Insufficient soil-available phosphorus (AP) inhibits photosynthesis and sucrose metabolism in the subtending leaves to cotton bolls, significantly reducing lint yield and fiber length. However, the mechanism by which P deficiency affects cotton fiber strength remains unclear. This study explored the influence of P deficiency on the fiber strength of two cotton varieties, CCRI-79 (low-P tolerant) and SCRC-28 (low-P sensitive). A two-year pool-culture experiment was conducted under three soil AP levels: P0 (3 +/- 0.5), P1 (6 +/- 0.5), and P2 (control, 15 +/- 0.5 mg kg-1). Soil AP deficiency (P1 and P0) decreased the activities of enzymes involved in fiber thickening, including soluble acid invertase (SAI), sucrose synthase (SuSy), sucrose phosphate synthase (SPS), and beta-1,3-glucanase. These reductions led to a 4.0%-16.9% decrease in cellulose content, which was a key substance for fiber strength formation. In addition, the reduced enzymatic activity decreased the maximum velocity of fiber thickening increment (VSmax), ultimately resulting in a 1.0-3.9 cN tex-1 decrease in fiber strength. The beta-1,3-glucan and cellulose levels, as well as SAI and beta-1,3-glucanase activities, showed more pronounced responsiveness to soil AP deficiency in SCRC-28 than in CCRI-79. This indicates that SCRC-28 fibers were more adversely affected under low soil AP levels. Cellulose, SAI, and beta-1,3-glucanase were the key factors affecting fiber strength under low soil AP conditions. This study contributes to a deeper understanding of how soil AP deficiency influences fiber carbohydrate metabolism and fiber strength, offering a scientific basis for breeding low-P tolerant cotton cultivars.
