High Affinity K+ Uptake in Maize Roots

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Abstract
We report here on the putative coupling between a high affinity K+ uptake system which operates at low external K+ concentrations (Km = 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+(86Rb+) 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+(86Rb+) 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.