Legume Genomics and Genetics 2025, Vol.16, No.6, 253-269 http://cropscipublisher.com/index.php/lgg 260 It is particularly worth mentioning that the optimization of dense planting and fertilization can also improve the uniformity between individuals in the group, reduce the occurrence of "small old seedlings" or "empty stalks", and enable almost all plants to produce pods and seeds. This means that the unstable loss of group biomass and yield is reduced, and the overall yield increase effect is more stable (Xu et al., 2021). The optimized combination can delay the aging process of plants by improving the nutritional status and group structure of plants. For example, the group closure formed by appropriate topdressing of nitrogen fertilizer and dense planting can often maintain a longer green leaf functional period (extend leaf aging, the so-called "greed for green"), continue photosynthesis in the late growth period and transport nutrients to the grains. Of course, excessive "greed for green" may lead to delayed maturity, but it can generally be regulated through variety selection and chemical control measures. In practical applications, the biological basis of the synergy between dense planting and fertilization is reflected in: increasing the photosynthetic area and efficiency of the entire group (dense planting effect), while ensuring the efficient conversion and distribution of each unit of photosynthetic products (fertilization effect). The former provides the "source" and the latter strengthens the "sink", and the two are connected through "flow" (transport and distribution of nutrients and assimilated products). If the management is proper, the three aspects of source, flow and sink are all enhanced, and a significant yield increase effect will be achieved. On the contrary, if the two aspects do not match (such as too high density and insufficient source or insufficient fertility and limited sink), the yield increase potential cannot be fully realized. Therefore, strengthening the monitoring and regulation of the relationship between crop source, flow and sink, so that dense planting and fertilization effects complement each other, is the key to achieving a coordinated increase in crop yields. 4.3 Analysis of crop adaptation response and plasticity to resource pressure As an annual crop, legume crops have a certain phenotypic plasticity to cope with density and nutrient changes. This study observed that soybean plants can adapt to resource pressure by changing morphological and physiological characteristics under different treatments. For example, under high density or low fertilizer conditions, plants give priority to top growth, which is manifested as longer stems and accelerated inflorescence differentiation, in order to seize space and reproduce successfully; while under low density or high fertilizer conditions, plants tend to increase the number of branches and leaves to make full use of resources and the environment (Cassel et al., 2025). This plastic response enables crops to buffer the stress caused by management measures within a certain range, thereby maintaining basic growth and yield. However, the degree of plasticity varies among different varieties, which determines their adaptability to dense planting and fertilization. Dense-tolerant varieties usually have weaker marginal branching habits and more upright plant types, with less yield loss under high density; while varieties that perform well in fertile soils often have a significant decrease in yield under poor conditions. Therefore, when promoting dense planting and fertilization optimization technology, corresponding cultivation strategies should be selected in combination with variety characteristics. On the other hand, crops can also be induced to produce favorable adaptive responses through reasonable management. For example, dense planting and nitrogen fertilizer regulation can moderately delay plant aging, increase chlorophyll content, form a "continuous reservoir" effect, and use more late photosynthetic products for grain filling. In the Qiqihar experiment, we noticed that in the treatment of controlled-release nitrogen fertilizer, the leaf function of soybean population was better in the later stage, and the SPAD value of leaves in the grain filling stage was 2-3 units higher than that of ordinary urea treatment, indicating that the plant's nutrient supply was more sustained, thereby improving the fullness of pods and grains. For example, in the high-density nitrogen reduction and potassium increase treatment in Zhumadian, although the number of branches of the plant decreased, the number of pods on the main stem increased significantly, and the proportion of the total number of pods increased from 60% of the control to 75%, reflecting the compensatory mechanism of the plant to concentrate resources on the main stem to be strong. It can be seen that crops themselves have a certain regulatory ability, and we should use this ability to guide the shaping of plants in a direction conducive to high yield through fine management.
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