Drought-induced change in carbon allocation between cotton root and the leaf subtending to cotton boll determines boll weight formation

Drought often has an effect on cotton yield by changing carbon metabolism during the flowering and boll-forming stage. In order to investigate how drought influences carbon allocation between cotton root and the leaf subtending to cotton boll (LSCB), thereby affecting boll weight formation, the pot experiment was conducted from 2021 to 2022 by using three drought levels with soil relative water content (SRWC) of (75 ± 5)% (CK), (60 ± 5)% (moderate drought), and (45 ± 5)% (severe drought) and three drought durations of 10, 24, and 38 days. Along with ¹³C isotope labeling, drought significantly inhibited the LSCB photosynthetic performance, leading to a reduction in photosynthetic carbon input and decreased ¹³C accumulation in both cotton root and LSCB. In LSCB, drought increased sucrose synthase (SuSy) and sucrose phosphate synthase (SPS) activities but decreased invertase (Acid-INV, Alkaline-INV) activities, leading to sucrose accumulation and restricted carbon transport to cotton boll. In cotton root, drought enhanced SuSy, SPS, and invertase activities, increasing soluble sugar (e.g., sucrose, glucose, fructose) for drought tolerance. Increased root carbon demand further limited carbon supply to cotton boll. Notably, re-watering after 10-day moderate drought rapidly restored LSCB photosynthesis, optimized carbon allocation, mitigated carbon competition between root and cotton boll, and consequently promoted boll weight recovery and maintained seedcotton yield. These results indicate that drought-induced change in carbon allocation between cotton root and LSCB, highlighting the beneficial effect of timely re-watering after moderate short-term drought on yield, thus providing new theoretical insights for optimizing cotton water management strategies.