Rice Genomics and Genetics 2025, Vol.16, No.2, 71-85 http://cropscipublisher.com/index.php/rgg 76 4.1 Changes in panicles, grain number, and 1 000-grain weight Planting density determines the number of effective panicles per unit area and is the primary factor affecting yield. Generally speaking, increasing density can significantly increase the number of effective panicles, while reducing the number of grains per panicle and the fruiting rate; conversely, reducing density increases the number of grains per panicle and grain weight, but reduces the number of panicles. This trade-off effect determines that the yield-density relationship is a quadratic curve of "first increase and then decrease". Many experiments have verified that the optimal density corresponds to the peak yield: Zhu et al. (2023) reported that the yield of drone-sown rice was the highest when the basic seedlings were 1.95 million plants/hectare, an increase of about 6% from 1.5 million plants/hectare, while the yield decreased when it continued to increase to 2.4 million plants/hectare. Hu et al. (2025) also found in the drone direct seeding experiment that both types of rice varieties achieved the highest yield at the intermediate density (about 30 plants/m2 for conventional japonica rice and about 12 plants/m2 for two-line hybrid rice), and their yield increased by 4%-6% compared with the low density and 6%-8% higher than the highest density. These results show that the combination of various elements of yield composition can be optimized by adjusting the density to maximize the yield. Specifically, density mainly affects the yield composition through the following channels: Effective panicle number: Increasing density usually linearly increases the number of effective panicles per unit area, which is one of the main contributing factors to the increase in yield by dense planting. Under mechanical transplanting conditions, when the basic seedling density was increased from 16 holes/m2 to 20 holes/m2, the number of ears increased by 8%-12%, and the yield increased accordingly (Hou et al., 2019). The high-density treatment produced 2-3 more ears per square meter than the conventional density, and the yield increased by about 5% (Tang et al., 2020). However, when the density is too high, the ability of some tillers to form panicles decreases, and even "spikelets" or deformed panicles appear in the population, resulting in the number of effective panicles no longer increasing or even decreasing with increasing density. Therefore, there is a critical density that makes the number of effective panicles reach the maximum value. Number of grains per panicle: dense planting tends to reduce the number of grains per panicle, including the number of spikelets and the number of fruiting grains per panicle on primary and secondary branches. Hu et al. (2025) observed that the total number of grains per panicle decreased from low to high density. For example, when the density of ordinary japonica rice was reduced from 15 plants/m2 to 30 plants/m2, the number of grains per panicle decreased from about 185 to 163, a decrease of about 12%. The two-line hybrid rice also decreased by a similar amount. This is because the vegetative growth in high-density populations is inhibited and the panicle type of individual plants becomes smaller. At the same time, the excessively high density leads to insufficient light in the panicle area, and the limited development of floral organs will also reduce the number of spikelets per panicle. In contrast, sparse planting can form large panicles because the individual plants have sufficient nutrition, and the number of branches per panicle, especially the secondary branches, increases. For example, Huang et al. (2018) found that the average number of spikelets per ear in the treatment of multi-seedling traditional machine transplanting (relatively low density) was about 15 more than that in the treatment of single-seedling high-density machine transplanting. However, the decrease in the number of grains per ear is often offset or even exceeded by the increase in the number of ears, so the total number of spikelets (the total number of grains per ear per square meter) still increases within a certain range of density. For example, when compared under low nitrogen treatment, the total spikelet number in the densely planted population was still 4.5% higher than that in the sparsely planted population, although the number of grains per ear was reduced ( Yang et al., 2019 ). Grain setting rate and grain weight: Due to fierce resource competition in high-density groups, grain filling may be affected, which is manifested as a decrease in grain setting rate and a slight decrease in thousand-grain weight. Chen et al. (2023) showed that increasing planting density would lead to a decrease in rice grain setting rate by 0.5-1.5 percentage points, but had little effect on thousand-grain weight. Yang et al. (2019) also reported that the fruit set rate of the dense planting treatment was slightly lower than that of the conventional treatment by about 2 percentage points, mainly because the small flowers at the bottom of the panicle were easily empty and barren due
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