Ionic and osmotic components of salt stress specifically modulate net ion fluxes from bean leaf mesophyll

Ionic mechanisms of salt stress perception were investigated by non-invasive measurements of net H⁺, K⁺, Ca²⁺, Na⁺, and Cl^- fluxes from leaf mesophyll of broad bean (Vicia faba L.) plants using vibrating ion-selective microelectrodes (the MIFE technique). Treatment with 90 mM NaCl led to a significant increase in the net K⁺ efflux and enhanced activity of the plasma membrane H⁺-pump. Both these events were effectively prevented by high (10 mM) Ca²⁺ concentrations in the bath. At the same time, no significant difference in the net Na⁺ flux has been found between low- and high-calcium treatments. It is likely that plasma membrane K⁺ and H⁺ transporters, but not the VIC channels, play the key role in the amelioration of negative salt effects by Ca²⁺ in the bean mesophyll. Experiments with isotonic mannitol application showed that cell ionic responses to hyperosmotic treatment are highly stress-specific. The most striking difference in response was shown by K⁺ fluxes, which varied from an increased net K⁺ efflux (NaCl treatment) to a net K⁺ influx (mannitol treatment). It is concluded that different ionic mechanisms are involved in the perception of the 'ionic' and 'osmotic' components of salt stress.