Threshold transitions of 15N fluxes reveal state-dependent root conductance and flux partitioning within steady-state nitrate isotherms

This study reexamines Epstein’s ionic isotherms, arguing that the traditional enzyme-substrate interpretation, which assumes a coherent, continuous, and saturated response, has bypassed significant transition thresholds in root ion fluxes. We integrate new and published 15 N tracer datasets obtained under homogeneous and heterogeneous nitrate supply conditions. The results show that transitions in root uptake ( v in and v app ) and export to the shoot ( v out ) are strongly context-dependent. These findings suggest that a three-compartment soil-root-shoot model better explains biphasic nitrate isotherms than commonly-cited two-compartment approaches. Under all tested experimental conditions, the relationships between 15 N fluxes and external concentrations exhibit clear, threshold-like regime switching. This is consistent with shifts in the dominance of radial versus longitudinal resistances within the root-shoot system. We propose that repeated radial units act as biological varistors - nonlinear, threshold-sensitive elements whose collective behavior depends on root architecture and nitrate distribution rather than fixed Km and Vm parameters. Varistor-like responses have been reported for X-QUAC channels and NRT1.1/NPF6.3-like transporters. Meanwhile, NRT2.1 and Ca 2+ -dependent CBL-CIPK signaling remain central to regime transitions. Using steady-state isotherms that focus on context-dependent regime changes, root structure, and threshold transitions provides a more realistic basis for interpreting nitrate uptake experiments, designing physiological studies, and developing breeding strategies that improve nitrogen use efficiency in variable environments.