		 Introduction ................................................................................................................... 22		 Materials and Methods .................................................................................................. 24		 Seed source and plant culture ................................................................................... 24		 Red/far-red ratio treatment ........................................................................................ 25		 Morphological response of plants to red/far-red ratio treatment .............................. 26		 Effect of red/far-red ratio treatment on root morphology and growth parameters ... 26		 Statistical analyses .................................................................................................... 27		 Results ........................................................................................................................... 28		 Effect of red/far-red ratio treatment on growth parameters ...................................... 28		 Effect of red/far-red ratio on root morphology and growth parameters ................... 32		 Effect of red/far-red ratio on plant allometry ............................................................ 32		 Discussion ..................................................................................................................... 32		 Conclusion .................................................................................................................... 38	Chapter 3: Effects of leaf position on reflectance, transmittance, and absorptance of red and far-red irradiance in leaves of tomato, lamb’s-quarters, and pigweed ...........................39		 Summary ....................................................................................................................... 39		 Introduction ................................................................................................................... 40		 Materials and Methods .................................................................................................. 41		 Seed source and plant culture ................................................................................... 41		 Effect of leaf position on reflectance (R), transmittance (T) and absorptance (A) at 660 and 730 nm ..................................................................................................................... 42		 Measurement of leaf chlorophyll content ................................................................. 44	ix  	 Specific leaf weight measurement ............................................................................ 44		 Statistical analyses .................................................................................................... 45		 Results ........................................................................................................................... 45		 Effect of leaf position on reflectance (R), transmittance (T), and absorptance (A) .. 45		 Effect of leaf position on red/far-red ratio of reflected and transmitted light ........... 48		 Effect of leaf position on chla+b, chla/b ratio, and SLW ............................................. 49		 Relationships of Rratio and Tratio with chla+b, chla/b, and SLW .................................... 49		 Discussion ..................................................................................................................... 53		 Conclusion .................................................................................................................... 56	Chapter 4: Changes in leaf optical properties during tomato, lamb’s-quarters, and pigweed plant development ........................................................................................................................58		 Summary ....................................................................................................................... 58		 Introduction ................................................................................................................... 59		 Materials and Methods .................................................................................................. 61		 Seed source and plant culture ................................................................................... 61		 Changes of reflectance (R), transmittance (T), and absorptance (A) at 660 and 730 nm during plant development ............................................................................................... 62		 Measurement of leaf chlorophyll content ................................................................. 62		 Specific leaf weight measurement ............................................................................ 63		 Statistical analyses .................................................................................................... 63		 Results ........................................................................................................................... 64		 Changes of reflectance (R), transmittance (T), and absorptance (A) during plant development .......................................................................................................................... 64	x  	 Changes of red/far-red ratio of reflected (Rratio) and transmitted (Tratio) lights during plant development ................................................................................................................. 67		 Changes of chlorophyll content (chla+b), chla/b ratio, and specific leaf weight (SLW) during plant development ..................................................................................................... 67		 Relationships of changes in Rratio and Tratio with chla+b, chla/b, and SLW .................. 70		 Discussion ..................................................................................................................... 72		 Conclusion .................................................................................................................... 75	Chapter 5: Influence of red to far-red ratio on response of plants to UV-B radiation .........76		 Summary ....................................................................................................................... 76		 Introduction ................................................................................................................... 77		 Materials and Methods .................................................................................................. 79		 Seed source and plant culture ................................................................................... 79		 Red/far-red ratio treatment ........................................................................................ 80		 UV-B radiation treatment ......................................................................................... 82		 Anthocyanin measurement ........................................................................................ 82		 Effect of UV-B radiation on plant growth parameters .............................................. 84		 Levels of UV-B-absorbing compounds, H2O2, and L-phenylalanine ammonia-lyase activity in corn leaves ........................................................................................................... 85		 Statistical analyses .................................................................................................... 86		 Results ........................................................................................................................... 87		 Effect of red/far-red ratio treatment on anthocyanin concentration in corn, lettuce, and redroot pigweed .............................................................................................................. 87	xi  	 Influence of red/far-red ratio pre-treatment on responses of corn plants to UV-B radiation ................................................................................................................................ 87		 Influence of red/far-red ratio pre-treatment on responses of lettuce and redroot pigweed plants to UV-B radiation ........................................................................................ 96		 Discussion ..................................................................................................................... 96		 Conclusion .................................................................................................................. 101	Chapter 6: General Conclusions...............................................................................................103	References ...................................................................................................................................107	 xii  List of Tables  Table 2.1. Effect of red/far-red ratio treatment on growth parameters of common lamb’s-quarters, redroot pigweed, and tomato plants………………………………….29 Table 2.2. Effect of red/far-red ratio treatment on biomass and leaf SPAD value in common lamb’s-quarters, redroot pigweed, and tomato plants……………………….....31 Table 2.3. Effect of red/far-red ratio treatment on root parameters of common lamb’s-quarters, redroot pigweed, and tomato plants…………………...………….….33 Table 2.4. Effect of red/far-red ratio treatment on allometry of common lamb’s-quarters, redroot pigweed, and tomato plants…………..………..………………………34 Table 3.1. Effect of leaf position on reflectance, transmittance, and absorptance of lamb’s-quarters, redroot pigweed, and tomato leaves at 660 nm and 730 nm wavelengths………………………………………………….……………...…46 Table 3.2. Effect of leaf position on chlorophyll content (Chla+b), Chla/b ratio, and specific leaf weight (SLW) of lamb’s-quarters, redroot pigweed, and tomato leaves………………………………………………………..………………....51 Table 3.3. Relationships of red/far-red ratio of the reflected (Rratio) and transmitted (Tratio) lights with Chla+b, Chla/b, and specific leaf weight (SLW) in lamb’s-quarters, redroot pigweed, and tomato……………………...…………………………...52 Table 4.1. Changes of reflectance, transmittance, and absorptance at 660 and 730 nm of the 3rd true leaf during the development of common lamb’s-quarters, redroot pigweed, and tomato plants……………………....……………….…………...65 xiii  Table 4.2. Changes of chlorophyll content per unit leaf area (chla+b), chlorophyll a/b ratio (chla/b), and specific leaf weight (SLW) of the 3rd true leaf of common lamb’s-quarters, redroot pigweed, and tomato during plant development……………………………………………………………….…..69 Table 4.3. Relationships of red/far-red ratios of the reflected (Rratio) and transmitted (Tratio) light with chlorophyll content (chla+b), chlorophyll a/b ratio (chla/b), and specific leaf weight (SLW) of the 3rd true leaf of common lamb’s-quarters, redroot pigweed, and tomato plant…………………………………………………..…71 Table 5.1. Red and far-red light intensities in different red/far-red ratio treatments............81 Table 5.2. Influences of enhanced UV-B radiation on plant height and leaf area of corn plants pre-exposed to 0.3 or 1.1 red/far-red light ratio…………...…………….91 Table 5.3. Influences of enhanced UV-B radiation on biomass allocation of corn plants pre-exposed to 0.3 or 1.1 red/far-red light ratio……………….............................…92 Table 5.4. Influence of enhanced UV-B radiation on some growth indices of corn plants pre-exposed to 0.3 or 1.1 red/far-red light ratio…………………………….…..94 Table 5.5. Influences of enhanced UV-B radiation on levels of H2O2, UV-B absorbing compounds, and phenylalanine ammonia lyase activity in corn plants pre-exposed to 0.3 or 1.1 red/far-red ratio……………………………………...….95 Table 5.6. Influence of enhanced UV-B radiation on length, width, and mass of leaves from lettuce plants pre-exposed to 0.3 or 1.1 red/far-red light ratio….……………....97 xiv  Table 5.7. Influence of enhanced UV-B radiation on height, leaf and shoot mass of redroot pigweed plants pre-exposed to 0.3 or 1.1 red/far-red light ratio…………..........98  xv  List of Figures  Fig. 2.1. Effect of red/far-red ratio on growth of (A) common lamb’s-quarters, (B) redroot pigweed, and (C) tomato plants……………………………………..…………….30  Fig. 3.1. CI-710 miniature leaf spectrometer…………………………………..…………...43 Fig. 3.2. Effect of leaf position on the red/far-red ratio of (A) the reflected (Rratio) and (B) the transmitted (Tratio) lights from leaves of common lamb’s-quarters, redroot pigweed, and tomato………………………………..……………………………………….50 Fig. 4.1. Changes of red/far-red ratio of the reflected (Rratio) (A) and transmitted (Tratio) (B) light from the 3rd true leaf during common lamb’s-quarters (○), redroot pigweed (●), and tomato (□) plant development……………………………………….….68 Fig. 5.1.   UV-B radiation treatment chambers in the greenhouse……………………….…..83 Fig. 5.2.   Effect of red/far-red ratio treatment on anthocyanin pigmentation in corn (A), redroot pigweed (B, C), and lettuce (D, E) plants………………………………....88 Fig. 5.3. Anthocyanin concentration in corn leaf sheath (A) after exposure to 0.3 or 1.1 red/far-red light ratio and (B) after UV-B treatment following red/far-red exposure…………………………………………………………………………..89  Fig. 5.4. Anthocyanin reflectance index (ARI) of leaf lamina of lettuce and redroot pigweed after low (0.3) or high (1.1) red/far-red ratio treatment……………………………90  xvi  List of Abbreviations A660                                                 Absorptance at 660 nm A730                                                 Absorptance at 730 nm ANOVA                                         Analysis of variance LAI                                                 Leaf area index LAR                                                Leaf area ratio LQ                                                  Common lamb’s-quarters Ls-mean                                          Least squares mean LWR                                               Leaf weight ratio PAR                                                Photosynthetically active radiation PAL                                                Phenylalanine ammonia lyase PW                                                  Redroot pigweed R                                                     Reflectance T                                                     Transmittance R660                                                 Reflectance at 660 nm R730                                                 Reflectance at 730 nm SE                                                   Standard error SLW                                               Specific leaf weight T660                                                 Transmittance at 660 nm T730                                                  Transmittance at 730 nm TCA                                                Trichloroacetic acid TM                                                  Tomato UV-B                                              Ultraviolet B xvii  Acknowledgements  I would like to offer my sincerest gratitude to my supervisor Dr. Mahesh K. Upadhyaya. My experience is invaluable during these past years and this would not have been possible without his patient guidance, encouragement, understanding, and support. I would also like to express appreciation to my advisory committee, Dr. Chris Chanway, Dr. David Clements and Dr. Roy Turkington for their valuable contributions. I am grateful to Dr. Clarence Swanton for offering guidance and encouragement, providing corn seeds for my experiment, and helping in the revision of Chapter 5.  I would also like to thank the Chinese Scholarship Council (CSC), Faculty of Land and Food Systems Graduate Award, Henry and Myrtle Eddie Scholarship, John M Yorston Memorial Prize, May L. Barnett Memorial Fund, Natural Science and Engineering Research Council of Canada (NSERC), Plant Science Graduate Scholarship in Faculty of Land and Food Systems, and Ursula Knight Abbott Travel Scholarship in Agricultural Sciences for financial support.  Many thanks to Dr. A. Kozak, Dr. V. Lemay and Rick White for help regarding statistical analyses, Peter Garnett and Jurgen Pehlke for help maintaining the growth chambers and experiment setup, David Kaplan and Melina Biron for support in the greenhouse and Seane Trehearne for help in Totem Field. Many thanks also go to Sylvia Leung and Lia Maria Dragan for their help and support. I would like to give my sincere appreciation to graduate student manager Shelley Small, who has been a special friend and greater supporter. I am so lucky to have her guidance and encouragement. xviii   My sincere appreciate also goes to Wenfang Hao, supervisor of my M.Sc. My life would be totally different without her help, encouragement, and support. I would also like to thank my friend Liang Zou, Cheng Peng, Guibo Yin for their help.  Special thanks go to my beloved husband, Kunzhong Jian for his unconditional love and support throughout my doctorate program. This achievement is impossible without his help, support, encouragement, and company.   xix  Dedication  I dedicate my dissertation work to my family who has always been there for me in these past years. I give special thanks to my two wonderful sons Botao Jian and Boyao Jian. I am forever grateful to my sister, Lina Ma for always supporting and believing in me. This thesis would not have been completed without their help and inspiration. I will always appreciate their contributions and support. 1  Chapter 1: Introduction   General Introduction  Light plays important roles in regulating plant growth and development. Light reaching plant leaves can be reflected, absorbed and/or transmitted. In the visible light spectrum, blue and red light are mostly absorbed while green and far-red light wavelengths are reflected or transmitted by leaves. Plants in natural populations are subjected to modified red/far-red ratios due to the selective absorbance of red light, and the transmittance and reflectance of far-red light by green foliage. While the red/far-red ratio of sunlight is approximately one, the red/far-red ratio ranges between 0.1 and 0.9 under a leaf canopy (Smith 1982). Red/far-red ratio plays an important role in signaling the impending competition from neighbouring plants (Ballaré et al. 1987, 1990; Smith et al. 1990; Ballaré 1999). When subjected to a reduced red/far-red ratio, plants express the shade avoidance syndrome, which includes stem elongation, increase in carbon allocation to the shoot, and change in leaf orientation (Ballaré et al. 1990; Maddonni et al. 2002; Rajcan et al. 2004; Liu et al. 2009) to increase light acquisition for photosynthesis. The shade avoidance responses induced by changes in red/far-red ratio could affect inter-plant interactions. In agroecosystems, the competitive interactions among crops and their associated weeds can be affected if they differ in their response to changes in red/far-red ratio. Knowledge of the growth and morphological responses of a crop and its associated weeds can provide important information for crop and weed management.   2  The red/far-red ratio in plant canopies is determined by leaf optical properties, which affect the amount of light reflected, transmitted and/or absorbed by leaves. The red/far-red light ratio can vary in a canopy profile if leaves at different positions on the stem differ in their optical properties. This has important implications for agricultural ecosystems where weeds emerge at different times in a crop and are therefore present at a different layer in a weed-crop canopy. Also, different species can signal different levels of potential competition if they differ in their leaf optical properties. When a crop is growing with several weeds, the signal for potential competition from these weeds can differ depending on their ability to alter the red/far-red ratio. Information on leaf optical properties can help weed managers know which weeds will exert a stronger signal for the upcoming competition in a weed-crop mixture. Growth rate, plant size and leaf area index (LAI) are generally considered important parameters of competitive ability in mixed populations. All leaves on an individual plant and of different species in a mixture are normally considered qualitatively similar when using LAI to assess the eco-physiological function of a vegetation cover in agroecosystems. However, it is of interest to know if leaves from different positions on the individual plant, of different age, and from different species differ in their optical properties. This knowledge could improve our understanding of eco-physiological underpinnings of plant-plant interactions within and among species.  Besides modified red/far-red ratio, plants in nature are also subjected to numerous other environmental stresses including drought, extreme temperatures, salinity, light intensity, pollution, and enhanced UV-B radiation (280-320 nm) (Smirnoff 1998). Increased UV-B radiation has been reported to affect plant growth and development (Dai et al. 1997; Mackerness et al. 1998; Mackerness 2000; Costa et al. 2002) and competitive balance between plant species (Barnes et al. 3  1988; Furness et al. 2005a, b). Low red/far-red light ratio has been reported to decrease the concentration of anthocyanins (Mancinelli 1990; Li and Kubota 2009; Afifi and Swanton 2012), which are water-soluble secondary phenolic plant protectants (Close and Beadle 2003). Commonly plants are exposed to modified red/far-red ratio and UV-B radiation either simultaneously or sequentially because of changes in plant density, shading, different filtration of light, and changes in season or cloudiness. Whether changes in red/far-red ratio and anthocyanin concentration induced by low red/far-red ratio modify the response of plants to enhanced UV-B radiation is not known. Information on these poorly understood aspects of light quality relationships is important to our understanding of ecophysiology of plants growing in ecosystems, where both the red/far-red ratio and UV-B radiation levels fluctuate.  Research objectives:  The objectives of the research described in this thesis are to:  1. characterize the responses of tomato, common lamb’s-quarters, and redroot pigweed plants to red/far-red light ratio, and determine if these species differ in this regard,   2. investigate if leaf position affects leaf optical properties at red and far-red wavelengths in tomato, common lamb’s-quarters, and redroot pigweed,  3. investigate if leaf optical properties at red and far-red wavelengths change during development of tomato, common lamb’s-quarters, and redroot pigweed plants, 4   4. determine if red/far-red light ratio and the associated change in anthocyanin influence the response of plants to subsequent exposure to elevated UV-B radiation.   Literature Review   Red/far-red ratio and plant ecology Light serves as both a source of energy for photosynthesis and a signal providing plants with important spatial and temporal information about the environment in their vicinity. Plants perceive and integrate all the environmental signals to adjust their growth and development to optimize photosynthetic activity and outcompete their neighbours. Since light reaching a plant population is selectively reflected, absorbed and/or transmitted by plant leaves, plants growing in canopies are subjected to reduced light intensity and altered light quality (Campbell and Norman 1998). In the visible light spectrum, blue and red light are preferentially absorbed by chlorophyll and carotenoid pigments while green and far-red light are mostly reflected or transmitted. Therefore, the red/far-red light ratio decreases due to the enrichment of far-red light in plant canopies. According to Smith (1982), the proportion of red to far-red light (655-665 nm/725-735 nm) in daylight is close to one (red/far-red ratio ≈ 1.15 ± 0.02), but this ratio decreases under plant canopies.   In plants, light signals are perceived by three classes of signal-transducing photoreceptors - red and far-red light-absorbing phytochrome, blue/UV-A light absorbing cryptochrome and 5  phototropin (Ballare and Casal 2000). Phytochromes are capable of sensing changes in red/far-red ratio. Seed dormancy, seed germination, seedling establishment, photosynthetic machinery development, vegetation architecture, flowering, responses to neighbour competition, and carbon allocation between plant organs can be controlled by phytochromes (Ballaré and Casal 2000). Five genes of phytochromes (PHYA-PHYE) have been identified and sequenced in Arabidopsis thaliana (Sharrock and Quail 1989; Clack et al. 1994). Phy A, phy B, and phy C, encoded by the genes PHYA, PHYB and PHYC, respectively, are three main types of phytochromes in angiosperms (Franklin and Whitelam 2007). Only phy A, phy B, and phy C are found in monocotyledonous plants (Mathews and Sharrock 1996, 1997). Phy B is the main photoreceptor that senses the red/far-red ratio and triggers the morphological and physiological changes in plants exposed to modified red/far-red ratio (Ballaré and Scopel 1997). Phytochromes exist in two photoconvertible forms, the biologically inactive red light-absorbing Pr form, and a biologically active far-red light-absorbing Pfr form (Smith 2000). Phytochromes are synthesized in the inactive Pr form and photo-transformed to the biologically active Pfr form (Smith 2000; Schäfer and Nagy 2006). These two forms of phytochrome usually coexist in equilibrium. The proportion of phytochromes in the form of Pfr (Pfr/Pr) is a direct function of the red/far-red ratio of incident light (Smith and Holmes 1977), which controls the shade avoidance responses in plants. Pfr/Pr can be altered by a change in red/far-red ratio caused by the scattered far-red light from neighboring vegetation (Smith et al. 1990). An approximately 60% Pfr/total phytochrome photoequlibrium is established in daylight but it can be as low as 10% in a canopy or in crowded plant communities (Smith 2000).   