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UBC Theses and Dissertations

Testing, advancing, and applying an isotope mass balance model to investigate nitrogen uptake and assimilation in poplar and willow Hu, Yi


Poplars and willows are widely distributed in temperate ecosystems and are important woody crops, but their growth depends on adequate (and often replete) supplies of water and nitrogen. Nitrogen isotope discrimination (∆¹⁵N) have utility as an indicator of nitrogen-use in plants, and an isotope mass balance (IMB) model has been proposed to estimate the fluxes across root plasma membranes (E/I), the proportion of inorganic nitrogen assimilated in roots (Proot) and translocation of inorganic nitrogen to shoots (Ti/Tt) under steady-state conditions. Good correspondence between model-estimated E/I and compartmental analysis of tracer efflux has been demonstrated. In this thesis, the IMB model was further validated and then, after adjusting the ∆¹⁵N value of nitrate reductase (∆enzyme), used to explore intraspecific variation in nitrogen-use in black cottonwood (Populus trichocarpa Torr. & Gray) and heart-leaved willow (Salix eriocephala Michx.). To test the IMB model, I measured concentrations and δ¹⁵N of inorganic and organic nitrogen in xylem sap extracted from stems of black cottonwood provisioned hydroponically with nitrate. Direct determinations of Ti/Tt and Proot compared very favorably with IMB model-derived estimates. The difference in δ¹⁵N between organic-N and NO3̄ -N fractions in xylem sap (24.9‰) suggests a ∆enzyme of 25.1‰, which is higher than previously assumed by the IMB model, but close to recent estimates that average ~22‰. There was significant variation in nitrogen-use related traits in both poplar and willow. In heart-leaved willow, variation in δ¹⁵N, δ¹³C and C/N ratios indicated differences in nitrogen uptake and assimilation, WUE, and NUE. Clinal variation in δ¹⁵N was found in field and hydroponic experiments, possibly related to water availability. The absence of a trade-off between E/I, WUE, and NUE in willow suggests that additive variation in these traits can be captured without compromise. Variation in growth, R:S ratio, ∆¹⁵N, and C/N ratio was also found in black cottonwood, but biogeoclimatic correlations were not strong. The GWAS study using root and leaf ∆¹⁵N and other traits did not find significant genes related to nitrate transport or assimilation. There was one for root ∆¹⁵N and several for R:S ratio, which was in turn correlated with whole-plant ∆¹⁵N and E/I.

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