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An isotope mass balance approach to measure variability in nitrogen fluxes, allocation and assimilation in balsam poplar (Populus balsamifera L.) and other plants Kalcsits, Lee Anthony


Populus species or their hybrids are being increasingly used as feedstock for the growing bioenergy industry. To optimize carbon uptake efficiencies, fertilizer inputs must be minimized while, at the same time, achieving rapid growth. The complexity of nitrogen uptake and assimilation in plants and environmental heterogeneity force simplifications that limit improvements in nitrogen-use efficiency. In particular, a lack of integration limits the applicability of many traditional nitrogen-use assays to whole plant nitrogen use. Net nitrogen isotope discrimination has potential to act as a time-integrated process indicator of nitrogen-use in plants grown under steady-state conditions. The objective of this thesis was to further develop an isotope discrimination-based integrated measure of nitrogen-use that reflects whole plant and organ level nitrogen use. Observed differences in nitrogen isotope discrimination were proposed to be a function of nitrogen isotope discrimination of the assimilatory enzymes, fluxes across the root plasma membrane and translocation of inorganic nitrogen to the shoot. As a test, nitrogen supply and demand was environmentally varied in black cottonwood (Populus trichocarpa (Torr & Gray)) and genetically manipulated with knockout lines of Arabidopsis (Arabidopsis thaliana L.). Changes in isotopic composition (δ¹⁵N) were interpretable within the context of the proposed model. Furthermore, efflux/influx (E/I) across the root plasma membrane calculated from the isotope mass balance approach was positively correlated with E/I measured using an established ¹⁵N compartmental analysis approach indicating that the isotope mass balance approach produced a reliable measure of E/I. The isotope mass balance approach was then used to determine intraspecific variability in balsam poplar (Populus balsamifera L.), a species used for hybrid poplar breeding. Nitrogen use traits were calculated for 25 genotypes from five climatically dispersed provenances grown hydroponically under steady-state nitrogen conditions with either ammonium or nitrate. Genotypic variation exceeded provenance level variation in most cases and significant variation was observed in growth, nitrogen isotope composition and calculated nitrogen-use traits indicating that there is potential for breeding for nitrogen-use using balsam poplar. The isotope mass balance model presented here provides a new approach for probing integrated nitrogen-use traits in plants, which are often technically difficult to measure.

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