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, I500) and limited (425 mm, I425)-and three different potassium fertilization levels (low-potassium K1: 48 kg ha-1; medium-potassium K2: 144 kg ha-1; and high-potassium K3: 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 (I425K2). 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 I500K1 treatment, and the fiber biomass and lint percentage significantly increased. Ultimately, the I425K2 treatment achieved the highest three-year mean lint yield, representing a 26.0% increase over the lowest-yielding I500K1 treatment, and simultaneously improved irrigation water use efficiency (IWUE) by 39.6% relative to I500K1. The I425K3 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.