Evidence for a sodium influx pump in sunflower roots

Transmembrane electrical potential differences in the cortical cells of the root of the sunflower (Helianthus annuus) have been measured using microelectrodes. The plants were grown in culture solution with a range of sodium concentrations. It was found that increasing the external sodium concentration had virtually no effect on the transmembrane potential. The vacuolar content of sodium did not change significantly with the age of the tissue indicating that sodium was in flux equilibrium in our experiments. This allowed the Nernst equation to be used to calculate the electrochemical potential gradient for sodium between the vacuole and the external solution. It was concluded that sodium was being transported into the vacuole against the electrochemical potential gradient. The location and role of the inwardly directed sodium pump implied by these results is discussed in relation to the efflux pumps for sodium reported for roots of other species. Potassium was also accumulated against the electrochemical potential gradient by these cells.Sodium was found to stimulate the growth of H. annuus when present in the culture solution at very low concentrations.