Analysis of plant root electropotentials

A model is proposed for the analysis of steady-state electropotentials in plant roots. The radial element of this model is discussed in terms of two parallel pathways, i.e. the symplasm and the extracellular space. The symplasm pathway is treated as a system of three compartments in series. The electrical potential difference across this pathway and its resistance are computed as a function of ion concentrations in the different compartments and the membrane properties. Similarly the extracellular pathway is analyzed as an ion-exchange membrane. Integration of both pathways into a parallel array analog and comparison with experimental data show that the widely used treatment of the root system as a single passive membrane is inadequate. It is also concluded that the occurrence of electrogenic pumps cannot be ascertained by straightforward inhibitor studies. However, the model proved to be insensitive to relatively wide variations in some of the parameters involved in the determination of its behavior. Analysis of the longitudinal axis of the root system predicts electrical potential difference (p.d.) along the root even in the absence of living cells or permselective cross walls. Conditions can be set where the longitudinal p.d. will not affect the overall root p.d. It is suggested that the p.d. measured between the bathing medium and the exudate of an excised root reflects only the properties of the uppermost segment of this system. The implications of the model to measurements of ion and water transport across the root are discussed.