Stage-specific high-temperature stress reduces grain yield by disrupting photosynthesis and antioxidant defenses in summer maize.

High-temperature stress has become increasingly frequent and intense in the Huanghuaihai summer maize region, posing a serious threat to maize production. In this study, two maize cultivars, Anke985 and Helian1589, were subjected to high-temperature stress at four key growth stages: large whorl (V12), tasseling (VT), blister (R2), and milk (R3), with ambient conditions as the control (CK). High-temperature stress was imposed at different growth stages, and key physiological and biochemical traits, including photosynthetic characteristics, chlorophyll fluorescence parameters, antioxidant enzyme activities, dry matter accumulation, and yield components were investigated. High-temperature stress significantly reduced grain yield by decreasing kernel number per ear, kernel number per row, and 100-kernel weight. Correspondingly, grain yield under V12-HT, VT-HT, R2-HT, and R3-HT treatments decreased by 40.03%, 45.13%, 21.72%, and 16.82%, respectively, compared with the control. High-temperature stress impaired photosynthetic performance and antioxidant defense, accompanied by reduced chlorophyll fluorescence efficiency and enhanced oxidative damage. Dry matter allocation to grains was also reduced, particularly under V12-HT and VT-HT, with reductions of 26.48% and 27.68%, respectively, indicating restricted assimilate transport to developing grains. Both maize cultivars showed consistent response patterns. Overall, the adverse effects were most pronounced when high-temperature stress occurred at the V12 and VT stages. Collectively, stage-specific high-temperature stress impaired photosynthesis and antioxidant defense systems, leading to accelerated leaf senescence, reduced dry matter accumulation, and restricted assimilate allocation to developing kernels, thereby ultimately decreasing grain yield.