Enhancement of grain yield in rice under combined drought and high-temperature stress conditions by maintaining photosynthesis through antioxidant enzyme activities by melatonin

Rice (Oryza sativa L.) is susceptible to drought or high-temperature stress, and the response of rice to combined drought and high-temperature stress was not studied in detail under field condition. Moreover, rice will be susceptible to combined drought and high-temperature stress during its sensitive phases. Melatonin is recognized as a multifunctional hormone that plays a pivotal role in plant stress defense mechanisms. This study aimed to evaluate the efficacy of melatonin under combined drought and high-temperature stress conditions in rice. A field experiment was conducted in a randomized block design with four replications. Five treatments viz., (i) absolute control (no stress and no melatonin application), (ii) stress control (combined drought and high-temperature stress and no melatonin spray), (iii) combined stress + application of 200 mu M melatonin as a seed treatment, (iv) combined stress + application of 200 mu M melatonin as a foliar spray, and (v) combined stress + application of 200 mu M melatonin as seed treatment and foliar spray. Combined drought and high-temperature stress were applied during the flowering stage for 15 days. Results showed that combined drought and high-temperature stress significantly decreased the chlorophyll index, photosystem II quantum yield, photosynthetic rate, stomatal conductance, and transpiration rate due to membrane damage. In contrast, the application of melatonin @ 200 mu M increased the antioxidant enzymes viz. superoxide dismutase, catalase, and ascorbate peroxidase activities. The enhanced antioxidant enzymatic activity prevented membrane damage and improved the gas exchange traits, nitrogen metabolism (nitrate reductase activity, proline, and soluble protein content), and yield-related traits in rice under combined drought and high-temperature stress conditions.