6  Red/far-red ratio is a far more reliable indicator of the vicinity of neighbouring plants than the amount of photosynthetically active radiation (PAR) penetrating a plant canopy (Smith 2000). It is strongly affected by canopy density and to a lesser extent by environmental changes (Casal and Smith 1989). Red/far-red ratio is also influenced by the spacing, architecture and optical properties of neighbouring plants. Species or varieties may differ in modifying red/far-red ratio in plant canopies due to differences in their canopy structure and foliar characteristics. Light reflected between neighbouring vegetation has a low red/far-red ratio due to the selective absorption of red relative to far-red light by chlorophyll and carries information about the proximity and spatial distribution of the neighbouring plants (Gundel et al. 2014). The reduction in the red/far-red ratio of light reflected from neighbouring plants can signal the presence, structure, and proximity of neighbouring vegetation, and enable the plants to initiate tolerance or avoidance strategies before the commencement of competition for light (Franklin 2008). Plants can distinguish potential and actual shading because of decreases in both red/far-red ratio and PAR, and increase in ethylene concentration under actual shading (Franklin 2008). Changes in red/far-red ratio influence plant growth and morphology (shade avoidance response) and have been considered as a signal of impending competition from neighbours (Ballaré et al. 1987, 1990; Smith et al. 1990; Ballaré 2009).   Many plant species, especially angiosperms, are able to perceive and respond to red/far-red ratio. Upon perceiving a low red/far-red ratio, plants express various responses, e.g. enhanced elongation growth, alteration of carbon allocation pattern (Liu et al. 2009), reduced leaf chlorophyll content and increased apical dominance (Smith and Whitelam 1997), more leaves on the upper position of the canopy, with the leaves turning away from neighbours (Maddonni et al. 2002), alteration of 7  their phytochemistry (Li and Kubota 2009), and a reduction of root biomass (Liu et al. 2009; Page et al. 2009). Besides the shade avoidance response, the red/far-red ratio can also affect seed germination in many species (Nolan and Upadhyaya 1988; Jankowska-Blaszczuk and Daws 2007; Górski et al. 2013). Far-red light transmitted through the leaf canopies was shown to inhibit the germination of 396 species (out of 487 tested) to various extents (Górski et al. 2013). Silvertown (1980) reported that germination was reduced significantly under a leaf canopy, compared to the dark, in 17 out of 27 species. A study on germination of Silene gallica and Brassica campestris seeds under an establishing wheat canopy showed that germination was inhibited at a very early stage (e.g. 15 days after crop emergence), when the canopy leaf area index was below one and the red/far-red ratio under the canopy was above 0.8 (Batlla et al. 2000). These studies suggest that seeds of these species can sense the potential threats of shading by a plant canopy and avoid germination if the environment is not suitable for successful seedling establishment (Batlla et al. 2000).  The success of competition between interacting plants is directly affected by their morphology (Robson et al. 2015). The most significant effects of red/far-red ratio on plant morphology are stem elongation and decrease in branching (Ballaré and Casal 2000), which are important in competition for light. The competitive balance among plant species will be shifted if they differ in their responses to modified red/far-red ratios. Therefore, the red/far-red ratio can affect plant-plant interactions among neighbouring plants by influencing plant morphology. It has been reported that the ability to detect changes in red/far-red ratio and adjust morphologically (e.g. elongated stem, altered biomass allocation, and leaf orientation) to maximize light interception is the mechanism by which corn plants respond to the presence of weeds (Rajcan et al. 2004). However, little 8  information on the relative response of crop and associated weed species to changes in the red/far-red ratio is available. Besides regulating the above-ground plant morphology, the red/far-red ratio can also affect below-ground activities, e.g. root growth and morphology, root exudation and interactions with beneficial soil microbes (Gundel et al. 2014).    Leaf optical properties The incident light reaching the leaf surface is reflected, transmitted and/or absorbed. Light interception is complex and depends mainly on the leaf optical properties and the arrangement of leaves in plant canopies (Myneni et al. 1989). Leaf optical properties (reflectance, transmittance, absorptance) affect the quantity and quality (e.g. red/far-red ratio) of light penetrating the leaf tissue and plant canopy. The incident light in the PAR (400-700 nm) region is mostly absorbed and far-red light (> 700 nm) is reflected and/or transmitted. Leaf optical properties could impact the photosynthetic process, morphogenesis as well as energy and water balance (Trigui 1990; Carter 1991; Smith 1994).   Leaf optical properties have also been used to estimate the biochemical and physiological characteristics of plants such as water content, photosynthetic pigments, leaf mass per area, and nitrogen concentration (Gitelson et al. 1999; Gitelson et al. 2003; Cheng et al. 2008; Moorthy et al. 2008) by remote sensing. Serbin et al. (2012) suggested that remote sensing is a promising method to characterize the photosynthetic status of a canopy through the use of leaf optical properties. Canopy reflectance models have been developed to predict the forage quality of legume-grass mixtures (Biewer et al. 2009). The leaf reflectance spectra can also be used to detect 9  the effect of environment stress on plants (Gitelson et al. 2002; Ainsworth et al. 2014) and to classify plant species (Knapp and Carter 1998). Leaf optical properties are being used by tropical biologists and ecologists to classify species (Castro-Esau et al. 2006).  Several factors are known to influence leaf optical characteristics. These include leaf water (Carter 1991; Baldini et al. 1997), chlorophyll, carotenoid (Asner and Martin 2008), and mineral (Adams et al. 1993, Masoni et al. 1996) contents, leaf thickness (Baldini et al. 1997; Knapp and Carter 1998), shade, and plant species (Souza and Válio 2003). Soil nitrogen content has also been reported to influence the reflectance characteristics of corn canopies (Walburg et al. 1982). Changes in leaf optical properties in Liriodendron tulipifera and Pinus strobus under increased atmospheric O3 and CO2 were found to be highly correlated with the decreases in leaf chlorophyll content, particularly for chlorophyll a (r = 0.82) (Carter et al. 1995). Various factors may interact in affecting the optical properties of leaves. Klančnik et al. (2014a) showed that while 60% of the spectra variability in reflectance was explained by leaf chemistry (e.g. the UV-absorbing compounds, chlorophyll a and b) and specific leaf area (SLA), 62% of the variability in transmittance was explained by leaf physical traits (e.g. thickness of the palisade mesophyll, SLA, and thickness of the lower and upper epidermis) and anthocyanin content in Sagittaria sagittifolia and Ranunculus lingua. In a study of 24 species, Davis et al. (2011) reported that leaf optical properties were modulated by light-dependent chloroplast movement and that the differences between species could be attributed, at least partially, to variations in the diameter of palisade cells located in the first layer. The epidermis characteristics can also affect spectral characteristics of leaves. While removal of the epidermis from the illuminated leaf surface of four herbaceous species increased absorptance and transmittance and decreased reflectance of PAR, removal from 10  the unilluminated side decreased both absorptance and reflectance and increased transmittance (Lin and Ehleringer 1983).   Since leaf optical properties can modify the light environment (e.g. red/far-red light ratio) in plant canopies by affecting the amount of light reflected, transmitted and /or absorbed by plant leaves, they have significant ecological implications in both natural and managed ecosystems. Red/far-red light ratio, a signal of impending competition from neighboring plants, can shift the competitive balance between species in a plant community. Therefore, leaf optical properties play an important role in plant-plant interactions by modifying the light environment in plant canopies.   Red/far-red light ratio also affects seed germination of some, especially small-seeded, species (Jankowska-Blaszczuk and Daws 2007; Górski et al. 2013). Plant-plant interactions and population dynamics can be affected if plants differ in their responses to the light environment modified by leaf optical properties. Knowledge of leaf optical properties of species in a population could help us understand eco-physiological underpinnings of plant-plant interactions within and among species. Unfortunately, little or no information on how leaf optical properties of crop and weed species affect the red/far-red ratio in plant canopies is available.    Ultraviolet-B radiation The ozone layer in the stratosphere protects living organisms from ultraviolet radiation by absorbing certain wavelengths of incoming solar ultraviolet (UV) radiation. Over the past few decades, stratospheric ozone has declined globally due to the production and release of 11  chlorofluorocarbons (CFCs) and halocarbons (Rowland 1990). This decrease has resulted in an increase in the level of ultraviolet-B (UV-B; 280-315 nm) radiation reaching the Earth’s surface, affecting human and animal health, as well as plant growth and biodiversity. Following the Montreal Protocol and its amendments, which restricted the production and use of ozone-depleting substances, the stratospheric ozone depletion from anthropogenic halogens is projected to recover by the middle or end of the current century (Eyring et al. 2010). However, it is very unlikely that the ozone level will recover to levels observed prior to 1980, when the large loss of ozone concentration was first measured over Antarctica (Solomon et al. 1986; Eyring et al. 2010). Enhanced UV-B radiation will continue to affect flora and fauna on earth for several decades. Furthermore, anthropogenic emission of greenhouse gasses and climate change can also affect the stratospheric ozone resulting in altered levels of UV-B radiation. For example, an atmospheric chemistry climate model predicted that climate change would cause a 4% and 20% increase of clear-sky ultraviolet radiation index in the tropics and southern high latitudes, respectively, in late spring and early summer between 1965 and 2095 (Hegglin and Shepherd 2009).  UV-B radiation is biologically active and can induce changes in molecular, physiology and plant architecture levels. Detrimental effects of UV-B radiation on plants include damage to cellular membranes, proteins, and DNA, inhibition of growth and photosynthesis, and alteration of gene expression (Greenberg et al. 1997). UV-B radiation can also induce acclimation responses in plants, e.g. increase in DNA repair, flavonoid biosynthesis, alteration in leaf cell division, etc. (Greenberg et al. 1997). UV-B radiation has been reported to increase activities of superoxide dismutase (SOD), catalase (CAT), ascorbate peroxidase (APX), peroxidase (POD), abscisic acid (ABA), salicylic acid (SA), malondialdehyde (MDA), hydrogen peroxide (H2O2), and superoxide 12  radical (O2•−) levels (Esringu et al. 2016). In addition, it can influence the expression of the phenylalanine ammonia lyase (PAL) gene, and decrease chlorophylls, carotenoid, gibberellic acid (GA), and indole-3-acetic acid (IAA) contents (Esringu et al. 2016).   UV-B radiation can interact with other abiotic factors in affecting plant growth and chemistry (Escobar-Bravo et al. 2017). It has been reported to interact with drought (Robson et al. 2015; Shen et al. 2015), temperature (Torre et al. 2012; Neugart et al. 2014), elevated CO2 (Koti et al. 2007; Gwynn-Jones et al. 2012), PAR (Cen and Bornman 1990; Deckmyn and Impens 1997; Bolink et al. 2001; Wargent et al. 2011, 2015), UV-A (Štroch et al. 2015), blue light (Adamse et al. 1994; Hoffmann et al. 2015) and far-red light (Tegelberg et al. 2004; Hayes et al. 2014). It has been reported that the concentration of quercetin 3-galactoside was higher in red light + UV-B treated leaves than in far-red light + UV-B treated leaves, but UV-B induced production of kaempferols, chlorogenic acids, and most quercetins were not modified by the red/far-red ratio (Tegelberg et al. 2004). The effect of UV-B radiation on the protective mechanisms in maize has been reported to be enhanced by the pre-treatment of S-methylmethionine, an exogenous amino acid (Rudnóy et al. 2015). Sodium nitroprusside pre-treatment has been reported to alleviate the inhibition of chlorophylls, carotenoid, GA, and IAA by exposure to UV-B radiation (Esringu et al. 2016).  Morphological changes induced by UV-B radiation include, among others, thicker and curling leaves, shorter petioles and stems, altered leaf shape and shoot: root ratios, and increased axillary branching (Greenberg et al. 1997; Robson et al. 2015). Such effects are dose-responsive, species specific and dependent on the experimental conditions. The morphological effects induced by UV-13  B radiation can affect plant-plant interactions as competitive success is mostly determined by shoot architecture and light interception (Barnes et al. 1996). It has been reported that wheat (Triticum aestivum L.) and wild oat (Avena fama L.) growing in mixture under field conditions were affected differentially by supplemental UV-B radiation on shoot morphology, canopy structure, light interception and stand photosynthesis (Barnes et al. 1988). Studies conducted on broccoli (Brassica oleracea var. italica) and LQ showed that both intra-and/or inter-specific competitive balances were affected by above-ambient UV-B radiation; the effect on interspecific competition was greater compared to that on intraspecific competition (Furness et al. 2005a). Besides above-ground competition, UV-B radiation can also modulate plant-plant interaction by affecting below ground processes (Zaller et al. 2002, 2004). Compared to ambient solar UV-B radiation, reduced UV-B radiation increased root length by 30% and decreased mycorrhizal colonization frequency by threefold in a fen in Tierra del Fuego (Argentina) (Zaller et al. 2002). The effect of UV-B radiation on root systems was species specific (Zaller et al. 2004). Therefore, UV-B radiation could alter the competitive balance and biodiversity of ecosystems in the long term by affecting both the above- and/or below ground plant-plant interactions. Our understanding of differential responses of plant species to UV-B radiation and its interactions with other environmental factors could help us modify competitive interactions among species in agricultural ecosystems.      14   Plant species used in this study  1.2.4.1 Common lamb’s-quarters (Chenopodium album L.) Common lamb’s-quarters (fat-hen, white goosefoot) (LQ) is a widespread, noxious, highly competitive agricultural weed (Conley et al. 2003; Moechnig et al. 2003; Kruger et al. 2009). It occurs in all provinces of Canada and can adapt to almost any environmental conditions, except the extreme desert (Holm et al. 1977). While it can infest a variety of crops, it is ranked first in abundance in potato (Solanum tuberosum L.) and sugar beet (Beta vulgaris ssp. vulgaris), and seventh in corn (Holm et al. 1977).  LQ is reproduced by seeds. Production of a large number of seeds, seed dormancy, and longevity, genetic heterogeneity and resistance of some biotypes to herbicides are important factors in the persistence of this weed. As many as 72,450 seeds can be produced by a single plant (Stevens 1932).  LQ is an upright and fast growing annual weed belonging to Chenopodiaceae family. While the average LQ plants are 100 cm tall, they can reach up to 200 cm in height. LQ leaves are alternate, 1 to 12 cm long and 0.5 to 8.0 cm wide, light green to whitish in color, ovate to triangular to lanceolate in shape, with slight lobes on edges (Basset and Crompton 1978). The distinguishing characteristic of LQ leaves is the white mealy coating, present mainly on the abaxial surface of the leaf. LQ flowers from late May to October. Its pollination is mainly mediated by wind, but insects can also assist (Basset and Crompton 1978).  15   The light environment in which the LQ plants grow affects germination and morphological  characteristics of their offspring. For example, seeds produced by the parent plants growing under long day length have low germination rates and thick seed coats whereas those produced by short-day mother plants have high germination rates and thin coats (Karssen 1970). The germination of LQ seeds can also be affected by environmental factors, e.g. the shift of photoperiod (Jursík et al. 2003), drought (Barrett and Peters 1976), and temperature (Chu et al. 1978; Weaver et al. 1988). It has been shown that LQ seeds that matured in August and October differed significantly in the duration and strength of dormancy (Jursík et al. 2003). While seeds ripened in August took 50 to 100 days to germinate, those ripened in October germinated in 50 days (Jursík et al. 2003). A field experiment on soil seed bank emergence for 22 site-years from Ohio to Colorado and Minnesota to Missouri conducted from 1991 to 1994 showed that the average emergence rate of LQ in a season was 2.7% (Forcella et al. 1997).  LQ can germinate and grow at low temperatures (Chu et al. 1978; Weaver et al. 1988), which offers it a competitive advantage by germinating early in the season. The yield loss caused by LQ interference has been well documented in various crops. In field corn, LQ at densities of 12, 16, and 20 plants per m2 has been reported to reduce corn yield loss by ~ 63, 70, and 75%, respectively (Sarabi et al. 2011). In soybean [Glycine max (L.) Merr.], LQ at the density of 64 plants per m2 has been reported to cause a 61% reduction in yield (Conley et al. 2003). Crook and Renner (1990) observed a 20% and 15% reduction in soybean yield when 32 and 16 LQ plants, respectively, were present per 10 m of row for the entire growing season.  16   LQ has been shown to exert allelopathic influences on plants. Qasem and Hill (1989) showed that fresh and dry weights and N, P, K, Ca and Mg concentrations of tomato shoots were reduced significantly by LQ leachates. Residues of LQ have been shown by Bhowmik and Doll (1984) to reduce the dry weight of corn and soybean. Alam et al. (1997) reported that aqueous leaf extracts of LQ alone or in combination with NaCl (salinity) reduced seedling growth and dry weight of rice. The combination of LQ leaf extract and NaCl (salinity) reduced wheat growth more than the treatment effect alone (Alam et al. 2002).  1.2.4.2  Redroot pigweed (Amaranthus retroflexus L.) Redroot pigweed (PW), also called amarante à racine rouge, green amaranth, pigweed, redroot, rough pigweed, tall pigweed, amarante réfléchie, armarante pied rouge (OMAFRA 2016), is an erect, broadleaved, summer annual weed belonging to the family Amaranthaceae. The name of redroot PW comes from the red color of its taproot. The lower part of the stem is usually thick, smooth, and pinkish, whereas the upper stem is often hairy (Burki et al. 1997).   PW reproduces by seed, producing over 250,000 seeds per plant (Sellers et al. 2003). More than 90% of the seeds are viable and form persistent seed banks in soil (Weaver and McWilliams 1980). It has been estimated that approximately 3 to 8% of the seeds in the soil seed bank emerge each year in arable fields (Forcella et al. 1997; Zhang et al. 1998).  17  PW plants can grow up to three meters in height. Lance-shaped leaves, alternately arranged along the stem, are green on the adaxial and green, pinkish to completely red with a distinct vein on the abaxial surface. Flowering occurs from June to November. It is pollinated mainly by wind but insects can also assist.   PW is a serious weed in many fields and horticultural crops in most provinces of Canada. It can reduce crop yield and quality by competing for resources, producing allelochemicals, hosting crop pathogens and pests, and causing toxicity to livestock and allergic reactions in humans (Costea et al. 2004). Serious yield losses caused by PW interference have been reported in many crops including corn (Knezevic et al. 1994), soybean (Légère and Schreiber 1989; Bensch et al. 2003); sorghum [Sorghum bicolor (L.) Moench] (Knezevic et al. 1997), potato (Vangessel and Renner 1990), cowpea [Vigna unguiculata (L.) Walp.] (Itulya et al. 1997) and green beans (Phaseolus vulgaris L.) (Mirshekari et al. 2010).  The effect of PW on crop yield depends both on density and time of emergence. The time of PW emergence relative to the crop has been reported to be more important than the density in influencing crop-PW competition (Vangessel and Renner 1990; Knezevic et al. 1994; 1997; Mirshekari et al. 2010). PW emerging after the 7-leaf stage of corn growth did not cause corn yield loss (Knezevic et al. 1994), but PW emerging before the 5.5-leaf stage of sorghum growth reduced the crop yield significantly (Knezevic et al. 1997). In a competition study, Vangessel and Renner (1990) found that as few as one PW plant per meter of row reduced marketable potato tuber yield by 19 to 33% when seeded in the row at the time of potato planting; four PW plants per meter planted 6 to 7 weeks after potato planting did not influence tuber yield. Competition from crops, 18  especially when grown in narrow rows, has been reported to reduce PW biomass (Légère and Schreiber 1989; McLachlan et al. 1993).  The relative competitive ability of PW and LQ, when grown in the same environment, depends on air and soil temperatures (Chu et al. 1978). The low threshold temperatures for development calculated for LQ and PW were 6.0 and 10.0°C, respectively (Wiese and Binning 1987). Chu et al. (1978) found that LQ had greater rates of germination, growth, and CO2-exchange rates at lower temperatures (day/night; 13/7°C), whereas PW performed better at higher temperatures (24/18°C). Therefore, if a field is prepared for crop production in late April or early May when the temperature is low, LQ germinates first and dominates other species. PW grows faster and becomes more problematic compared to LQ if the crop is sown in late May when the weather is warmer.  1.2.4.3 Tomato (Solanum lycopersicum L.) Tomato (TM), from the family Solanaceae (nightshade), is a herbaceous perennial plant grown as an annual in temperate regions (Carver 2007). This crop is native to South America and was first domesticated in Mexico. Tomato, introduced to Europe in the mid-16th century, was reintroduced back to America in the 18th century and since then it has become an important crop (Carver 2007). The name Solanum lycopersicum for this crop has largely replaced Lycopersicon esculentum. Tomato plants typically grow up to 1 to 3 meters in height. Vegetative tissues of tomato are covered with fine glandular hairs. Most tomato plants have compound, petiolated leaves, 10-25 cm long, with five to nine leaflets.   19  TM is now grown all over the world as a vegetable with fruits of a variety of sizes, shapes, and colors. According to the FAOSTAT (2016), about 170 million tons of tomatoes were produced in the world in 2014. Canada grows about 5,018 hectares of TM crop, producing 368,449 t of tomatoes per year (FAOSTAT 2016). In BC, greenhouse production of TM has become a big industry in recent years. TM is important for our nutrition because of its lycopene and various phytonutrient (e.g. vitamin C, biotin, folate, Vitamin A and potassium) content. The importance of TM is continuously increasing worldwide, both for consumption as a fresh crop, for processing, and for research on plant growth and development (Hobson and Grierson 1993).  The critical period of weed interference in transplanted TM is between 24 and 36 days after transplanting (Friesen 1979; Weaver and Tan 1983). Weed interference is primarily due to shading (Weaver and Tan 1983; McGiffen et al. 1992). Compared to black nightshade (Solanum nigrum), eastern black nightshade (Solanum ptycanthum) at the same density caused more yield loss in tomatoes because it is taller than TM plants, reducing PAR reaching the top of TM canopy (McGiffen et al. 1992). Growing TM at higher densities can also suppress weed growth (Norris et al. 2001). In a two-year barnyardgrass (Echinochloa crus-galli (L.) Beauv) and TM competition study, Norris et al. (2001) showed that while competition from TM reduced barnyardgrass plant growth by 70-80% and seed production by 30% at low weed density (< 5 plants per m of row), plant growth and seed production were reduced by 60-80% and 20-30%, respectively at high weed density (> 50 weeds per m of row).   LQ, PW and TM plants were used in Chapter 2, 3 and 4 because LQ and PW are important agricultural weeds and TM is the second most important vegetable crop of the world. Upright and 20  fast growth also made these species desirable for this study. For research described in Chapter 5, which aimed to determine if red/far-red light ratio and the associated change in anthocyanin influence the response of plants to subsequent exposure to elevated UV-B radiation, corn, lettuce and pigweed were used due to presence of anthocyanin pigment in these species.   21  Chapter 2: Effect of red/far-red light ratio on lamb’s-quarters, pigweed, and tomato plants   Summary Red/far-red ratio, a signal of impending competition from neighbouring plants, fluctuates in plant canopies due to selective absorption of red light, and transmission and reflection of far-red light by leaves. Differential response of a crop and associated weeds to changes in red/far-red ratio could, therefore, influence competitive interaction among them. In a growth chamber study, growth, morphological and allometric responses of common lamb’s-quarters (Chenopodium album L.), redroot pigweed (Amaranthus retroflexus L.) and tomato (Solanum lycopersicum L.) seedlings to changes in red/far-red ratio were investigated. Low red/far-red ratio increased plant height, stem length below the first node, stem diameter, leaf length and stem/shoot weight ratio, and decreased leaf greenness and leaf/shoot weight, and leaf/stem weight ratios in some or all of these species; the species differed significantly in this regard. Total length and surface area of tomato root were greater under low red/far-red ratio. Red/far-red ratio did not affect root parameters of lamb’s-quarters and pigweed. The observed differential response to red/far-red treatment suggests that fluctuations in the red/far-red ratio in a canopy of mixed populations could impact interactions among these species.   