Ethephon regulates the SS/SPS activity balance in cotton to coordinate assimilate transport and promote fiber quality in bolls

Ethephon application significantly accelerates boll opening and promotes synchronized maturation in cotton. In heat-limited regions, it coordinates the trade-off between rapid boll opening and fiber quality development, which is a key strategy for achieving high-quality and high-yield cotton production. In this study, cotton cultivars with contrasting boll periods were used, and ethephon was applied to investigate its regulatory effects on carbon partitioning in cotton bolls and fiber quality formation. The results showed that ethephon application significantly shortened the boll period and increased fiber length and strength by an average of 0.48 mm and 0.42 cN tex −1 , respectively. In boll walls, amylase activity increased by 17.92 −21.29%, and starch content decreased by a maximum of 8.46 mg g −1 DW. Sucrose phosphate synthase (SPS) activity increased by 21.84%, whereas the SS/SPS activity ratio decreased by 30.43%. In fibers, sucrose synthase (SS) activity increased by 45.03%, while SPS activity decreased significantly by 39.59% ( P = 0.034), and the SS/SPS activity ratio increased by 117.11%. Furthermore, sucrose content in fibers decreased significantly by 1.34 −5.18 mg g −1 DW, while fructose content increased by 2.56 −5.80 mg g −1 DW. These findings suggest that ethephon application enhances sucrose export in boll walls, while accelerating the conversion of sucrose to cellulose in fibers. The contrasting patterns in fructose content between boll walls and fibers reflected the coordinated partitioning of carbon flux within the boll. Therefore, strategic application of ethephon can facilitate assimilate accumulation by coordinating sucrose metabolism between boll walls and fibers. This practice maintains stable fiber quality while shortening the boll development period, thereby providing a theoretical basis for promoting uniform boll opening and efficient mechanical harvesting in cotton regions with limited heat resources.