Potassium, Sodium and Chloride Ion Concentrations in Leaves and Isolated Chloroplasts of the Halophyte Suaeda australis R. Br

Abstract

Seedlings of S. australis were grown hydroponically in nutrient solutions with 0-600 mM NaCl added. The plants grew poorly in the absence of added NaCl and optimal growth, based on fresh or dry weight, occurred at 50-150 mM NaCl. Higher concentrations of NaCl decreased growth although the plants continued to grow even at 600 mM NaCl. The osmotic potential of the leaf sap decreased with increasing salinity and this was mostly the result of accumulation of NaCl in the leaves. Leaf K⁺ concentration decreased with increasing salinity while both Na⁺ and Cl^- increased, reaching 800 and 500 mM, respectively, on a tissue water basis. Intact chloroplasts were isolated by mechanically disrupting the leaves and the chloroplasts were purified on a two-step Percoll gradient. The osmotic potential of the isolation medium was adjusted so as to be isotonic with the leaf sap in each case. The chloroplasts were generally more than 90% intact and exhibited rates of CO₂-dependent oxygen evolution of 70-130 µmol (mg chlorophyll) ^-1 h^-1. The concentration of K⁺ in the chloroplast decreased with increasing salinity and was similar to the concentration of K⁺ in the leaf as a whole. For plants grown without added NaCI, leaf Na⁺ and Cl^- concentrations were low (less than 10 mM) but these ions were accumulated in the chloroplasts, which contained 70-140 mM Na⁺ and 80-120 mM Cl^-. With increasing salinity, the concentration of Na⁺ and Cl^- in the chloroplast did not increase despite the more than 70-fold increase in the concentration of these two ions in the leaf. The results suggest that the concentration of Na⁺ and Cl^- in the chloroplasts of Suaeda australis is regulated, with these ions being accumulated in the chloroplast at low leaf levels and excluded from the chloroplast when leaf Na⁺ and Cl^- are high.