Effect of Sodium on Potassium Fluxes at the Cell Membrane and Vacuole Membrane of Red Beet

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Abstract
Slices of red beet (Beta vulgaris) washed for 5 to 6 days are known to accumulate Na+ in preference to K+ from solutions containing both ions. The present work, using ion concentrations of 1.0 mm or less, with Ca2+ added in some cases, shows that Na+ strongly inhibits K+ influx at the cell membrane (plasmalemma) while K+ efflux is increased to a lesser extent. This result from compartmental analysis is confirmed by short (15-minute) influx experiments, which indicate an immediate inhibitory effect of Na+ on K+ influx at the cell membrane. It is concluded that cation selectivity, even when Na+ is favored for uptake, is primarily determined at the cell membrane. Nevertheless, a high level of K+ in the cytoplasm is maintained during Na+ influx, by an inhibition of K+ transfer to the vacuole.