Potassium-driven coordination of carbon and nitrogen metabolism to enhance cotton fiber development under limited drip irrigation conditions

Introduction Cotton is the world’s dominant natural fiber crop, and its production in arid oasis systems is widely constrained by drought. Potassium supplementation can alleviate water-deficit damage in cotton. However, the core mechanism linking potassium signaling to fiber carbon–nitrogen coordination and cellulose biosynthesis remains uncharacterized fiber. Methods Field experiments were conducted from 2021 to 2023 under two distinct drip irrigation regimes-conventional (500 mm, I 500 ) and limited (425 mm, I 425 )—and three different potassium fertilization levels (low-potassium K 1 : 48 kg ha -1 ; medium-potassium K 2 : 144 kg ha -1 ; and high-potassium K 3 : 240 kg ha -1 ). Key enzyme activities, metabolite contents, cellulose accumulation levels, and cotton fiber carbon and nitrogen metabolism yields were measured. Results Under limited drip irrigation conditions, the activities of sucrose synthase (SS), sucrose phosphate synthase (SPS), and β-1,3-glucanase increased by 11.1–21.2%, 9.9–21.4%, and 5.8–27.2%, respectively, in the medium-potassium treatment (I 425 K 2 ). These changes promoted the accumulation of sucrose and β-1,3-glucan during the critical period of fiber development and provided sufficient substrates for cellulose synthesis. Concurrently, the activities of nitrate reductase (NR) and glutamine synthetase (GS) increased by 9.7–33.2% and 7.3–30.1%, respectively, increasing the free amino acid and soluble protein contents and optimizing nitrogen metabolism. This treatment significantly reduced the fiber sucrose-to-soluble-protein (S/P) ratio during the secondary wall thickening period (40–50 days), indicating a more coordinated allocation between carbon substrates and nitrogenous metabolites in developing fibers. The cellulose content increased by 22.7% compared with that in the I 500 K 1 treatment, and the fiber biomass and lint percentage significantly increased. Ultimately, the I 425 K 2 treatment achieved the highest three-year mean lint yield, representing a 26.0% increase over the lowest-yielding I 500 K 1 treatment, and simultaneously improved irrigation water use efficiency (IWUE) by 39.6% relative to I 500 K 1 . The I 425 K 3 treatment ranked second in lint yield. Multivariate correlation, Mantel test and principal component analyses further showed that coordinated carbon/nitrogen enzyme activity, a reduced S/P ratio, and enhanced cellulose accumulation were associated with elevated lint yield and IWUE. Discussion We conclude that moderate potassium fertilization under regulated limited drip irrigation synergistically activates core carbon- and nitrogen-metabolic enzymes, optimizes the balance between fiber sucrose supply and soluble protein availability, and accelerates cellulose synthesis to simultaneously increase fiber biomass and lint output. This water-potassium co-regulation framework delivers a field-deployable, water-saving high-yield agronomic strategy tailored to arid cotton-growing regions, and establishes a mechanistic foundation for optimizing source-sink carbon-nitrogen partitioning under chronic water limitation.