Field Crop 2025, Vol.8, No.6, 284-292 http://cropscipublisher.com/index.php/fc 287 inadequate. The seedlings will grow slowly during the seedling stage, and the yield in the later stage will be impossible to catch up. In some years, some sensitive varieties still encountered low temperatures at the heading stage after late sowing, and the seed setting rate decreased particularly significantly (Gao et al., 2024; Feng, 2025). There is also the issue of rain. If it rains too heavily during the sowing days, the seedlings may not grow at all. On the contrary, if it is too dry, no seedlings will emerge. Whether the sowing time is chosen well or not sometimes determines the success or failure of a season of rice. 4 Regulation of Yield and Population Structure by Planting Density 4.1 Effects on photosynthetic efficiency and leaf area index Whether photosynthetic efficiency is affected or not, planting density is often an unavoidable variable. Leaf area index (LAI) is closely related to density. When density increases, LAI generally increases as well, and the ability to intercept sunlight also improves accordingly. However, it's not the case that the higher the density, the better. Too many leaves block each other, and the lower leaf surfaces cannot get sunlight, which instead slows down the net photosynthetic rate. On the contrary, a density that is too low is also not good. Although there is sufficient light, the overall photosynthetic output cannot be increased. The conclusions given by several studies are actually quite consistent: Moderate density is the safest and can enable the crop population to have a good performance in both biomass and dry matter accumulation (Zhao et al., 2024). 4.2 Mechanisms affecting panicle number, grain number, and seed-setting rate When the density goes up, the number of ears will indeed increase, but this is only the aspect that seems to indicate a stable yield. In fact, in a high-density environment, water, fertilizer and light all need to be seized. After the competition intensifies, the number of grains per spike often drops, and the seed setting rate is also affected accordingly (Wang et al., 2022). Under low-density planting, although the total number of panicles is slightly smaller, the number of grains that each plant can support is actually greater, and each panicle can produce more seeds, which can somewhat make up for the loss in the number of panicles. However, one should not be overly optimistic. If the density is too low, there is also an upper limit to this "panicle grain supplementation" mechanism. Ultimately, it is still necessary to find that "critical point" - to keep both the number of panicles and the number of grains per panicle in an optimal state, so that the yield can be more stable. 4.3 Advantages and disadvantages under high and low density conditions Under high density, the plants are densely distributed, and the field looks very "full". The growth is fast in the early stage, the biomass accumulation is large, and the yield is sometimes indeed high (Hou et al., 2019). However, the problems are not small either: excessive shading leads to poor grain filling, slow grain development, and it is also prone to lodging and pests and diseases. In the case of low density, although the number of panicles decreased somewhat, the grains were plump, the ventilation was good, the stem strength was improved, and the overall lodging rate was low (Bueno et al., 2023). However, it should be noted that the density cannot be reduced infinitely; otherwise, the total output may be held back due to insufficient number of ears. Ultimately, the setting of density depends on the variety, as well as the plot and climate. Neither too high nor too low is good. What matters most is suitability. Generally, slightly increasing the density can not only maintain the number of panicles but also help improve the grain filling rate. Most importantly, it is less likely to cause problems such as lodging or a drop in the photosynthetic efficiency of the population. 5 Interaction Effects Between Sowing Date and Planting Density 5.1 Mechanisms of joint regulation on growth period The growth process of rice is ultimately driven by temperature and light conditions, but the adjustment of sowing time and planting density will play a role of "adding fuel to the fire" or "stepping on the brake" on this basis. Early sowing can prolong the vegetative growth period and is inherently beneficial for tillering. When a medium to high density is added, the population in the field will thrive, and the number of tillers and biomass will also increase accordingly. However, once sowing is delayed, the duration of sunlight shortens and the temperature fails to keep up, thus compressing the entire growth period significantly. At this point, even if you increase the density further, the tillering number is still prone to drop, especially in groups with intense competition for light and heat.
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