Computation of Surface Electrical Potentials of Plant Cell Membranes

A Gouy-Chapman-Stern model has been developed for the computation of surface electrical potential (ψ(0)) 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 ψ(0) 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), where I(Ζ) represents an ion of charge Ζ. Binding constants for the negative site are 21,500 m(−1) for H(+), 20,000 m(−1) for Al(3+), 2,200 m(−1) for La(3+), 30 m(−1) for Ca(2+) and Mg(2+), 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 ψ(0), 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 ψ(0), ion binding, ion concentrations, and ion activities at membrane surfaces may be requested from the authors.