Involvement of Long-Distance Na+ Transport in Maintaining Water Potential Gradient in the Medium-Root-Leaf System of a Halophyte Suaeda altissima

Abstract

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.

Physiological Response of Halophyte (<i>Suaeda altissima</i> (L.) Pall.) and Glycophyte (<i>Spinacia oleracea</i> L.) to Salinity

“…When the salt concentration in the medium is further increased to 750 mM, organ mass decreases 4 -5-fold (Figures 1(a) and (c)). These results support the known data that NaCl concentration of 250 mM in the medium is optimal for seablite growth and development [9]. In the glycophyte, an increase in NaCl concentration in the medium up to 250 mM leads to a slight decrease in fresh biomass (Figure 1(b)), while dry mass does not change significantly (Figure 1(d)).…”

“…Seablite is a salt-accumulating halophyte. It adapts to high salinity by accumulating high quantities of ions in vacuoles of shoot cells in order to create a gradient of water potential along the axis of the plant [9]. These data indicate the presence of an efficient mechanism of Na + loading into the xylem in the halophyte.…”

“…However, they do not explain the diversity of reactions in whole organisms from different ecological groups. Studies on the physiological functions in the whole plant conducted previously [9,10], as well as data presented in this work on the accumulation and distribution of Na …”

“…Plants were grown with solution culture, as described in [9,16], with the use of Robinson and Dounton mineral solution [17] On the forth week the sodium chloride was added to the growth medium every 2 -3 days so that NaCl concentration in a pot increased by no more than 50 mM. Finish sodium chloride concentration in the solution was 0.5, 150, 250 mM for spinach and 0.5, 250, 750 mM for seablite.…”

“…High salt tolerance in dicotyledonous halophytes, for example, from the Chenopodiaceae family, is due to their ability to accumulate up to 1 -1.5 M of sodium ions in shoot vacuoles in high salinity conditions, as well as their ability to use Na + to maintain turgor and act instead of K + [9,10]. In high salinity conditions, the halophytes show increase in growth rate, caused by the effect of sodium on cell extension growth and plant water balance.…”