Foliar zinc-sorbitol regulates dry-matter partitioning and photosynthetic performance to sustain cotton yield and irrigation water productivity under brackish water irrigation

Long-term brackish water (BW) irrigation is a major constraint on cotton production in arid oasis regions, yet agronomic options for mitigating the resulting salinity stress remain limited. A three-year (2023–2025) field split-plot experiment was conducted in southern Xinjiang, China, with irrigation water quality (freshwater, FW, 0.74 g·L⁻¹; BW, 2.79 g·L⁻¹) as the main-plot factor and five foliar zinc–sorbitol formulation concentrations (0, 0.5, 1.0, 1.5, and 2.0 g·L⁻¹; designated as Zn0–Zn4, corresponding to 0, 70, 140, 210, and 280 mg·L⁻¹ elemental Zn) as subplots, to test whether zinc–sorbitol regulates the linked chain of ion homeostasis, endogenous hormones, photosynthesis, dry-matter partitioning and yield formation. Relative to FWZn0, BWZn0 raised the leaf Na⁺/K⁺ ratio by 524.5–528.7% and abscisic acid by 12.6–18.6%, suppressed the growth-promoting hormones indole-3-acetic acid and gibberellin, and lowered the maximum net photosynthetic rate (Pn max ) by 35.6–36.2% and yield by 4.1–5.0%. Under BW, 1.5 g·L⁻¹ zinc–sorbitol (Zn3) was consistently the most effective treatment across the three seasons: it reduced the leaf Na⁺/K⁺ ratio by 60.8–64.3%, and the accompanying relief of stomatal limitation increased stomatal conductance by 79.2–95.0% and Pn max by 49.2–51.1%. These leaf-level gains were associated with preferential dry matter allocation to reproductive organs, increasing yield by 7.4–9.6% and the fiber quality index by 22.9–41.6%. Random Forest and partial least squares path modelling indicated that the dominant correlates of yield shifted from growth-related traits under FW to antioxidant and ion homeostasis traits under BW. Quadratic regression identified higher optimal concentrations under BW (1.41–1.69 g·L⁻¹) than under FW (1.36–1.55 g·L⁻¹). Under the conditions tested, foliar zinc–sorbitol is a practical option for supporting yield and fiber quality when BW is used in arid oasis cotton production systems.