Heat stress is a major constraint to current and future maize production at the global scale. Male and female reproductive organs both play major roles in increasing seed set under heat stress at flowering, but their relative contributions to seed set are unclear. In this study, a 2-year field experiment including three sowing dates in each year and 20 inbred lines was conducted. Seed set, kernel number per ear, and grain yield were all reduced by more than 80% in the third sowing dates compared to the first sowing dates. Pollen viability, silk emergence ratio, and anthesis–silking interval were the key determinants of seed set under heat stress; and their correlation coefficients were 0.89***, 0.65***, and –0.72***, respectively. Vapor pressure deficit (VPD) and relative air humidity (RH) both had significant correlations with pollen viability and the silk emergence ratio. High RH can alleviate the impacts of heat on maize seed set by maintaining high pollen viability and a high silk emergence ratio. Under a warming climate from 2020 to 2050, VPD will decrease due to the increased RH. Based on their pollen viability and silk emergence ratios, the 20 genotypes fell into four different groups. The group with high pollen viability and a high silk emergence ratio performed better under heat stress, and their performance can be further improved by combining the improved flowering pattern traits.
