Molecular Plant Breeding 2025, Vol.16, No.4, 211-220 http://genbreedpublisher.com/index.php/mpb 214 reproductive period precisely at the hottest time, such as the V12 to R1 stage. This can easily lead to a reduction in kernels and a decrease in yield (Li et al., 2024). 4.2 Photoperiod sensitivity and stress susceptibility Maize is very sensitive to changes in temperature and light. Early sowing can avoid high temperature and drought in the later stage, reduce heat damage during flowering and pollination, and make it easier to ensure kernel setting and quality (Cao et al., 2024; Li et al., 2024). Conversely, if sown late, maize may encounter problems such as high temperature and little rainfall during the reproductive period. As a result, pollen vitality decreases, filaments age faster, and eventually normal pollination is affected, and the seed setting rate is also low (Zhu et al., 2022). In addition, different sowing times will also affect the nutrition of the kernels. For example, the contents of protein and fiber will change accordingly (Liu et al., 2023). 4.3 Field observations on pollen viability and silk receptivity The observations in the field also support these viewpoints. Appropriate sowing time can make pollen release and filamentous ejection more synchronous, which is conducive to improving pollination effect and seed setting rate. When sown early, both pollen vitality and filament state are relatively good, and it is less likely to have pollen death or pollination failure (Cao et al., 2024; Tsafack et al., 2024). Sowing too late can easily lead to poor pollen vitality and premature aging of filaments, thereby affecting normal pollination and kernel formation. 5 Plant Density and Reproductive Resource Allocation 5.1 Canopy structure and light penetration Increasing the planting density will significantly change the canopy structure of maize. When the density is high, both the leaf area index (LAI) and the light energy received by the population (TPAR) will increase. However, if the density is too high, the lower leaves will be unable to carry out photosynthesis normally due to too weak light (Yang et al., 2021). Under high-density conditions, the area and Angle of the upper layer leaves become smaller, allowing the middle and lower layer leaves to receive more light, and the attenuation of light also becomes slower (the KL value decreases), which helps the entire maize plant to make better use of light energy (Tian et al., 2022). In addition, as long as the variety combination is reasonable and the planting density is properly controlled, dry matter accumulation and yield can also be increased. 5.2 Source-sink balance and nutrient partitioning to kernels Planting density also directly affects the “source-reservoir relationship” of maize, that is, the way plants produce and transport nutrients. When the density is high, more dry matter and nitrogen can be accumulated per square meter, but the nutrients allocated to the roots and kernels per plant will decrease instead, the ratio of roots to stems will be lower, and the harvest index (HI) will also decline (Shao et al., 2024b). High density will concentrate more nutrients in the stems and leaves, resulting in less distribution to the kernels. Therefore, the number and weight of kernels per plant will decrease, but the overall population yield will still increase (Zhang et al., 2020). In this case, if nitrogen fertilizer is appropriately increased, it can help improve the efficiency of nitrogen transport from stems and leaves to kernels, and also enhance the utilization rate of nitrogen fertilizer (Duan et al., 2023; Shao et al., 2024a). 5.3 High-density stress effects on pollination efficiency When the density is high, the competition for resources among plants will also be more intense. As a result, reproductive growth is prone to be affected. For example, the time for filaments to emerge becomes later, male and female are out of sync, pollination effect deteriorates, and eventually leads to a reduction in the number of kernels (Borrás and Vitantonio-Mazzini, 2018). Some varieties can enhance reproductive capacity at high density, such as growing multiple spikes, which can ensure yield as much as possible under the condition of limited nutrients (Ross et al., 2020). However, when the environmental stress is high, the nutrient distribution and filamentesis of the second spike (E2) are both restricted, which will affect its pollination and fruiting (D’Andrea et al., 2022; Parco et al., 2022). Therefore, to maintain synchronized pollination and healthy kernel development under high density, it is necessary to select the right variety, control the planting density well, and ensure the supply of nutrients during the critical period.
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