Wirkung von K+ auf die Fluxe und den Transport von Na+ in Gerstenwurzeln, K+-stimulierter Na+-Efflux in der Wurzelrinde

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Abstract
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 22Na+ 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.