Mitigative Effects And Mechanisms Of Exogenous Calcium On Rice Seedlings Under Salt Stress

Two rice (Oryza sativa L.) cultivars, i.e. Wuyujing No.3 (salt-tolerant) and IR36 (salt-sensitive), were used to investigate the effects of exogenous calcium on photosynthesis, chlorophyll content, uptake, transport and distribution of Na+ and K+, active oxygen radicals and antioxidative defenses, and osmotic adjustment substances in salt-stressed rice seedlings. The main results were presented as follows:Net photosynthetic rate, stomatal conductance and transpiration rate were decreased in two rice cultivars with the prolongation of salt stress. Intercellular CO2 concentration decreased in the beginning and then increased, but the reverse was observed for stomatal limitation value. The photosynthesis and photosynthetic efficiency of the leaves of salt-stressed rice were significantly enhanced with the inclusion of calcium. The levels of net photosynthetic rate, stomatal conductance and transpiration rate in the rice seedlings treated with exogenous Ca were increased more significantly than in those without Ca2+. The degree of increase or decrease of above mentioned parameters was smaller in salt-tolerant cultivar than in salt-sensitive one. Treatment with higher Ca (10 mmol/L Ca2+) was observed to perform better alleviating effect than that with lower Ca (5 mmol/L Ca2+) in the two contrasting rice cultivars differing in salt tolerance. The reduction of net photosynthetic rate in rice under salt stress might be mainly attributed to stomatal limitation in short-term salt treatment but to nonstomomatal limitation in long-term salt treatment. Addition of Ca2+ increased the contents of chlorophyll, which promoted to maintain a higher photosynthetic efficiency of salt-stressed rice.The absorption and the root-to-shoot transportation of Na+ and K+ were effectively adjusted by the inclusion of calcium under salt stress. Exogenous calcium decreased Na+ but increased uptake and transport of K. The lower uptake and accumulation of Na+ mitigated the salt damage and enhanced the growth of salt-stressed rice. The accumulation of Na+ in the root and shoot was higher in salt-sensitive cultivar than in salt-tolerance one,but the reverse was found for K+. Furthermore, the treatment with higher Ca (10 mmol/L Ca2+) was observed to perform better effect than that with lower Ca (5 mmol/L Ca2+).The concentration of H2O2 and the generative rate of O2- were increased in two rice cultivars under salt stress. The concentration of MDA as well as membrane permeability was also enhanced. The degree of increment of above mentioned parameters was higher in salt-sensitive rice than in salt-tolerance one. However, all the above parameters were decreased by the inclusion of calcium. In the same way, the effect of the treatment with 10mmol/L Ca2+ was more distinct than that with 5 mmol/L Ca2+.Salt stress decreased the activities of antioxidant enzymes and the contents of antioxidants. Exogenous calcium remarkably enhanced the activities of superoxide dismutase (SOD), peroxidase (POD), catalase (CAT) and glutathione reductase (GR). Compared to the treatment with salt only, addition of Ca to the salt treatment significantly increased the contents of proline and soluble sugar. Exogenous calcium also enhanced the contents of antioxidants such as ascorbic acid (AsA) and glutathione (GSH). The activities of antioxidant enzymes and the contents of antioxidants and osmotic adjustment substances in salt-tolerant rice cultivar were higher than in salt-sensitive one.