Physiological responses and adaptive strategies of tomato plants to salt and alkali stresses

In this study, seedlings of tomato were treated with salt stress or alkali stress. The growth, photosynthesis and concentrations of solutes and inorganic ions in tissue sap of stressed seedlings were measured to investigate the physiological adaptive mechanisms by which tomato tolerates alkali stress. The alkali stress clearly inhibited growth and photosynthesis. With increasing salinity, the Na+ concentration and Na+/K+ ratio in leaves increased, with greater degrees of increase under alkali than under salt stress. This indicated that high-pH caused by alkali stress might affect the control of roots on Na+ uptake and increased the intracellular Na+ to a toxic level, which might be the main cause of reductions of stomatal conductance and net photosynthetic rates under alkali stress. Under salt stress, organic acids (OAs), Na+, K+ and Cl- were the main osmolytes in both roots and leaves. Under alkali stress, roots and leaves revealed different mechanisms of ion balance and osmotic regulation. Under alkali stress, in roots, OAs and Na+ were the main osmolytes, and the osmotic role of K+ was small; however, in leaves, OAs, Na+ and K+ all played important osmotic roles. The mechanisms governing ionic balance under both stresses were different. Under salt stress, the contribution of inorganic ions to keep ion balance was greater than that of OAs. However, under alkali stress, Cl-, H2PO4- and SO42- concentrations decreased, and tomato might have enhanced OA synthesis to compensate for the shortage of inorganic anions. (C) 2011 Elsevier B.V. All rights reserved.