Plasticity of sympodial leaves in the cotton boll–leaf system governs single boll weight by modulating photosynthetic assimilate supply

In the formation of cotton yield, the source-sink relationship exhibits inherent complexity, resulting in persistent controversy over photosynthetic organ contributions to single boll weight. In modern cultivars under high-density planting conditions, the boll-leaf system (BLS), which serves as the fundamental unit of yield formation, better elucidates the contribution of major photosynthetic organs to boll weight. Considering the pronounced environmental sensitivity of sympodial leaves in BLS, we hypothesize that this plasticity governs the variation in the single boll weight. To test this hypothesis, we implemented controlled simulations quantifying the photosynthetic source organ's contribution in the BLS to single boll weight development. Cotton varieties with indeterminate branch types exhibited a significantly greater photosynthetic area, photosynthetic rate, and single boll weight than nulliplex branch varieties. For indeterminate branch types, increasing the number of sympodial leaves enhanced the photosynthetic area and rate of the BLS, prolonged the photosynthetic functionality and boll dry matter accumulation duration, and accelerated the boll dry matter accumulation rate, increasing the single boll weight. This trend was not affected by girdling treatment on the target BLS. Mathematical fitting revealed that sympodial leaves in the BLS with an indeterminate branch type contributed a maximum of 48.8% to single boll weight, exceeding the contributions from the main-stem leaf and non-leaf organs. These results were further demonstrated by the removal of sympodial leaves from girdled BLS in the field. Therefore, optimizing the photosynthetic performance of the BLS by regulating sympodial leaf plasticity can maximize boll dry matter accumulation and the single boll weight.