High Affinity K+ Uptake in Maize Roots

We report here on the putative coupling between a high affinity K(+) uptake system which operates at low external K(+) concentrations (K(m) = 10-20 micromolar), and H(+) efflux in roots of intact, low-salt-grown maize plants. An experimental approach combining electrophysiological measurements, quantification of unidirectional K(+)((86)Rb(+)) influx, and the simultaneous measurement of net K(+) and H(+) fluxes associated with individual cells at the root surface with K(+)- and H(+)-selective microelectrodes was utilized. A microelectrode system described previously (IA Newman, LV Kochian, MA Grusak, and WJ Lucas [1987] Plant Physiol 84: 1177-1184) was used to quantify net ion fluxes from the measurement of electrochemical potential gradients for K(+) and H(+) ions within the unstirred layer at the root surface. No evidence for coupling between K(+) uptake and H(+) efflux could be found based on: (a) extremely variable K(+):H(+) flux stoichiometries, with K(+) uptake often well in excess of H(+) efflux; (b) dramatic time-dependent variability in H(+) extrusion when both fluxes were measured at a particular location along the root over time; and (c) a lack of pH sensitivity by the high affinity K(+) uptake system (to changes in external pH) when net K(+) uptake, unidirectional K(+)((86)Rb(+)) influx, and K(+)-induced depolarizations of the membrane potential were determined in uptake solutions buffered at pH values from pH 4 to 8. Based on the results presented here, we propose that high affinity active K(+) absorption into maize root cells is not mediated by a K(+)/H(+) exchange mechanism. Instead, it is either due to the operation of a K(+)-H(+) cotransport system, as has been hypothesized for Neurospora, or based on the striking lack of sensitivity to changes in extracellular pH, uptake could be mediated by a K(+)-ATPase as reported for Escherichia coli and Saccharomyces.