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We studied the effects of melatonin on soybean photosynthesis and seed growth at different developmental stages under drought stress induced by polyethylene glycol in the growth medium. Experimental groups were supplied with a normal nutrient solution after seven days of continuous treatment; overall, five treatments were completed when the early maturity stage was reached. Under normal water supply, melatonin supplementation had no significant effect on the photosynthetic characteristics or soybean seed growth. However, drought stress reduced leaf relative water content, chlorophyll content, and relative photosynthetic gas-exchange parameters, while it increased a leaf content of osmoregulatory substances, antioxidant-related enzyme activities, and seed growth-related indicators. Compared to the drought-stressed group, the melatonin treatment under drought stress improved the above indicators; specifically, melatonin effectively increased the photosynthetic rate, 100-seed mass, and total seed mass per plant. All the effects were most significant during the first seven days after the early seed-filling stage.

Drought stress reduces soybean yield. Exploring the mechanism of improving drought tolerance and reducing yield loss is of great significance for soybean production. Melatonin application can alleviate the growth inhibition and oxidative damage of plants under drought stress. In this experiment, the effects of foliar application of melatonin on photosynthesis, stress resistance, carbon and nitrogen metabolism and yield of soybean during seed filling stage under drought stress were studied in 2017-2018. The application exogenous melatonin increased the antioxidant enzyme activity, inhibited the production of reactive oxygen species, decreased cell membrane damage under drought stress, alleviated the inhibition of photosynthetic capacity by drought stress, improved the carbon and nitrogen assimilation ability, and alleviated the yield loss caused by drought stress. Compared with drought stress, the treatment of melatonin increased the number of pods per plant, the grain number per plant and the hundred grain weight by 2.9%, 0.8%, and 17.2% on average of two years, respectively, and the yield (grain weight per plant) increased by 14.7%.

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