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

P>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 42K+, 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, 42K+ efflux was inhibited by the channel blockers barium (Ba2+), caesium (Cs+), tetraethylammonium (TEA+), and lanthanum (La3+). Ammonium and nitrate (10 mM) stimulated and inhibited 42K+ 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+].