Exogenous melatonin alleviates post-flowering natural high-temperature stress in maize (Zea mays L.) by promoting photosynthesis and antioxidant defense responses

Introduction: High-temperature stress severely restricts the growth, development and yield formation of summer maize. Foliar application of exogenous melatonin has been widely demonstrated to enhance stress tolerance and alleviate abiotic stress in crops. However, the physiological and molecular mechanisms underlying melatonin-mediated thermotolerance in summer maize under post-flowering high-temperature stress remain unclear. Methods: In this study, field experiments were conducted using foliar application of melatonin at different concentrations on maize. By integrating agronomic, physiological and transcriptomic approaches, we systematically investigated the mechanisms by which melatonin improves the adaptation of summer maize to post-flowering high-temperature stress. Results and Discussion: The results showed that exogenous melatonin increased the net photosynthetic rate of ear leaves and maintained high photosynthetic capacity. Melatonin treatment also significantly increased the activities of key antioxidant enzymes and the contents of antioxidant substances in the maize ear leaves, thereby enhancing the antioxidant defense capacity of the plant. Transcriptomic analysis further revealed that melatonin upregulated the expression of key genes involed in the photosynthetic and antioxidant defense systems of maize. In summary, exogenous melatonin application improved the resistance of summer maize to natural post-flowering high-temperature stress by optimizing photosynthetic function and reinforcing the antioxidant defense system, effectively reducing the damage of high-temperature to maize growth, biomass accumulation and grain yield. The 150 µmol L-1 melatonin treatment exerted the optimal mitigating effect under field conditions. These findings help to elucidate the key mechanisms by which exogenous melatonin enhances thermotolerance in maize under post-flowering high-temperature stress, and provide a theoretical basis for research on of maize stress resistance, disaster reduction, yield improvement, and efficient chemical regulation strategies.