Salinity induced alterations in photosynthetic and oxidative regulation are ameliorated as a function of salt secretion.

Abstract

Ion secretion facilitates recretohalophytes to tolerate saline and drought conditions but its relative contribution to the survival of many species remains poorly understood. Tamarix chinensis has high potential for restoration of saline deteriorated lands. The water management and high salt tolerance of the plant have highlighted the need to determine the strategies that govern these mechanisms. Here we report the selectivity of this halophyte to transport, utilize, and secrete different cations and anions under various NaCl (0, 100, 200 and 400mM) concentrations. Plant growth, photosynthesis and antioxidant defense responses were also determined to relate them with the function of ion secretion. Results reflected two different sets of strategies adopted by plants to survive low and high salinities. Exposure to highly saline conditions caused reduction in photosynthesis due to stomatal and biochemical limitations. The decreased content of photosynthetic pigments exposed plants to excessive light energy that accelerated production of ROS (i.e., hydrogen peroxide H2O2) and caused damage to cellular membranes. The increased activities of anti-oxidative enzymes (superoxide-dismutase, catalase, ascorbate-peroxidase, and glutathione-reductase) were insufficient to detoxify H2O2. In contrast, plants treated with low salinity did not face stomatal limitations while the photosynthetic pigments increased. As no damage to membranes was detected, the increased content of H2O2 was postulated for its messenger role. The assimilation of essential nutrients was affected due to increased content of toxic ions (Na+ and Cl-) in the growing medium and within the plants. However, the ability to regulate K+ facilitated plants to improve water use efficiency under hyper-osmotic environment. The removal of toxic ions from the photosynthesizing tissues demands high energy, which was evident in the compromised growth of plants. This study offers a window to physiological mechanisms, e.g., potassium retention that ensure salt secretion as a beneficial strategy for prolonged survival of T. chinensis.