Quantitative characterization and photosynthetic manifestations of water-nutrient balance in Xinjiang's winter wheat

[Context] In the extremely arid oasis agroecosystems of Xinjiang, wheat production is constrained by limited water and nutrient resources. Yield depends not only on individual inputs but also on the balance between water and nutrients. However, how water–nutrient imbalances propagate through wheat photosynthesis to affect yield remains unclear. [Objective] This study aims to propose a water–nutrient balance assessment framework for wheat in arid oases of southern Xinjiang, explore potential pathways linking imbalance to winter wheat yield, and develop flexible management strategies under resource-limited conditions. [Methods] A two-year field experiment was conducted in southern Xinjiang using drip-irrigated winter wheat. Four irrigation levels (60%, 80%, 100%, 120% of crop evapotranspiration) were combined with four N–P₂O₅–K₂O rates (70–65–25, 140–130–50, 210–195–75, 280–260–100 kg ha⁻¹). A water–nutrient balance index ( WNBI ) was established based on available water and optimal fertilizer rates to classify imbalance scenarios within the observed water-supply and fertilizer range: water deficit/nutrient excess and nutrient deficit/water excess. The effects of imbalances on water use, leaf area index ( LAI ), leaf photosynthetic traits, and grain yield were evaluated. Partial least squares path modeling (PLS-PM) was used to explore candidate photosynthetic pathways associated with yield limitation under different imbalance scenarios. [Results] Within the observed co-limitation range, each additional 1 mm of irrigation corresponded to an empirical increase of 2.08 kg ha⁻¹ of NPK (14:13:5) to maintain balance. Near-optimal balance ( WNBI ≈1) stabilized stomatal conductance and chlorophyll content, maintained high net photosynthesis during reproduction, and maximized yield under resource constraints. PLS-PM suggested that LAI reduction due to absolute water–fertilizer input and decreased photosynthetic efficiency under imbalance were the main yield-limiting associations. Water-nutrient imbalance suppressed photosynthesis via different pathways: nutrient excess/water deficit ( WNBI > 1) was associated with stomatal limitation through soil water depletion, while water surplus/nutrient deficit ( WNBI < 1) was associated with reduced chlorophyll content. [Conclusions] Under limited resource conditions in this drip-irrigated oasis system, co-management of water and fertilizer to approach water–nutrient balance can improve photosynthesis and yield while reducing resource waste.