Salinity Effects on the Activity of Plasma Membrane H+ and Ca2+ Transporters in Bean Leaf Mesophyll: Masking Role of the Cell Wall

Net fluxes of H+ and Ca2+ were measured in the mesophyll tissue of broad bean (Vicia faba L.) leaves and in protoplasts derived from these cells. NaCl at 90 mm enhanced H+ extrusion in both protoplasts and tissue, but in different ways. Proton extrusion was inhibited by vanadate, suggesting the involvement of the plasma membrane H+-ATPase in cell responses to salinity. There was virtually no effect of NaCl on the net Ca2+ flux in protoplasts, while in the tissue a large transient Ca2+ efflux followed the salt treatment. Salt-induced Ca2+ efflux was essentially independent of external Ca2+ concentrations in the range 0.1 to 10 mm. Also, Ca2+ flux responses were 'saturated' above 50 mM NaCl. It is suggested that almost all the measured Ca2+ flux originates from Na+/Ca2+ and H+/Ca2+ ion exchange in the cell wall. This conclusion was supported by the results of modelling cation exchange in the cell wall. (C) 2000 Annals of Botany Company.