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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 1 M. 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 1 M 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 accumulation 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.

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The contents of Na + , K + , water, and dry matter were measured in leaves and roots of euhalophytes Salicornia europaea L. and Climacoptera lanata (Pall.) Botsch featuring succulent and xeromorphic cell structures, respectively, as well as in saltbush Atriplex micrantha C.A. Mey, a halophyte having bladder-like salt glands on their leaves. All three species were able to accumulate Na + in their tissues. The Na + content in organs increased with elevation of NaCl concentration in the substrate, the concentrations of Na + being higher in leaves than in roots. When these halophytes were grown on a NaCl-free substrate, a trend toward K + accumulation was observed and was better pronounced in leaves than in roots. Particularly high K + concentrations were accumulated in Salicornia leaves. There were no principal differences in the partitioning of Na + and K + between organs of three halophyte species representing different ecological groups. At all substrate concentrations of NaCl, the total content of Na + and K + in leaves was higher than in roots. This distribution pattern persisted in Atriplex possessing salt glands, as well as in euhalophytes Salicornia and Climacoptera. The physiological significance of such universal pattern of ion accumulation and distribution among organs in halophytes is related to the necessity of water absorption by roots, its transport to shoots, and maintenance of sufficient cell water content in all organs under high soil salinity.