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Soil sodium, while toxic to most plants at high concentrations, can be beneficial at low concentrations, particularly when potassium is limiting. However, little is known about Na uptake in this 'high-affinity' range. New information is provided here with an insight into the transport characteristics, mechanism, and ecological significance of this phenomenon. High-affinity Na and K fluxes were investigated using the short-lived radiotracers 24Na and 42K, under an extensive range of measuring conditions (variations in external sodium, and in nutritional and pharmacological agents). This work was supported by electrophysiological, compartmental, and growth analyses. Na uptake was extremely sensitive to all treatments, displaying properties of high-affinity K transporters, K channels, animal Na channels, and non-selective cation channels. K ,

NH4 , and Ca2 suppressed Na transport biphasically, yielding IC50 values of 30, 10, and <5 [mu]M, respectively. Reciprocal experiments showed that K influx is neither inhibited nor stimulated by Na . Sodium efflux constituted 65% of influx, indicating a futile cycle. The thermodynamic feasibility of passive channel mediation is supported by compartmentation and electrophysiological data. Our study complements recent advances in the molecular biology of high-affinity Na transport by uncovering new physiological foundations for this transport phenomenon, while questioning its ecological relevance.

(C) Society for Experimental Biology 2012. Published by Oxford University Press. All rights reserved.