Exogenous melatonin application mitigates the adverse impact of drought stress at panicle initiation and anthesis stages in rice (Oryza sativa)

Foliar application of plant growth regulators (PGRs) and nutrients are being employed increasingly as a management option in recent years. Such a new compound to be explored for stress mitigation is melatonin. Melatonin, an indoleamine compound, plays an important role in plant stress defense. Studies reveal that exogenous treatment or ectopic overexpression of melatonin biosynthetic genes improve the resistance against a series of stressors in crops. But there is still opacity about the mechanisms through which melatonin confers drought tolerance in rice. To clear this ambiguity, we studied the impact of foliar application of melatonin in rice under water-stressed conditions. A glasshouse experiment was conducted at the Department of Crop Physiology, Tamil Nadu Agricultural University, Coimbatore during kharif 2021. The study focussed on standardizing the optimum melatonin concentration and disseminating the physiological traits involved in conferring melatonin-mediated drought tolerance in rice. The trial was laid in a factorial completely randomized design with six treatments imposed at panicle initiation and anthesis stages in a separate set of plants replicated four times. Drought stress was imposed at panicle initiation and anthesis by withholding water for seven days, while the absolute control was watered regularly. Foliar spray of 200 and 250 µM melatonin was given, when the absolute control and control plants were left untreated. The results interpreted that drought stress significantly altered the chlorophyll content, leaf water status, lipid peroxidation and electrolyte leakage. The exogenous application of melatonin alleviated the adversative effects of drought stress by improving the osmolyte accumulation and antioxidant enzyme activity, including catalase, superoxide dismutase and ascorbate peroxidase. Therefore, it is concluded that foliar application of 200 µM melatonin was most effective in mitigating the negative influences of drought stress in rice by enhancing the antioxidant system which effectively scavenges the drought-induced reactive oxygen species.