The yield of cotton (Gossypium hirsutum L.) can be effectively maximized through high planting density. Estimating the yields from single-leaf photosynthesis under various plant densities is difficult as the source-sink relationship of cotton is complicated. By considering the boll-leaf system (BLS: including the main-stem leaf, sympodial leaf and non-leaf organs) as the basic unit of the cotton canopy, the present study asserts that the photosynthetic function of BLS determines single boll weight. This hypothesis was tested by establishing plant densities to explore changes in photosynthesis of the cotton BLS and the consequent effects on boll weight. The results showed that high plant density increased total boll number per unit ground area, whereas single boll weight remained relatively stable, thereby increasing cotton yield. High plant density significantly limited the biomass of the BLS and reduced the distribution ratio of assimilate to sympodial leaves, whereas the dry matter proportion of cotton bolls increased. Although the leaf area and photosynthetic rate of both the main-stem and sympodial leaves of the BLS decreased with increasing plant density, the extent of decrease in the latter was greater. CO₂ assimilation within the BLS decreased slightly with increasing plant density, and significant decreases in CO₂ assimilation were only evident in extremely high plant densities. A significant positive linear correlation was observed between CO₂ assimilation of the BLS and single boll weight. Therefore, photosynthesis of the BLS was identified as the primary factor maintaining boll weight under various plant densities. The main-stem leaf plays a key role in maintaining photosynthesis of the BLS, whereas BLS photosynthesis can be improved by plastically regulating the photosynthetic performance in sympodial leaves.

棉花（陆地棉）的产量L.）可以通过高种植密度有效地最大化。由于棉花的源库关系很复杂，因此很难估计各种植物密度下单叶光合作用的产量。通过将棉铃叶系统（BLS：包括主茎叶，共生叶和非叶器官）作为棉花冠层的基本单位，本研究断言，BLS的光合功能决定了单铃重。通过建立植物密度来研究该假设，以探索棉花BLS光合作用的变化及其对铃重的影响。结果表明，高植物密度增加了单位地面面积的总铃数，而单铃重量却保持相对稳定，从而提高了棉花产量。较高的植物密度显着限制了BLS的生物量，并降低了同化物到子叶的分配比例，而棉铃的干物质比例却增加了。尽管随着植物密度的增加，BLS的主干和共生叶片的叶面积和光合速率均随植物密度的增加而降低，但后者的降低程度更大。一氧化碳随着植物密度的增加，BLS中_2的同化作用略有降低，而只有在极高的植物密度下，CO _2的同化作用才明显减少。观察到BLS的CO ₂同化与单铃重之间存在显着的线性正相关。因此，BLS的光合作用被确定为在各种植物密度下维持铃重的主要因素。主茎叶在维持BLS的光合作用中起关键作用，而BLS的光合作用可以通过塑性调节sym叶的光合性能而得到改善。