COMPUTATION OF SURFACE ELECTRICAL POTENTIALS OF PLANT-CELL MEMBRANES - CORRESPONDENCE TO PUBLISHED ZETA POTENTIALS FROM DIVERSE PLANT SOURCES

A Gouy-Chapman-Stern model has been developed for the computation of s urface electrical potential (psi(0)) of plant cell membranes in respon se 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 psi(0) that correlate highly with publ ished zeta potentials of protoplasts and plasma membrane vesicles from diverse plant sources. The model assumes ion binding to a negatively charged site (R- = 0.3074 mu mol m(-2)) and to a neutral site (P-0 = 2 .4 mu mol m(-2)) according to the reactions R- + I-Z reversible arrow RIZ-1 and P-0 + I-Z reversible arrow PIZ, where I-Z represents an ion of charge Z. Binding constants for the negative site are 21,500 M-1 fo r H+, 20,000 M-1 for Al3+, 2,200 M-1 for La3+, 30 M-1 for Ca2+ and Mg2 +, and 1 M-1 for Na+ and K+. Binding constants for the neutral site ar e 1/180 the value for binding to the negative site. ion activities at the membrane surface, computed on the basis of psi(0), appear to deter mine many aspects of plant-mineral interactions, including mineral nut rition and the induction and alleviation of mineral toxicities, accord ing to previous and ongoing studies. A computer program with instructi ons for the computation of psi(0), ion binding, ion concentrations, an d ion activities at membrane surfaces may be requested from the author s.