Steady-state isotopic fractionation in branched pathways using plant uptake of NO3- as an example.

A steady-state isotopic discrimination model is developed for material transfer between a single source and two distinct sinks arising from an internal branching of the uptake pathway. Previous analyses of isotopic discrimination in multistep processes are extended to include the effects of the interacting sinks. The theory is first developed as a set of generic expressions allowing for flexibility in the definition of intermediate pools in all parts of the branched transport pathways, and then applied to a case study of nitrate uptake by plants. The isotopic composition of assimilated nitrate may be evaluated with the model for either contrasting root versus shoot assimilate pools, each with a unique isotopic signature, or as a single mean value for whole-plant nitrate reduction. The theory is further developed to indicate how isotopic measurements may be used to infer (i) efflux:influx ratios at the root plasma membrane, (ii) partitioning of assimilate capture between root and shoot reduction sites, and (iii) mixing of root and shoot assimilate pools in sink tissues due to whole-plant circulation of organic nitrogen in both xylem and phloem.