Water Potential of the Apoplast in Substomatal Cavity of the Suaeda altissima (L.) Pall. Leaf under Salt Stress

The technique that enables simultaneous evaluation of water potential in the substomatal cavity of the intact leaf (psi(wa)) and rate of its photosynthetic CO2/H2O gas exchange was applied to a halophyte Suaeda altissima (L.) Pall. Formation of a water-potential gradient in a whole plant, rate of CO2 uptake, rate of transpiration, and concentrations of Na+ and Cl- ions in the plant organs were determined under conditions of chloride-sodium salinization of a nutrient solution. It was found that the salinization decreases not only biological productivity of the plants but also their capacity to accumulate Cl- in the amounts equivalent to Na+ accumulation. High salinity also diminished the gradient of water potential between a nutrient solution and the apoplast of the cells in the leaf substomatal cavity due to the increase in psi(wa) and, respectively, decrease in the water stream from roots to leaves and the rate of CO2/H2O gas exchange of the leaf. It was shown that the decrease in the water potential in the interface between liquid and gaseous phases in the apoplast of the substomatal cavity (expressed in the psi(wa) value) plays an essential role in the regulation of water uptake under salinization conditions. It is supposed that the NaCl-induced increase in psi(wa) is a consequence of the suppression of photosynthesis and a resultant osmolyte shortage together with the decrease in plant productivity due to the stress impact exerted by NaCl.