Maize Genomics and Genetics 2025, Vol.16, No.3, 129-138 http://cropscipublisher.com/index.php/mgg 132 will lead to dehydration and wilting under high temperature conditions, loss of mucus on the surface, and ultimately affect the grain setting rate and grain yield (Waqas et al., 2021). High temperature stress changes the sugar composition of silk, and the supply of hexose inhibits the growth of pollen tubes (Wang et al., 2023). 3.2 Heat-induced disturbances in pollination and fertilization High temperature stress has a destructive effect on the pollination and fertilization process of corn. When the temperature is higher than 32 ℃-35 ℃, it is not conducive to pollination. The interaction stage between pollen and stigma and the early grain development stage are highly sensitive to high temperature stress, which directly leads to a decrease in crop yield (Mitchell and Petolino, 1988). In the post-pollination stage, high temperature stress inhibits the early growth of pollen tubes. The growth process of pollen tubes in the style/filament and ovary transport bundle tissue is more sensitive to high temperature, which in turn causes grain sterility (Lizaso et al., 2018). When the pollinated spikelets are exposed to a high temperature environment exceeding 36°C, continuous high temperature stress can cause abortion of plant fruiting organs during flowering and maturity, and the fertilization rate is significantly reduced. This phenomenon shows that high temperature stress has a serious negative impact on the fertilization process. 3.3 Hormonal and molecular dysregulation in female tissues High temperature stress can cause changes in hormone and molecular levels in female reproductive tissues of maize. Superoxide dismutase (SOD) first decreases and then increases, and the activities of peroxidase (POD) and catalase (CAT) continue to increase, thus affecting their development and function. The accumulation of reactive oxygen species (ROS) and significant changes in the levels of plant hormones (such as abscisic acid and auxin) can affect the growth of pollen tubes. High temperature stress can also induce the synthesis of heat shock proteins (HSPs) in female tissues. Under normal conditions, the content of Hsps is less than 5% of the total protein, but under adverse external environmental conditions, especially when stimulated by heat stress, it will be synthesized in large quantities, accounting for about 15% of the total protein. This is one of the important defense mechanisms of plants to cope with heat stress (Dupuis and Dumas, 1990). 4 Impact of Heat Stress on Seed Setting Rate and Yield Formation 4.1 Reduction in kernel number and grain filling High temperature stress significantly reduces the number of maize grains and affects the grain filling process. High temperature accelerates the dry weight accumulation in the early stage of grain filling, but reduces the dry weight of grains in the mature stage, thus having a negative impact on yield formation. In key reproductive stages such as pollen development and flowering, high temperature stress can cause pollen abortion and reduced stigma receptivity, resulting in reduced fertilization rate and fewer grains (Lizaso et al., 2018; Waqas et al., 2021). Among them, the period from meiosis of pollen mother cells to the early stage of microspore formation is sensitive to high temperature stress. High temperature affects the meiotic process of pollen mother cells, resulting in increased frequency of homologous chromosome crossing and recombination, abnormal chromosome separation, and the formation of incomplete tetrads and microspores with abnormal chromosome ploidy, leading to male sterility and reduced grain number. High temperature stress also impairs grain filling efficiency by interfering with the activities of bound starch synthase (GBSS), soluble starch synthase (SSS), sucrose synthase (SS) and sucrose synthase phosphatase (SPS), resulting in a significant decrease in corn yield (Bheemanahalli et al., 2022). 4.2 Timing and duration of stress exposure In recent years, the frequency of high temperature weather has increased and the duration of stress has prolonged, and the impact on corn has become increasingly serious. During sensitive developmental stages, such as pollen development and the microspore tetrad stage of flowering (especially shortly after pollination), even a short temperature peak can cause severe grain loss (Lv et al., 2024). The reproductive stage, especially the critical period after pollination, is highly sensitive to high temperature stress, and a short temperature increase can cause significant grain abortion (Mitchell and Petolino, 1988). Therefore, it is necessary to clarify the specific exposure time of high temperature stress and develop strategies to alleviate its negative impact on yield, so as to increase crop yield (Hill and Li, 2022; Zenda et al., 2022).
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