Phosphorus-induced improvement of photosynthate synthesis and transport in the leaf subtending to cotton boll provided sufficient sucrose for fiber thickening during the crucial period and improved fiber bundle strength

Cotton fiber is an important natural textile raw material, and its quality directly affects spinning performance. Phosphorus (P) deficiency reduces fiber quality, particularly fiber strength. Fiber strength formation relies on sucrose supplied by the leaf subtending to cotton boll (LSCB), and low P may affect it by regulating sucrose supply and utilization in LSCB and fiber. Our study aimed to investigate how P regulates the formation of fiber bundle strength through essential carbohydrate metabolic pathways. A three-year experiment on Lu 54 (low-P sensitive) and Yuzaomian 9110 (low-P tolerant) was carried out to study the influence of P on sucrose synthesis and export in LSCB, sucrose absorption and utilization in fiber under 0 (Deficient P), 100 (Critical P), and 200 (Excessive P) kg P2O5 ha−1 in soil with 16.9 mg kg−1 available P. P application significantly increased fiber bundle strength (8.9 %−14.6 %), primarily attributed to the enhanced sucrose supply and utilization in LSCB and fiber during the crucial period for fiber bundle strength formation (17–24 days post anthesis). During this period, P application promoted the sucrose synthesis in LSCB by improving the net photosynthetic rate (9.0 %−16.8 %), and sucrose phosphate synthase (6.3 %−12.4 %) and sucrose synthase (10.0 %−17.4 %) activities. Concomitantly, P application up-regulated the expressions of sucrose transporter genes in LSCB and fiber, promoting sucrose transport and increasing the sucrose content (28.1 %−54.0 %) in fiber. Furthermore, P application markedly enhanced the sucrose utilization in fiber by increasing sucrose synthase (9.7 %−19.2 %) and invertase (14.0 %−35.7 %) activities, improving cellulose content (6.7 %−11.5 %) and fiber bundle strength. Lu 54 exhibited a greater sensitivity to P in the carbohydrate metabolism of LSCB and fiber than Yuzaomian 9110, making its cellulose content and fiber bundle strength increase more. Incremental effects of unit P on the above measurements attenuated markedly with P rates. The critical P content in LSCB (LPC) required for effective fiber bundle strength formation was 0.40 % for Lu 54 versus 0.36 % for Yuzaomain 9110, on the basis of the content of sucrose (the key substrate) in LSCB and fiber. P application increased cotton fiber bundle strength mainly by enhancing sucrose synthesis in LSCB, sucrose transport between LSCB and fiber, and sucrose utilization in fiber during the crucial period. This report revealed the crucial carbohydrate metabolic path by which P regulated the fiber bundle strength formation and quantified its LPC demand during the crucial period, providing theoretical support for P diagnosis and regulation in cotton cultivation.