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N-fertilization effects on water- and nitrogen-use efficiency in Picea glauca families Pritchard, Emily S.


Water-use efficiency (WUE) and nitrogen-use efficiency (NUE) are typically negatively correlated across environmental variation in N fertilization or drought. The contribution of genotypic and species NH₄⁺ preference in uptake to WUE and NUE was tested using ten fullsibling families of white spruce (Picea glauca (Moench) Voss). The families were raised hydroponically in solutions containing 100 μM N as NH₄⁺, NO₃⁻ or NH₄NO₃ and after eight weeks, growth parameters, gas exchange, C:N ratio and stable C and N isotope composition (δ¹³C and δ¹⁵N, respectively) were analyzed. NH₄⁺ treatment significantly increased biomass which correlated negatively with NUE (C:N ratio). Although the significant difference in treatment mean biomass indicated a preference for NH₄⁺, there was no associated relationship with NUE or δ¹³C, and a negative correlation between NUE and WUE (as measured by gas exchange and δ¹³C) did not occur across treatments. Potential reasons for the lack of correlation are the significant variation between treatment replicates or an unknown effect of hydroponics on gas exchange in this species. In contrast to treatment effects, families were significantly different in biomass, C:N ratio and δ¹³C, and each family maintained its rank in the measured parameters. This indicated that there are high uptake and low uptake families regardless of the form of nitrogen supplied. There was no evidence of a genotypic trade-off between WUE and NUE presumably because of the interaction of genetic control of N assimilation and allocation to growth versus non-growth related N compounds. Mean Δδ¹⁵N was significantly different in each of the treatments and positively correlated with biomass. A potential explanation for treatment differences relates to the efflux/influx ratio associated with each of the N forms. Slower uptake and assimilation of NO₃⁻ over NH₄⁺ combined with a low storage capacity, could cause greater efflux of 1 5N yielding more negative Δδ¹⁵ values in the NO₃⁻ treatment than in the NH₄⁺ and NH₄NO₃ treatments. Stable N isotope composition was also shown to be genetically controlled and correlated positively with δ¹³C. This correlation may be a result of genetically determined uptake rates and sink strength, such that if a family has a high uptake rate and allocates a greater proportion of assimilated N to Rubisco, this will yield less negative δ¹³C and Δδ¹⁵N values. A second experiment was designed to test the physiological control of N isotope fractionation. The experimental design was similar to the first experiments but the treatments imposed were steady-state and draw-down supply rates of 200 μM NH₄⁺ + . There was no significant difference between treatments in total biomass, but the ratio of root dry matter to shoot dry matter was higher in the draw-down treatment indicating a higher level of nutrient stress. Treatments also significantly affected C:N ratio, δ¹³C and C[sub i]/C[sub a] , resulting in a positive environmental correlation between NUE and δ¹³C. Treatment means for Δδ¹⁵N were not significantly different possibly because the ¹⁵N enrichment of the medium overcame enzymatic discrimination. Genotypic control of Δδ¹⁵N was significant and may reflect genetic control of the balance of N efflux/influx at the roots. This balance of efflux/influx may be related to genetic control of N uptake, assimilation, allocation and demand.

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