Dynamics of pollen fertility in summer maize under short-term heat stress during anthesis

Heat stress has become a major constraint on crop production under global warming, often occurring as short-term heat waves. However, the precise time point at which maize pollen begins to respond to heat stress remains unclear. In this study, we used a heat-tolerant maize variety (Zhengdan958, ZD-tol) and a heat-sensitive variety (Xianyu335, XY-sens) to investigate pollen responses under heat stress. Plants were exposed to 36±2°C/28±2°C (day/night) for seven consecutive days during anthesis, with 29±2°C/20±2°C (day/night) as the control. Pollen reactive oxygen species (ROS), callose deposition, lipid droplets, and fertility were monitored over time to determine the onset of heat effects. In both maize varieties, heat stress triggered oxidative stress and irregular callose deposition within 30 min but had no significant effect on pollen fertility. After 2 h, pollen lipid droplets decreased, and viability and germination significantly declined by 18% and 29%, respectively, resulting in a 34% reduction in grains per ear. Thus, the 2-h of heat exposure represented a critical window for the onset of pollen fertility decline during anthesis. Compared with ZD-tol, XY-sens exhibited a significantly reduced anther dehiscence area, higher oxidative accumulation in pollen, more severe irregular callose deposition, greater decrease in lipid droplet number, larger losses in pollen quantity and fertility, and more severe yield reduction. Application of ROS inhibitors significantly alleviated heat-induced damage and reduced yield losses. Notably, heat stress during pollen germination caused more severe fertility impairment than stress before germination. These findings highlight the temporal dynamics of pollen responses to heat stress and provide insights for improving maize heat tolerance.