Legume Genomics and Genetics 2026, Vol.17, No.1, 68-79 http://cropscipublisher.com/index.php/lgg 72 highlight that managing planting density is critical for optimizing soybean growth traits such as plant height, stem diameter, LAI, canopy structure, and dry matter dynamics—all of which collectively influence final yield outcomes. Balancing these factors through appropriate density selection tailored to local environmental conditions and cultivar characteristics can improve soybean productivity sustainably. 4 Effects of Planting Density on Photosynthetic Characteristics and Physiological Indices 4.1 Changes in photosynthetic rate and chlorophyll content Planting density has a notable impact on the photosynthetic rate and chlorophyll content of soybean leaves, which are critical for biomass accumulation and yield formation. Increasing planting density generally enhances leaf area index (LAI) and canopy coverage, leading to improved light interception and photosynthesis at the population level. For example, doubling plant density from 160,000 to 320,000 plants per hectare significantly increased leaf chlorophyll content and net photosynthetic rate, contributing to higher aboveground biomass and seed yield (Liao et al., 2022). However, excessively high densities can induce shading stress within the canopy, reducing individual leaf photosynthetic capacity due to lower light availability and altered microclimate conditions. Chlorophyll content measured by SPAD values tends to increase with planting density up to an optimal point, reflecting enhanced nitrogen status and photosynthetic pigment concentration in leaves. Studies have shown that moderate densities improve chlorophyll content during key growth stages such as flowering and pod filling, supporting sustained photosynthesis (Zhang et al., 2015). Yet, at very high densities, chlorophyll content may decline or plateau as competition for nutrients intensifies. Thus, an optimal planting density balances increased canopy photosynthesis with maintenance of leaf physiological health to maximize productivity. 4.2 Analysis of light use efficiency in the population Light use efficiency (LUE), defined as the conversion efficiency of intercepted light into biomass or yield, is influenced by planting density through its effects on canopy structure and light distribution. Increasing density raises LAI and intercepted photosynthetically active radiation (IPAR), which generally enhances total biomass production (Liao et al., 2022). However, beyond a certain threshold, dense canopies suffer from self-shading that reduces light penetration to lower leaves, decreasing overall LUE at the population scale (Zhang et al., 2021). This decline in LUE at very high densities is linked to reduced net photosynthetic rates per leaf area despite greater total light interception. Uniform plant spacing combined with optimal density improves canopy light interception uniformity and reduces gaps that cause inefficient light use. Research indicates that uniform distribution increases dry matter accumulation by enhancing LUE compared to non-uniform spacing under similar densities (Xu et al., 2021). Additionally, cultivars tolerant to higher densities maintain better canopy architecture that optimizes light distribution and sustains higher LUE under dense planting conditions (Zhang et al., 2021). Therefore, managing both planting density and spatial arrangement is essential for maximizing LUE in soybean populations. 4.3 Responses of key physiological indicators Key physiological indices such as SPAD value (relative chlorophyll content), transpiration rate (Tr), stomatal conductance (Gs), and intercellular CO2 concentration (Ci) respond dynamically to changes in planting density. Moderate increases in density often elevate SPAD values due to improved nitrogen uptake efficiency per unit land area and enhanced leaf chlorophyll synthesis (Liao et al., 2022). Transpiration rate typically shows a peak at intermediate densities corresponding with maximum stomatal conductance during critical growth stages like flowering or pod filling but declines at very high densities due to stomatal closure induced by stress factors such as shading or water limitation (Zhang et al., 2015). Stomatal conductance follows a similar pattern; it increases initially with rising density but decreases when crowding limits gas exchange efficiency. Intercellular CO2 concentration often decreases with increasing density up to an optimum level due to enhanced photosynthetic activity but may rise again under excessive crowding when photosynthesis is inhibited (Zhang et al., 2015). These physiological responses reflect complex interactions
RkJQdWJsaXNoZXIy MjQ4ODYzNA==