22   Introduction Light is an important factor in the regulation of plant growth and development. Light quantity and quality are two signals that help plants gather information about the light environment in their vicinity (Ballaré 2009). Light quality, particularly the red/far-red ratio, plays an important role in sensing impending competition from neighbouring plants (Ballaré 1999). Red/far-red ratio decreases in a plant canopy because of the selective absorption of red light (655 - 665 nm) and transmission and reflection of far-red light (725 - 735 nm) by green foliage. While the proportion of red to far-red light in solar irradiance is approximately similar (red/far-red ratio ≈ 1.15  0.02), it ranges from 0.1 to 0.9 under a leaf canopy (Smith 1982). Changes in the red/far-red ratio are detected by the phytochrome pigment, which converts reversibly between the active (Pfr) and inactive (Pr) forms (Smith 1982; Casal and Smith 1989). It should be noted that a small change in red/far-red within the canopy can result in a disproportionate change in Pfr/Ptotal as the phytochrome photoactivity curve is not linear (Smith 1982). Plants can detect the proximity of neighbours by perceiving the increase of back-scattered far-red light using phytochromes even before the commencement of canopy closure (Ballaré 1999). PhyA, phyB, and phyC, encoded by the PHYA, PHYB and PHYC genes, are the three major phytochromes that exist in angiosperms (Franklin and Whitelam 2007). PhyB is the main photoreceptor that detects the proximity of neighbours and triggers the morphological and physiological changes in plants (Ballaré and Scopel 1997).  When subjected to a reduced red/far-red ratio, plants express the shade avoidance syndrome, which includes stem elongation, increase in carbon allocation to the shoot, increase in leaf area (Rajcan et al. 2004; Liu et al. 2009), more leaves in the upper position of the canopy, and orientation of 23  leaves away from neighbours (Girardin and Tollenaar 1994; Maddonni et al. 2002) to optimize light acquisition for photosynthesis. Photosynthetic capacity per unit leaf area and the partitioning of leaf nitrogen have also been reported to be influenced by reduced red/far-red ratio (Hoad and Leakey 1994; Frak et al. 2002). Shade avoidance syndrome is ecologically significant because it can affect plant fitness. For example, transgenic tobacco (Nicotiana tabacum) plants, which displayed delayed shade avoidance syndrome in response to low red/far-red ratio, had lower competitive ability compared with the wild-type (Schmitt et al. 1995; Pierik et al. 2003). Plant fitness can also be influenced by the quality of light to which its maternal plant is exposed (Causin 2004). Plants exhibiting greater shade avoidance response may have an advantage in competing with their neighbours and a better chance to survive and reproduce in canopies with low red/far-red ratio.   Common lamb’s-quarters (Chenopodium album L.) (LQ) and redroot pigweed (Amaranthus retroflexus L.) (PW) are fast-growing, highly competitive weeds that cause significant yield loss to crops (Harrison 1990; Costea et al. 2004; Kruger et al. 2009; Mirshekari et al. 2010). These weeds are a serious problem in tomato (TM) (Robinson et al. 2006), the second most important vegetable crop in the world (FAOSTAT 2013). Red/far-red ratio fluctuates in canopies of these weeds and the crop that they infest (Smith 1982; Skálová et al. 1999). While shade avoidance response has been studied in individual species or in the presence of competition for resources (Brainard et al. 2005; Weijschede et al. 2006; Markham and Stoltenberg 2010), little information comparing responses of a crop and its associated weeds to changes in red/far-red ratio alone in absence of competition for resources [including photosynthetically active radiation (PAR)] is available. This information is important because even small differences in morphological response 24  (e.g., stem elongation) of different species to changes in the red/far-red ratio in a mixture before the commencement of competition for resources could subsequently impact plant interactions when competition for resources occurs. The objectives of this study, therefore, were to characterize the growth and morphological responses of TM, LQ, and PW to different red/far-red ratios and to determine if they differ in their response to this treatment.    Materials and Methods   Seed source and plant culture LQ, PW or TM (cv. Gold Nugget Cherry) seeds were sown 0.5 cm deep in Dutch Treat growth medium (Eddi’s Wholesale Garden Supplies, Surrey, BC, Canada) in 10 cm × 10 cm × 9 cm (height) plastic pots. LQ and PW seeds were collected in September 2010 from a field population at the Totem Field Laboratory of the University of British Columbia, and TM seeds were bought from West Coast Seeds Ltd. (Delta, BC, Canada).   In a separate study, the effect of changes in the red/far-red ratio on root growth parameters was investigated. Plants were grown in 12.5 cm diameter and 12 cm tall plastic pots in silica sand (Lane Mountain Company, Valley, WA, USA) to facilitate easy removal and cleaning of roots. A layer of plastic mesh screen was placed at the bottom of each pot to prevent the leakage of sand. TM seeds were sown five days prior to LQ and PW to synchronize seedling emergence of the three species. Seedlings were thinned to one per pot at the first true leaf stage. All plants were fertigated on alternate days with a fertilizer solution containing macro- (N, 8-15; P, 1-1.8; K, 6-12; Ca, 6-12; 25  Mg, 2-3; and S, 1.9-2.9 mmol/L) and micro- nutrients (Cl, 2-12; Na, <0.01-0.03; Si, 0.1-0.2 mmol/L; B, 30-80; Fe, 8-20; Mn, 5-10; Zn, 4-8; Cu, 1-3; and Mo, 0.5-1 µmol/L).   Red/far-red ratio treatment After emergence, plants were exposed to either a 0.3, 0.6 or 1.1 red/far-red ratio, hereafter called low, medium and high red/far-red ratios respectively, in Conviron growth chambers (25/20°C day/night temperature, 16 h photoperiod) as described below. Each growth chamber was divided into three sections with aluminum foil and the three red/far-red ratio treatments were assigned to these sections randomly. There were four plants of each species per treatment in each of the three growth chambers for shoot growth and morphology, and three plants for root growth study. Red/far-red ratios were achieved by supplementing FR light from Lumigrow FR bulbs (740 nm peak; ECC-FR, LumiGrow, Inc. Novato, CA, USA) with additional FR LED tubes (730 nm, Shenzhen Gehl Lamps Co., Ltd, Shenzhen, China). Plants in all treatments received a similar amount of red light and PAR. PAR [provided by Phillips F72T8/TL841/HO tubes (Phillips Lighting Co., Somerset, NJ, USA) and 40 and 60 W incandescent bulbs] was 210 µmol m-2 s-1. PAR was measured using LI-COR LI-185B light meter (LI-COR Inc., Lincoln, NE, USA) and LI-COR quantum sensor (Q 9674; LI-COR Inc.). Red/far-red ratios were measured using an SKR 110 sensor and SKR 100 meter (SKY Instruments Ltd. Llandrindod Wells, Powys, UK). Red/far-red ratios and PAR were measured at the floor level. Plants were re-randomized every week to minimize effects of micro-environmental variations. The experiment was repeated once.  26   Morphological response of plants to red/far-red ratio treatment Plants were harvested 20 days after the start of the red/far-red ratio treatments. At harvest, primary growth and morphological parameters were measured. Three Conviron growth chambers (three blocks) were used in this study. The average of four measurements taken on four plants (one per pot) of a treatment in each block was considered a replicate. Plant height from the soil surface to the shoot tip, stem length between the soil surface and the first true leaf, length of the longest leaf (from the base of the petiole to the leaf tip) of each plant, leaf greenness (using a SPAD-502 chlorophyll meter, Minolta Camera Co., Ltd., Japan) and the diameter of the stem right above cotyledons (using a Mastercraft digital caliper, Canadian Tire, BC, Canada) were measured. Plant roots were gently separated from the growth medium, washed under running water, and the surface moisture was touch-dried using paper towels. Leaves, stems, and roots were dried at 70°C to a constant weight and weighed. Dry weights of plant parts were used to construct the following growth indices: shoot/root ratio [shoot (stem + leaf) dry weight/root dry weight]; leaf weight ratio (leaf dry weight/shoot dry weight); stem weight ratio (stem dry weight/shoot dry weight); and leaf/stem ratio (leaf dry weight/stem dry weight) to study the effect of red/far-red ratio treatment on biomass allocation (allometry) in these species.   Effect of red/far-red ratio treatment on root morphology and growth parameters  Plants grown in silica sand in pots for root analysis were harvested two weeks after the start of red/far-red ratio treatments. Three Conviron growth chambers (three blocks) were used in this study. The average of three measurements taken on three plants (one per pot) of a treatment in each block was considered a replicate. Sand was removed carefully by washing with running water 27  to minimize root damage. Roots were washed and scanned using an Epson Perfection V700 Photo Scanner (Seiko Epson Corp., Suwa, Nagano, Japan). Images were analyzed to measure total root length, surface area, and volume using the WinRHIZO Pro V2009c software (Regent Instruments Canada Inc., Ville de Québec, QC, Canada).   Statistical analyses A randomized complete block design with three blocks was used. SAS software (SAS Institute, Inc., Cary, NC, USA) was used for statistical analysis. Assumptions of homogeneity of variance and normality were tested before subjecting the data to the analysis of variance (ANOVA) and the data were transformed as needed. LQ plant height, height below the first node, biomass data were log transformed, and root total length, surface area, and shoot/root ratio data were sine, cosine and cubic transformed, respectively. PW plant height, biomass, root total length, root surface area, root volume, and leaf/stem and shoot/root ratio data were log transformed. TM plant height data were square transformed, stem weight ratio data were fifth-power transformed, and root volume and leaf/stem ratio data were log transformed. Data from the two experiments were analyzed using the PROC MIXED model in SAS with experiment and block treated as random effects for each species. Means were generated using LSMEANS. Pairs of means t-test, with α level adjusted according to the number of pairwise comparisons (α’ = 0.05/3), was used to compare each pair of means.    28   Results   Effect of red/far-red ratio treatment on growth parameters Red/far-red ratio treatments influenced several growth and morphological parameters of the three species employed in this study and the species differed in their response to this light signal (Table 2.1). The LQ, PW and TM plants grown under low red/far-red ratio were 75, 90 and 33% taller, respectively compared with those grown under high red/far-red ratio (Table 2.1, Fig. 2.1). Height below the first node also increased at low, compared with high, red/far-red ratio in all three species. The magnitude of increase was 57, 91 and 40% in LQ, PW, and TM, respectively. Low red/far-red ratio increased leaf length in LQ and TM, but not in PW. Compared with the high red/far-red ratio, the stem diameter of the plants exposed to low red/far-red ratio were 18% and 19% higher in LQ and PW, respectively (p ≤ 0.05). The stem diameter of TM plants was not affected. With the exception of leaf length, the morphological response to low red/far-red ratio was the largest in PW followed by LQ and TM (Table 2.1).  Plant biomass was not affected by red/far-red treatment in all the three species (Table 2.2). Leaves produced under low red/far-red ratio had 11, 12 and 22% lower SPAD reading in LQ, PW, and TM, respectively compared with those developed under high red/far-red ratio.     29    Table 2.1. Effect of red/far-red ratio treatment on growth parameters of common lamb’s-quarters, redroot pigweed, and tomato plants. Species Red/far-red Plant height (cm) Height below first node (cm) Leaf lengthb (cm) Stem diameter (mm) Common lamb’s-quarters 0.3 18.15a a 3.97 a 9.84 a 2.86 a 0.6 15.39 a 3.14 b 9.30 ab 2.75 ab 1.1 10.35 b 2.53 c 8.49 b 2.43 b Redroot pigweed 0.3 26.96 a 9.17 a 9.19 a 4.71 a 0.6 22.87 b 7.29 b 9.73 a 4.63 ab 1.1 14.21 c 4.81 b 9.32 a 3.95 b Tomato 0.3 52.07 a 9.60 a 19.93 a 5.61 a 0.6 45.13 b 7.95 b 18.84 b 5.72 a 1.1 39.29 b 6.88 b 17.63 b 5.71 a aMeans of two experiments with three replicates each. Values followed by a different letter(s) within a column for each species are significantly different (p ≤ 0.05, n = 6). Multiple comparisons between means were conducted using pairs of means t-test with adjusted α level (α’ = 0.05/3). bPetiole + lamina  30  Fig. 2.1. Effect of red/far-red ratio on growth of (A) common lamb’s-quarters, (B) redroot pigweed, and (C) tomato plants. In each figure, plants from left to right developed under 0.3 (T1), 0.6 (T2) or 1.2 (T3) red/far-red ratio, respectively.       A C B T T1 T2 T3 T1 T1  T2  T2 T3 T3  31    Table 2.2. Effect of red/far-red ratio treatment on biomass and leaf SPAD value in common lamb’s-quarters, redroot pigweed, and tomato plants. Species Red/far-red  Biomass (g) SPAD meter readingb Common lamb’s-quarters 0.3                   0.62a a                35.60 a 0.6                   0.60 a  39.33 ab 1.1                   0.38 a 39.97 b Redroot pigweed 0.3 0.77 a 25.55 a 0.6 0.78 a   28.39 ab 1.1 0.57 a 29.14 b Tomato 0.3 3.02 a  35.04 a 0.6 2.91 a    41.30 ab 1.1 2.39 a   45.20 b aMeans of two experiments with three replicates each. Values followed by a different letter(s) within a column for each species are significantly different (p ≤ 0.05, n = 6). Multiple comparisons between means were conducted using pairs of means t-test with adjusted α level (α’ = 0.05/3). bSPAD-502 chlorophyll meter reading32    Effect of red/far-red ratio on root morphology and growth parameters  Total length and surface area of TM root decreased by 13 and 17% respectively, in high compared with low red/far-red ratio (Table 2.3). Root volume of TM was not significantly affected by changes in red/far-red ratio. The red/far-red ratio treatment did not affect LQ and PW root growth parameters (Table 2.3).   Effect of red/far-red ratio on plant allometry  Leaf weight ratio and leaf/stem ratio increased with increasing red/far-red ratio in LQ and PW (Table 2.4). The magnitude of increase was greater in PW. Compared with low red/far-red ratio, leaf weight ratio and leaf/stem ratio in PW under high red/far-red ratio increased by 25% and 105 %, respectively. The increase in the case of TM was not statistically significant. Stem weight ratio of LQ and PW plants grown under high, compared with low, red/far-red ratio were 25% and 38% lower respectively. Shoot/root ratio was affected by changes in red/far-red ratio only in LQ (Table 2.4); it decreased with decreasing red/far-red ratio.    Discussion  Red/far-red ratio, which fluctuates in canopies of weed-crop mixtures, can impact inter-plant interactions among species. While the shade avoidance response of plants to low red/far-red ratio are well described for individual species (Morgan and Smith 1981; Causin and Wulff 2003; Rajcan et al. 2004; Franklin and Whitelam 2007; Mahoney and Swanton 2008; Liu et al. 2009; Markham  and Stoltenberg 2010), little information comparing responses of a crop and its associated weeds   33    Table 2.3. Effect of red/far-red ratio treatment on root parameters of common lamb’s-quarters, redroot pigweed, and tomato plants. Species Red/far-red Total length (cm) Surface area (cm2) Volume (cm3) Common lamb’s-quarters 0.3 196.37a a 21.84 a 0.20 a 0.6  210.64 a 24.45 a 0.23 a 1.1  162.45 a 18.07 a 0.16 a Redroot pigweed 0.3 123.36 a 15.43 a 0.15 a 0.6 139.79 a 18.55 a 0.20 a 1.1 115.34 a 14.47 a 0.15 a Tomato 0.3 621.72 a         113.43 a 1.67 a 0.6         580.41 ab 106.08 ab 1.62 a 1.1 541.67 b  93.63 b  1.31 a aMeans of two experiments with three replicates each. Values followed by a different letter(s) within a column for each species are significantly different (p ≤ 0.05, n = 6). Multiple comparisons between means were conducted using pairs of means t-test with adjusted α level (α’ = 0.05/3).   34  Table 2.4. Effect of red/far-red ratio treatment on allometry of common lamb’s-quarters, redroot pigweed, and tomato plants. Species Red/far-red Leaf weight ratiob Leaf/stem ratioc Stem weight ratiod Shoot/root ratioe Common lamb’s-quarters 0.3 0.76a a 3.32 a 0.24 a 7.69 a 0.6 0.79 a   3.95 ab 0.21 a    8.70 ab 1.1 0.82 b 4.67 b 0.18 b   9.81 a Redroot pigweed 0.3 0.61 a 1.65 a 0.39 a 11.33 a 0.6 0.66 a 2.21 a 0.33 a 11.59 a 1.1 0.76 b 3.38 b 0.24 b 12.64 a Tomato 0.3 0.59 a 1.49 a 0.41 a  8.44 a 0.6 0.61 a 1.58 a 0.39 a   8.71 a 1.1 0.63 a 1.72 a 0.37 a   8.64 a aMeans of two experiments with three replicates each. Values followed by a different letter(s) within a column for each species are significantly different (p ≤ 0.05, n = 6). Multiple comparisons between means were conducted using pairs of means t-test with adjusted α level (α’ = 0.05/3). bLeaf dry weight/shoot dry weight. cLeaf dry weight/stem dry weight. dStem dry weight/shoot dry weight.  eShoot dry weight/root dry weight. 35  to changes in red/far-red ratio, achieved by supplementing with far-red light only in absence of resource competition is available. Species-specific differences in response to changes in red/far-red ratio before the start of competition could potentially alter subsequent competitive interactions between species. This research characterizes the response of TM and two weeds, LQ and PW, to changes in red/far-red light ratio and shows that these species differ in several ways in their response to this light signal. For example, plant height and the height below the first node increased under low, compared with high, red/far-red ratio and the species differed significantly in this regard (Table 2.1). PW showed the greatest elongation, followed by LQ and TM. This suggests that under lower red/far-red ratio, LQ and PW seedlings can have a competitive advantage because of their greater elongation growth. This will enable these weeds to place their foliage higher, reaching out for more PAR, thereby outcompeting the less responsive TM plants. I focused on growth in the early part of the life cycle (20 days) of these species in this study because even a small growth advantage at an earlier growth stage could be pivotal to the establishment of subsequent competitive relationship among species in a mixture (Page et al. 2012). Moreover, it has been reported that LQ plants become insensitive to changes in red/far-red ratio during the later stages of growth (Causin and Wulff 2003; Mahoney and Swanton 2008).   As stems elongated in LQ and PW under low, compared with high, red/far-red ratio, more dry weight was allocated to the plant stem as shown by an increase in the stem weight ratio (stem dry weight/total shoot dry weight) (Table 2.4). The stimulation of elongation growth by low red/far-red ratio is therefore associated with a greater allocation of the shoot dry weight to the stem. Interestingly, leaf weight ratio (leaf dry weight/shoot dry weight) and leaf/stem ratio (leaf dry 36  weight/stem dry weight) decreased (Table 2.4) as plants elongated with the lowering of red/far-red ratio. This suggests that relatively less carbon was allocated to leaves as the stems elongated.  In TM, on the other hand, the leaf weight ratio and leaf/stem ratio were unaffected. Relatively less compartmentalization of dry matter into leaves in LQ and PW suggest lesser photosynthate allocation for leaf area production, potentially resulting in lesser carbon fixation by photosynthesis. However, while LQ and PW plants elongated more under low red/far-red ratio, and had relatively more dry matter allocated for the stem growth and lesser for the leaves, their total biomass was not affected (Table 2.2).   Interestingly, while plant biomass did not change with changing red/far-red ratio, the SPAD meter reading of plant leaves, an estimator of leaf chlorophyll content, decreased under low red/far-red ratio (Table 2.2). This anomaly may suggest the involvement of factors other than chlorophyll concentration in the regulation of photosynthesis. Light quality can modify leaf anatomical features such as thickness of leaves as well as palisade and spongy mesophyll tissues (Schuerger et al. 1997), and/or may increase the amount of major chloroplast components, namely, ribulose 1,5-biphosphate carboxylase, adenosine triphosphate synthase, and cytochrome b6f complex relative to chlorophyll (Chow et al. 1990). A decrease in chlorophyll content by low red/far-red ratio treatment without any change in the rate of photosynthesis has been reported in Phaseolus vulgaris L. leaves (Bartoli et al. 2009). It should also be pointed out that the relationship between the SPAD meter reading and leaf chlorophyll content can be significantly influenced by factors such as chloroplast movement (Nauš et al. 2010), anthocyanin concentration (Hlavinka et al. 2013), and the heterogeneity in chlorophyll distribution in leaves with high chlorophyll content 37  (Jifon et al. 2005; Uddling et al. 2007). Therefore, the SPAD meter reading is not necessarily always consistent with biomass production.  Larger root length and surface area are characteristics that aid a plant’s competitiveness by increasing access to below-ground resources. The results of my study show that root growth parameters (e.g., root total length and surface area) were significantly influenced by red/far-red ratio treatment in TM (Table 2.3). However, roots of LQ and PW were not affected (Table 2.3), presumably because of a larger variance in the case of these two genetically heterogenous weeds compared with the TM crop, and/or to damage of their fine roots during root washing and cleaning. Larger total root length and surface area under reduced red/far-red ratio may offer a competitive advantage to TM plants by increasing its access to soil-borne resources. The effect of exposure of shoots to red/far-red ratio treatment on root growth could be because of a greater supply of the photosynthate from the shoot to the root.   This research shows several differences in the response of TM and its associated weeds, LQ and PW, to changes in red/far-red ratio in the absence of resource competition, which could influence interactions among these species. A species with greater shade avoidance response (e.g., stem elongation) is expected to have a competitive advantage over species with a lesser response. While this study provided useful information on differential responses of plant species to red/far-red ratio treatments, experiments to study competitive interactions among species in weed crop mixtures under field conditions are needed.   38   Conclusion This research shows several differences in the response of TM and its associated weeds LQ and PW to red/far-red ratio treatment, which could influence interactions among these species. A species with greater shade avoidance response in terms of stem elongation may produce a taller plant when exposed to low red/far-red ratio and outcompete other species with a lower response. While this study provided useful information on differential responses of plant species to red/far-red ratio, field experiments with red/far-red ratio treatments given to species grown in mixtures are needed.       39  Chapter 3: Effects of leaf position on reflectance, transmittance, and absorptance of red and far-red irradiance in leaves of tomato, lamb’s-quarters, and pigweed   Summary Leaf optical properties can play an important role in determining the red/far-red ratio, a signal of impending competition, in plant canopies. Knowledge of leaf optical properties and factors affecting them is important to our understanding of the ecophysiology of agroecosystems. Effects of leaf position on the plant stem on their optical properties (reflectance and transmittance at 660 and 730 nm) were studied in tomato (Solanum lycopersicum L.) and its two weeds common lamb’s-quarters (Chenopodium album L.) and redroot pigweed (Amaranthus retroflexus L.) using a leaf spectrometer. Absorptance was calculated by subtracting reflectance and transmittance percentage from 100%. Leaf position on stem strongly influenced leaf optical properties. Reflectance and transmittance were generally lower for the lamb’s-quarters and pigweed leaves at higher positions on the stem, except for reflectance at 730 nm in lamb’s-quarters, which did not change. Reflectance was not affected in tomato. Transmittance generally decreased at higher leaf positions. Red/far-red ratios of reflected (Rratio) and transmitted (Tratio) light generally decreased in all species, except for Rratio in tomato, where it increased slightly at higher positons. These effects were greater in pigweed compared to lamb’s-quarters and tomato. Changes in these ratios were partly explained by chlorophyll content and specific leaf weight. The results show that leaf position on plant stem influences leaf optical properties in tomato and the two weeds (lamb’s-quarters and pigweed) and these species differ in this regard. These influences and the differences among 40  species could modify red/far-red ratios in canopies of populations comprising these species, which could influence their growth and inter-plant interactions. This should be considered while assessing eco-physiological functions of a vegetation cover.   Introduction Low red/far-red light ratio, a signal of impending competition, induces a series of morphological and physiological changes in plants (Hoad and Leakey 1994; Tegelberg et al. 2004; Page et al. 2010). Plants express shade-avoidance response when exposed to low red/far-red ratio, which may enhance stem elongation, alter carbon allocation pattern (Liu et al. 2009), turn leaves away from neighbors (Maddonni et al. 2002), and reduce leaf area and shoot and root biomass (Liu et al. 2009; Page et al. 2009). Changes in red/far-red ratio, resulting from differences in amounts of reflected, transmitted or absorbed red and far-red lights by leaves, may influence plant-plant interactions. Competitive balance between plants growing in mixed populations can be altered by changes in the red/far-red ratio (Ballare and Casal 2000; Rajcan and Swanton 2001), which are affected by the proximity, architecture and the leaf optical properties of neighboring plants.  