Rice Genomics and Genetics 2025, Vol.16, No.2, 71-85 http://cropscipublisher.com/index.php/rgg 77 to light and nutrient restrictions. The sparse planting group has a stronger assimilation capacity per plant, and the fruit set is sufficient, and the thousand-grain weight is often slightly higher. But in general, compared with the changes in the number of panicles and the number of grains, the density has a smaller effect on the fruit set rate and grain weight. In actual production, the thousand-grain weight is more determined by the genetics of the variety, and it is difficult to significantly change the grain weight by adjusting the density. Therefore, the change in yield with density is mainly reflected by the increase and decrease of the number of panicles and the number of grains per panicle. There is an obvious trade-off relationship between planting density and the three elements that constitute rice yield. Density increases → number of panicles increases → number of grains per panicle decreases → yield presents an inverted U-shaped curve. In order to obtain the highest yield, it is necessary to choose a balance point so that the product of the number of panicles and the number of grains per panicle is maximized and the fruit set rate is guaranteed. A large number of studies and production practices have shown that this equilibrium density usually corresponds to the suitable range of basic seedlings under different cultivation modes: for example, ordinary hybrid rice cultivated by transplanting is 16-20 holes/m2, japonica rice is 20-30 plants/m2, and direct-seeded rice is about 1-1.5 million seedlings/hectare (equivalent to 15-22.5 plants/m2). Beyond this range, some aspects of the group or individual will become limiting factors. For example, Jiang et al. (2021) pointed out in their study on hybrid rice under high temperature and high humidity that reducing nitrogen fertilizer combined with moderately dense planting (basic seedlings are about 30 plants/m2) can improve the coordination of panicle-grain structure, thereby increasing both yield and nitrogen fertilizer utilization efficiency. On the contrary, blindly dense planting at the same nitrogen level will significantly reduce the fruit set rate and grain weight, although the number of panicles will increase, and the final yield will decrease instead of increase. 4.2 Nonlinear yield-density relationships In addition to yield, planting density also affects indicators such as rice quality and fertilizer utilization efficiency. Generally speaking, dense planting will reduce the rice quality to a certain extent. The main reason is that the supply of assimilates in the later stage of the dense planting population is relatively insufficient, resulting in poor grain enrichment, decreased grain weight and increased chalkyness. Chen et al. (2023) found that high-density treatment reduces the appearance quality of rice and the cooking flavor quality, such as the rice rate and amylose content, both of which have decreased. Hu et al. (2017) also reported that increasing density under machine insertion conditions reduced the gel consistency and protein content of the grains, and the rice quality slightly worsened. This reminds you to pay attention to quality changes while pursuing output. It is worth noting that density affects quality often interacts with fertilization level: high-density populations are prone to produce more serious problems of chalky rice and excessive protein under high nitrogen conditions, while moderate reduction of nitrogen fertilizers combined with dense planting can partially alleviate the decline in quality. Therefore, in high-yield cultivation, it is often achieved by optimizing the fertilizer and density combination to achieve "both high-yield and high-quality". In terms of nutrient utilization, appropriate density increases are usually beneficial to improving nitrogen fertilizer utilization efficiency (NUE). Close transplantation can increase the total amount of nitrogen absorption in the population and increase nitrogen absorption per unit area. Hou et al. (2019) showed that under mechanical transplantation conditions, the partial productivity of nitrogen fertilizers and nitrogen recovery rate were significantly improved under higher density (25% increase in basic seedlings per hole) treatment. Jiang et al. (2021) also pointed out that tight planting and nitrogen reduction can achieve high yield and high efficiency by "smashing nitrogen supplementation": in the southern double-season rice area, the yield of nitrogen fertilizer reduced by 15% under high temperature and high humidity conditions and increased by 20% of basic seedlings is the same as usual, but the nitrogen fertilizer utilization rate has increased by more than 8%. It can be seen that density optimization is one of the important measures to achieve efficient fertilizer saving. In addition, density affects the mass water transpiration and soil nutrient consumption rhythm, thus related to the efficiency of water and fertilizer utilization. Generally, dense planting groups consume water and fertilizer faster in the early stage,
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