[Effect of K(+) on Na (+) fluxes and transport in barley roots: K(+)-stimulated Na (+) efflux in the root cortex].

Barley roots grown on a nutrient solution containing 1 mM Na(+) but no K(+) are capable of a considerable Na(+) transport via the symplasm of the root and the xylem vessels. K(+) added to the medium surrounding the root cortex severely inhibits this transport after a lag period at a high rate constant (Fig. 3).It is likely that the fluxes of Na(+) are changed drastically during this transition from low to high K(+) status. Although originally limited to steady state fluxes, the extended method of efflux analysis for excised roots (Pitman, 1971) has been applied to the non-steady fluxes which occur upon the addition of K(+) to the roots. It is shown that besides other changes the efflux of (22)Na(+) through the cortex of barley roots is stimulated instantaneously (Fig. 5) by the addition of K(+) and presumably by an influx of K(+) ions. From this a transient, K(+)-stimulated Na(+) efflux at the plasmalemma of the cortical cells can be estimated. It amounts to 10.9 μ moles/g fw · h compared to the control efflux of 3.3 μ moles/g fw · h without K(+).The stimulated efflux is attributed to a Na(+) efflux pump at the plasmalemma and is thus related to the K-Na-selectivity of barley plants. The inhibition of the Na(+) transport by K(+) is probably a consequence of this increased efflux of Na(+) from the symplasm through the root cortex.