Leaf optical properties (reflectance, transmittance and absorptance) affect the amount of light reflected, transmitted and/or absorbed by plant leaves. These properties are wavelength (e.g. red and far-red) specific and have been used to evaluate plant moisture status, ribulose-1,5-bisphosphate carboxylase activity, and nitrogen level (Solari et al. 2008; Serbin et al. 2012). Knowledge of leaf optical properties of species in a population could help us understand eco-physiological underpinnings of plant-plant interactions within and among species. 41   Leaf optical properties are affected by several factors including the soil nitrogen level (Walburg et al. 1981), leaf water (Carter 1991), chlorophyll, carotenoid (Asner and Martin 2008), and mineral (Adams et al. 1993, Masoni et al. 1996) contents, leaf thickness (Knapp and Carter 1998), shade, and plant species (Souza and Válio 2003). These factors may interact in affecting leaf optical properties.  Little information on effects of leaf position on plant stem on reflectance, transmittance, and absorptance of red and far-red lights in a crop and its associated weeds is available. The objective of this chapter, therefore, was to study the effect of leaf position on the stem on leaf optical properties with regard to red and far-red light in tomato (Solanum lycopersicum Mill., cv. Gold Nugget Cherry) (TM) and two common weeds, common lamb’s-quarters (Chenopodium album L.) (LQ) and redroot pigweed (Amaranthus retroflexus L.) (PW). The objective of this study is to improve our understanding of the role of leaf optical properties in plant-plant interactions in agroecosystems.   Materials and Methods   Seed source and plant culture LQ and PW seeds, collected in September 2010 from the Totem Field at the University of British  Columbia (UBC) and TM seeds obtained from West Coast Seeds Ltd. (Delta, BC, Canada) were used. Ten LQ or PW seeds were sown 0.5 cm deep in 10 cm × 10 cm × 9 cm (height) plastic pots 42  filled with a 70% peat moss and 30% perlite potting mix (West Creek Farms, Fort Langley, BC, Canada) in Conviron growth chambers (25/20°C day/night temperature, 16 h photoperiod) in the Faculty of Land and Food Systems at UBC. Two TM seeds were sown in each pot filled with the same potting mix. TM seeds were sown five days prior to sowing PW and LQ seeds in order to synchronize seedling emergence of the three species. Seedlings were thinned to one uniform plant per pot for each species at the first true leaf stage. Plants were re-randomized once a week to minimize effects of micro-environmental variations. There were six replicates for each species and the experiment was conducted twice.   Effect of leaf position on reflectance (R), transmittance (T) and absorptance (A) at 660 and 730 nm The 3, 5, 7, 9 and 11th true leaves of PW and LQ were used for optical property measurements; the first true leaf of these weeds had fallen at the 11th true leaf stage and could not be used. For TM, the 1, 3, 5 and 7th true leaves were used, when the plants were at the 8th true leaf stage.  R and T at 660 nm and 730 nm were measured using CI-710 Miniature Leaf Spectrometer (CID Bio-Science, Inc., Camas, WA, USA) (Fig. 3.1), which allows measurement of optical properties while the leaves are still attached to the plant. The built-in light source of the spectrometer provides radiation in the range of 400 to 1000 nm. The adaxial surface was irradiated by the light source of    43     Fig. 3.1 CI-710 miniature leaf spectrometer.   44   the spectrometer and the R and T at red (660 nm) and far-red (730 nm) wavelengths were measured. A at 660 and 730 nm were calculated by subtracting the R and T percentage values from 100%. Leaves were used to determine chlorophyll content and the specific leaf weight (SLW) following optical property measurements, as described below.   Measurement of leaf chlorophyll content Chlorophyll in discs (7 mm diam.) punched from leaves at the 3, 5, 7, 9 and 11th positions for LQ and PW, and the 1, 3, 5, 7th positions for TM was extracted with 3 ml of 80% acetone in Erlenmeyer flasks on a rotary shaker at 90 rpm in darkness for 24 h; this extraction time yielded the same amount of chlorophyll as with 48 or 92 h extractions. Chlorophyll a, chlorophyll b, and total chlorophyll (chla+b) concentration, an indicator of photosynthesis rate (Lobato et al. 2010) were calculated using the formula described by Porra et al. (1989). The ratio of chlorophyll a to b (chla/b), an indicator of the amount of light harvesting complex in chloroplast (Alberte 1976; Chow et al. 1990), was also calculated.   Specific leaf weight measurement Following excision of 7 mm diam. discs for chlorophyll measurement as described above, area of the remaining lamina was measured using a LI-COR LI-3000 portable leaf area meter (Li-Cor Inc., Lincoln, NE, USA). The lamina was then dried at 70℃ and weighed. The specific leaf weight (mg cm-2), an indicator of leaf thickness, was calculated.  45   Statistical analyses SAS software (version 9.4) was used for statistical analyses (SAS Institute, Inc., Cary, NC, USA). The assumptions of normality and homogeneity of variance were tested before subjecting the data to ANOVA, and the data were transformed as needed. Primary data of LQ chla+b was log transformed, and SLW data were sine transformed. PW T660 and Tratio primary data were log transformed, and A660 and R730 data were transformed using a power transformation (power = 3). TM A660 data were transformed using a power transformation (power = 5). Data from the two experiments were analyzed using PROC MIXED procedure with experiment and plant treated as random effects. Multiple comparisons between each pair of means were conducted using pairs of means t-test with α level adjusted according to the number of pairwise comparisons. All descriptive statistics for variables are means of original measures. Multiple linear regression analysis was carried out to study relationships of red/far-red ratios of the reflected and transmitted lights with chla+b, chla/b ratio and SLW using PROC REG procedure. A t-test was used to evaluate the significance of each variable.    Results    Effect of leaf position on reflectance (R), transmittance (T), and absorptance (A) The results showed R, T and A of leaves at red and far-red wavelengths changed with their positions on the stem and the three species employed in this study differed in this regard (Table 3.1). Reflectance at 660 nm (R660) of LQ leaves at the 5th to the 9th position was significantly lower  46  Table 3.1. Effect of leaf position on reflectance, transmittance, and absorptance of lamb’s-quarters, redroot pigweed, and tomato leaves at 660 nm and 730 nm wavelengths. Species Leaf position Percent of irradiation R660 T660 A660 R730       T730 A730  Common lamb’s-quarters 3       4.63a a 2.01 a       93.44 a 43.25 a     47.34 a         9.43 a 5       3.96 bc 1.36 b   94.68 bc 42.57 a  44.19 bc  13.27 c 7 3.80 c 1.03 c       95.16 d 43.79 a 43.26 c   12.99 c 9 3.87 c 1.04 c   95.09 cd 43.99 a 43.51 c    12.53 bc  11 4.26 b     1.20 bc  94.53 b 44.31 a  45.53 b    10.19 ab  Redroot pigweed 3       8.36 a   10.47 a 81.78 a 44.61 a 54.63 a         2.10 a 5       6.70 b 5.51 b 87.79 b  42.57 ab 47.33 b  10.10 b 7       6.40 bc 3.31 c 90.29 c 41.94 b  44.68 cd   13.38 bc 9       6.19 bc 2.44 d  91.37 c 41.33 b 43.42 d  15.25 c 11 5.87 c   2.85 cd  91.28 c 41.49 b   46.14 bc  12.38 b   Tomato 1 3.96 a 3.55 a  92.51 a 42.11 a 51.65 a    6.25 a 3 3.92 a 2.55 b  93.53 b 42.51 a  49.30 ab    8.19 a 5 3.90 a 2.11 c  93.99 b 41.54 a  47.24 bc   11.23 b 7 4.20 a 1.88 c  93.80 b 41.68 a 45.35 c   12.97 b aLs-means based on 12 replicates pooled from two experiments. Values followed by a different letter(s) within a column for each species are significantly different (p ≤ 0.05).47    compared to that at the 3rd and 11th position. Reflectance at 730 nm (R730) was not affected (Table 3.1).  Transmittance at 660 (T660) and 730 nm (T730) of the 3rd leaf of LQ was significantly higher compared to the 5 to 11th leaf. Percent decrease in T660 with increasing leaf position was greater compared to that for T730. R660, T660, T730 values for the leaf at the 11th position were higher, though not always significantly, compared to the leaf at the 9th position. Absorptance at 660 nm (A660) increased slightly but significantly from the 3rd to 7th leaf and then decreased. Absorptance at 730 nm (A730) of the 5th and 7th leaves were greater compared to the leaves at the 3rd and 11th positions.  In the case of PW, R660 of the 3rd leaf was 25 to 40% greater compared to the leaves at higher positions (Table 3.1). Both T660 and T730 declined gradually from the 3rd to 9th leaf; the value for the leaf at the 11th position was slightly higher compared to those at the 9th position but this difference was significant only for T730. A660 increased from ~82% for leaves at the 3rd to ~91% for the 11th position. A730 at higher leaf positions was significantly greater (up to 6.26-fold for the 9th leaf) compared to the 3rd leaf. While A730 declined for leaves at the 11th position, it was still ~5-fold greater compared to that for leaves at the 3rd position.  Neither R660 nor R730 in TM was influenced by leaf position but T660 and T730 decreased gradually as leaf position increased (Table 3.1). Compared to leaves at the 1st position, the magnitude of the decrease in T660 and T730 for leaves at the 7th position, were 47% and 12%, respectively. No change in reflectance but a decline in transmittance with higher leaf positions resulted in a steady increase 48  in A660 and A730. A730 for leaves at the 7th position was about double that for leaves at the 1st position.  The effect of leaf position on optical properties was the greatest in PW followed by LQ and TM (Table 3.1). For example, compared to the 3rd leaf, T660 of the 7th leaf decreased by 68, 49 and 26% in PW, LQ, and TM, respectively. Compared to the 3rd leaf, T730 of the 7th decreased by 18, 9 and 8% and A730 increased by almost 537, 38 and 58% in PW, LQ, and TM, respectively (Table 3.1).   Effect of leaf position on red/far-red ratio of reflected and transmitted light  Due to changes in R and T of red and far-red light with leaf position, the red/far-red ratio of the reflected (Rratio) and transmitted (Tratio) lights changed with leaf position (Fig. 3.2A, B). The effects of leaf position on Rratio and Tratio were greater in PW compared to LQ and TM (Fig. 3.2A, B). In PW, Rratio decreased by 26% from the 3rd to the 11th leaf (Fig. 3.2A) and Tratio decreased by 68% from the 3rd to the 9th leaf (Fig. 3.2B). There was no difference in Tratio between the 9th and 11th leaf (Fig. 3.2B). The Rratio for LQ leaf at the 3rd position was slightly but significantly greater compared to those at the 5, 7, 9 and 11th positions (Fig. 3.2A). Tratio decreased by ~45% from the 3rd to the 7th leaf and then leveled off (Fig. 3.2B). In TM, Rratio did not change as the leaf position increased from the 1st to the 5th position; Rratio at the 7th position was slightly but significantly greater than the leaves at lower positions. Tratio, on the other hand, decreased by 40% with increasing leaf position (Fig. 3.2B).  49   Effect of leaf position on chla+b, chla/b ratio, and SLW  In LQ, chla+b of the leaves at the 5th and 7th positions were significantly greater than the 3rd position (Table 3.2). For PW, chla+b content of the leaves at the 5th to 11th positions were about double that of the leaf at the 3rd position. In the case of TM, chla+b level increased continuously with increasing leaf position. It was 68% higher for the leaf at the 11th position compared to the leaf at the 3rd position. There was no consistent effect of leaf position on chla/b ratio.  SLW also affected by leaf position in both LQ and PW (Table 3.2). In LQ, the 3rd leaf had the least and the 7th to the 11th leaf had the greatest SLW. For leaves at the 11th position, it was 42% greater compared to the leaf at the 3rd position. For PW, SLW increased from the 3rd to the 7th position by 39% and then decreased from the 7th to the 11th position by 13%. SLW at the 5th to the 11th position was significantly greater compared to the 3rd position. In the case of TM, SLW did not change with leaf position (Table 3.2).    Relationships of Rratio and Tratio with chla+b, chla/b, and SLW Multiple linear regression analysis was carried out to study relationships of Rratio and Tratio with chla+b, chla/b, and SLW. Both Rratio and Tratio were related with chla+b, chla/b ratio and SLW in LQ and PW (p < 0.0001) (Table 3.3). The Tratio showed a stronger relationship (R2 = 0.72 – 0.80) with chla+b, chla/b ratio and SLW compared to Rratio (R2 = 0.54 – 0.60) in LQ and PW (Table 3.3). Both chla+b and SLW had negative relationships with Rratio and Tratio in LQ and PW. Chla/b ratio had a positive relationship with Rratio and Tratio in LQ and negative relationship in PW. For TM, only the chla+b significantly accounted for the variation in Rratio (p = 0.0017) and Tratio (p < 0.0001). It related  50         	 Fig. 3.2. Effect of leaf position on red/far-red ratio of (A) the reflected (Rratio) and (B) the transmitted (Tratio) lights from leaves of common lamb’s-quarters, redroot pigweed, and tomato. Values are ls-means based on 12 replicates pooled from two experiments. Different letters next to the data points of a species denote a significant difference between leaf positions (p ≤ 0.05, n = 12).   0.000.050.100.150.200.250.301 3 5 7 9 11Lamb's-quartersPigweedTomatoARed/far-red ratio0.000.050.100.150.200.250.301 3 5 7 9 11Bacdbda b c ca b c c c l          l          l         l          l          l Leaf position a b b bc c a a a b b b c bc  l           l          l          l          l          l a 51     Table 3.2. Effect of leaf position on chlorophyll content (Chla+b), Chla/b ratio, and specific leaf weight (SLW) of lamb’s-quarters, redroot pigweed, and tomato leaves. Species Position Chla+b (nmol cm-2) Chla/b SLW (mg cm-2) Common lamb’s-quarters 3 41.63a a  3.45 a     2.91 a 5         49.57 b  3.33 a            3.24 b 7         50.66 b  3.33 a            3.88 c 9  45.25 ab  3.33 a    4.12 c 11  44.97 ab  3.43 a    4.13 c Redroot pigweed 3         16.41 a  4.19 a    3.02 a 5         29.21 b  4.26 a     3.87 bc 7         33.68 b  4.21 a   4.18 c 9 34.60 b    4.37 ab   4.12 c 11 30.31 b  4.60 b           3.64 b  Tomato  1         26.33 a  3.09 a   3.40 a 3         36.94 b  3.09 a   3.18 a 5  42.17 bc  3.00 b  3.31 a 7         44.10 c   3.04 ab  3.64 a aLs-means based on 12 replicates pooled from two experiments. Values followed by a different letter(s) within a column for each species are significantly different (p ≤ 0.05).   52   Table 3.3. Relationships of red/far-red ratio of the reflected (Rratio) and transmitted (Tratio) lights with Chla+b, Chla/b, and specific leaf weight (SLW) in lamb’s-quarters, redroot pigweed, and tomato.   Common lamb’s-quarters   Intercept Chla+b Chla/b SLW R2 p Rratio 0.10947 (7.43)a -0.0005 (0.0017) 0.00938 (0.0071) -0.00711 (<0.0001) 0.5424  <0.0001  Tratio -0.01159 (0.538) -0.00108 (<0.0001) 0.03772 (<0.0001) -0.00977 (<0.0001) 0.804  <0.0001    Redroot pigweed   Intercept Chla+b Chla/b SLW R2       p Rratio     0.31761 (<0.0001)   -0.0006   (0.0198)   -0.0229 (<0.0001)   -0.0114    (0.012) 0.5995  <0.0001  Tratio   0.47364 (<0.0001) -0.0032 (<0.0001)   -0.0377  (0.0067) -0.0325 (0.0047) 0.7165  <0.0001    Tomato   Intercept Chla+b Chla/b SLW R2 p Rratio     0.03623   (0.3158)   0.00046   (0.0017)   0.01272   (0.2388)   0.00084   (0.6921)    0.2057    0.0166 Tratio   0.11478 (0.0111) -0.0015 (<0.0001) 0.0025 (0.848) -0.0041 (0.1226)  0.6605 <.0001 aValues in brackets below the regression parameters are p-values for the corresponding estimated parameters.   53  positively with Rratio and negatively with Tratio (Table 3.3). SLW and chla/b ratio were not significant for Rratio and Tratio in TM.    Discussion The results of this study showed several significant effects of leaf position on leaf optical properties. These include differences in the R, T, and A at 660 and 730 nm and the red/far-red ratio of the light reflected (Rratio) and transmitted (Tratio) from leaves. Interestingly, the species employed in this study also differed in this regard. My observation showing differences in optical properties of leaves at different positions on the stem has significant ecological implications. Leaf area index (LAI; total leaf area per unit of ground area) (Bréda 2003) has been used as a parameter for assessing functions of vegetation cover (Baez-Gonzalez et al. 2005). All leaves on an individual plant as well as leaves of different species in a mixture are considered qualitatively similar in assessing the competitive ability of a vegetation cover in agroecosystems. The results of my study suggest that this assumption may not be correct. These results indicate that the same area of leaves at different positions on the stem and of different species may differ in their impact on the light environment of a canopy. Among the species employed in my study, the magnitude of leaf position effect on optical properties was the greatest in PW followed by LQ and TM (Table 3.1). Smith (1982) showed that Pfr/Ptotal ratio is particularly sensitive to the red/far-red ratio in the range of ~0.05-1.15 found in plant canopies. Foliage of different species and at different profiles in a canopy would exert different signals of competitive pressure on associated plants. Therefore, differences in leaf optical properties due to both species and the position of leaf on the stem must be considered while assessing competitive interactions among species. The observed differences in leaf optical 54  properties may also be important in determining germination of light-requiring weed seeds present on the soil surface (Silvertown 1980; Nolan and Upadhyaya 1988), as well as in the selection of species for cover crops and mixed cropping.   Differences in optical properties of leaves at different positions on the stem as well as the species-specific differences could be due to differences in leaf chemistry, particularly the chlorophyll content, and the SLW. Chlorophyll content in PW and TM leaves and the thickness of leaves from LQ and PW generally increased with increasing leaf position (Table 3.2). It must be pointed out that the observed effect of leaf position on their optical properties could be due to differences in exposure to solar radiation at different positions, and/or to changes in leaf chemistry and/or morphology during leaf development. Leaves present at the higher positions on the stem receive more light compared to leaves at lower positions. This may cause differences in leaf optical properties by influencing leaf physiology and chemistry. Light intensity has been reported to influence leaf water, chlorophyll and soluble carbohydrate contents, chloroplast ultrastructure, stomatal density and leaf thickness (Lichtenthaler et al. 1981). These effects could contribute to the differences in the leaf optical properties. Leaf epidermal characteristics also play an important role in determining the reflectance, transmittance, and absorptance of a leaf (Lin and Ehleringer 1983). The adaxial leaf surface of PW has little or no epicuticular wax whereas LQ leaf has a thick layer of crystalline epicuticular wax (Bitterlich and Upadhyaya 1990). Chemical composition and ultrastructure of leaf epicuticular wax are also known to change with species as well as the leaf age (Wirthensohn and Sedgley 1996; Jetter and Schäffer 2001). Furthermore, the number of glandular trichomes present on LQ leaves decrease as leaves mature (Sanyal et al. 2006). My preliminary studies monitoring reflectance, transmittance, and absorbance of the 3rd true leaf with 55  time have shown changes in its optical properties as it developed and senesced (Chapter 4). The role of leaf surface characteristics in the observed differences in optical properties of leaves of these species remains to be determined.  Rratio and Tratio changed with leaf position in all three species employed in this study. Thus, leaves from different positions on the plant stem are expected to differ in their ability to modify the light environment, conveying a different signal of potential competition. Seedling establishment is influenced by the early morphological response of seedlings to alteration in red/far-red ratio (Kasperbauer and Karlen 1994; Kasperbauer 1999). In agroecosystems, weeds that emerge at different times in a season may be present at different layers in a weed-crop canopy and may receive different red/far-red signals depending on their position relative to associated plants. Interestingly, the three species employed in this study differed with regard to effects of leaf position on Rratio and Tratio (Fig. 3.1A, B). The magnitude of position effect was greater in PW compared to LQ and TM (Fig. 3.1A, B). This finding is important for cropping systems because it suggests that different species could differ in their ability to signal shade avoidance response (Rajcan et al. 2004; Green-Tracewicz et al. 2011), which may influence their competitive interactions, even in the absence of competition for resources (Ballare and Casal 2000; Rajcan and Swanton 2001). In a crop with several weeds, the seriousness of weeds will differ depending on their ability to alter the red/far-red ratio in the canopy and its influence on species in the mixture, including the crop. This information can help weed managers know which weeds will exert a stronger competition signal on a crop based on their ability to alter the red/far-red ratio.   56  As chla+b content in PW and TM increased with increasing leaf position (Table 3.2), T660 declined (Table 3.1). However, while R660 declined with increasing leaf position in PW, it did not change in TM. There was no consistent change in LQ. This suggests that the change in R660 was possibly due to a change in leaf characteristics other than a decline in chla+b content. Changes in red/far-red ratio cannot be fully explained by chlorophyll concentration, chla/b ratio and leaf thickness (Table 3.3). Other factors, which were beyond the scope of this study, may be involved. Anthocyanin, Carotenoid, nutrient, and leaf water contents have been shown to influence leaf optical properties (Baldini et al. 1997; Neill and Gould 2000; Asner and Martin 2008). Baldini et al. (1997) reported the influence of lamina thickness and water content on leaf optical properties in five woody species. Leaf optical properties could also be affected by the concentration, location, and type of biominerals (Klančnik et al. 2014b, c). Calcium concentration has been reported to affect reflectance, and Si, Mg and Ca concentrations transmittance properties in leaves of Phragmites australis, Phalaris arundinacea, Molinia caerulea, Deschampsia cespitosa, and Carex elata (Klančnik et al. 2014c). Investigation of influences of leaf position on leaf anatomy and surface characteristics (e.g. wax layer, epidermal cells, cuticular thickness and trichome) in crop and associated weeds, and how these influences affect leaf optical properties could advance our understanding of physiological underpinnings of plant-plant interactions among species in agroecosystems.   Conclusion Results of this study show several significant effects of leaf position on leaf optical properties in LQ, PW, and TM. These include differences in the R, T, and A at 660 and 730 nm and the red/far-57  red ratio of the light reflected (Rratio) and transmitted (Tratio) from leaves. Interestingly, the species employed in this study (TM, LQ, and PW) also differed in this regard. This research suggests that leaf position and species could modify the light environment in canopies of plant communities by affecting the leaf optical properties, which in turn could influence plant-plant interaction. This information could be useful for weed scientists in managed cropping systems. Generally, growth rate, plant size, and LAI are considered to be the most important parameters of competitive ability of a species in mixed populations. My results suggest species-specific differences in leaf optical properties, as well as leaf position effects, should be taken into consideration in assessing eco-physiological functions of a vegetation cover. The observed differences in leaf optical properties may also be important in determining germination of light requiring weed seeds and may be useful in selection of species for cover crops and mixed cropping. Since changes in the red/far-red ratio of light reflected and transmitted through plant leaves could not be explained fully by chlorophyll content and SLW, the role of other factors affecting leaf optical properties cannot be ruled out. The effect of leaf position on the stem on their optical properties reported in this study could be a combined effect of leaf position on the stem, which determines the environment to which a leaf is exposed, and the leaf age. The next chapter of this thesis examines the optical properties of a leaf during its development. 58  Chapter 4: Changes in leaf optical properties during tomato, lamb’s-quarters, and pigweed plant development   Summary Leaf optical properties influence the red: far-red light ratio (a signal of potential competition) in plant canopies, which could alter inter-plant interactions among species comprising a plant community. Information on leaf optical properties and the factors affecting them is, therefore, important to understand underpinnings of plant-plant interaction in natural and managed ecosystems. To investigate changes in leaf optical properties during plant development, reflectance (R) and transmittance (T) of red (660 nm) and far-red (730 nm) lights for the 3rd true leaf were measured at 4, 8 and 12th leaf stages of common lamb’s-quarters (Chenopodium album L.) (LQ) and redroot pigweed (Amaranthus retroflexus L.) (PW) plants and at the 4, 6 and 8th leaf stages of tomato (Solanum lycopersicum L.) (TM) plant growth using a CI-710 Miniature Leaf Spectrometer. Percentage absorptance (A) was calculated as 100% - (% reflectance + % transmittance). Potential relationships of leaf optical properties with specific leaf weight and chlorophyll content were also investigated. Compared to older leaf stages, reflectance and transmittance at 660 nm (R660, T660) and 730 nm (R730, T730) were the largest, and absorptance at 660nm (A660) and 730 nm (A730) were the least at the 4th leaf stage of LQ plant growth. Changes in TM leaves followed a trend similar to LQ, except for R660 and A660. In PW, R660, T660, and R730 were the largest, and A660 was the least when plants were at the 12th, compared to younger leaf stages of plant growth. T730 was the least and A730 the largest at the 8th leaf stage of PW plant growth. A730 and A660 showed the largest and the smallest change with plant development, 59  respectively. Red/far-red ratios of reflected (Rratio) and transmitted (Tratio) light from the 3rd true leaf also changed with plant development in all three species. Rratio and Tratio significantly correlated with chlorophyll content and specific leaf weight (R2 = 0.32-0.87). These results show that leaf optical properties at red and far-red light wavelengths change with the development of TM plant and its two weeds LQ and PW, and the three species differ in this regard. Chlorophyll content and specific leaf weight can only partly explain differences in leaf optical properties of these species. These differences could influence growth and intra- and/or inter-species interactions in plant communities comprising these species.   