Rice Genomics and Genetics 2025, Vol.16, No.2, 71-85 http://cropscipublisher.com/index.php/rgg 74 significantly reduced the plant height and the length of the internodes below the ear, making the plant morphology more compact, but at the same time the mechanical strength of the stem at the base decreased. Hu et al. (2025) also found that the increase in the density of direct seeding by drones will lead to thinning of the internodes at the base of rice plants and reduction of the thickness of the internode walls. Although it can reduce plant height and the probability of lodging to a certain extent, the ability of stems to resist bending will decrease instead of increase when planted too densely. The spatial arrangement of leaves in densely planted groups is more overlapping, especially the lower leaves are prone to being in a weak light environment. If the density is appropriate, the canopy of the group can present an "ideal plant type" with upright upper leaves and moderately curved middle and lower leaves, which enhances the upper light interception and lower light transmission; when the density is too high, the lower part of the canopy is prone to premature aging and the functional period of leaves is shortened. For example, Wang et al. (2019) compared the canopy characteristics of groups under different plant spacing treatments. The results showed that the leaf area and dry weight of individual plants in the 18 cm plant spacing (moderate density) treatment were significantly higher than those in the 16 cm high density treatment, and the aging rate of functional leaves was slower, indicating that excessive density weakened the development of individual leaf area and functional maintenance. Therefore, density regulation changes the spatial pattern of photosynthetic production of the group by affecting plant type shaping and leaf area distribution. 3.3 Leaf area index (LAI) and photosynthetic efficiency Density has an important influence on photosynthetic performance and material accumulation. Appropriate dense planting can improve the photosynthetic potential and net productivity of the group. Ling et al. (2024) reported that transplanting strong seedlings cultivated at high density helps to improve the photosynthetic capacity of the group after transplanting. The net photosynthetic rate during the tillering and jointing stages is 2%-5% higher than that of conventional treatments, and the dry matter accumulation is significantly increased. However, excessive dense planting will lead to the obstruction of photosynthetic production in the later stage due to the limited light of the lower leaves and increased respiratory consumption (Figure 1). Liu et al. (2017) found that the group photosynthetic rate of the high density treatment (30 holes/m2) decreased more than that of the moderate density treatment in the late filling period, which was related to the premature senescence of the lower leaves and the decrease in the light transmittance of the group. On the other hand, the sparsely planted group has many tillers and less nutrient competition, and the individual photosynthetic organs are well developed, and the photosynthetic intensity of a single leaf is usually high, but the total photosynthetic amount of the group may be low due to insufficient leaf area. The results of Wang et al. (2019) showed that under the same nitrogen application level, the 18 cm plant spacing treatment maintained a higher population growth rate and photosynthetic potential during the jointing-heading stage, and the net assimilation rate of the population was more than 5% higher than that of the 16 cm and 20 cm treatments, respectively, thereby accumulating more dry matter. This shows that the photosynthetic efficiency of the population is the highest when the density is moderate. In general, density changes the supply pattern of photosynthetic products by affecting the leaf area and leaf function of the population. In order to give full play to the photosynthetic potential of individuals and populations, a density level should be selected that ensures sufficient photosynthetic area of the population without reducing the light efficiency of a single leaf. Density also affects rice lodging resistance and the occurrence of diseases and insect pests. In general, sparsely planted plants have strong individuals, thick and hard stems, and strong lodging resistance, but the overall number of panicles in the population is small and the center of gravity is high, which may also lead to lodging (especially when the plants grow excessively when fertilizer and water are sufficient). Dense planting can reduce the center of gravity of individual panicles, but if the stems are thin and weak and the ventilation and light transmission are poor, the population is more prone to wind lodging and disease spread. Li (2020) compared the effects of different sowing densities on the lodging of direct-seeded rice in a double-season early rice experiment in Nanchang. The results showed that appropriately reducing the sowing density (from 25 kg per mu to 15 kg) significantly increased the stem bending resistance and base internode tensile strength of the group, and the lodging rate dropped from 15.3% of the control to 3.8%, proving that excessive density weakens the lodging resistance of the plant. Other studies have pointed out that under high-density planting conditions, the field canopy humidity is
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