title: Nitrogen isotope discrimination as an integrated measure of nitrogen fluxes, assimilation and allocation in plants.
abstract: Fractionation of nitrogen isotopes between a plant and its environment occurs during uptake and assimilation of inorganic nitrogen. Fractionation can also occur between roots and the shoot. Under controlled nitrogen conditions, whole-plant and organ-level nitrogen isotope discrimination (Δ(15) N) is suggested to primarily be a function of three factors: nitrogen efflux back to the substrate relative to gross influx at the root (efflux/influx), the proportion of net influx assimilated in the roots and the export of remaining inorganic nitrogen for assimilation in the leaves. Here, an isotope discrimination model combining measurements of δ(15) N and nitrogen content is proposed to explain whole-plant and organ-level variation in δ(15) N under steady-state conditions and prior to any significant retranslocation. We show evidence that nitrogen isotope discrimination varies in accordance with changes to nitrogen supply or demand. Increased whole-plant discrimination (greater Δ(15) N or more negative δ(15) N relative to the source nitrogen δ(15) N) indicates increased turnover of the cytosolic inorganic nitrogen pool and a greater efflux/influx ratio. A greater difference between shoot and root δ(15) N indicates a greater proportion of inorganic nitrogen being assimilated in the leaves. In addition to calculations of integrated nitrogen-use traits, knowledge of biomass partitioning and nitrogen concentrations in different plant organs provides a spatially and temporally integrated, whole-plant phenotyping approach for measuring nitrogen-use in plants. This approach can be used to complement instantaneous cell- and tissue-specific measures of nitrogen use currently used in nitrogen uptake and assimilation studies.
authors: Department of Forest and Conservation Sciences, University of British Columbia, Vancouver, BC, Canada; Department of Biology, Centre for Forest Biology, University of Victoria, Victoria, BC, Canada.
authors: Lee A
authors: LA
authors: Kalcsits
authors: Hannah A
authors: HA
authors: Buschhaus
authors: Robert D
authors: RD
authors: Guy
mesh_terms: D001692:Biological Transport; D008954:Models, Biological; D009584:Nitrogen / Q000378:metabolism*; D009587:Nitrogen Isotopes / Q000378:metabolism; D018515:Plant Leaves / Q000378:metabolism*; D018517:Plant Roots / Q000378:metabolism*; D010944:Plants / Q000378:metabolism*
chemical_list: D009587:Nitrogen Isotopes; D009584:Nitrogen