Introduction Leaf optical properties influence red/far-red light ratio in plant canopies. Changing red/far-red ratio is a signal for impending inter-plant interaction (Ballaré 1999). Low red/far-red ratio induces shade-avoidance responses in plants, which can influence plant growth and competitive balance among species in a population (Ballaré and Casal 2000; Rajcan and Swanton 2001). Knowledge of factors affecting leaf optical properties is, therefore, important to our understanding of underpinnings of inter- and intra-species interactions in mixed populations.  Leaf optical properties can be affected by exposure to PAR, UV radiation (Klem et al. 2012) and other environmental stressors (Carter and Knapp 2001; Grzesiak et al. 2009), leaf morphology, and leaf chemistry (Klančnik et al. 2014a; Mershon et al. 2015). The partitioning of incident light into reflected, transmitted, or absorbed components by leaves can also be influenced by various factors, including species (Gausman and Allen 1973; Souza and Válio 2003), leaf surface 60  characteristics (e.g. leaf hairs and waxes), internal structures (e.g. arrangement of chloroplasts and mesophyll cells) (Gausman et al. 1969; Grant et al. 1987), and leaf chemistry (Woolley 1971; Klančnik et al. 2014a; Mershon et al. 2015).  From a plant-plant interaction perspective, entire leaf area of a plant is considered qualitatively similar and is used as an indicator of the plant’s ability to compete. Results presented in Chapter 3 showed that leaves from different positions on plant stem differ in their optical properties for red and far-red wavelengths in TM, LQ, PW, and the three species differed in this regard. This suggested that the same leaf area from leaves on different nodes on the stem and from different species can modify the light environment in a plant canopy differently, thereby having different influences on the growth of its neighbours. As described in the previous chapter, the effect of leaf position on its optical properties can be due to exposure of upper leaves to a different light environment and stresses compared to the lower (shaded) leaves, and/or to changes associated with leaf aging.  Leaf age is known to influence leaf surface characteristics, e.g. epicuticular wax, cutin, and pectin, number of stomata, trichomes, and glands (Chachalis et al. 2001; England and Attiwill 2006; Sanyal et al. 2006; Yamawo et al. 2014). Changes in leaf cuticle thickness (England and Attiwill 2006), leaf chemistry (e.g. aucubin, catalpol, iridoid glycosides, nitrogen, fiber, cellulose and mineral contents) ( Gomide et al. 1969a, b; Bowers and Stamp 1993), morphology, anatomy, and physiology (Bond 2000; Richardson et al. 2000; Day et al. 2001; Apple et al. 2002) with plant age have been reported.  61   Knowledge of changes in leaf optical properties of a crop and associated weeds with their development is important to our understanding of the underpinnings of plant-plant interactions among species in agroecosystems. It is also important to understand influences of a vegetation cover on germination of light-requiring weed seeds in soil-surface seed banks. This knowledge can provide criteria for selecting species for use as cover crops in mixed cropping. The objective of the research described in this chapter was to study changes in leaf optical properties with regard to red (660 nm) and far-red (730 nm) light during the development of TM and two common weeds LQ and PW.   Materials and Methods   Seed source and plant culture LQ and PW seeds, collected in September 2010 from the Totem Field at the University of British Columbia (UBC) and TM seeds obtained from West Coast Seeds Ltd. (Delta, BC, Canada) were used. Ten LQ or PW, or two TM seeds were sown 0.5 cm deep in 10 × 10 × 9 cm (height) plastic pots filled with a 70% peat moss and 30% perlite potting mix (West Creek Farms, Fort Langley, BC, Canada) in Conviron growth chambers (25/20°C day/night temperature, 16 h photoperiod) in the Faculty of Land and Food Systems at UBC. TM seeds were sown five days prior to PW and LQ seeds in order to synchronize seedling emergence of the three species. Seedlings were thinned to one uniform plant per pot at the first true leaf stage. Plants were re-randomized once a week to minimize effects of micro-environmental variations. 62    Changes of reflectance (R), transmittance (T), and absorptance (A) at 660 and 730 nm during plant development R, T and A of the 3rd true leaf were studied at the 4, 8 and 12th true leaf stage for LQ and PW; both species reached at these leaf stages at 13, 18 and 23 days after seedling emergence, respectively. Since flower buds started to appear at the 6th true leaf stage in TM, the 3rd true leaf at only the 4, 6 and 8th true leaf stages of plant development was used for this species. TM plants reached the 4, 6, and 8th true leaf stages at 12, 17, and 24 days after seedling emergence, respectively. The average of measurements for the 2nd and 3rd leaflets of the 3rd true leaf of TM was used to study changes in leaf optical properties with plant development. These two leaflets were selected because of the suitability of their size and shape for optical property measurements.  R and T at 660 nm and 730 nm were measured using CI-710 Miniature Leaf Spectrometer as described in Chapter 3. Absorbance at 660 and 730 nm was calculated by subtracting the R and T percentages from 100%. After optical property measurements, the 3rd leaf was used to determine chlorophyll content and the specific leaf weight (SLW), as described below.   Measurement of leaf chlorophyll content Chlorophyll from leaf discs (7 mm diam.) punched from the 3rd leaf at the 4, 8 and 12th leaf stages of LQ and PW plant development, and 4, 6 and 8th leaf stages of plant development for TM were extracted using the procedure described in Chapter 3. Chla+b concentration, an indicator of 63  photosynthesis rate (Lobato et al. 2010) and chla/b, an indicator of the amount of light harvesting complex in chloroplast (Alberte 1976; Chow et al. 1990), were calculated.   Specific leaf weight measurement The lamina area after removal of 7 mm diam. discs for chlorophyll measurement as described above was measured using a LI-COR LI-3000 portable leaf area meter (Li-Cor Inc., Lincoln, NE, USA). The leaves were then dried at 70℃ and weighed. The specific leaf weight (mg cm-2), an indicator of leaf thickness, was calculated.   Statistical analyses A completely randomized experimental design with six replicates was used and the experiment was repeated once. All statistical analyses were conducted using the SAS software (SAS software, ver. 9.4; SAS Institute, Inc., Cary, NC, USA). The assumptions of homogeneity of variance and normality were tested before subjecting the data to ANOVA. Primary data were transformed as needed. LQ Chla+b and Tratio data were log transformed. PW A660, R730 data were transformed using a power transformation (power = 3), and T660, A730, Rratio, Tratio and SLW data were log transformed. For TM, T660 data were log transformed, A660 were transformed using a power transformation (power = 3), and SLW were reciprocal transformed. Data from the two experiments were analyzed using PROC MIXED procedure with experiment treated as a random effect. Pair-wise comparisons of means were conducted using pairs of means t-test with the alpha level adjusted according to the number of pairwise comparisons (α’ = 0.05/3). Data presented are least squares means (ls-means) of original data. Multiple linear regression analysis was carried out to 64  study relationships of red/far-red ratios of the reflected and transmitted lights with chla+b concentration, chla/b ratio and SLW using the PROC REG procedure (SAS ver. 9.4). A t-test was used to evaluate the significance of variables.   Results   Changes of reflectance (R), transmittance (T), and absorptance (A) during plant development The results showed that leaf optical properties of the 3rd true leaf at 660 nm and 730 nm changed with plant development in LQ, PW, and TM, and the three species differed in this regard (Table 4.1). With exception of R660 in TM, generally less light was reflected or transmitted at 660 nm and 730 nm in the 3rd leaf of LQ and TM as the plants developed. Leaves of LQ and TM showed the largest change in A730 and the smallest change in A660 as plants grew. A730 at the 8th leaf stage of LQ plant development was almost four-fold of that at the 4th leaf stage of growth; it decreased by 21% from the 8th to the 12th leaf stage. A660 increased only by 3% from the 4th to the 8th leaf stage and then leveled off to the 12th leaf stage of plant growth. Both A660 and A730 were the lowest at the 4th, compared to the older leaf stages of plant growth, except A660 in TM. A660 had the lowest value at the 6th leaf stage of TM plant development; there were no differences between the 4th and the 8th leaf stages of growth. A730 at the 8th true leaf stage of plant growth was approximately seven-fold greater than that at the 4th leaf stage. More than 90% of the light at 660 nm, but little at 730 nm was absorbed by the 3rd true leaf of LQ and TM plants at the growth stages employed in  65  Table 4.1. Changes of reflectance, transmittance, and absorptance at 660 and 730 nm of the 3rd true leaf during the development of 1 common lamb’s-quarters, redroot pigweed, and tomato plants. 2 Species Leaf stage Percent of irradiation R660 T660 A660 R730 T730 A730 Common  lamb’s-quarters 4 5.41a a 3.41 a 91.18 a 44.58 a 52.14 a  3.28 a 8 4.64 b 1.47 b 93.89 b 41.67 b 46.29 b 12.03 b 12 4.54 b 1.99 c 93.47 b   43.20 ab 47.28 b   9.54 c Redroot pigweed 4 5.65 a 7.11 a 87.22 a 39.62 a 55.93 a   4.33 a 8 5.33 a 4.75 b 89.92 b 38.48 a 47.36 b 14.16 b 12 8.36 b 10.48 c 81.18 c 44.58 b 54.59 a   1.62 a Tomato 4 3.94 a 3.66 a 92.80 a 43.93 a 54.70 a  1.36 a 6 3.43 b 2.09 b 94.48 b 42.14 b 49.72 b  8.15 b 8 3.92 a 2.55 b 93.53 a 42.51 b 49.30 b  8.19 b aLs-means based on 12 replicates pooled from two experiments. Values followed by a different letter(s) within a column for each 3 species are significantly different (p ≤ 0.05). 4 66  this study. Compared to the 4th leaf stage, R660 and T730 decreased as LQ plants became older. T660 and R730 had the lowest value for the mid-aged plants (8th leaf stage) (Table 4.1). In TM, R660 decreased at the 6th and then increased from the 6th to the 8th true leaf stage of plant growth. T660, R730, and T730 decreased as plants developed from the 4th to the 6th leaf stage and then leveled off at the 8th leaf stage of growth.   Unlike in LQ and TM, R660, R730, and T660 of the 3rd true leaf of PW plants were significantly higher at the 12th true leaf stage compared to younger leaf stages. No significant differences were found between the 4th and the 8th true leaf stages in R660 and R730. T660 decreased by 33% at the 8th and then increased by 121% from the 8th to the 12th true leaf stage of plant growth. A660 increased slightly when PW plants grew from the 4th to the 8th leaf stage, followed by a decrease from the 8th to the 12th leaf stage. T730 and A730 values were the smallest and greatest respectively, at the 8th true leaf stage; there was no difference between the 4th and the 12th true leaf stages (Table 4.1).   In all the three species employed in this study, more than 80% of the light at 660 nm was absorbed with only little reflected and/or transmitted. On the other hand, most of the light at 730 nm was reflected and/or transmitted and little was absorbed. R660 and T660 values range from ~1% to 10%, and R730 and T730 range from ~38% to 54%. While ~90% of light was absorbed at 660 nm, only ~1% to 14% was absorbed at 730 nm.  67   Changes of red/far-red ratio of reflected (Rratio) and transmitted (Tratio) lights during plant development Rratio and Tratio of the 3rd true leaf changed with plant development and the three species differed in  this regard (Fig. 4.1A, B). Rratio of the 3rd true leaf did not change up to the 8th leaf stage of PW plant growth but increased significantly at the 12th leaf stage (Fig. 4.1A). In LQ, Rratio declined consistently as plants grew from the 4th to the 12th leaf stage (Fig. 4.1A). In TM, Rratio declined by 9% from the 4th to the 6th, followed by an increase of 14% at the 8th leaf stage of growth (Fig. 4.1A). Rratio was the greatest in PW and the lowest in TM at all the plant growth stages.  Tratio declined by 21% as PW plants grew from the 4th to the 8th leaf stage and then increased significantly at the 12th true leaf stage of plant growth (Fig. 4.1B). In LQ, Tratio decreased from the 4th to the 8th leaf stage and then increased slightly but significantly to the 12th leaf stage of plant growth (Fig. 4.1B). Tratio decreased by about 31% as TM plants grew from the 4th to the 6th leaf stage and then increased by 23% at the 8th leaf stage of growth. Tratio was greater in PW compared to LQ and TM; it was about two-fold of that for LQ and/or TM at the 4th true leaf stage, and ~two and three-fold of that for TM and LQ, respectively at the 8th true leaf stage of plant growth. At the 12th true leaf stage of PW plant growth, it was almost four-fold of that for LQ (Fig. 4.1B).   Changes of chlorophyll content (chla+b), chla/b ratio, and specific leaf weight (SLW) during plant development  Chla+b content per unit area of the 3rd true leaf increased from the 4th to the 6th (TM), or 8th (LQ and PW) leaf stage of plant growth and then declined (Table 4.2). The magnitude of changes in the   68        Fig. 4.1. Changes of red/far-red ratio of the reflected (Rratio) (A) and transmitted (Tratio) (B) light from the 3rd true leaf during common lamb’s-quarters (○), redroot pigweed (●), and tomato (□) plant development. Different letters next to the data points for each species denote a significant difference between plant ages (p ≤ 0.05, n = 12).0.000.050.100.150.200.250.304 6 8 10 12Lamb's-quartersPigweedTomatoa aba b bab aR ratio0.000.050.100.150.200.250.304 6 8 10 12Babcab cab cT ratioleaf stage of plant growth l            l             l             l            l   A l            l             l             l           l 69       Table 4.2. Changes of chlorophyll content per unit leaf area (chla+b), chlorophyll a/b ratio (chla/b), and specific leaf weight (SLW) of the 3rd true leaf of common lamb’s-quarters, redroot pigweed, and tomato during plant development. Species Leaf stage    Chla+b (nmol cm-2)      Chla/b SLW (µg cm-2) Common lamb’s-quarters 4 30.14a a 3.87 a 2.13 a 8 46.93 b 3.40 b 2.86 b 12 41.27 c 3.43 b 2.90 b Redroot pigweed 4 26.09 a 4.70 a 1.55 a 8 42.10 b 4.16 b 2.59 b 12 16.26 c 4.09 b 3.04 c Tomato 4 33.18 a 3.42 a 1.66 a 6 42.76 b 3.09 b 2.35 b 8 36.94 a 3.09 b 3.18 c aLs-means based on 12 replicates pooled from two experiments. Values followed by a different letter(s) within a column for each species are significantly different (p ≤ 0.05). The third true leaf from each species was used.   70  three species, however, was different. As plants grew from the 4th to the 8th leaf stage, chla+b increased by 56% and 61%, respectively, and then declined by 12% and 61% from the 8th to the 12th leaf stage of plant growth in LQ and PW, respectively. TM plants had the highest chla+b content at the 6th true leaf stage of growth; there was no difference between the 4th and the 8th leaf stages. Chla/b of the 3rd true leaf was the greatest when plants were at the 4th leaf stage of development in all three species. There were no differences between the two later stages of development (Table 4.2).  SLW of the 3rd true leaf also changed as plants of the three species employed in this study grew, and the species differed in this regard. SLW increased by 34% as LQ plants grew from the 4th to the 8th leaf stage and then leveled off to the 12th leaf stage (Table 4.2). In PW, SLW increased by 96% as plants grew from the 4th to 12th leaf stage. Similarly, SLW of TM plants increased consistently (up to 92%) from the 4th to the 8th leaf stage of growth.   Relationships of changes in Rratio and Tratio with chla+b, chla/b, and SLW  Multiple linear regression analysis was conducted to test relationships of Rratio and Tratio with chla+b, chla/b ratio, and SLW for LQ, PW, and TM and a t-test was used to evaluate the significance of regression coefficients. The regressions were significant for all the three species (Table 4.3). For LQ, chla+b and SLW accounted for 60% of the variation for Rratio; 85% of Tratio was explained by chla+b and chla/b ratio. For PW, both Rratio and Tratio were related negatively with chla+b and positively with SLW, with R2 = 0.82 and 0.87 for Rratio and Tratio, respectively. The weakest relationship   71   Table 4.3. Relationships of red/far-red ratios of the reflected (Rratio) and transmitted (Tratio) light with chlorophyll content (chla+b), chlorophyll a/b ratio (chla/b), and specific leaf weight (SLW) of the 3rd true leaf of common lamb’s-quarters, redroot pigweed, and tomato plants.   Common lamb’s-quarters   Intercept Chla+b Chla/b SLW R2 p Rratio 0.09958 0.00043 0.01099 -0.0164 0.5786 <.0001   (0.0038)a (0.0350) (0.0854) (0.0121)          Tratio 0.0155 -0.001 0.0241 -0.0055 0.8459 <.0001   (0.6917) (0.0001) (0.0033) (0.4716)     Redroot pigweed   Intercept Chla+b Chla/b SLW R2 p Rratio 0.17249 -0.0015 -0.0042 0.01786 0.8150 <.0001   (<.0001) (<.0001) (0.4171) (<.0001)          Tratio 0.21316 -0.0029 -0.0068 0.0149 0.8740 <.0001   (<.0001) (<.0001) (0.3023) (0.0013)     Tomato   Intercept Chla+b Chla/b SLW R2 p Rratio 0.02932 -0.0003 0.01727 0.00541 0.3201 0.0058  (0.29570) (0.1853) (0.0159) (0.0037)          Tratio 0.09913 -0.0014 0.00313 -0.0026 0.7001 <.0001   (0.0051) (<.0001) (0.7074) (0.1569)   aValues in brackets below the regression parameters are p-values for the corresponding estimated parameters. 72  (R2 = 0.3201) was observed for Rratio versus chla/b and SLW in TM. The Tratio in TM leaves was highly related (R2 = 0.7) with chla+b (Table 4.3).   Discussion Leaf optical properties play important role in determining the red/far-red ratio (a signal of impending competition) of the light reflected and/or transmitted by leaves, which could influence plant growth and inter-plant interactions. Studies of leaf optical properties and the factors affecting them could provide valuable information for understanding the eco-physiology and function of vegetation cover. Results described in Chapter 3 of this thesis showed that leaves present at different positions on the stem differ in their optical properties and that LQ, PW, and TM differ in this regard. This effect could be due to a variety of factors including differences in leaf age or the physical position of leaves on the plant stem, which could influence its environment of exposure. The research in this chapter monitored changes in optical properties of a single leaf (the 3rd true leaf) with plant development, defined in terms of the leaf stages of plant growth.  Results showed that leaf optical properties (reflectance, transmittance, and absorptance at 660 nm and 730 nm) of TM and its two weeds change as the plants develop (Table 4.1). Red/far-red ratios of light reflected and/or transmitted through the 3rd true leaf changed due to changes in leaf optical properties with plant development (Fig 4.1). Low red/far-red ratio, a signal of impending competition, can trigger early detection of neighboring plants even before canopy closure (Ballaré et al. 1990). Since red/far-red ratio plays important roles in affecting plant growth, photomorphogenesis and seed yield (Browns et al. 1995; Goins et al. 1997), these findings have 73  interesting ecological implications in cropping systems. The results of this study suggest that leaves at different stages of plant growth will differ in their influences on the light environment in plant canopies, e.g. red/far-red ratio. Therefore, even though the area of the leaf might stay the same, its ability to influence light quality in the canopies would change during plant development conveying different signals of potential competition.   This study also showed that species differ in their abilities to affect the red/far-red ratio of light reflected or transmitted through leaves (Fig. 4.1). In agricultural ecosystems, crops grow in mixtures with various associated weed species, which emerge at different times. Weeds that have less effect on light quality are expected to be less problematic in terms of signaling potential competition compared to the other species, and the relative significance of a weed in a crop could change as plants grow. My study showed the red/far-red light ratio and the magnitude of changes in this ratio during plant development were greater in PW compared to LQ, suggesting that different species would induce different shade avoidance response during plant development. Plants will perceive different levels of potential competition depending on the species growing with them in a mixture, as well as their stage of plant growth. This could impact competitive interactions within a plant community. It has been reported that altered red/far-red ratio could affect plant-plant interactions by influencing plant morphology (Ballaré and Casal 2000, Rajcan and Swanton 2001). In addition to effects on plant growth and plant-plant interaction, a change in red/far-red ratio in canopy could also influence germination of light-sensitive seeds lying on the soil surface (Shinomura et al. 1996; Jankowska-Blaszczuk and Daws 2007), which could influence dynamics of soil seed banks. The finding that the relative importance of weeds in a multi-species mixture could change as their leaf optical properties change during plant development has 74  significant implications for weed management in cropping system. Differences in the ability to alter the light environment in a canopy due to both the species and plant development should be considered when assessing the competitive interactions among species.  The differential changes of leaf optical properties with the development of different species could be due to a variety of factors, including the variations in leaf chlorophyll content and SLW. Results of this study showed that changes in the red/far-red ratio of light reflected and/or transmitted from the leaf could partially be explained by chla+b, chla/b and SLW, with R2 ranging from 0.32 to 0.87 (Table 4.3). Other leaf changes associated with aging during plant development (e.g. changes in leaf surface morphology, physiology, chemistry, light environment, etc.) which were beyond the scope of this study, could play important role in affecting leaf optical properties. Using data collected from 162 tropical forest species from Australia, Asner and Martin (2008) reported that chlorophylls, carotenoids, and specific leaf area were highly correlated with leaf reflectance (r = 0.90 to 0.91). Epidermis characteristics can affect leaf reflectance, transmittance, and absorptance (Lin and Ehleringer 1983). It should be noted that the species employed in my study differ in their leaf surface morphology; for example, LQ leaves have crystalline epicuticular wax but PW leaves have little or none (Bitterlich and Upadhyaya 1990). The differences in leaf surface structure among species may also change with plant development. Holloway (1970) reported that the amount, as well as the ultrastructure of leaf epicuticular wax, is influenced by species and leaf age. Balloon-shaped glands on LQ leaf surface have been reported to disappear as the plant ages (Sanyal et al. 2006). Change in leaf chemistry with age could also influence leaf optical properties (Holloway 1970; Bower and Stamp 1993). Leaf optical properties have also been reported to be highly correlated with leaf nutrient and water contents (Asner and Martin 2008), anthocyanin and 75  biomineral concentration and distribution (Neill and Gould 1999; Klančnik et al. 2014b, c). Investigation of changes in these leaf characteristics with plant development and the influence of these changes on leaf optical properties in weedy as well as crop species is an interesting area for further studies.    Conclusion The results presented in this chapter show that the leaf optical properties (reflectance, transmittance, and absorptance) at 660 nm and 730 nm change during the development of plants, and that LQ, PW, and TM differ in this regard. These findings are significant to our understanding of how red/far-red light ratio in a canopy change during the development of plants growing in a mixture. The results suggest that the ability of the same area from a leaf to alter the light environment may change during development of plants. Thus, while using LAI to assess functions of vegetation cover, differences in growth stages of plants as well as species should be considered. Knowledge of changes in leaf optical properties with plant development in a mixture, and factors affecting these properties could help agronomists and weed managers in making weed management decisions. Leaf changes associated with plant development (e.g. leaf anatomical, biochemical characteristics), which could affect leaf optical properties, would be interesting topics for future study. 