Cotton has many leaves and even more bolls, which results in a complicated source–sink relationship. Under water stress, the single boll weight (SBW) of cotton remains relatively stable, while both the leaf area and leaf photosynthetic rate decrease greatly. It is therefore difficult to understand how the formation of SBW is regulated under water stress solely by considering single-leaf photosynthesis. Considering the cotton boll–leaf system (BLS: including the main-stem leaf, sympodial leaves, and non-leaf organs) as the basic unit of the cotton canopy, we speculated that the formation of SBW may depend on photosynthesis in the corresponding BLS under water stress. To verify this hypothesis, five water treatments were set up in the field. The results showed that with increasing water stress, the relative water content (RWC) of the main-stem and sympodial leaves decreased gradually, and the decrease in the sympodial leaves was more obvious. The SBW and the number of BLSs decreased slightly with increasing water stress, while the number of bolls per plant decreased significantly. The area of the BLS decreased gradually with increasing water stress, and the area of sympodial leaves decreased more than that of the main-stem leaves. Gas exchange showed that the photosynthetic rate of the BLS (Pn_(BLS)) decreased gradually with increasing water stress. In addition, the single-leaf photosynthesis and carboxylation efficiency (CE) decreased progressively and rapidly with the increase of water stress. Compared with the main-stem leaf, the photosynthetic function of the sympodial leaf decreased more. Further analysis showed that compared with leaf photosynthetic rate, there was a better correlation between Pn_(BLS) and SBW. Thus, the formation of SBW mainly depends on Pn_(BLS) under water stress, and the increase of BLS to boll is also helpful to maintain SBW to some extent. In BLS, the photosynthesis of the main-stem leaf plays a very important role in maintaining the stability of SBW, while the photosynthetic performance in sympodial leaves may be regulated plastically to influence SBW.

棉花叶子多，棉铃多，源库关系复杂。在水分胁迫下，棉花单铃重（SBW）保持相对稳定，而叶面积和叶片光合速率均大幅下降。因此，仅通过考虑单叶光合作用很难理解 SBW 的形成如何在水分胁迫下受到调节。考虑到棉铃叶系统（BLS：包括主茎叶、合叶和非叶器官）作为棉花冠层的基本单位，我们推测SBW的形成可能依赖于相应BLS中的光合作用在缺水压力下。为了验证这一假设，在现场设置了五种水处理方法。结果表明，随着水分胁迫的增加，主茎和合生叶的相对含水量（RWC）逐渐降低，合生叶的下降更为明显。随着水分胁迫的增加，SBW和BLS的数量略有下降，而单株铃数显着下降。随着水分胁迫的增加，BLS的面积逐渐减小，合叶面积的减少大于主茎叶的面积。气体交换表明 BLS 的光合速率 (Pn 合叶面积比主茎叶面积减少的多。气体交换表明 BLS 的光合速率 (Pn 合叶面积比主茎叶面积减少的多。气体交换表明 BLS 的光合速率 (Pn_(BLS) ) 随着水分胁迫的增加而逐渐降低。此外，随着水分胁迫的增加，单叶光合作用和羧化效率（CE）逐渐下降。与主茎叶相比，合叶叶的光合功能下降较多。进一步分析表明，与叶片光合速率相比，Pn _（BLS）与SBW之间存在更好的相关性。因此，SBW 的形成主要取决于 Pn _(BLS)在水分胁迫下，铃铃BLS的增加在一定程度上也有助于维持SBW。在BLS中，主茎叶的光合作用对维持SBW的稳定性起着非常重要的作用，而合叶的光合性能可能受到塑性调节以影响SBW。