Rice Genomics and Genetics 2025, Vol.16, No.2, 71-85 http://cropscipublisher.com/index.php/rgg 73 individual development. This verifies that density optimization requires a balance between group advantages and individual vitality to achieve a match between the source (leaf photosynthetic products) and the sink (grain capacity). Density also involves the theory of rice population quality. High-yield populations usually require sufficient seedlings in the early stage, reasonable reduction of ineffective tillers in the middle stage, and maintenance of functional leaf activity in the later stage, that is, "enough seedlings in the early stage, stable spikelets in the middle stage, and strong seeds in the later stage" (Huang et al., 2024). If the density is too low, it is difficult to form a sufficient seedling population in time, and if it is too high, the canopy will be closed in the middle stage and prone to premature aging. Chen et al. (2014) pointed out that there is an interaction between planting density and nitrogen application level, which has a significant impact on population dynamics: moderate density combined with medium nitrogen fertilizer level can establish a high-yield population structure with appropriate number of panicles and slow decay of population leaf area. This theoretical basis shows that density optimization often needs to be combined with fertilizer management, and the optimal configuration between panicle number and panicle size can be achieved by cultivating strong seedlings and controlling tillering. The theoretical basis for optimizing rice planting density is: by adjusting the basic number of seedlings, the population structure is promoted to develop in the direction of having sufficient panicles without damaging individual capabilities, so as to achieve the unity of "maximum productivity of the population" and "optimal function of the individual". This process involves multiple mechanisms such as light energy utilization, growth resource allocation, and source-sink coordination. It is necessary to comprehensively consider variety characteristics and environmental conditions to determine the optimal density range. 3 Analysis of the Impact on Growth and Development Planting density directly determines the individual growth environment and group ecological conditions of rice, and has a significant impact on the morphological construction, physiological characteristics and growth and development process of the plant. It is mainly reflected in tillering dynamics, plant structure, photosynthetic performance and lodging resistance. 3.1 Tiller dynamics and productive panicles Density affects tillering occurrence and ear formation rate. Generally speaking, reducing planting density (sparse planting) is conducive to increasing the number of tillers per plant, but too many tillers are often difficult to all ear, resulting in an increase in the proportion of ineffective tillers. On the contrary, increasing density (dense planting) can inhibit the number of tillers per plant, prompting limited tillers to stagnate earlier, thereby increasing the tillering ear formation rate. In the study of Yang et al. (2019), the number of tillers per plant of machine-transplanted late rice was significantly reduced under low nitrogen combined with dense planting treatment, but the total number of ears in the group was equivalent to that of conventional treatment, and the ear formation rate increased by about 5 percentage points, indicating that dense planting helps control ineffective tillers and optimize the group ear structure. Under mechanical transplanting conditions, close planting is often combined with the "one seedling per hole" technology to reduce the number of basic seedlings per pile but increase the number of holes. It is reported that compared with the conventional sparse planting of multiple seedlings per hole, single dense planting can reduce the maximum number of tillers per plant by about 20%, while the number of effective ears is only slightly lower or equivalent, so the ear efficiency (effective tillering rate) is significantly improved. It can be seen that close planting achieves the effect of streamlining individual tillers and improving ear formation efficiency by "controlling tillers with groups". 3.2 Plant height, lodging, and canopy structure Density changes plant type and canopy structure. Under sparse planting conditions, the plant spacing is large, individual competition is small, and individual plants can form a relatively open plant type and thick stems. When densely planted, the plants tend to be upright and compact, which is manifested by increased plant height, thinner stem base, and narrower and upright leaves. Hu et al. (2023) showed that increasing the planting density
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