Effects of Exogenous Proline on the Ascorbate-Glutathione Cycle in Roots of Cucumis melo Seedlings under Salt Stress

To study the effects of exogenous proline on the ascorbate-glutathione cycle in roots of melon seedlings under salt stress, two melon cultivars (Cucumis melo ‘Yuhuang’ and ‘Xuemei’) under differing salt tolerances were investigated in a nutrient solution culture system. The results showed that:(1) The content of reduced ascorbic acid (ASA) and reduced glutathione (GSH) decreased and oxidized glutathione (GSSG) increased in the two melon cultivars under salt stress, with C. melo ‘Xuemei’ showing a larger range than C. melo ‘Yuhuang’. (2) Exogenous proline had greater effects on ‘Xuemei’ than on ‘Yuhuang’ in increasing the content of ASA and GSH while decreasing the content of GSSG, and therefore the ratio of GSH/GSSG increased. (3) After 3 days salt treatment, ‘Xuemei’ showed a markedly larger decreasing range in the activities of ascorbate peroxidase (APX), dehydroascorbate reductase (DHAR) and glutathione reductase (GR). Five days later, enzyme activity increased in ‘Yuhuang’ but decreased in ‘Xuemei’ with the extension of stress time. (4) Exogenous proline had larger impact on ‘Xuemei’ than on ‘Yuhuang’ in regards to increasing the activities of APX, DHAR and GR. These results indicated that exogenous proline can clean reactive oxygen species (ROS) caused by salt stress through increasing the activities of antioxidant enzyme and non-enzymatic antioxidant content, such as ASA and GSH, and can thus improve the ascorbate-glutathione cycle to protect melon seedlings from damage caused by salt stress.