Mechanism of Salinity Stress and Its Countermeasures in Crop Plants

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When the effects of salinity stress and water stress were compared in rice, damages in chloroplasts were more prominent in salinity stress. Typical symptom of salinity stress was swelling of the thylakoids. Damages in chloroplasts by salinity were severer in light than in the dark even if Na and Cl concentrations and Na/K ratio in the leaf tissues were comparable or higher in the dark. Therefore, the damages in chloroplasts induced by salinity were dependent on light and the involvement of reactive oxygen species (ROS) was suggested. Pretreatment of leaf tissues with ascorbate (versatile scavenger of ROS including H_2O_2) and benzoate (scavenger of ・OH) was effective to reduce damages in chloroplasts by salinity. It is therefore suggested that the damages were caused by ・OH and/or its precursor H_2O_2. Growth inhibition of rice plants by salinity was ameliorated by Ca^<2+> in hydroponic culture but K^+ was more effective in soil culture. Especially, KNO_3 showed ameliorative effects on the yield of rice. There was a strong correlation between growth inhibition and Na/K ratio in the shoot. Therefore, cultivation practice to reduce Na/K ratio in the shoot is effective to reduce salinity stress. Pretreatment of leaf tissues with a low concentration of methyl viologen (ROS producing herbicide) reduced damages in chloroplasts by salinity and increased the activity of ascorbate peroxidase in the leaf. Therefore, treatments to reduce H_2O_2 concentration in the leaf are effective to ameliorate salinity stress.