K-42 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-42(+) 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 (Ca2+). As well, K+ efflux was strongly stimulated by KCl, NH4Cl and mannitol, and NaCl also stimulated (NH4+)-N-13 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 (Zn2+) and lanthanum (La3+). 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.