Regulation and mechanism of potassium release from barley roots: an in planta 42 K + analysis

Summary• Potassium (K + ) flux into plant cells is a well‐characterized ion transport phenomenon. By contrast, little is known about the mechanisms and regulation of K + flux from the cell. Here, we present a radioisotopic analysis of K + fluxes from roots of intact barley ( Hordeum vulgare ), in the context of recent discoveries in the molecular biology and electrophysiology of this process. • Plants were labelled with 42 K + , and kinetics of its release from roots were monitored at low (0.1 mM) or high (1.0 mM) external K concentration, [K + ] ext , and with the application of channel modulators and nutrient shifts. • At 0.1 (but not 1.0) mM [K + ], where K + efflux is thought to be mediated by K + ‐outward‐rectifying channels, 42 K + efflux was inhibited by the channel blockers barium (Ba 2+ ), caesium (Cs + ), tetraethylammonium (TEA + ), and lanthanum (La 3+ ). Ammonium and nitrate (10 mM) stimulated and inhibited 42 K + efflux, respectively, while 10 mM [K + ] ext or [Rb + ] ext decreased it. No evidence for the involvement of ATP‐binding cassettes, nonselective cation channels, or active K + ‐efflux pumps was found. • Our study provides new evidence for the thermodynamic transition between high‐ and low‐affinity transport, from the efflux perspective, identifying the operation of channels at low [K + ], and the cessation of transmembrane efflux at high [K + ].