Legume Genomics and Genetics 2025, Vol.16, No.6, 253-269 http://cropscipublisher.com/index.php/lgg 255 adaptation caused by dense planting has a "double-edged sword" effect: moderate plant height increase and canopy compactness are conducive to improving the photosynthetic capacity of the group, but excessive leggy growth and reduced branching will weaken individual productivity. Therefore, in dense planting cultivation, the variety characteristics (such as plant compactness and density tolerance) should be combined to regulate the group structure so that the plant morphological adaptation and high yield formation are coordinated. 2.2 Group competition and photosynthetic efficiency regulation mechanism The main purpose of increasing planting density is to enhance the utilization rate of light energy, space and nutrient resources by the group. Under dense planting conditions, the number of plants per unit area increases, the interception of light by the group is significantly improved, and the utilization of Photosynthetically Active Radiation (PAR) is more sufficient. Studies have shown that the canopy interception rate of high-density soybean populations in the middle growth period can be close to 95%-98%, which is significantly higher than the level of less than 96% under conventional density. Higher interception rate leads to an increase in total photosynthetic production: the dry matter accumulation of the population under dense planting treatment is about 15% higher than that of the control, and the advantage is particularly obvious in the late growth period (R5 of grain filling stage to R7 of maturity stage) (Maitree and Toyota, 2017). However, there is also fierce light competition and interlayer shading within the dense planting population. The upper leaves often obtain too much light and become light saturated, while the lower leaves have reduced photosynthetic efficiency due to long-term low light. This uneven light will increase the growth differences of individuals in the population, which is not conducive to yield stability. By uniform planting (that is, ensuring that the plants are evenly distributed in the field rather than clustered), the coefficient of variation of light interception and assimilation capacity between plants can be significantly reduced, thereby improving the consistency and synergy of the population. The field experiment of Xu et al. (2021) showed that the coefficient of variation of the grain weight of soybeans planted in uniform strips at high density was 71.5% lower than that of non-uniform distribution, and the group yield was higher and more stable. The shade environment caused by dense planting will also induce a series of physiological responses, such as changes in chlorophyll fluorescence parameters and regulation of hormone levels, to adapt to the low light intensity environment. Studies have shown that under low light conditions, soybean leaves improve light energy utilization efficiency by reducing the light saturation point and maximum photosynthetic rate, while allocating more carbon to stem elongation to break through the upper light limitation. These adaptation strategies alleviate the light energy utilization conflict caused by dense planting to a certain extent, enabling the group to maintain a high net photosynthetic productivity. However, when the density is too high, excessive light competition will cause premature aging of the lower leaves and a decrease in net assimilation, which may eventually offset the benefits of dense planting and increased yield. Therefore, to achieve dense planting and increased yield, it is necessary to take measures (such as breeding dense-tolerant varieties, optimizing row spacing and sowing methods) to reduce excessive competition between plants and maintain coordinated growth of the group while improving the photosynthetic efficiency of the group. 2.3 Dynamic trade-off between pod number and yield per unit area The impact of dense planting on the yield components of legume crops is complex and critical. Generally speaking, as the planting density increases, the resources available to each plant decrease, resulting in a decrease in the number of pods per plant, the number of grains per pod, and the grain weight; but as the number of plants per unit area increases, the total number of pods and grains in the population may increase, thereby increasing the yield per unit area. This reflects the trade-off between single-plant yield and population yield. A large amount of experimental data supports this point: within a certain density range, crop yield increases with increasing density until it reaches a plateau or peak; after exceeding the optimal density, the yield does not increase but decreases. Taking soybeans as an example, their single-plant yield (mainly determined by the number of pods per plant and grain weight) is extremely sensitive to increased density. Under dense planting treatment, the number of effective branches and full pods per soybean plant decreased significantly, the number of complete pods decreased by more than 20% at low density, and the proportion of unfilled pods increased. At the same time, the thousand-grain
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