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

The electrical potentials at membrane surfaces (ψ₀) strongly influence the physiological responses to ions. Ion activities at membrane surfaces may be computed from ψ₀, and physiological responses to ions are better interpreted with surface activities than with bulk-phase activities. ψ₀ influences the gating of ion channels and the driving force for ion fluxes across membranes. ψ₀ may be computed with electrostatic models incorporating the intrinsic surface charge density of the membrane (σ₀), 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. σ₀ is less well established, although it has been estimated by several methods, including computation from the near-surface electrical potentials [zeta (ζ) potentials] measured by electrophoreses. Computation from ζ 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 ζ potentials was noted. The preponderance of evidence supports the suitability of σ₀= -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.