Effects of high temperature stress during pollination on plant morphology, leaf photosynthetic characteristics, and yield of summer maize

High temperatures during the flowering period seriously affect the safe production of summer maize in the Huang-Huai-Hai region. In this experiment, the heat-sensitive variety ‘Xianyu 335’ was used as the test material; the effects of high temperature stress during pollination on plant morphology, leaf photosynthetic characteristics, dry matter accumulation and distribution, and yield of summer maize were studied by setting up normal field temperature treatment (CK) and high temperature treatment during pollination (HT). The results showed that in 2021 and 2022, the number of days with maximum canopy temperatures exceeding 40 ℃ in the HT treatment group was 7 and 8 d, respectively, and the maximum canopy temperatures in the HT treatment group were higher than those in the CK group by 1.7−6.8 ℃ and 1.5−4.6 ℃, respectively. HT treatment significantly increased the plant height and ear height of summer maize but had no significant effect on stem diameter and green leaf area during the high-temperature treatment period. However, HT treatment delayed leaf senescence in the late reproductive stage of maize, and the green leaf area at maturity was 34.69% and 163.72% higher than that in the CK group in 2021 and 2022, respectively. During the high-temperature treatment period, leaf stomatal conductance, transpiration rate, and intercellular CO₂ concentration were significantly higher and leaf carboxylation efficiency, stomatal limitation value, and water use efficiency were significantly lower in the HT treatment group than in the CK treatment group. The net photosynthetic rate of maize leaf in the HT treatment group varied with the treatment temperature: it significantly reduced when compared with that in the CK treatment group only when HT treatment temperature was too high (generally >40 ℃). High-temperature stress during pollination led to a decrease in the overall photosynthetic performance of the maize leaves. After exposure to 10 d of high temperature stress during pollination, the dry weights of maize stems, leaves, bracts, cobs, and individual plants decreased significantly. The dry weight of the cobs decreased the most, while those of the male ears and filaments increased significantly. HT treatment resulted in an increase in the partitioning of dry matter to stems, leaves, male ears, and filaments, and a significant decrease in partitioning to the cobs. At maturity, HT treatment significantly reduced the dry weights of maize grains and plant by 48.32% and 16.71%, respectively, while those of maize stems and leaves increased by 35.01% and 9.48%. HT treatment resulted in a significant decrease of 54.43% and 53.19% in the seed setting rate and grain number per ear, respectively; a significant increase of 10.13% in 100-grain weight; and a significant decrease of 46.82% in the grain yield. In conclusion, high temperature stress during pollination enhanced the stomatal transpiration of maize leaves, increased intercellular CO₂ concentration, and decreased leaf carboxylation efficiency and water use efficiency. It also led to a decline in the overall photosynthetic performance of the plant and restricted the accumulation of photosynthetic products and their transfer and partitioning to the ear, resulting in a significant decrease in seed setting rate and grain number per ear. This decline also restricted the post-flowering transport of photosynthesized assimilated compounds from the “source” (stems and leaves) to the “sink” (grains), which ultimately led to a significant decrease in grain yield.