Exogenous melatonin enhances heat tolerance of maize at the seedling stage by coordinating light and dark reactions

In recent years, frequent extreme high-temperature events have severely limited maize production in China. Exogenous melatonin (Met) has been shown to enhance crop stress tolerance by improving antioxidant capacity; however, its role in regulating carbon assimilation and primary photochemical efficiency in maize leaves under heat stress remains poorly understood. In this study, the heat-sensitive maize cultivar Xianyu 335 (XY) and heat-tolerant cultivar Denghai 605 (DH) were used to investigate the effects of exogenous Met on photosynthetic responses and chlorophyll fluorescence kinetic curves under heat stress conditions (38℃/28℃) at the six-leaf stage. Compared with water treatment, exogenous Met significantly promoted the accumulation of soluble sugars and proline in maize leaves under heat stress, enhanced plant water content, and increased the activities of superoxide dismutase (SOD), peroxidase (POD), and catalase (CAT), thereby reducing oxidative damage and lowering malondialdehyde (MDA) levels. These improvements alleviated the rise in relative fluorescence at the K phase (300 μs) and L phase (200 μs), mitigated heat-induced damage to the oxygen-evolving complex, relieved electron transport blockages, and increased the number of PSII reaction centers per unit area. Under Met treatment, the maximum photochemical efficiency (<italic>ϕ</italic>_Po) increased by 29.1% in XY and 17.2% in DH. Furthermore, Met enhanced the activities of ribulose-1,5-bisphosphate carboxylase/oxygenase (Rubisco) and phosphoenolpyruvate carboxylase (PEPC), especially in the heat-sensitive XY. Photosynthetic response analysis showed that under Met treatment, the maximum net photosynthetic rate (<italic>P</italic>_nmax) increased by 38.6% in XY and 30.8% in DH, while photosynthetic capacity (<italic>P</italic>_c) increased by 30.2% and 19.2%, respectively. In addition, Met reduced the photorespiration rate (<italic>R</italic>_p) and increased the light saturation point (<italic>I</italic>_sat), CO₂ saturation point (<italic>C</italic>_isat), and maximum carboxylation efficiency (<italic>V</italic>_cmax). Overall, these findings indicate that exogenous Met enhances osmotic adjustment and antioxidant defense in maize leaves, synergistically improves both light and dark reactions of photosynthesis, and effectively mitigates heat stress-induced damage to net photosynthesis (<italic>P</italic>_n) and early-stage plant growth.