Control of Sodium Transport in Sunflower Roots

Abstract

Electrochemical potential differences (driving forces) for sodium distributed between the outside solution and the exuding sap of water-culture-grown sunflower plants (Helianthus annuus) have been determined. The results indicated that sodium was moving from the outside solution to the xylem against the electrochemical potential gradient at external concentrations below approximately 0-30 mM Na. At higher external concentrations sodium appeared to be actively excluded from the xylem. An electrical potential difference between the exuding sap and the external solution of approximately 30 mV was observed. It was unaffected by the external sodium concentration. Use of a short-circuiting technique indicated that the trans-root potential resides at the plasmalemma of the cortical cells. Driving forces on sodium distributed between the external solution and the root and between the xylem sap and the root were calculated. They indicated that the root is able to accumulate sodium actively both from the external solution and the xylem sap. It is con cluded that sodium transport to the xylem in this species is controlled by the balance of these two opposing forces.