K analysis of sodium-induced potassium efflux in barley: mechanism and relevance to salt tolerance

• Stimulation of potassium (K) efflux by sodium (Na) has been the subject of much recent attention, and its mechanism has been attributed to the activities of specific classes of ion channels. • The short-lived radiotracer K was used to test this attribution, via unidirectional K-flux analysis at the root plasma membrane of intact barley (Hordeum vulgare), in response to NaCl, KCl, NH4Cl and mannitol, and to channel inhibitors. • Unidirectional K efflux was strongly stimulated by NaCl, and K influx strongly suppressed. Both effects were ameliorated by elevated calcium (Ca). As well, K efflux was strongly stimulated by KCl, NH4Cl and mannitol , and NaCl also stimulated NH4 + efflux. The Na-stimulated K efflux was insensitive to cesium (Cs) and pH 4.2, weakly sensitive to the K-channel blocker tetraethylammonium (TEA) and quinine, and moderately sensitive to zinc (Zn) and lanthanum (La). • We conclude that the stimulated efflux is: specific neither to Na as effector nor K as target; composed of fluxes from both cytosol and vacuole; mediated neither by outwardly-rectifying K channels nor nonselective cation channels; attributable, alternatively, to membrane disintegration brought about by ionic and osmotic components; of limited long-term significance, unlike the suppression of K influx by Na, which is a greater threat to K homeostasis under salt stress.