Exogenous melatonin (MT) enhances salt tolerance of okra ( Abelmoschus esculentus L.) plants by regulating proline, photosynthesis, ion homeostasis and ROS pathways

Melatonin (MT) is a ubiquitous molecule with pleiotropic roles in plant adaption to stress. However, physiological and biochemical mechanisms of enhancing salt tolerance of okra plants based on application of MT is poorly understood. In this study, the application of the investigated 100 µM MT significantly improved salt tolerance of okra plants based on the germination rates and culture pot experiment under salt stress. Based on optimal concentration of MT inducing salt tolerance of okra plants, application of 5 mM DPI and 100 µM MT + 5 mM DPI, respectively, significantly increased salt tolerance of okra plants. Assays on the content of component matter indicated the significant increases in proline, photosynthetic pigment, K + , relative water, glutathione (GSH) and ascorbic acid (AsA) content and the significant decreases in Na + , hydrogen peroxide (H 2 O 2 ), superoxide radicals (O 2− ) and malondialdehyde (MDA) content and electrolyte leakage rates were observed in okra plants with 100 µM MT under salt stress. Enzymatic activities in terms of pyrroline-5-carboxylate synthase (P5CS), superoxide dismutase (SOD), glutathione peroxidase (GPX), catalase (CAT), glutathione reductase (GR), ascorbate peroxidase (APX) and peroxidase (POD) activities were significantly increased in okra plants by applied MT under salt stress. Furthermore, the marked decrease of H 2 O 2 , O 2− and MDA content, and the significant increase of CAT, GR and APX activities and GSH and ASA content involved in ASA-GSH cycle were observed in MT-okra, DPI-okra and MT + DPI-okra plants under salt stress. H 2 O 2 acted a toxic accumulation of ROS rather than as a second messenger for signal transduction in okra plants. These results demonstrate that exogenous MT increases the accumulation of proline and ratio of K + /Na + , which enhance salt tolerance of okra plants, by regulating osmotic balance and ion homeostasis, protecting membrane integrity and photosynthesis and activating ROS scavenging system. As a conclusion, treating okra plants with 100 µM MT could alleviate the harmful effects of salinity stress.