High temperatures are a major concern that limit rice germination and plant growth. Although previous studies found that melatonin can promote seed germination, the physiological regulation mechanism by which exogenous melatonin mediates high temperature tolerance during rice seed germination is still largely unknown. In order to overcome these challenges, the present study investigates the effects of melatonin on the characteristics of rice seed germination as well as on antioxidant properties, under different high temperature conditions. The results show that 100 μM melatonin seed-soaking treatment under high temperature conditions effectively improves the germination potential, the germination index, and the vigor index of rice seeds; increases the length of the shoot and the root; improves the activity of the antioxidant enzymes; and significantly reduces the malondialdehyde content. The gray relational grade of the shoot peroxidase activity and the melatonin soaking treatment was the highest, which was used to evaluate the effect of melatonin on the heat tolerance of rice. The subordinate function method was used to comprehensively evaluate the tolerance, and the results show that the critical concentration of melatonin is 100 μM, and the critical interactive treatment is the germination at 38 °C and followed by the recovery at 26 °C for 1 day + 100 μM. In conclusion, 100 μM of melatonin concentration improved the heat resistance of rice seeds by enhancing the activity of the antioxidant enzymes.

Drought is a severe abiotic stress that imposes a serious threat to rice productivity. Although previous studies have found that melatonin can promote seed germination, the physiological regulation mechanism of drought tolerance in rice seed germination mediated by exogenous melatonin is still unclear. In order to overcome these challenges, polyethylene glycol 6000 (PEG6000) at concentrations of 20% and 35% was used to simulate osmotic stress. Rice seeds were treated with different concentrations of melatonin (i.e., 0, 20, 100, and 500 μM) to study the effects on germination characteristics, growth and development, superoxide dismutase (SOD) activity, peroxidase (POD) activity, catalase (CAT) activity, malondialdehyde (MDA) content, and soluble protein content. The results showed that the seed soaking treatment using melatonin at a concentration of 100 μM under drought stress effectively promoted the germination rate and improved the biomass of rice seed shoots and roots. Meanwhile, this treatment reduced MDA content to alleviate the oxidative damage of rice seeds caused by drought stress. The two-way ANOVA showed that the effect of single melatonin soaking treatment on rice seed germination was more significant than that of single drought stress and the interaction of drought stress and melatonin (p < 0.05). Using the membership function method, it was shown that the critical gradient of rice seeds under drought stress was 35%, and the critical treatment of interactive treatment was 35% + 100 μM. Through grey correlation analysis, it was found that germination rate (7 d) had the highest grey correlation with melatonin seed soaking treatment to evaluate the mitigation effect of melatonin on drought stress. This study provides a theoretical basis for light and simple cultivation technology for the dry direct seeding of rice.