The high genetic yield of Xinjiang cotton is associated with improvements in boll-leaf system photosynthesis

Despite significant advancements in cotton breeding, there is ongoing debate regarding the mechanism of the yield increase in early-maturing cotton (Gossypium hirsutum L.) through photosynthesis of single leaves in Xinjiang, China. This study aims to investigate the role of the boll-leaf system (BLS) involving the main-stem leaf, the sympodial leaf, and the boll, in cotton yield formation. Our hypothesis was that genetic breeding enhances cotton yield by improving BLS photosynthesis. To examine the relationship between BLS photosynthesis and yield, we conducted a comprehensive literature review spanning 70 years to assess yield changes in the Xinjiang cotton-growing region. We also conducted a series of field experiments in 2020 and 2021 using main cultivars to further investigate the connection between BLS photosynthesis and yield. Literature data analysis revealed a consistent increase in boll density, boll weight, and yield of the main cotton cultivars in Xinjiang since the 1950s, particularly after the 1990s. The succession of cultivars showed greater improvements in boll density than boll weight. Our experiments supported these findings by demonstrating that the gradual genetic enhancement in cotton cultivars also led to increased photosynthetic area (PA) and photosynthetic rate (Pn) of the boll-leaf system (BLS). When the BLS was isolated by stem girding, we observed a significant rise in PA, Pn, and boll weight during the genetic improvement. Notably, the boll weight of the cultivars from the 2010s showed the least impact from girdling. These results highlight the prominent enhancement of BLS photosynthesis and subsequent boll growth throughout the succession of Xinjiang cotton cultivars. Further analysis revealed that genetic breeding improved the PA and Pn of both the main-stem leaf and sympodial leaf within the BLS. Specifically, the Pn of the main-stem leaf exhibited a stronger association with boll density than boll weight. Conversely, the Pn of the sympodial leaf showed a significant association with boll weight but not with boll density. The gradual genetic improvement of cotton cultivars significantly enhanced BLS photosynthesis, resulting to improvements in both boll density and boll weight. The improvement in photosynthesis of the main-stem leaf contributed to enhanced boll density, while the improvement in photosynthesis of the sympodial leaf promoted greater boll weight.