Canopy-Level Regulation of Within-Boll Cotton Yield and Fiber Quality Under Staged Saline Water Supplemental Irrigation in Xinjiang

Freshwater scarcity severely limits sustainable cotton production in arid regions. This study aimed to establish the optimal salinity threshold for staged saline water supplemental irrigation (SWSI) and elucidate its canopy-level mechanisms in optimizing within-boll yield components and fiber quality. A two-year field trial (2023–2024) was conducted in Awati County, Xinjiang, using mulched drip irrigation at five SWSI levels (3.5–9.5 g L−1) and a freshwater control (CK). Compared with CK, 3.5 g L−1 treatment significantly increased lint yield by 31.4%, boll number per plant by 22.45%, and fibers per seed by 6.01–10.59%, while fiber length and strength rose by 6.98–10.38% and 2.69–6.00%, respectively. When salinity reached 8.0 g L−1, yield declined by 8.5%, and a salinity of 9.5 g L−1 reduced yield by 24.52%. Spatially, mid-fruiting branches (nodes 4–6) remained stable, maintaining high lint mass per seed even under high salinity, whereas upper branches (≥node 7) were most sensitive; at 9.5 g L−1, the boll number (0.36) was 56.6% lower than at 3.5 g L−1 (0.83), and the Q-score decreased by 6.7%. These results demonstrate that SWSI with ≤5.0 g L−1 salinity (optimum 3.5 g L−1) simultaneously enhances lint yield and fiber quality, providing a practical strategy for efficient saline water use in arid cotton regions.