Melatonin improves tolerance to salt stress and boron toxicity in soybean and pepper plants

The role of melatonin (MT) in promoting tolerance to salt stress and boron toxicity in soybean and pepper plants was investigated. The treatments comprised: Control (C), salt stress (SS; 100 mM NaCl), boron toxicity (BT; 2 mM B) and SS + BT. Harmful effects of combined stresses resulted in a greater decrease in plant biomass and chlorophyll contents in both plants compared to those of a single stress. However, SS and BT increased electron leakage, and contents of H2O2, proline and MDA, while an increase was observed in the activities of POD, SOD and CAT. Foliar applied MT (100 μM) caused an increase in fresh and dry mass and photosynthetic pigments of plants of both crops under both stresses imposed singly or in combination. However, melatonin reduced electron leakage, and the contents of H2O2 and MDA. Proline contents in the pepper plants increased with MT application, while they decreased in the soybean plants. The activities of vital antioxidant enzymes, POD, SOD and CAT were found to be increased with MT application in both plants.