76  Chapter 5: Influence of red to far-red ratio on response of plants to UV-B radiation   Summary In nature, plants are exposed to a variety of stressors either simultaneously or sequentially. Exposure to one stressor can modify plant response to another stressor. Plants exposed to low red/far-red light ratio in a canopy may experience enhanced UV-B radiation at subsequent growth stages. This study investigates if pre-exposure to low red/far-red light ratio influences plant responses to increasing levels of ultraviolet-B (UV-B) radiation and if a low red/far-red light ratio-induced decrease in anthocyanin concentration is involved in this effect. Corn (Zea mays L.), containing anthocyanin in the leaf sheath, and lettuce (Lactuca sativa L.) and redroot pigweed (Amaranthus retroflexus L.), containing anthocyanin in their lamina, were used. Seedlings of all three species were exposed to either 0.3 (low) or 1.1 (high) red/far-red light ratio in growth chambers. These ratios were achieved by providing additional far-red light using far-red LEDs; all treatments received the same amount of photosynthetically active radiation. Following red/far-red light ratio treatment, anthocyanin concentration in the leaf sheath (corn) or lamina (lettuce and pigweed) was measured and plants were exposed to three levels of UV-B radiation, achieved by filtering radiation from UVB-313 fluorescent tubes with one (high UV-B), two (medium UV-B) and/or three (low UV-B) layers of cellulose acetate film in a greenhouse. The effect of red/far-red light ratio pretreatment on the response of plants to enhanced UV-B exposure and the possible role of low red/far-red ratio induced anthocyanin pigmentation in this effect were studied. Enhanced UV-B radiation significantly affected a variety of plant growth and allometric parameters in corn, 77  lettuce, and redroot pigweed. However, exposure to different red/far-red light ratios did not influence the response of these plants to subsequent UV-B radiation treatment. While pre-exposure to low, compared to high, red/far-red light ratio, reduced anthocyanin concentration by 60% in the corn leaf sheath, and 31% and 100%, respectively in the lettuce and pigweed laminae, this influence did not affect the plant response to UV-B radiation treatment. Effect of pre-exposure to low red/far-red ratio on plant response to elevated UV-B radiation could impact plant-plant interaction. However, my study showed that plant response to UV-B radiation was not influenced by red/far-red ratio pre-treatment. This is significant to our understanding of effects of these environmental stressors on plant-plant interaction in agroecosystems where both red/far-red light ratio and UV-B radiation levels fluctuate.   Introduction Light reaching the plant canopy can be reflected, transmitted and/or absorbed by leaves. The absorbance of red light by leaf chlorophyll and reflection and transmittance of far-red light can modify the red/far-red ratio in plant canopies. The red/far-red light ratio, which is around 1.15 for daylight, can range from 0.1 to 0.9 in plant canopies (Smith 1982). Changes in red/far-red light ratio trigger a series of phytochrome-mediated morphological and physiological changes in plants (Smith 1982; Kwesiga et al. 1986; Casal and Smith 1989; Barreiro et al. 1992; Rajcan et al. 2004; Ballare 2009; Page et al. 2010; Kegge et al. 2015).   In nature, plants are exposed to a variety of stressors, such as drought, extreme temperatures, salinity, light intensity, pollution, or enhanced UV-B irradiation (Smirnoff 1998), either 78  simultaneously or sequentially. Exposure to one stressor can modify plant response to another stressor. Plants exposed to low red/far-red light ratios in a canopy may experience enhanced UV-B radiation at subsequent stages of growth. Changes in plants exposed to low red/far-red ratio including greater shoot: root ratio, change in carbon allocation and leaf orientation (Rajcan et al. 2004), reduced photosynthetic rate and leaf chlorophyll concentration (Hoad and Leakey 1994), increased concentrations of total chlorogenic acids (Tegelberg et al. 2004), enhanced lignin and ethylene biosynthesis (Afifi and Swanton 2012), etc., could modify plant responses to enhanced UV-B radiation. Increases in UV-B radiation caused by depletion of the ozone layer in the stratosphere (McKenzie et al. 2007) have been reported to affect plant growth, development and productivity (Dai et al. 1997; Mackerness et al. 1998; Mackerness 2000; Costa et al. 2002), and also the competitive balance between species (Barnes et al. 1988; Furness et al. 2005a). Even slight differences in shoot morphological responses induced by increased UV-B exposure can alter canopy structure thereby influencing light interception and relative competitiveness (Barnes et al. 1996; Furness et al. 2005b; Robson et al. 2015).  Anthocyanins are water-soluble secondary phenolic compounds, which have antioxidant properties (Prior et al. 1998). These pigments serve as phyto-protectants against insect pests, diseases and harmful radiation (e.g. UV-B) (Close and Beadle 2003).  Phytochrome pigments are involved in the regulation of anthocyanin synthesis in plants (Warnasooriya et al. 2011). Afifi and Swanton (2012) showed a decrease in anthocyanin concentration in maize (Zea mays L.) grown under a low red/far-red light ratio. A similar decline has been reported in lettuce (Lactuca sativa L. cv. Red Cross) (Li and Kubota 2009), cabbage (Brassica oleracea L.) (Mancinelli 1990) and other plant species (Alokam et al. 2002). 79   Whether changes in red/far-red light ratio and the associated changes in anthocyanin concentration can modify the response of plants to enhanced UV-B radiation at subsequent growth stages is not known. This information is important to our understanding of the eco-physiology of plants growing in ecosystems, where both the red/far-red light ratio and UV-B radiation levels fluctuate. Two separate studies were conducted in 2014 and 2015 to determine if exposure to different red/far-red light ratios influences the susceptibility of plants to subsequent UV-B radiation and if a low red/far-red ratio-induced change in anthocyanin concentration is involved in this effect. In the first study (2014), corn (Zea mays L.) seedlings that have anthocyanin in the leaf sheath were used. In the second study (2015), lettuce (Lactuca sativa L., cv. Maiko) seedlings that have anthocyanin pigmentation on the adaxial leaf surface, and redroot pigweed (Amaranthus retroflexus L.) seedlings that have anthocyanin pigmentation in the abaxial leaf surface, were used.   Materials and Methods   Seed source and plant culture In the first study, corn (hybrid CG 108 × CG 102) seeds were sown 1 cm deep in Redi-earth growth medium (Eddi’s Wholesale Garden Supplies, Surrey, BC, Canada) in 10 × 10 × 9 cm (height) plastic pots in the Horticulture Greenhouse of the University of British Columbia (UBC). In the repeat of this experiment, corn plants were grown in Dutch Treat media (Eddi’s Wholesale Garden Supplies, Surrey, BC, Canada) because the Redi-earth media was unavailable.  In the second study, seeds of redroot pigweed, collected in September 2010 from the Totem Field 80  Laboratory at UBC, and lettuce, obtained from West Coast Seeds Ltd. (Delta, BC, Canada) were sown 0.5 cm deep in 10 × 10 × 9 cm (height) plastic pots filled with 70% peat moss and 30% perlite potting mix (West Creek Farms, Fort Langley, BC, Canada) in the greenhouse. Seedlings were thinned to one per pot at the first true leaf stage.    Red/far-red ratio treatment Soon after emergence, seedlings of all species were exposed to either high (1.1) or low (0.3) red/far-red light ratio in Conviron growth chambers (25/20°C day/night temperature, 16 h photoperiod) (Table 5.1). Each of three growth chambers was divided into two sections using aluminum foil in order to separate the high and low red/far-red light ratio treatments. Different red/far-red ratios were achieved by supplementing far-red radiation using Lumigrow far-red bulbs (740 nm peak; ECC-FR, LumiGrow, Inc., Novato, CA, USA) and far-red LED tubes (Shenzhen Gehl Lamps Co., Ltd., Shenzhen, China). All plants received a similar amount of red light and photosynthetically active radiation (PAR; 210 µmol m-2 s-1) provided by Phillips F72T8/TL841/HO tubes (Phillips Lighting Co., Somerset, NJ, USA) and incandescent bulbs. Corn seedlings were exposed to red/far-red ratio treatment for a week, and lettuce and pigweed seedlings were exposed for two weeks. Red/far-red ratios were measured using an SKR 110 sensor and SKR 100 meter (SKY Instruments Ltd., Llandrindod Wells, Powys, UK). PAR was measured with a LI - COR quantum sensor (Q 9674) and LI-COR LI-185B light meter (LI-COR Inc., Lincoln, NE, USA). Both PAR and red/far-red ratios were measured at the growth chamber floor level. A randomized completely block design was used with three blocks (three chambers) and two red/far-red ratios in each block. Plants were re-randomized every week to minimize effects of 81         Table 5.1. Red and far-red light intensities in different red/far-red ratio treatments. Treatment Red (µmol m-2 s-1) Far-red (µmol m-2 s-1) Red/far-red ratio T1 5.50a 17.90 0.3 T2 5.54 4.87 1.1   aMean of three replicates.   82  micro-environmental variations. All experiments were repeated once.   UV-B radiation treatment After receiving red/far-red light ratio treatments, seedlings were exposed to different levels of UV-B radiation as described below. Plants were placed in small frames (110 × 34 × 60 cm high) covered with one, two or three layers of cellulose acetate film (diacetate type, 0.127 mm) (McMaster-Carr Supply Co., OH, USA) to achieve three levels of UV-B radiation, henceforth called the high, medium and low UV-B levels, respectively (Fig. 5.1). These frames were placed in larger metal frames (1.2 × 1.2 × 1.25 m), which had ten 40 W UVB - 313 fluorescent tubes mounted 1.1 m above the greenhouse bench, and were enclosed with Mylar film (Type D, 0.127 mm thick) (Cadillac Plastics Ltd., Burnaby, BC, Canada) to confine all UV radiation within the frames.    Anthocyanin measurement  Anthocyanin concentration in the corn leaf sheath under the second leaf collar was assayed following the red/far-red ratio treatment (prior to UV-B exposure) using the procedure of Afifi and Swanton (2012). Frozen leaf sheath tissue (0.1 g) was ground using a mortar and pestle and extracted in 0.5 ml of acidified-methanol (1% HCl) overnight at 4℃ in dark. Extracts were centrifuged (10 min, 8160 g). The supernatant was re-centrifuged after addition of 0.835 ml of chloroform to separate the anthocyanin and chlorophyll pigments. The absorbance of the aqueous fraction at 657 nm and 530 nm were measured using a UV160U UV-visible recording   83                   Fig. 5.1.  UV-B radiation treatment chambers in the greenhouse.    84  spectrophotometer (Shimadzu Scientific Co., Ltd., Kyoto, Japan). The total anthocyanin content was determined by subtracting the A657 from A530 as described by Jia et al. (2015).   Anthocyanin contents of lettuce and pigweed leaves were estimated non-destructively using the anthocyanin reflectance index (ARI), which allows an accurate estimation of anthocyanin accumulation in intact leaves (Gitelson et al. 2001). ARI = (R550)-1 - (R700)-1, where (R550)-1 and (R700)-1 are inverse reflectances at 550 and 700 nm, respectively. Leaf reflectance was measured with a CI-710 Miniature Leaf Spectrometer (CID Bio-Science, Inc., Camas, WA, USA). This non-destructive procedure was used so that plants could be exposed to UV-B radiation following red/far-red ratio treatments.   Effect of UV-B radiation on plant growth parameters Four corn plants from each UV-B level were harvested ten days after the start of UV-B treatment. One randomly selected plant was frozen in liquid nitrogen and stored at -60°C for biochemical analysis. The remaining three plants were used to measure various growth parameters. After measuring the shoot height, plant roots were gently washed and touch-dried using paper towels. Leaf blades were scanned using a Dell AIO 810 Scanner (Dell, Xiamen, China) and their area was calculated using the Adobe Photoshop CS2 Version 9.0 software (Adobe Systems Inc., San Jose, CA, USA). Leaf blades, leaf sheaths and stem, and roots were dried at 70°C to constant weight and their biomass recorded. Dry weights were used to construct the following growth indices: leaf area ratio (LAR) [leaf area/shoot dry weight], specific leaf weight (SLW) [leaf blade dry weight/leaf area], and leaf weight ratio (LWR) [leaf blade weight/shoot dry weight]. 85   Two lettuce and two pigweed plants from each UV-B treatment level were harvested 8 days after the start of UV-B treatments in experiment 1. Shoot height of pigweed plants from the soil surface to the shoot apex, and the length and width of the largest leaf of lettuce plants were measured. Leaves and stems were separated and dried at 70°C to a constant weight and their biomass was recorded. Since pigweed plants were found to be very sensitive to UV-B radiation in the first experiment, plants were exposed to this radiation for only five days in the second experiment to reduce the level of UV-B damage.    Levels of UV-B-absorbing compounds, H2O2, and L-phenylalanine ammonia-lyase activity in corn leaves The methanol-extractable UV-B-absorbing capacity of leaves was measured using the procedure of Cuadra et al. (2004). Frozen leaf blade tissue (0.1 g) was ground and extracted with 5 ml of acidified-methanol (79:20:1 v/v, CH₃OH, H2O, HCl) in a water bath (60°C for 1h). After cooling the extract to room temperature, its absorbance at 310 nm was measured using a UV160U UV-visible spectrophotometer (Shimadzu Scientific Co., Ltd., Kyoto, Japan).  The hydrogen peroxide (H2O2) level was measured using the procedure of Alexieva et al. (2001). Frozen leaf blade tissue (0.1 g) was ground and homogenized in 1 ml of cold 0.1% (w/v) trichloroacetic acid (TCA). The homogenate was centrifuged at 12000 g for 15 min at 4°C.  A 0.5 ml aliquot of the supernatant and 1 ml of 1 M KI was added to 0.5 ml of 100 mM potassium phosphate buffer (pH 7.0). TCA (0.1%) without the leaf extract was used as the blank. After 86  incubation for 1 h in darkness, the absorbance of the mixture at 390 nm was read. The amount of H2O2 was determined using a H2O2 standard curve.  L-phenylalanine ammonia-lyase (PAL) activity was measured using the procedure of Liu and McClure (1995). A mixture of borate buffer (1 ml, 100 mM, pH 8.8), L-phenylalanine (0.25 ml, 50 mM) and enzyme extract (0.25 ml) was incubated at 30°C in a water bath for 60 min. The reaction was stopped by adding 0.1 ml of 6 M HCl and the absorbance at 290 nm was measured. The increase in A290 by 0.01 per hour was considered a unit of the enzyme activity.   Statistical analyses A randomized block design with three blocks and one replicate per block (three samples per replicate for corn and two for lettuce and pigweed) was used. All statistical analyses were conducted using SAS software (SAS Institute, Inc., Cary, NC, USA). Assumptions of homogeneity of variance and normality were tested before subjecting the data to ANOVA. Data were transformed as needed. Corn plant biomass, root mass, SLW, and H2O2 and UV-B absorbing compounds (A310) concentration data were log transformed, and LWR and PAL data were square root transformed. Lettuce leaf width and leaf mass and pigweed plant height, leaf mass and total plant mass data were log transformed. Data from the two experiments were analyzed using PROC MIXED model with experiment and block treated as random effects for each species. Means were generated using LSMEANS. Pairs of means t-test, with α level adjusted according to the number of pairwise comparisons (α’ = 0.05/3), was used to compare each pair of means.  87   Results   Effect of red/far-red ratio treatment on anthocyanin concentration in corn, lettuce, and redroot pigweed Corn: anthocyanin concentration decreased significantly in plants grown under the low red/far-red light ratio (Fig. 5.2A and 5.3). Compared to the high red/far-red ratio, anthocyanin concentration was 60% lower in the leaf sheaths of corn plants exposed to the low red/far-red ratio (p ≤ 0.05) (Fig. 5.3A). The magnitude of this effect decreased during subsequent UV-B exposure – by the end of the UV-B exposure, the magnitude was only 24% (Fig. 5.3B).   Lettuce and redroot pigweed: anthocyanin pigmentation decreased by 31% and 100% in the laminae of lettuce and pigweed plants grown under 0.3 compared to 1.1, red/far-red ratio, respectively (p ≤ 0.05) (Fig. 5.2B-E and 5.4).   Influence of red/far-red ratio pre-treatment on responses of corn plants to UV-B radiation Primary growth parameters  Plant height, leaf area, and stem, leaf, root and total plant biomass decreased with increasing levels of UV-B radiation (Tables 5.2 and 5.3). Compared to the low UV-B radiation level, the height of corn plants exposed to high UV-B radiation level decreased by 7%, leaf area by 24%, and stem, leaf, root, and plant dry biomass by 21, 15, 25, and 19%, respectively. All of these effects were   88         	  1.1 0.3 1.1 0.3 A B C D E 0.3 1.1 Fig. 5.2. Effect of red/far-red ratio (0.3 or 1.1) treatment on anthocyanin pigmentation in corn (A), redroot pigweed (B, C), and lettuce (D, E) plants.  	89               Fig. 5.3. Anthocyanin concentration in corn leaf sheath (A) after exposure to 0.3 or 1.1 red/far-red light ratio and (B) after UV-B treatment following red/far-red exposure. Results are pooled from two experiments. *Denotes a significant difference (p ≤ 0.05). Anthocyanin pigmentation was measured using acidified-methanol extraction.   0.00.10.20.30.3 1.1Anthocyanin(A530-A657)*A00.10.20.30.3 1.1*BRed/far-red ratio 90    	                Fig. 5.4. Anthocyanin reflectance index (ARI) of leaf lamina of lettuce and redroot pigweed after low (0.3) or high (1.1) red/far-red ratio treatment. Anthocyanin pigmentation was estimated using leaf reflectance in this study. ARI = (R550)-1 - (R700)-1, where (R550)-1 and (R700)-1 are inverse reflectances at 550 and 700 nm, respectively. * above each species denotes significant differences (p ≤ 0.05).    -3 0369121518Lettuce Pigweed0.31.1**Anthocyanin Reflectance Index 91      Table 5.2. Influences of enhanced UV-B radiation on plant height and leaf area of corn plants pre-exposed to 0.3 or 1.1 red/far-red light ratio. UV-B Height (cm) Leaf areaa (cm2) 0.3b 1.1 0.3 1.1 Lowc 78.8d 77.8 334.6 371.6 Medium 75.3 76.0 319.4 338.4 High 73.5 72.3 267.7 269.1 aLeaf blade area. bRed/far-red light ratio treatment. cLow, medium and high UV-B levels were obtained by passing the radiation from UV-B tubes through three, two, and one layer of cellulose acetate film. dLs-means based on six replicates pooled from two experiments.      There were no interactions between red/far-red and UV-B (p > 0.05).    92      Table 5.3. Influences of enhanced UV-B radiation on biomass allocation of corn plants pre-exposed to 0.3 or 1.1 red/far-red light ratio. UV-B Stem (mg) Leaf (mg) Root (mg) Total plant (mg) 0.3a   1.1 0.3 1.1 0.3 1.1 0.3 1.1 Lowb 304c 319 590 668 254 283 1148 1270 Medium 299 308 599 634 266 280 1164 1222 High 247 243 522 543 197 206 965 992 aRed/far-red light ratio treatment. bLow, medium and high UV-B levels were obtained as described in Table 5.2.  cLs-means based on six replicates pooled from two experiments. There were no interactions between red/far-red and UV-B (p > 0.05).     93  statistically significant (p ≤ 0.05). While enhanced UV-B radiation level significantly (p ≤ 0.05) affected various growth parameters in corn, pre-treatment with different red/far-red ratios did not influence effects of enhanced UV-B radiation on any of these growth parameters. Interactions between red/far-red ratio treatment and UV-B effects were not statistically significant (p > 0.05). Allometric indices LWR, an indicator of compartmentation of plant biomass in leaves, was slightly lower under the low, compared to high red/far-red ratio (Table 5.4). Red/far-red ratio pre-treatment had no effect on SLW, a measure of leaf thickness and/or density and LAR, an index of leafiness (p > 0.05) (Table 5.4).  There was a 20% increase in SLW and a slight but significant increase in LWR in corn grown under the high, compared to low, UV-B radiation level (p ≤ 0.05) (Table 5.4). LAR was not significantly affected by the UV-B radiation treatment (p > 0.05) (Table 5.4). These responses to increasing levels of UV-B radiation were not influenced by pre-exposure to different red/far-red ratios; red/far-red ratio and UV-B treatments interactions were not statistically significant (p > 0.05). H2O2, UV-B absorbing compounds, and PAL activity Neither the red/far-red ratio pre-treatment nor the subsequent exposure to enhanced UV-B radiation influenced H2O2, UV-B absorbing compound concentrations or PAL activity in corn leaves (Table 5.5). Interactions between red/far-red ratio and UV-B radiation treatment effects were not significant. 94      Table 5.4. Influence of enhanced UV-B radiation on some growth indices of corn plants pre-exposed to 0.3 or 1.1 red/far-red light ratio. UV-B   SLWa        LWRb LARc 0.3d        1.1 0.3 1.1 0.3 1.1 Lowe  0.79f 0.90 0.67 0.68 392  380  Medium 0.97 0.88 0.67 0.68 362  372  High 1.02 1.00  0.69 0.70 350  348 aSpecific leaf weight (leaf blade dry weight/leaf area). bLeaf weight ratio (leaf blade weight/shoot dry weight).  cLeaf area ratio (leaf area/shoot dry weight). dRed/far-red light ratio. eLow, medium and high UV-B levels were obtained as described in Table 5.2.  fLs-means based on six replicates pooled from two experiments. There were no interactions between red/far-red ratio and UV-B (p > 0.05).  95     Table 5.5. Influences of enhanced UV-B radiation on levels of H2O2, UV-B absorbing compounds, and phenylalanine ammonia lyase activity in corn plants pre-exposed to 0.3 or 1.1 red/far-red ratio. UV-B H2O2 (µmol g-1 FW) UV-B absorbing compounds (A310)a Phenylalanine ammonia lyase (Unit g-1 FW h-1) 0.3b 1.1 0.3 1.1 0.3 1.1 Lowc 1.99d 2.02 0.47 0.54 5259 5339 Medium 2.10 2.22 0.53 0.49 5448 5544 High 2.04 1.89 0.50 0.49 5371 5560 aAmount of UV-B absorbing compounds was estimated by the absorbance at 310 nm (A310). bRed/far-red light ratio. cLow, medium and high UV-B levels were obtained as described in Table 5.2.  dLs-means based on six replicates pooled from two experiments. There were no interactions between red/far-red ratio and UV-B (p > 0.05).    96   Influence of red/far-red ratio pre-treatment on responses of lettuce and redroot pigweed plants to UV-B radiation Primary growth parameters  Low red/far-red ratio treatment prior to UV-B exposure increased lettuce leaf length and width, and pigweed plant height (data not shown). Compared to 1.1 red/far-red ratio, lettuce plants grown under 0.3 ratio had greater leaf length (by 23%) and width (by 10%) at the time of transfer to UV-B treatment. Similarly, height of pigweed plants pretreated with 0.3 red/far-red ratio was twice that of the plants pretreated with 1.1 red/far-red ratio. Enhanced UV-B radiation level strongly inhibited seedling growth of both species (p ≤ 0.05). Leaf length, width, and biomass of lettuce under the high, compared to low UV-B radiation, decreased by 16, 18, and 26%, respectively (Table 5.6). Pigweed plant height, and leaf and shoot biomass decreased by 30, 52, and 54%, respectively with increasing UV-B levels (Table 5.7). While both red/far-red ratio pre-treatment and UV-B radiation exposure affected the growth of lettuce and pigweed (p ≤ 0.05), pre-exposure to different red/far-red ratios did not modify the effect of the UV-B radiation treatment. There were no significant interactions between UV-B and red/far-red ratio treatments (p > 0.05).   Discussion Plants developed in the canopies of mixed populations can experience different red/far-red ratios, which can affect their morphology and physiology (Ballare and Casal 2000; Rajcan and Swanton 2001) and potentially influence their response to enhanced UV-B radiation, thereby influencing plant-plant interactions. In this chapter, I investigated if pre-treatment with red/far-red light ratio and the associated low red/far-red ratio-induced decrease in anthocyanin concentration modify the 97       Table 5.6. Influence of enhanced UV-B radiation on length, width, and mass of leaves from lettuce plants pre-exposed to 0.3 or 1.1 red/far-red light ratio. UV-B Leaf length (cm) Leaf width (cm) Leaf mass (mg) 0.3a 1.1 0.3 1.1 0.3 1.1 Lowb  13.5c 11.9 8.3 7.8 481 485 Medium 12.8 10.4 7.6 6.9 450 361 High 11.5   9.9 6.7 6.5 378 340 aRed/far-red light ratio treatment. bLow, medium and high UV-B levels were obtained as described in Table 5.2.  cLs-means based on six replicates pooled from two experiments. There were no interactions between red/far-red ratio and UV-B (p > 0.05).    98      Table 5.7. Influence of enhanced UV-B radiation on height, leaf and shoot mass of redroot pigweed plants pre-exposed to 0.3 or 1.1 red/far-red light ratio. UV-B Height (cm) Leaf mass (mg) Shoot mass (mg) 0.3a 1.1 0.3 1.1 0.3 1.1 Lowb    15.4c    10.1    158     147     240      193 Medium 14.1 7.9 120 138 187      172 High 11.5 6.3   58   89   90      108 aRed/far-red light ratio treatment. bLow, medium and high UV-B levels were obtained as described in Table 5.2.  cLs-means based on six replicates pooled from two experiments. There were no interactions between red/far-red ratio and UV-B (p > 0.05).    99    response of plants to enhanced UV-B radiation levels in two systems: (1) corn seedlings, which contain anthocyanin in the leaf sheath, and (2) lettuce and redroot pigweed seedlings, which contain anthocyanin in the leaf laminae. As expected, exposure to UV-B radiation affected various growth parameters in corn, lettuce, and pigweed. Corn plant height and leaf area, dry biomass of stem, leaf, root and shoot decreased significantly under high, compared to low UV-B radiation level (Tables 5.2 and 5.3). Lettuce leaf length, width and biomass, and pigweed plant height, leaf mass, and aboveground biomass also decreased with increasing UV-B radiation levels (Table 5.6 and 5.7). These effects of enhanced UV-B radiation, however, were not affected by the pre-treatment of red/far-red light ratio; there were no interactions between red/far-red pre-treatment and response to enhanced UV-B radiation levels. Interactions between red/far-red ratio and UV-B were shown for Arabidopsis plants when supplemental far-red light and UV-B radiation were applied simultaneously (Hayes et al. 2014). Hayes et al. (2014) found that UV-B perceived by UVR8 UV-B photoreceptor strongly inhibited shade avoidance response (e.g. petiole elongation and leaf elevation) by antagonizing the phytohormones auxin and gibberellin. Findings from my study of three plant species suggest that pre-exposure to low red/far-red ratio, which could be caused by the presence of neighbouring plants, changes in planting density, and shading in a plant canopy, may not affect the response of plants to enhanced UV-B radiation.   