Close coupling between extrusion of H(+) and uptake of K (+) by barley roots.

Extrusion of H(+) by intact barley (Hordeum vulgare L.) roots was automatically titrated. Simultaneously, uptake of K(+) into the roots, transport of K(+) through the roots, and (as a residual term) accumulation of K(+) within the root tissue were determined. When no monovalent cation was present in the medium the steady rate of H(+) release was close to zero. Addition of K(+) stimulated H(+) extrusion within less than 1 min. The stimulation of H(+) release was apparently limited only by the movement of K(+) through the apoplast of the roots. The steady rate of H(+) extrusion depended on the availability of external K(+) and saturated at a K(+) concentration of about 100 μmol· dm(-3). Half-maximum rates of net K(+) uptake and H(+) extrusion were reached at a K(+) concentration of about 10 μmol·dm(-3). With (slowly absorbable) sulfate as the only anion present, the stoichoimetry between H(+) release and net K(+) uptake was one. In conclusion, the uptake of K(+) across the plasmalemma of the cells of the root cortex is electrically coupled to H(+) extrusion.