Can unidirectional influx be measured in higher plants? A mathematical approach using parameters from efflux analysis

A comprehensive and pragmatic approach to the design of unidirectional ion transport experiments in plants is presented here, revising and simplifying classical models. The kinetic constant for cytosolic ion exchange (k(c)) is critical to the understanding of the interrelated flux processes occurring simultaneously at the cellular level. This constant is most effectively estimated using the compartmental analysis by efflux method, which, by providing values for additional kinetic parameters (i.e. unidirectional influx and efflux) can be used to determine the extent of distortion inherent in assessments of influx at, or close to, the steady state. Focusing on the kinetics of nitrogen exchange, with tracer efflux experiments in barley (Hordeum vulgare) using (NH4+)-N-13, conducted under perturbation conditions, it was demonstrated that a transitional state was rapidly established following a concentration shift, characterized by a restoration of the preperturbational (steady-state) k(c) value. It is concluded that a reasonably accurate estimate of unidirectional influx can be made when influx measurements are conducted subsequent to the establishment of this transitional state. A mathematical treatment of unidirectional flux processes allows the exact determination of errors caused by ionic counterfluxes under steady-state conditions.