It should be pointed out that pigweed plants grown under low, compared to high, red/far-red ratio pre-treatment were taller, and the length and width of lettuce leaves were greater prior to the commencement of UV-B treatment. This effect was carried over to the end of UV-B treatment (Table 5.7). However, the interactions between the effect of red/far-red ratio pre-treatment and 100  UV-B radiation treatment were not statistically significant. Tegelberg et al. (2004) showed that the leaf petioles and stems of far-red light treated silver birch seedlings were longer than those of red light treated seedlings, but they were not modified by UV-B radiation even when these treatments were applied simultaneously.  This study showed that anthocyanin concentration significantly decreased in corn leaf sheaths, and lettuce and redroot pigweed laminae in plants grown under low, compared to high, red/far-red ratio (Fig 5.2 to 5.4). However, this decrease in anthocyanin concentration induced by low red/far-red ratio did not affect the responses of these plants to subsequent exposure to increasing UV-B radiation in my study. This lack of protection by anthocyanin against UV-B radiation could be because the anthocyanin concentration was either not high enough to offer protection or the magnitude of red/far-red ratio pre-treatment effect on this pigment was not maintained throughout the duration of UV-B exposure. In corn, low red/far-red ratio reduced anthocyanin concentration by 60% compared to high red/far-red ratio, prior to UV-B exposure (Fig. 5.3a). This reduction was only 24% by the end of the UV-B treatment (Fig. 5.3b). Continuous exposure to high (1.1) red/far-red ratio may be necessary to maintain anthocyanin level high enough to offer protection against UV-B exposure.   Plants may synthesize UV-B absorbing compounds in leaf epidermal cells when exposed to this radiation to protect leaves from UV-B damage (Pinto et al. 1999; Dai et al. 2004). In my study, UV-B treatment did not influence the level of methanol-extractable UV-B absorbing compounds in corn leaves (Table 5.5), which may make corn more susceptible to this radiation. A study on three bean cultivars (Arroz, Pinto, and Vilmorin) showed that UV-B absorbing compounds 101  increased significantly only in cultivars Pinto and Vilmorin, but not in Arroz, which is the most sensitive to UV-B radiation (Pinto et al. 1999). Smith et al. (2000) reported that four out of seven crop species showed an increase in UV-B absorbing compounds in response to an elevated level of UV-B radiation, one showed a decrease and two showed no change. Similarly, the activity of PAL, an important enzyme of phenolic biosynthesis (Liu and McClure 1995), was also not affected by enhanced UV-B radiation in my study (Table 5.5). These results suggest that these cellular effects were not involved in protecting plants from UV-B radiation damage in my study. UV-B radiation is known to increase the concentration of H2O2, which causes cellular damage (Dai et al. 1997; Gao and Zhang 2008). However, increases in H2O2 level in response to enhanced UV-B radiation were not observed in my study (Table 5.5). This could be due to the increase in antioxidant activities during the 10 days of UV-B exposure. Increases in antioxidants (SOD, CAT, ASA, and GSH) activities by enhanced UV-B radiation, especially during the early few days of exposure (i.e. at days 7 and 14), have been reported by Dai et al. (1997).   Conclusion Pre-exposure to different red/far-red light ratios and the associated decrease in anthocyanin pigments in the leaf sheaths of corn, and the laminae of lettuce and pigweed seedlings did not modify the effects of increasing UV-B radiation on the various growth, allometric and biochemical parameters measured in this study. This indicates that changes in red/far-red ratio, which can result from the presence of neighbouring plants, changes in planting density, shading, and the associated change in anthocyanin pigmentation, may not influence the response of corn, lettuce and pigweed seedlings to subsequent exposure to elevated UV-B radiation levels. Since fluctuation in the 102  red/far-red light ratio and/or UV-B radiation levels can play important role in affecting plant-plant interaction, observations that pre-exposure to low red/far-red ratio did not influence the susceptibility of plants to elevated UV-B radiation are significant to our understanding of the eco-physiology of the mixed-cropping system.  103  Chapter 6: General Conclusions  Light is an important factor in the regulation of plant growth and development. Plants in canopies are exposed to altered light quality (red/far-red ratio) due to the selective absorbance of red light (655-665 nm) and transmittance and reflectance of far-red light (725-735 nm) by green foliage. Light quality, particularly the red/far-red ratio, plays an important role in signaling the impending competition from neighbouring plants (Ballaré et al. 1987, 1990; Smith et al. 1990; Ballaré 1999) and triggers shade avoidance responses in plants. A change in red/far-red ratio can affect plant competitiveness in the weed-crop association if plants differ in their responses to changing red/far-red ratios. Red/far-red ratio in plant canopies is affected by leaf optical properties, which determine the amount and quality of light penetrating plant canopies. Knowledge of leaf optical properties of species in populations could help us understand eco-physiological underpinnings of plant-plant interactions within and among species. Plants in nature are exposed to reduced red/far-red ratio and elevated UV-B radiation either simultaneously or sequentially. Pre-exposure to low red/far-red light might affect the susceptibility of plants to UV-B radiation, which in turn can affect the competitive status of plants in mixed populations.   In this thesis, I investigated eco-physiology of effects of red/far-red ratio on a crop TM and two associated weeds, LQ and PW, in growth chamber and greenhouse experiments. More specifically, I characterized the growth and morphological responses of TM, LQ and PW to red/far-red ratio (Chapter 2), and investigated how leaf reflectance, transmittance and absorptance for red (660 nm) and far-red (730 nm) wavelengths change with leaf position and during plant development (Chapter 3 and 4). In addition, I determined if red/far-red ratio pre-treatment influences responses 104  of corn (Zea mays L.), lettuce (Lactuca sativa L.), and PW plants to UV-B radiation exposure (Chapter 5).    Results presented in Chapter 2 of this thesis shows that red/far-red treatments influence several primary growth (e.g. plant height, stem length below the first node, stem diameter, plant biomass, leaf greenness, leaf length and biomass, and total root length and surface area) as well as allometric (e.g. leaf/shoot weight, stem/shoot weight, and leaf/stem weight ratios) parameters in these species and the species differed in this regard. The species-specific differences in the response to red/far-red light ratio have significant ecological implications because they could alter inter- and/or intra-species interactions among these species. For example, the species with greater shade avoidance response in terms of stem elongation will produce taller plants when exposed to low red/far-red ratio and outcompete less responsive species. While this study provided useful information on differential responses of plant species to red/far-red ratio in growth chambers, field experiments with red/far-red ratio treatments given to species grown in mixtures, are needed.  Leaf optical properties play an important role in determining the red/far-red ratio in plant canopies. Results from Chapter 3 showed that leaf position on plant stem influence R, T and A at 660 nm and 730 nm in TM, LQ, and PW leaves. This could modify the light environment in canopies of plant communities comprising these species. The effect of leaf position on the stem on their optical properties could be a combined effect of leaf position on the stem and leaf age. Therefore, I monitored the optical properties of an individual leaf (the 3rd true leaf) during plant development (Chapter 4). The results showed that leaf optical properties change with time as plants develop. Interestingly, the species (TM, LQ, and PW) employed in this study differed in this regard. For 105  example, the effect on light quality (red/far-red light ratio) was the greater in PW followed compared with LQ and TM. The results of these two chapters improve our understanding of changes in leaf optical properties with leaf position and during plant development, and of species-specific differences in this regard. This information has important implications for plant-plant interactions in plant communities and could be useful for weed managers. Generally, growth rate, plant size, and LAI are considered most important parameters of competitive ability in mixed populations. These results suggest species-specific differences in leaf optical properties, as well as leaf position and age effects, should be taken into consideration in assessing eco-physiological functions (including germination of light-sensitive weed seeds on the soil surface) of a vegetation cover and in the selection of suitable species for cover crops. The variation in the red/far-red ratio of light reflected and transmitted through plant leaves correlated highly with chlorophyll content and SLW. However, these variations could not be explained fully by chlorophyll content and SLW; the role of other factors affecting leaf optical properties, therefore, cannot be ruled out. Factors like leaf chemistry, anatomy and surface characteristics (e.g. wax layer, epidermal cells, cuticular thickness and trichome) could be involved.  Plants developed under different red/far-red ratio might experience enhanced levels of UV-B radiation in their later stages of growth. Results of Chapter 5 showed that changes in red/far-red ratio, which can result from the presence of neighbouring plants, changes in planting density, shading, and the associated changes in anthocyanin pigmentation, may not influence the susceptibility of corn, lettuce and pigweed seedlings to subsequent exposure to elevated UV-B radiation levels. This finding is significant to our understanding of the eco-physiology of agro-ecosystems, where both red/far-red light ratio and UV-B radiation levels fluctuate. 106   Experiments in this research were conducted under growth chamber and greenhouse conditions. While this research provides some useful information enhancing our understanding of weed ecophysiology, it opens new avenues for future studies. Studies on the effect of red/far-red light ratio treatment on plant-plant competitions both within and between plant species under field conditions can provide valuable information for crop and weed management in real life situations. However, this was beyond the scope of this study. Such studies could be interesting if resources to provide precise red/far-red light ratio treatment under field conditions were available. Investigation on influences of leaf position and plant development on leaf chemistry, anatomy and surface characteristics (e.g. wax layer, epidermal cells, cuticular thickness and trichome) in crop and associated weeds, and how these influences affect leaf optical properties could be productive avenues for future studies. Findings of these studies could improve our understanding of the underpinning of changes in leaf optical properties and hence the light environment in plant canopies. Moreover, studies on the interactions between altered red/far-red ratios and other environmental factors, e.g. water stress, light stress, herbivory, extreme temperatures, salinity can improve our understanding of ecology in agro-ecosystems, where plants are exposed to various biotic and abiotic stresses that can alter interactions among weed and crop species.  107  References  Adams ML, Norvell WA, Peverly JH, Philpot WD. 1993. Fluorescence and reflectance characteristics of manganese deficient soybean leaves: effect of leaf age and choice of leaflet. Plant and Soil 155/156: 235-238. Adamse P, Britz SJ, Caldwell CR. 1994. Amelioration of UV-B damage under high irradiance II: role of blue light photoreceptors. Photochemistry and Photobiology 60: 110-115. Afifi M, Swanton CJ. 2012. Early physiological mechanisms of weed competition. Weed            Science 60: 542-551. Ainsworth EA, Serbin SP, Skoneczka JA, Townsend PA. 2014. Using leaf optical properties to detect ozone effects on foliar biochemistry. Photosynthesis Research 119: 65-76. Alam SM, Azmi AR, Naqvi SSM, Khan MA, Khanzada B. 1997. Effect of aqueous leaf extract of common lambsquarters (Chenopodium album L.) and NaCI on germination and seedling growth of rice. Acta Physiologiae Plantarum 19: 91-94. Alam SM, Khan MA, Mujtaba SM, Shereen A. 2002. Influence of aqueous leaf extract of common lambsquarters and NaCl salinity on the germination, growth, and nutrient content of wheat. Acta Physiologiae Plantarum 24: 359-364. Alberte RS, Mcclure PR, Thornber JP. 1976. Photosynthesis in trees: organization of chlorophyll and photosynthetic unit size in isolated gymnosperm chloroplasts. Plant Physiology 59: 351-353. Alexieva V, Sergiev I, Mapelli S, Karanov E. 2001. The effect of drought and ultraviolet radiation on growth and stress markers in pea and wheat. Plant, Cell and Environment 24: 1337-1344. 108  Alokam S, Chinnappa CC, Reid DM. 2002. Red/far-red light mediated stem elongation and anthocyanin accumulation in Stellaria longipes: differential response of alpine and prairie ecotypes. Canadian Journal of Botany 80: 72-81. Ammer C. 2003. Growth and biomass partitioning of Fagus sylvatica L. and Quercus robur L. seedlings in response to shading and small changes in the R/FR-ratio of radiation. Annals of Forest Science 60: 163-171. Apple M, Tiekotter K, Snow M, et al. 2002. Needle anatomy changes with increasing tree age in Douglas-fir. Tree Physiology 22: 129-136. Asner GP, Martin RE. 2008. Spectral and chemical analysis of tropical forests: scaling from leaf to canopy levels. Remote Sensing of Environment 112: 3958-3970. Baez-Gonzalez AD, Kiniry JR, Maas SJ, et al. 2005. Large-area maize yield forecasting using leaf area index based yield model. Agronomy Journal 97: 418-425. Baldini E, Facini O, Nerozzi F, Rossi F, Rotondi A. 1997. Leaf characteristics and optical properties of different woody species. Trees 12: 73-81. Ballaré CL. 1999. Keeping up with the neighbours: phytochrome sensing and other signalling mechanisms. Trends in Plant Science 4: 97-102. Ballaré CL. 2009. Illuminated behaviour: phytochrome as a key regulator of light foraging and plant anti-herbivore defence. Plant, Cell and Environment 32: 713-725. Ballaré CL, Casal JJ. 2000. Light signals perceived by crop and weed plants. Field Crops Research 67: 149-160. Ballaré CL, Sánchez RA, Scopel AL, Casal JJ, Ghersa CM. 1987. Early detection of neighbour plants by phytochrome perception of spectral changes in reflected sunlight. Plant, Cell and Environment 10: 551-557. 109  Ballaré CL, Scopel AL. 1997. Phytochrome signaling in plant canopies: testing its population-level implications with photoreceptor mutants of Arabidopsis. Ecology 11: 441-450. Ballaré CL, Scopel AL, Sánchez RA. 1990. Far-red radiation reflected from adjacent leaves: an early signal of competition in plant canopies. Science 247: 329-331. Barnes PW, Ballaré CL, Caldwell MM. 1996. Photomorphogenic effects of UV-B radiation on plants: consequences for light competition. Journal of Plant Physiology 148: 15-20. Barnes PW, Jordan PW, Gold WG, Flint SD, Caldwell MM. 1988. Competition, morphology and canopy structure in wheat (Triticum aestivum L.) and wild oat (Avena fatua L.) exposed to enhanced ultraviolet-B radiation. Functional Ecology 2: 319-330. Barreiro R, Guiamét JJ, Beltrano J, Montaldi ER. 1992. Regulation of the photosynthetic capacity of primary bean leaves by the red: far-red ratio and photosynthetic photon flux density of incident light. Physiologia Plantarum 85: 97-101. Barrett M, Peters RA. 1976. Germination of corn, lambsquarters and fall panicum under simulated drought. Proceedings of the Northeastern Weed Science Society 30: 98-103. Bartoli CG, Tambussi EA, Diego F, Foyer CH. 2009. Control of ascorbic acid synthesis and accumulation and glutathione by the incident light red/far red ratio in Phaseolus vulgaris leaves. Federation of European Biochemical Societies Letters 583: 118-122. Bassett IJ, Crompton CW. 1978. The biology of Canadian weeds: 32 Chenopodium album L. Canadian Journal of Plant Science 58: 1061-1072. Batlla D, Kruk BC, Benech-Arnold RL. 2000. Very early detection of canopy presence by seeds through perception of subtle modifications in red: far red signals. Functional Ecology 14: 195-202. 110  Bensch CN, Horak MJ, Peterson D. 2003. Interference of redroot pigweed (Amaranthus retroflexus), Palmer amaranth (A. palmeri), and common waterhemp (A. rudis) in soybean. Weed Science 51: 37-43. Bhowmik PC, Doll JD. 1984. Allelopathic effects of annual weed residues on growth and nutrient uptake of corn and soybeans. Agronomy Journal 76: 383-388. Biewer S, Fricke T, Wachendorf M. 2009. Development of canopy reflectance models to predict forage quality of legume-grass mixtures. Crop Science 49: 1917-1926. Bitterlich I, Upadhyaya MK 1990. Leaf surface ultrastructure and susceptibility to ammonium nitrate injury. Canadian Journal of Botany 68: 1911-1915. Bolink EM, Van Schalkwijk I, Posthumus F, Van Hasselt PR. 2001. Growth under UV-B radiation increases tolerance to high-light stress in pea and bean plants. Plant Ecology 154: 147-156. Bond BJ. 2000. Age-related changes in photosynthesis of woody plants. Trends in Plant Science 5: 349-353. Bowers MD, Stamp NE. 1993. Effects of plant age, genotype and herbivory on Plantago performance and chemistry. Ecology 74: 1778-1791. Brainard DC, Bellinder RR, DiTommaso A. 2005. Effects of canopy shade on the morphology, phenology, and seed characteristics of powell amaranth (Amaranthus powellii). Weed Science 53: 175-186. Bréda NJJ. 2003. Ground-based measurements of leaf area index: A review of methods, instruments and current controversies. Journal of Experimental Botany 54: 2403-2417. 111  Brown CS, Schuerger AC, Sager JC. 1995. Growth and photomorphogenesis of pepper plants under red light-emitting diodes with supplemental blue or far-red lighting. Journal of the American Society for Horticultural Science 120: 808-813. Burki HM, Schroeder D, Lawrie J, et al. 1997. Biological control of pigweeds (Amaranthus retroflexus L., A. Powellii S. Watson and A. bouchonii Thell.) with phytophagous insects, fungal pathogens and crop management. Integrated Pest Management Reviews 2: 51-59. Campbell GS, Norman JM. 1998. An introduction to environmental biophysics, 2nd edn. New York: Springer. Carter GA. 1991. Primary and secondary effects of water content on the spectral reflectance of leaves. American Journal of Botany 78: 916-924. Carter GA, Knapp AK. 2001. Leaf optical properties in higher plants: linking spectral characteristics to stress and chlorophyll concentration. American Journal of Botany 88: 677-684. Carter GA, Rebbeck J, Percy KE. 1995. Leaf optical properties in Liriodendron tulipifera and Pinusstrobus as influenced by increased atmospheric ozone and carbon dioxide. Canadian Journal of Forest Research 25: 407-412.  Carver GW. 2007. Introduction. In: Jones Jr JB. ed. Tomato plant culture: in the field, greenhouse, and home garden, 2nd edn. Boca Raton: CRC Press, pp 1-54. Casal JJ, Smith H. 1989. The function, action and adaptive significance of phytochrome in light-grown plants. Plant, Cell and Environment 12: 855-862. 112  Castro-Esau KL, Sánchez-Azofeifa GA, Rivard B, Wright SJ, Quesada M. 2006. Variability in leaf optical properties of Mesoamerican trees and the potential for species classification. American Journal of Botany 93: 517-530. Causin HF. 2004. Responses to shading in Chenopodium album: the effect of the maternal environment and the N source supplied. Canadian Journal of Botany 82: 1371-1381. Causin HF, Wulff RD. 2003. Changes in the responses to light quality during ontogeny in Chenopodium album. Canadian Journal of Botany 81: 152-163. Cen YP, Bornman JF. 1990. The response of bean plants to UV-B radiation under different irradiances of background visible light. Journal of Experimental Botany 41: 1489-1495. Chachalis D, Reddy KN, Elmore CD. 2001. Characterization of leaf surface, wax composition, and control of redvine and trumpetcreeper with glyphosate. Weed Science 49: 156-163. Cheng YB, Ustin SL, Riano D, Vanderbilt VC. 2008. Water content estimation from hyperspectral images and MODIS indexes in Southeastern Arizona. Remote Sensing of Environment 112: 363-374.  Chow WS, Anderson JM, Melis A. 1990. The photosystem stoichiometry in thylakoids of some Australian shade-adapted plant species. Australian Journal of Plant Physiology 17: 665-674. Chu C, Ludford PM, Ozbun JL, Sweet RD. 1978. Effects of temperature and competition on the establishment and growth of redroot pigweed and common lambsquarters. Crop Science 18: 308-310.  113  Clack T, Mathews S, Sharrock RA. 1994. The phytochrome apoprotein family in Arabidopsis is encoded by five genes: the sequences and expression of PHYD and PHYE. Plant Molecular Biology 25: 413-427. Close DC, Beadle CL. 2003. The ecophysiology of foliar anthocyanin. The Botanical Review 69: 149-161. Conley SP, Stoltenberg DE, Boerboom CM, Binning LK. 2003. Predicting soybean yield loss in giant foxtail (Setaria faberi) and common lambsquarters (Chenopodium album) communities. Weed Science 51: 402-407. Costa H, Gallego SM, Tomaro ML. 2002. Effect of UV-B radiation on antioxidant defense system in sunflower cotyledons. Plant Science 162: 939-945. Costea M, Weaver SE, Tardif FJ. 2004. The biology of Canadian weeds. 130. Amaranthus retroflexus L., A. powellii S. Watson and A. hybridus L. Canadian Journal of Plant Science 84: 631-668. Crook TM, Renner KA. 1990. Common lambsquarters (Chenopodium album) competition and time of removal in soybeans (Glycine max). Weed Science 38: 358-364. Cuadra P, Herrera R, Fajardo V. 2004. Effects of UV-B radiation on the Patagonian Jaborosa magellanica brisben. Journal of Photochemistry and Photobiology B: Biology 76: 61–68. Dai Q, Furness NH, Upadhyaya MK. 2004. UV-absorbing compounds and susceptibility of weedy species to UV-B radiation. Weed Biology and Management 4: 95-102. Dai Q, Yan B, Huang S, et al. 1997. Response of oxidative stress defense systems in rice (Oryza sativa) leaves with supplemental UV-B radiation. Physiologia Plantarum 101: 301-308. 114  Davis PA, Caylor S, Whippo CW, Hangarter RP. 2011. Changes in leaf optical properties associated with light-dependent chloroplast movements. Plant, Cell and Environment 34: 2047-2059. Day ME, Greenwood MS, White AS. 2001. Age-related changes in foliar morphology and physiology in red spruce and their influence on declining photosynthetic rates and productivity with tree age. Tree Physiology 21: 1195-1204. Deckmyn G, Impens I. 1997. The ratio UV-B/photosynthetically active radiation (PAR) determines the sensitivity of rye to increased UV-B radiation. Environmental and Experimental Botany 37: 3-12. England JR, Attiwill PM. 2006. Changes in leaf morphology and anatomy with tree age and height in the broadleaved evergreen species, Eucalyptus regnans F. Muell. Trees 20: 79-90. Escobar-Bravo R, Klinkhamer PGL, Leiss KA. 2017. Interactive effects of UV-B light with abiotic factors on plant growth and chemistry, and their consequences for defense against arthropod herbivores. Frontiers in Plant Science 8: 278. Esringu A, Aksakal O, Tabay D, Kara AA. 2016. Effects of sodium nitroprusside (SNP) pretreatment on UV-B stress tolerance in lettuce (Lactuca sativa L.) seedlings. Environmental Science and Pollution Research 23:589-597.  Eyring V, Cionni I, Lamarque JF, et al. 2010. Sensitivity of 21st century stratospheric ozone to greenhouse gas scenarios. Geophysical Research Letters 37: L16807. FAOSTAT. 2013. Rankings of food and agricultural commodities production/commodities by regions. http://faostat3.fao.org/browse/rankings/commodities_by_regions/E. Retrieved 8 August 2016. 115  FAOSTAT. 2016. Crops. http://www.fao.org/faostat/en/#data/QC/visualize. Retrieved 5 January 2017. Forcella F, Wilson RG, Dekker J, et al. 1997. Weed seedbank emergence across the corn belt. Weed Science 45: 67-76. Frak E, Le Roux XL, Millard P, et al. 2002. Spatial distribution of leaf nitrogen and photosynthetic capacity within the foliage of individual trees: disentangling the effects of local light quality, leaf irradiance, and transpiration. Journal of Experimental Botany 53: 2207-2216. Franklin KA. 2008. Shade avoidance. New Phytologist 179: 930-944. Franklin KA, Whitelam GC. 2007. Red: far-red ratio perception and shade avoidance. In Whitelam GC and Halliday KJ eds. Annual Plant Reviews Volume 30: Light and plant development. Blackwell Publishing Ltd., Oxford, U.K. pp. 211-234. Friesen GH. 1979. Weed interference in transplanted tomatoes (Lycopersicon esculentum). Weed Science 27: 11-13. Furness NH, Jolliffe PA, Upadhyaya MK. 2005a. Competitive interactions in mixtures of broccoli and Chenopodium album grown at two UV-B radiation levels under glasshouse conditions. Weed Research 45: 449-459. Furness NH, Jolliffe PA, Upadhyaya MK. 2005b. Ultraviolet-B radiation and plant competition: experimental approaches and underlying mechanisms. Photochemistry and Photobiology 81: 1026-1037. Gao Q, Zhang L. 2008. Ultraviolet-B-induced oxidative stress and antioxidant defense system responses in ascorbate-deficient vtc1 mutants of Arabidopsis thaliana. Journal of Plant Physiology 165: 138-148. 116  Gausman HW, Allen WA. 1973. Optical parameters of leaves of 30 plant species. Plant Physiology 52: 57-62. Gausman HW, Allen WA, Myers VI, Cardenas R. 1969. Reflectance and internal structure of cotton leaves, Gossypium hirsutum L. Agronomy Journal 61: 374-376. Girardin P, Tollenaar M. 1994. Effects of intraspecific interference on maize leaf azimuth. Crop Science 34: 151-155. Gitelson AA, Gritz Y, Merzlyak MN. 2003. Relationships between leaf chlorophyll content and spectral reflectance and algorithms for non-destructive chlorophyll assessment in higher plant leaves. Journal of Plant Physiology 160: 271-282. Gitelson AA, Merzlyak MN, Chivkunova OB. 2001. Optical properties and non-destructive estimation of anthocyanin content in plant leaves. Photochemistry and Photobiology 74: 38-45. Gitelson AA, Zur Y, Chivkunova OB, Gillon D, Houssard C, Joffre R. 1999. Using near-infrared reflectance spectroscopy to predict carbon, nitrogen and phosphorus content in heterogeneous plant material. Oecologia 118: 173-182. Gitelson AA, Zur Y, Chivkunova OB, Merzlyak MN. 2002. Assessing carotenoid content in plant leaves with reflectance spectroscopy. Photochemistry and Photobiology. 