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.