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 + , Ca 2+ , 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 m M 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 m M ) Ca 2+ 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 Ca 2+ 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.