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¹⁵N discrimination as an indicator of nitrogen dynamics in Populus trichocarpa Buschhaus, Hannah Ariel Elizabeth

Abstract

The current understanding of nitrogen stable isotope ratios in plant tissues increasingly emphasizes the relationship between plant growth and nitrogen nutrition in determining plant δ15N. This demand relative to supply is thought to influence the plant's ability to discriminate against the heavier 15N. Discrimination (Dplant) is then linked to the ratio of efflux to influx. Factors which influence either of these thus affect Dplant- This thesis examines genotypic differences and physiological manipulations in Populus trichocarpa to further test the proposed efflux/influx model for Dplant. Substrate depletion experiments used ramets grown in hydroponic media containing 200µM NH4 + . Root pruning to reduce the plant's capacity to supply N correspondingly increased the rate of NH4 + uptake and decreased Dplant. Shoot pruning and genotypic variation did not appear to play a significant role in determining Dplant as assessed by this method. Substrate depletion experiments also allowed us to calculate the root NH4 + influx and efflux from the δ15N and the net uptake rate. This novel application of the efflux/influx model for discrimination generated efflux and influx values that corroborated existing radiolabelled 13N studies. The ability to accurately calculate efflux and influx using stable isotope methods at natural abundance levels provides a new, non-radioactive approach for further nutrient-uptake efficiency studies. In steady-state experiments, ramets were grown at either ambient (400ppm) or elevated (800ppm) atmospheric CO2 concentrations in either 200µM or 400µ.M NH4 + hydroponic media. Within the treatments, Dplant corresponded to the relative growth rate responses, signifying its dependence on physiological growth factors. Genotypic differences in the discrimination values of P. trichocarpa provenances could be manipulated by changing the supply/demand regimes. Plant tissue δ15N revealed an unexpected but distinct foliar enrichment. These data prompted the development of a revised efflux/influx model that accounts for translocation and subsequent assimilation of NH4 + in the leaves. This newest model now provides testable hypotheses for future NH4 + translocation and assimilation studies.

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