75: 272. Goins GD, Yorio NC, Sanwo MM, Brown CS. 1997. Photomorphogenesis, photosynthesis, and seed yield of wheat plants grown under red light-emitting diodes (LEDs) with and without supplemental blue lighting. Journal of Experimental Botany 48: 1407-1413. Gomide JA, Noller CH, Mott GO, Conrad JH, Hill DL. 1969a. Effect of plant age and nitrogen fertilization on the chemical composition and in vitro cellulose digestibility of tropical grasses. Agronomy Journal 61: 116-120. 117  Gomide JA, Noller CH, Mott GO, Conrad JH, Hill DL. 1969b. Mineral composition of six tropical grasses as influenced by plant age and nitrogen fertilization 1 2. Agronomy Journal 61: 120-123. Gorski T, Gorska K, Stasiak H. 2013. Inhibition of seed germination by far-red radiation transmitted through leaf canopies. Polish Journal of Agronomy 13: 10-38. Greenberg BM, Wilson MI, Huang XD, et al. 1997. The effects of ultraviolet-B radiation on higher plants. In: Plants for Environmental Studies eds. Wang WJ, Gorsuch J, Hughes JS. CRC Press, Boca Raton, Florida, pp. 1-35. Green-Tracewicz E, Page ER, Swanton CJ. 2011. Shade avoidance in soybean reduces branching and increases plant-to-plant variability in biomass and yield per plant. Weed Science 59: 43-49.  Green-Tracewicz E, Page ER, Swanton CJ. 2012. Light quality and the critical period for weed control in soybean. Weed Science 60: 86-91. Grzesiak MT, Hura T, Grzesiak S, Pilarski J. 2009. Effect of drought stress on leaf optical properties in drought-resistant and drought-sensitive maize and triticale genotypes. Photosynthetica 47: 635-637. Gundel PE, Pierik R, Mommer L, Ballaré CL. 2014. Competing neighbors: light perception and root function. Oecologia 176: 1-10. Gwynn-Jones D, Jones AG, Waterhouse A, et al. 2012. Enhanced UV-B and elevated CO2 impacts sub-arctic shrub berry abundance, quality and seed germination. Ambio 41: 256-268. Harrison SK. 1990. Interference and seed production by common lambsquarters (Chenopodium album) in soybeans (Glycine max). Weed Science 38: 113-118. 118  Hayes S, Velanis CN, Jenkins GI, Franklin KA. 2014. UV-B detected by the UVR8 photoreceptor antagonizes auxin signaling and plant shade avoidance. Proceedings of the National Academy of Sciences of the United States of America 111: 11894-11899. Hegglin MI, Shepherd TG. 2009. Large climate-induced changes in ultraviolet index and stratosphere-to-troposphere ozone flux. Nature Geoscience 2: 687-691. Hlavinka J, Nauš J, Špundová M. 2013. Anthocyanin contribution to chlorophyll meter readings and its correction. Photosynthesis Research 118: 277-295. Hoad SP, Leakey RRB. 1994. Effects of light quality on gas exchange and dry matter partitioning in Eucalyptus grandis W. Hill ex Maiden. Forest Ecology and Management 70: 265-273. Hobson G, Grierson D. 1993. Tomato. In: Seymour G, Taylor J, Tucker G, ed. Biochemistry of fruit ripening. 1st ed. Dordrecht: Springer Science+Business Media, pp. 405-434. Hoffmann AM, Noga G, Hunsche M. 2015. High blue light improves acclimation and photosynthetic recovery of pepper plants exposed to UV stress. Environmental and Experimental Botany 109: 254-263. Holloway PJ. 1970. Surface factors affecting the wetting of leaves. Pesticide Management Science 1: 156-163. Holm LG, Plucknett DL, Pancho JV, Herberger JP. 1977. The world's worst weeds. University Press of Hawaii, Honolulu, Hawaii. pp. 84-91. Itulya FM, Mwaja VN, Masiunas JB. 1997. Collard-cowpea intercrop response to nitrogen fertilization, redroot pigweed density, and collard harvest frequency. HortScience 32: 850-853. 119  Jankowska-Blaszczuk M, Daws MI. 2007. Impact of red: far red ratios on germination of temperate forest herbs in relation to shade tolerance, seed mass and persistence in the soil. Functional Ecology 21: 1055-1062. Jetter R, Schäffer S. 2001. Chemical composition of the Prunus laurocerasus leaf surface. Dynamic changes of the epicuticular wax film during leaf development. Plant Physiology 126: 1725-1737. Jia X, Shen J, Liu H, et al. 2015. Small tandem target mimic-mediated blockage of microRNA858 induces anthocyanin accumulation in tomato. Planta 242: 283-293. Jifon JL, Syvertsen JP, Whaley E. 2005. Growth environment and leaf anatomy affect nondestructive estimates of chlorophyll and nitrogen in Citrus sp. leaves. Journal of the American Society for Horticultural Science 130: 152-158. Jursík M, Soukup J, Venclová V, Holec J. 2003. Seed dormancy and germination of Shaggy soldier (Galinsoga ciliata Blake.) and Common lambsquarter (Chenopodium album L.). Plant, Soil and Environment 49: 511-518. Karssen CM. 1970. The light promoted germination of the seeds of Chenopodium album L. III. effect of the photoperiod during growth and development of the plants on the dormancy of the produced seeds. Acta Botanica Neerlandica 19: 81-94. Kasperbauer MJ. 1999. Cotton seedling root growth responses to light reflected to the shoots from straw-covered versus bare soil. Crop Science 39:164-167. Kasperbauer MJ, Karlen DL. 1994. Plant spacing and reflected far-red light effects on phytochrome-regulated photosynthate allocation in corn seedlings. Crop Science 34:1564-1569. 120  Kegge W, Ninkovic V, Glinwood R, Welschen RAM, Voesenek, LACJ, and Pierik R. 2015. Red: far-red light conditions affect the emission of volatile organic compounds from barley (Hordeum vulgare), leading to altered biomass allocation in neighbouring plants. Annals of Botany 115: 961-970.  Klančnik K, Pančić M, Gaberščik A. 2014a. Leaf optical properties in amphibious plant species are affected by multiple leaf traits. Hydrobiologia 737: 121-130. Klančnik K, Vogel-Mikuš K, Gaberščik A. 2014b. Silicified structures affect leaf optical properties in grasses and sedge. Journal of Photochemistry and Photobiology B: Biology 130: 1-10. Klančnik K, Vogel-Mikuš K, Kelemen M, et al. 2014c. Leaf optical properties are affected by the location and type of deposited biominerals. Journal of Photochemistry and Photobiology B: Biology 140: 276-285. Klem K, Ač A, Holub P, Kováč D, Špunda V, Robson TM, Urban O. 2012. Interactive effects of PAR and UV radiation on the physiology, morphology and leaf optical properties of two barley varieties. Environmental and Experimental Botany 75: 52- 64. Knapp AK, Carter GA. 1998. Variability in leaf optical properties among 26 species from a broad range of habitats. American Journal of Botany 85: 940-946. Knezevic SZ, Horak MJ, Vanderlip RL. 1997. Relative time of redroot pigweed (Amaranthus retroflexus L.) emergence is critical in pigweed-sorghum [Sorghum bicolor (L.) Moench] competition. Weed Science 45: 502-508. Knezevic SZ, Weise SF, Swanton CJ. Interference of redroot pigweed (Amaranthus retroflexus) in corn (Zea mays). Weed Science 42: 568-573. 121  Koti S, Reddy KR, Kakani VG, et al.  2007. Effects of carbon dioxide, temperature and ultraviolet-B radiation and their interactions on soybean (Glycine max L.) growth and development. Environmental and Experimental Botany 60: 1-10. Kruger GR, Johnson WG, Weller SC, et al. 2009. U.S. grower views on problematic weeds and changes in weed pressure in glyphosate-resistant corn, cotton, and soybean cropping systems. Weed Technology 23: 162-166. Kwesiga FR, Grace J, Sandford AP. 1986. Some photosynthetic characteristics of tropical timber trees as affected by the light regime during growth. Annals of Botany 58: 23-32. Lavola A, Julkunen-Tiitto R, de la Rosa TM et al. 2000. Allocation of carbon to growth and secondary metabolites in birch seedlings under UV-B radiation and CO₂ exposure. Physiologia Plantarum 109: 260-267. Légère A, Schreiber MM. 1989. Competition and canopy architecture as affected by soybean (Glycine max) row width and density of redroot pigweed (Amaranthus retroflexus). Weed Science 37: 84-92. Li Q, Kubota C. 2009. Effects of supplemental light quality on growth and phytochemicals of baby leaf lettuce. Environmental and Experimental Botany 67: 59-64. Lichtenthaler HK, Buschmann C, Döll M, et al. 1981. Photosynthetic activity, chloroplast ultrastructure, and leaf characteristics of high-light and low-light plants and of sun and shade leaves. Photosynthesis Research 2: 115-141. Lim T, Loh W. 1996. A comparison of tests of equality of variances. Computational Statistics and Data Analysis 22: 287-301. Lin ZF, Ehleringer J. 1983. Epidermis effects on spectral properties of leaves of four herbaceous species. Physiologia Plantarum 59: 91-94. 122  Liu JG, Mahoney KJ, Sikkema PH, Swanton CJ. 2009. The importance of light quality in crop-weed competition. Weed Research 49: 217-224. Liu L, McClure JW. 1995. Effects of UV-B on activities of enzymes of secondary phenolic metabolism in barley primary leaves. Physiologia Plantarum 93: 734-739. Lobato AKS, Gonçalves-vidigal MC, Vidigal Filho PS, Andrade CAB, Kvitschal MV, Bonato CM. 2010. Relationships between leaf pigments and photosynthesis in common bean plants infected by anthracnose. New Zealand Journal of Crop and Horticultural Science 38: 29-37. Mackerness SAH. 2000. Plant responses to ultraviolet-B (UV-B: 280-320 nm) stress: what are the key regulators? Plant Growth Regulation 32: 27-39. Mackerness SAH, Surplus SL, Jordan BR, Thomas B. 1998. Effects of supplementary ultraviolet-B radiation on photosynthetic transcripts at different stages of leaf development and light levels in pea (Pisum sativum L.): role of active oxygen species and antioxidant enzymes. Photochemistry and Photobiology 68: 88-96. Maddonni GA, Otegui ME, Andrieu B, et al. 2002. Maize leaves turn away from neighbors. Plant Physiology 130: 1181-1189. Mahoney KJ, Swanton CJ. 2008. Exploring Chenopodium album adaptive traits in response to light and temperature stresses. Weed Research 48: 552-560. Mancinelli AL. 1990. Interaction between light quality and light quantity in the photoregulation of anthocyanin production. Plant Physiology 92: 1191-1195. Markham MY, Stoltenberg DE. 2010. Corn morphology, mass, and grain yield as affected by early-season red: far-red light environments. Crop Science 50: 273-280. 123  Masoni A, Ercoli L, Mariotti M. 1996. Spectral properties of leaves deficient in iron, sulfur, magnesium, and manganese. Agronomy Journal 88: 937-943. Mathews S, Sharrock RA. 1996. The phytochrome gene family in grasses (Poaceae): a phylogeny and evidence that grasses have a subset of the loci found in dicot angiosperms. Molecular Biology and Evolution 13: 1141-1150. Mathews S, Sharrock RA. 1997. Phytochrome gene diversity. Plant, Cell and Environment 20: 666-671. McGiffen Jr. ME, Masiunas JB, Hesketh JD. 1992. Competition for light between tomatoes and nightshades (Solanum nigrum or S. ptycanthum). Weed Science 40: 220-226. McKenzie RL, Aucamp PJ, Bais AF, Björn LO, Ilyas M. 2007. Changes in biologically active ultraviolet radiation reaching the Earth's surface. Photochemical and Photobiological Sciences 6: 218-231. McLachlan SM, Tollenaar M, Swanton CJ, Weise SF. 1993. Effect of corn-induced shading on dry matter accumulation, distribution, and architecture of redroot pigweed (Amaranthus retroflexus). Weed Science 41: 568-573. Mershon JP, Becker M, Bickford CP. 2015. Linkage between trichome morphology and leaf optical properties in New Zealand alpine Pachycladon (Brassicaceae). New Zealand Journal of Botany 53: 175-182. Merzlyak MN. 2002. Assessing carotenoid content in plant leaves with reflectance spectroscopy. Photochemistry and Photobiology 75: 272-281. Middleton L. 2001. Shade-tolerant flowering plants: adaptations and horticultural implications. Acta Horticulturae 552: 95-102. 124  Mirshekari B, Javanshir A, Arbat HK. 2010. Interference of redroot pigweed (Amaranthus retroflexus) in green bean (Phaseolus vulgaris). Weed Biology and Management 10: 120-125. Moechnig MJ, Stoltenberg DE, Boerboom CM, Binning LK. 2003. Empirical corn yield loss estimation from common lambsquarters (Chenopodium album) and giant foxtail (Setaria faberi) in mixed communities. Weed Science 51: 386-393.  Moorthy I, Miller JR, Noland TL. 2008. Estimating chlorophyll concentration in conifer needles with hyperspectral data: an assessment at the needle and canopy level. Remote Sensing of Environment 112: 2824-2838. Morgan DC, Smith H. 1981. Control of development in Chenopodium album L by shadelight – the effect of light quantity (total fluence rate) and light quality (red-far-red ratio). New Phytologist 88: 239-248. Myneni RB, Ross J, Asrar G. 1989. A review on the theory of photon transport in leaf canopies.  Agricultural and Forest Meteorology 45: 1-153. Nauš J, Prokopová J, Rebíček J, Spundová M. 2010. SPAD chlorophyll meter reading can be pronouncedly affected by chloroplast movement. Photosynthesis Research 105: 265-271.  Neill S, Gould KS. 2000. Optical properties of leaves in relation to anthocyanin concentration and distribution. Canadian Journal of Botany 77: 1777-1782. Neugart S, Fiol M, Schreiner M, et al. 2014. Interaction of moderate UV-B exposure and temperature on the formation of structurally different flavonol glycosides and hydroxycinnamic acid derivatives in kale (Brassica oleracea var. sabellica). Journal of Agricultural and Food Chemistry 62: 4054-4062. 125  Nolan RG, Upadhyaya MK. 1988. Primary seed dormancy in diffuse and spotted knapweed. Canadian Journal of Plant Science 68: 775-783. Norris RF, Elmore CL, Rejmánek M, and Akey WC. 2001. Spatial arrangement, density, and competition between barnyardgrass and tomato: II. Barnyardgrass growth and seed production. Weed Science 49: 69-76. OMAFRA. 2016. Ontario weeds: Redroot pigweed. Ontario Ministry of Agriculture, Food and Rural Affairs, Toronto, ON. Publication 505. http://www.omafra.gov.on.ca/english/crops/facts/ontweeds/redroot_pigweed.htm. Accessed April 7, 2017. Page ER, Cerrudo D, Westra P, et al. 2012. Why early season weed control is important in maize. Weed Science 60: 423-430. Page ER, Tollenaar M, Lee EA, Lukens L, Swanton CJ. 2009. Does the shade avoidance response contribute to the critical period for weed control in maize (Zea mays)? Weed Research 49: 563-571.  Page ER, Tollenaar M, Lee EA, Lukens L, Swanton CJ. 2010. Shade avoidance: an integral component of crop-weed competition. Weed Research 50: 281-288.  Pierik R, Visser EJW, De Kroon H, Voesenek LACJ. 2003. Ethylene is required in tobacco to successfully complete with proximate neighbours. Plant, Cell and Environment 26: 1229-1234. Pinto ME, Casati P, Hsu TP, Ku MS, Edwards GE. 1999. Effects of UV-B radiation on growth, photosynthesis, UV-B-absorbing compounds and NADP-malic enzyme in bean (Phaseolus vulgaris L.) grown under different nitrogen conditions. Journal of Photochemistry and Photobiology. B: Biology 48: 200-209. 126  Porra RJ, Thompson WA, Kriedemann PE. 1989. Determination of accurate extinction coefficients and simultaneous equations for assaying chlorophylls a and b extracted with four different solvents: verification of the concentration of chlorophyll standards by atomic absorption spectroscopy. Biochimica et Biophysica Acta 975: 384-394. Prior RL, Cao G, Martin A, et al. 1998. Antioxidant capacity as influenced by total phenolic and anthocyanin content, maturity, and variety of Vaccinium species. Journal of Agricultural and Food Chemistry 46: 2686-2693. Qasem JR and Hill TA. 1989. Possible role of allelopathy in the competition between tomato, Senecio vulgaris L. and Chenopodium album L. Weed Research 29: 349-356. Rajcan I, Chandler KJ, Swanton CJ. 2004. Red-far-red ratio of reflected light: a hypothesis of why early-season weed control is important in corn. Weed Science 52: 774-778. Rajcan I, Swanton CJ. 2001. Understanding maize-weed competition: resource competition, light quality and the whole plant. Field Crops Research 71: 139-150. Richardson AD, Berlyn GP, Ashton PM, Thadani R, Cameron IR. 2000. Foliar plasticity of hybrid spruce in relation to crown position and stand age. Canadian Journal of Botany 78: 305-317. Robinson DE, Soltani N, Hamill AS, Sikkema PH. 2006. Weed control in processing tomato (Lycopersicon esculentum) with rimsulfuron and thifensulfuron applied alone or with chlorothalonil or copper pesticides. HortScience 41: 1295-1297. Robson TM, Hartikainen SM, Aphalo PJ. 2015.  How does solar ultraviolet-B radiation improve drought tolerance of silver birch (Betula pendula Roth.) seedlings? Plant, Cell and Environment 38: 953-967. Robson TM, Klem K, Urban O, Jansen MAK. 2015. Re-interpreting plant morphological 127  responses to UV-B radiation. Plant, Cell and Environment 38: 856-866. Rowland FS. 1990. Stratospheric ozone depletion by chlorofluorocarbons. Ambio 19: 281-292. Rudnóy S, Majláth I, Pál M, et al. 2015. Interactions of S-methylmethionine and UV-B can modify the defence mechanisms induced in maize. Acta Physiologiae Plantarum 37: 148-159. Sanyal D, Bhowmik PC, Reddy KN. 2006. Leaf characteristics and surfactants affect primisulfuron droplet spread in three broadleaf weeds. Weed Science 54: 16-22. Sarabi V, Mahallati MN, Nezami A, Mohassel MHR. 2011. Effects of the relative time of emergence and the density of common lambsquarters (Chenopodium album) on corn (Zea mays) yield. Weed Biology and Management 11: 127-136. Schäfer E, Nagy F. 2006. Physiological basis of photomorphogenesis. In: Schäfer E, Nagy F, eds. Photomorphogenesis in plants and bacteria, 3rd edn. Dordrecht, the Netherlands: Springer, pp. 1-23. Schmitt J, McCormac AC, Smith H. 1995. A test of the adaptive plasticity hypothesis using transgenic and mutant plants disabled in phytochrome-mediated elongation responses to neighbors. The American Naturalist 146: 937-953. Schuerger AC, Brown CS, Stryjewski EC. 1997. Anatomical features of pepper plants (Capsicum annuum L.) grown under red light-emitting diodes supplemented with blue or far-red light. Annals of Botany 79: 273-282. Sellers BA, Smeda RJ, Johnson WG, Kendig JA, Ellersieck MR. 2003. Comparative growth of six Amaranthus species in Missouri. Weed Science 51: 329-333. Serbin SP, Dillaway DN, Kruger EL, Townsend PA. 2012. Leaf optical properties reflect variation in photosynthetic metabolism and its sensitivity to temperature. Journal of Experimental Botany 63: 489-502.  128  Sharrock RA, Quail PH. 1989. Novel phytochrome sequences in Arabidopsis thaliana: structure, evolution, and differential expression of a plant regulatory photoreceptor family. Genes and Development 3: 1745-1757. Shen X, Dong Z, Chen Y. 2015. Drought and UV-B radiation effect on photosynthesis and antioxidant parameters in soybean and maize. Acta Physiologiae Plantarum 37: 25.  Shinomura T, Nagatani A, Hanzawa H, Kubota M, Watanabe M, Furuya M. 1996. Action spectra for phytochrome A- and B-specific photo induction of seed germination in Arabidopsis thaliana. Proceeding of the National Academy of Sciences of United States of America 93: 8129-8133. Silvertown J. 1980. Leaf-canopy-induced seed dormancy in a grassland flora. New Phytologist 85: 109-118 Skálová H, Krahulec F, During HJ, Hadincová V, Pecháčková S, Herben T. 1999. Grassland canopy composition and spatial heterogeneity in the light quality. Plant Ecology 143: 129-139. Smirnoff N. 1998. Plant resistance to environmental stress. Current Opinion in Biotechnology 9:  214-219.  Smith H. 1982. Light quality, photoperception, and plant strategy. Annual Review of Plant Physiology 33: 481-518. Smith H. 1994. Sensing the light environment: the functions of the phytochrome family. In: Kendrick RE, Kronemberg GHM (eds) Photomorphogenesis in plants. 2nd edn.  Kluver Academic, Doordrecht, pp. 377-416. Smith H. 2000. Phytochromes and light signal perception by plants - an emerging synthesis. Nature 407: 585-590. 129  Smith H, Casal JJ, Jackson GM. 1990. Reflection signals and the perception by phytochrome of tlie proximity of neighbouring vegetation. Plant, Cell and Environment 13: 73-78.  Smith H, Holmes MG. 1977. The function of phytochrome in the natural environment-III. Measurement and calculation of phytochrome photoequilibria. Photochemistry and Photobiology 25: 547-550. Smith H, Whitelam GC. 1997. The shade avoidance syndrome: multiple responses mediated by multiple phytochromes. Plant, Cell and Environment 20: 840-844. Smith JL, Burritt DJ, Bannister P. 2000. Shoot dry weight, chlorophyll and UV-B-absorbing compounds as indicators of a plant's sensitivity to UV-B radiation. Annals of Botany 86: 1057-1063. Solari F, Shanahan J, Ferguson R, Schepers J, Gitelsond A. 2008. Active sensor reflectance measurements of corn nitrogen status and yield potential. Agronomy Journal 100: 571-579. Solomon S, Garcia RR, Rowland FS, Wuebbles DJ. 1986. On the depletion of Antarctic ozone. Nature 321: 755-758. Souza RP, Válio IFM. 2003. Leaf optical properties as affected by shade in saplings of six tropical tree species differing in successional status. Brazilian Journal of Plant Physiology 15: 49-54. Stevens OA. 1932. The number and weight of seeds produced by weeds. American Journal of Botany 19:784-794. Štroch M, Materová Z, Vrábl D, Karlický V, Šigut L, Nezval J, et al. 2015. Protective effect of UV-A radiation during acclimation of the photosynthetic apparatus to UV-B treatment. Plant Physiology and Biochemistry 96: 90-96. 130  Suzuki A, Suriyagoda L, Shigeyama T, et al. 2011. Lotus japonicus nodulation is photomorphogenetically controlled by sensing the red/far red ratio through jasmonic acid (JA) signaling. Proceedings of the National Academy of Sciences of USA 108: 16837-16842. Tegelberg R, Julkunen-Tiitto R, Aphalo PJ. 2004. Red: far-red light ratio and UV-B radiation: their effects on leaf phenolics and growth of silver birch seedlings. Plant, Cell and Environment 27: 1005-1013. Torre S, Roro AG, Bengtsson S, Mortensen LM, Solhaug KA, Gislerod HR, Olsen JE. 2012. Control of plant morphology by UV-B and UV-B-temperature interactions. Acta Horticulturae 956: 207-214. Trigui A. 1990. Optical properties of the olive trees: from the leaf to the canopy and its surroundings. Acta Horticulturae 286: 315-318. Uddling J, Gelang-Alfredsson J, Piikki K, Pleijel H. 2007. Evaluating the relationship between leaf chlorophyll concentration and SPAD-502 chlorophyll meter readings. Photosynthesis Research 91: 37-46. Vangessel MJ, Renner KA. 1990. Redroot pigweed (Amaranthus retroflexus) and barnyardgrass (Echinochloa crus-galli) interference in potatoes (Solanum tuberosum). Weed Science 38: 338-343. Walburg G, Bauer ME, Daughtry CST, Housley TL. 1982. Effects of nitrogen nutrition on the growth, yield and reflectance characteristics of corn canopies. Agronomy Journal 74: 677-683. 131  Wargent JJ, Elfadly EM, Moore JP, Paul ND. 2011. Increased exposure to UV-B radiation during early development leads to enhanced photoprotection and improved long-term performance in Lactuca sativa. Plant, Cell and Environment 34: 1401-1413. Wargent JJ, Nelson BC, Mcghie TK, Barnes PW. 2015. Acclimation to UV-B radiation and visible light in Lactuca sativa involves up-regulation of photosynthetic performance and orchestration of metabolome-wide responses. Plant, Cell and Environment 38: 929-940. Warnasooriya SN, Porter KJ, Montgomery BL. 2011. Tissue- and isoform-specific phytochrome regulation of light-dependent anthocyanin accumulation in Arabidopsis thaliana. Plant Signaling and Behavior 6: 624-631. Weaver SE, McWilliams EL. 1980. The biology of Canadian weeds: 44. Amaranthus retroflexus L., A. powellii S. Wats. and A. hybridus L. Canadian Journal of Plant Science 60: 1215-1234. Weaver SE, Tan CS. 1983. Critical period of weed interference in transplanted tomatoes (Lycopersicon esculentum): growth analysis. Weed Science 31: 476-481. Weaver SE, Tan CS, Brain P. 1988. Effect of temperature and soil moisture on time of emergence of tomatoes and four weed species. Canadian Journal of Plant Science 68: 877-886. Weijschedé J, Martínková J, de Kroon H, Huber H. 2006. Shade avoidance in Trifolium repens: costs and benefits of plasticity in petiole length and leaf size. New Phytologist 172: 655-666. Wiese AM, Binning LK. 1987. Calculating the threshold temperature of development for weeds. Weed Science 35: 177-179. 132  Wirthensohn MG, Sedgley M. 1996. Epicuticular wax structure and regeneration on developing juvenile eucalyptus leaves. Australian Journal of Botany 44: 691-704. Woolley JT. 1971. Reflectance and transmittance of light by leaves. Plant Physiology 47: 656-662. Yamawo A, Tagawa J, Suzuki N. 2014. Two Mallotus species of different life histories adopt different defense strategies in relation to leaf age. Plant Species Biology 29: 152-158. Zaller JG, Caldwell MM, Flint SD, Scopel AL, Sala O, Ballaré CL. 2002. Solar UV-B radiation affects below-ground parameters in a fen ecosystem in Tierra del Fuego, Argentina: implications of stratospheric ozone depletion. Global Change Biology 8: 867-871. Zaller JG, Searles PS, Caldwell MM, Flint SD, Scopel AL, Sala OE. 2004. Growth responses to ultraviolet-B radiation of two Carex species dominating an Argentinian fen ecosystem. Basic and Applied Ecology 5: 153-162. Zhang J, Hamill AS, Gardiner IO, Weaver SE. 1998. Dependence of weed flora on the active soil seedbank. Weed Research 38: 143-152.  

Cite

Citation Scheme:

        

Citations by CSL (citeproc-js)

Usage Statistics

Share

Embed

Customize your widget with the following options, then copy and paste the code below into the HTML of your page to embed this item in your website.
                        
                            <div id="ubcOpenCollectionsWidgetDisplay">
                            <script id="ubcOpenCollectionsWidget"
                            src="{[{embed.src}]}"
                            data-item="{[{embed.item}]}"
                            data-collection="{[{embed.collection}]}"
                            data-metadata="{[{embed.showMetadata}]}"
                            data-width="{[{embed.width}]}"
                            async >
                            </script>
                            </div>
                        
                    
IIIF logo Our image viewer uses the IIIF 2.0 standard. To load this item in other compatible viewers, use this url:
https://iiif.library.ubc.ca/presentation/dsp.24.1-0355872/manifest

Comment

Related Items