Effect of sodium on potassium fluxes at the cell membrane and vacuole membrane of red beet.

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 Ca(2+) 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.