42K 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 42K+ 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 13NH4+ 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. [ABSTRACT FROM AUTHOR]