Computation of Surface Electrical Potentials of Plant Cell Membranes

A Gouy-Chapman-Stern model has been developed for the computation of surface electrical potential (ψ0) of plant cell membranes in response to ionic solutes. The present model is a modification of an earlier version developed to compute the sorption of ions by wheat (Triticum aestivum L. cv Scout 66) root plasma membranes. A single set of model parameters generates values for ψ0 that correlate highly with published ζ potentials of protoplasts and plasma membrane vesicles from diverse plant sources. The model assumes ion binding to a negatively charged site (R  − = 0.3074 μmol m−2) and to a neutral site (P  0 = 2.4 μmol m−2) according to the reactions R  − +I  Ζ ⇌RI  Ζ−1 and  P  0 + I  Ζ⇌ PI  Ζ, whereI  Ζ represents an ion of charge Ζ. Binding constants for the negative site are 21,500m  −1 for H+, 20,000m  −1 for Al3+, 2,200m  −1 for La3+, 30m  −1 for Ca2+ and Mg2+, and 1 m  −1 for Na+ and K+. Binding constants for the neutral site are 1/180 the value for binding to the negative site. Ion activities at the membrane surface, computed on the basis of ψ0, appear to determine many aspects of plant-mineral interactions, including mineral nutrition and the induction and alleviation of mineral toxicities, according to previous and ongoing studies. A computer program with instructions for the computation of ψ0, ion binding, ion concentrations, and ion activities at membrane surfaces may be requested from the authors.