Application of Exogenous 24-Epibrassinolide at the Silking Stage Alleviates the Effects of Post-Silking Heat Stress on Photosynthetic Performance of Waxy Maize

Heat stress (HS) during the grain-filling stage severely limits yield in waxy maize by impairing leaf physiology and suppressing photosynthetic capacity. Although exogenous brassinosteroids are recognized for enhancing thermotolerance, their specific role in sustaining photosynthetic performance in waxy maize under HS has not been thoroughly investigated. This study investigated whether exogenous 24-epibrassinolide (BR) application could alleviate HS-induced damage in two waxy maize hybrids. Plants were exposed to HS with/without BR treatment over two growing seasons. Yield components, photosynthetic parameters, chloroplast ultrastructure, antioxidant enzyme activities, and physiological traits were analyzed. HS during the grain-filling stage significantly reduced maturity yields (SYN5: −42.8%; YN7: −39.0%) by impairing photosynthetic efficiency, chloroplast integrity, antioxidant capacity, and the translocation amount and rate of vegetative organs photosynthate after pollination. Chloroplasts exhibited structural disorganization and pronounced swelling. Photosynthetic pigment content and enzyme activities (ribulose-1,5-bisphosphate carboxylase/oxygenase, phosphoenolpyruvate carboxylase) declined, while reactive oxygen species accumulation increased. Exogenous BR substantially restored yields by preserving the chloroplast ultrastructure, enhancing photosynthetic function, reactivating antioxidant enzymes (ascorbate, catalase, superoxide dismutase), and reducing oxidative damage. BR also optimized hormone balance (reduced abscisic acid, increased indole-3-acetic acid) and elevated soluble protein/sugar contents. Meanwhile, BR reversed the negative effects of HS on dry matter accumulation and translocation. SYN5 suffered greater HS damage but exhibited stronger BR-mediated recovery than YN7. In conclusion, BR application mitigates HS by protecting the chloroplast structure, boosting photosynthetic resilience, and activating antioxidant defenses, offering a strategy to safeguard waxy maize productivity under HS.