Stage-specific soil moisture regulation improves grain yield and water productivity of drip-irrigated winter wheat: Associations with stem carbohydrate dynamics and grain filling

Water scarcity limits wheat productivity in arid agricultural regions, where improving water productivity (WP) requires optimizing irrigation timing according to crop physiological demand. However, the mechanisms linking irrigation timing with internal carbon allocation and grain formation remain unclear. A three-year field experiment was conducted under drip irrigation in northwestern China to evaluate the effects of stage-specific soil moisture regulation on wheat growth, carbon metabolism, assimilate partitioning, yield formation, and WP. Six irrigation strategies were established by regulating lower soil moisture thresholds corresponding to 50%, 65%, and 80% of field capacity (FC) during vegetative and reproductive stages. Wheat responses to soil moisture regulation were strongly stage-dependent, with the heading-to-grain filling period showing the highest sensitivity to water management. Using 80% FC as the irrigation initiation threshold from heading to grain filling (W3) enhanced photosynthetic activity, promoted dry matter allocation to reproductive organs, and improved grain formation. Across three growing seasons, W3 achieved grain yields of 10.3–10.7 t ha −1 , increasing yield by 12.85% and WP by 10.86% compared with conventional irrigation, with only a slight increase in irrigation input. Furthermore, optimized reproductive-stage irrigation enhanced stem carbohydrate accumulation and redistribution potential. Stem starch exhibited the highest predictive importance among organ-derived carbon reserves, and the ratio of net stem dry matter (DM) decline during grain filling to final grain DM ranged from 14.36% to 25.04%. These findings indicate that optimized irrigation timing was associated with sustained photosynthetic performance, more favorable stem carbohydrate dynamics, and improved grain filling. These responses provide a physiological basis for improving wheat productivity and water use under arid conditions.