The aim of this study was to determine the range of NaCl concentrations in the nutrient solution that allow Suaeda altissima (L.) Pall., a salt-accumulating halophyte, to maintain the upward gradient of water potential in the "medium-root-leaf' system. We evaluated the contribution of Na+ ions in the formation of water potential gradient and demonstrated that Na+ loading into the xylem is involved in this process. Plants were grown in water culture at NaCl concentrations ranging from zero to 1M. The water potential of leaf and root cells was measured with the method of isopiestic thermocouple psychrometry. When NaCl concentration in the growth medium was raised in the range of 0-500 mM (the medium water potential was lowered accordingly), the root and leaf cells of S. altissima decreased their water potential, thus promoting the maintenance of the upward water potential gradient in the medium-root-leaf system. Growing S. altissima at NaCl concentrations of 750 mM and 1M disordered water homeostasis and abolished the upward gradient of water potential between roots and leaves. At NaCl concentrations of 0-250 mM, the detached roots of S. altissima were capable of producing, the xylem exudate. The concentration of Na' in the exudate was 1.3 to 1.6 times higher than in the nutrient medium; the exudate pH was acidic and was lowered from 5.5 to 4.5 with the rise in the salt concentration. The results indicate that the long-distance Na+ transport and, especially, the mechanism of Na+ loading into the xylem play a substantial role in the formation of water potential gradient in S. altissima. The accu- mulation of Na' in the xylem and acidic pH values of the xylem sap suggest that Na' loading into the xylem is carried out by the Na+/H+ antiporter of the plasma membrane in parenchymal cells of the root stele.
