The surface charge density of plant cell membranes (σ): an attempt to resolve conflicting values for intrinsic σ

The electrical potentials at membrane surfaces (psi0) strongly influence the physiological responses to ions. Ion activities at membrane surfaces may be computed from psi0, and physiological responses to ions are better interpreted with surface activities than with bulk-phase activities. psi0 influences the gating of ion channels and the driving force for ion fluxes across membranes. psi0 may be computed with electrostatic models incorporating the intrinsic surface charge density of the membrane (sigma0), the ion composition of the bathing medium, and ion binding to the membrane. Some of the parameter values needed for the models are well established: the equilibrium constants for ion binding were confirmed for several ions using multiple approaches, and a method is proposed for the computation of other binding constants. sigma0 is less well established, although it has been estimated by several methods, including computation from the near-surface electrical potentials [zeta (zeta) potentials] measured by electrophoreses. Computation from zeta potentials yields values in the range -2 mC m(-2) to -8 mC m(-2), but other methods yield values in the range -15 mC m(-2) to -40 mC m(-2). A systematic discrepancy between measured and computed zeta potentials was noted. The preponderance of evidence supports the suitability of sigma0=-30 mC m(-2). A proposed, fully paramatized Gouy-Chapman-Stern model appears to be suitable for the interpretation of many plant responses to the ionic environment.