A model for cation content of plants based on surface potentials and surface charge densities of plant membranes

The model presented takes into account the interaction between the negatively charged membranes and macromolecules and the cations. A central postulate is that a constant average surface charge density (σ) as well as a constant average surface potential (Ψ) is conserved under different ionic conditions. The model makes it possible to predict the size of σ and Ψ from measurements of Na, K, Mg and Ca content in plant tissues of the same age but grown under two different ionic conditions (e.g. high and low K+). Assumptions were made about the relative amounts of free and bound Ca2+ and σ and Ψ were calculated from values in the literature. In all cases σ (and Ψ) are predicted to be higher for shoot (−29 to −96 mC m−2) than for root membranes (−14 to −27 mC m−2). In most cases the predicted σ falls within the range determined experimentally for biological membranes.