Exogenous melatonin application stimulates growth, photosynthetic pigments and antioxidant potential of white beans under salinity stress

Melatonin (N-acetyl-5-methioxytryotamine) is a nontoxic biological signaling molecule and powerful antioxidiant that can improve plants growth under saline condition. This study investigated the ability of melatonin to alleviate salinity stress in white beans (Phaseolus vulgaris L.) seedlings. Exogenous melatonin with 0, 25, 50, and 100 µM concentrations were applied to white beans growing in non-saline (0 mM NaCl) and saline (150 mM NaCl) conditions. Results indicated that 150 mM NaCl reduced the leaf area by (35%), shoot length (29%), root length (56%), increased in (45%) Na+ influx and higher (50%) membrane damage (MDA) than non-saline control plants. However, melatonin treatments ameliorated the negative impacts of salinity stress. The increasing melatonin concentrations were directly proportional to the plant stress tolerances in which 100 µM melatonin showed prominent results in improving growth parameters and antioxidant capacity. The 100 µM melatonin in salt stressed plants enhanced the leaf canopy by 45%, total seedling length by 55%, and total seedling weight by 54% compared to untreated control plants. Plants tested with melatonin (especially 100 µM) increased the accumulation the non-enzymatic antioxidants e,g polyphenol by (37%), proline by (27%), and ascorbic acid by (38%) as compared to control. In addition, exogenous melatonin (100 µM) also stimulated the activities of antioxidant enzymes defense system by upregulating the superoxide dismutase by 50%, peroxidase by 68%, catalase by 63%, and ascorbate peroxidase enzymes by 68% as compared to 0 µM melatonin. Similarly, the Na+ influxes was significantly reduced to subtoxic levels, and the K+ levels and the K+/Na+ ratio increased at higher exogenous melatonin treatments. In conclusion, the application of melatonin improved salinity stress resistance by enhancing antioxidant capacity and maintaining ion homeostasis.