Melatonin improves salinity stress tolerance of Phaseolus vulgaris L. cv. Pak by changing antioxidant enzymes and photosynthetic parameters

Plants face many stresses in their natural environment. Different types of phytohormones help the plant to adapt to stressful conditions. Melatonin is a newly known substance in the plant kingdom, that is effective in relieving stress for plants. In this study, the effect of melatonin on Phaseolus vulgaris L. cv. Pak that was exposed to salinity stress at concentrations of 100 and 200 mM NaCl was investigated. Treatment of stressed plants and control group (0 mM NaCl) with concentrations of 100, 200 μM melatonin improved effects of salinity on the dry weight of shoot, the root and the photosynthetic pigments, net photosynthesis rate, leaf stomatal conductance, transpiration rate, K+, Na+, and Ca2+ content, and the ratio of K+/Na+. Melatonin application increased proline and sugar content while decreasing malondialdehyde and H2O2 content that were increased by salinity stress. Also, melatonin increased the activity of antioxidant enzymes of catalase, peroxidase, ascorbate peroxidase, polyphenol oxidase, and superoxide dismutase. Overall, lower concentrations of melatonin had significant ameliorating effects on salinity stress-induced damage to the bean plant. While, the use of 400 μM melatonin could somewhat exacerbate the effects of salinity in P. vulgaris L. cv. Pak.