Photosynthetic limitations of a halophyte sea aster (Aster tripolium L) under water stress and NaCl stress

To understand the mechanisms of salt tolerance in a halophyte, sea aster (Aster tripolium L.), we studied the changes of water relation and the factors of photosynthetic limitation under water stress and 300 mM NaCl stress. The contents of Na+ and Cl- were highest in NaCl-stressed leaves. Leaf osmotic potentials (Psi(s)) were decreased by both stress treatments, whereas leaf turgor pressure (Psi(t)) was maintained under NaCl stress. Decrease in Psi(s) without any loss of Psi(t) accounted for osmotic adjustment using Na+ and Cl- accumulated under NaCl stress. Stress treatments affected photosynthesis, and stomatal limitation was higher under water stress than under NaCl stress. Additionally, maximum CO2 fixation rate and O-2 evolution rate decreased only under water stress, indicating irreversible damage to photosynthetic systems, mainly by dehydration. Water stress severely affected the water relation and photosynthetic capacity. On the other hand, turgid leaves under NaCl stress have dehydration tolerance due to maintenance of Psi(t) and photosynthetic activity. These results show that sea aster might not suffer from tissue dehydration in highly salinized environments. We conclude that the adaptation of sea aster to salinity may be accomplished by osmotic adjustment using accumulated Na+ and Cl-, and that this plant has typical halophyte characteristics, but not drought tolerance.