In tomato plants, salt stress has been induced to improve the quality of fruit. In general, plants under salt stress produce reactive oxygen species (ROS). Plants have their own ROS scavenging systems (antioxidant systems). In tomato plants, salt-induced changes in antioxidant systems have been examined extensively in leaves and roots; however, detailed information about salt-stressed fruits is not available. We examined the salt-induced changes in the antioxidant systems of the pericarp (containing epidermis) and pulp (containing seeds, placenta, and locule) during fruit ripening. Salt treatments were applied by adding 100 mM NaCl to the nutrient solution. In the pericarp and pulp, lipid peroxidation and hydrogen peroxide content were not increased by salt stress during ripening, indicating the absence of salt-induced oxidative stress. In the pericarp, the activities of superoxide dismutase (EC 22.214.171.124) and ascorbate (ASA)-glutathione (GSH) cycle-related enzymes increased with salt stress at the turning stage. Thus, at the turning stage, the antioxidant system may contribute to the enzymatic reaction involved ASA-GSH cycle. However, during the red and over-ripe fruit stages, salt stress produces little effect on antioxidant enzymes. In addition, the concentrations of antioxidants, such as the reduced form of ASA and GSH, increased during ripening in the control fruit, but those in the salt-stressed fruit remained unchanged. Therefore, the antioxidant system may contribute to the nonenzymatic reactions such as ASA and GSH taking place during the red and over-ripe fruit stages. In contrast, in the pulp, salt stress produces little effect on antioxidants and antioxidant enzymes. These results indicate that the salt-stressed fruit has protection mechanisms against salt-induced oxidative stress during ripening in both the pericarp and pulp. Salt-induced changes in antioxidant systems differed between the pericarp and pulp.
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