A theory for 15N/14N fractionation in nitrate-grown vascular plants

We present a theory describing how the δ15N values of the nitrogen (N) pools in a vascular plant depend on that of its source N (nitrate), on 15N/14N fractionations during N assimilation, and on N transport within and N loss from the plant. The theory allows measured δ15N values to be interpreted in terms of physiological processes. The δ15N values of various N pools are calculated using three rules: (1) when a pool divides without transformation, there is no change in the δ15N values of the N entering the resulting pools; (2) when nitrate is assimilated by nitrate reductase, the δ15N values of the resulting pools (product and residual substrate) are described by a Rayleigh equation; (3) when two N pools mix, the δ15N value of the mixture is a weighted average of the δ15N values of the component pools. The theory is written as a spreadsheet and solved numerically. Potentially, it has multiple solutions. Some contravene physiological reality and are rejected. The remainder are distinguished, where possible, using additional physiological information. The theory simulated independent measurements of δ15N in N pools of Brassica campestris L. var. rapa (komatsuna) and Lycopersicon esculentum Mill. cv. T-5 (tomato).