Temporal dynamics of pre-flowering dry matter remobilization and post-flowering photosynthetic compensation in wheat under combined high temperature and drought stress

Pre-flowering total dry matter remobilization (PTDR) and post-flowering photosynthetic accumulation (PTPA) are critical for grain-filling under abiotic stress, yet their temporal dynamics and organ-specific contributions under combined high temperature (HT) and drought stress (DS) remain poorly understood. Through a two-year field study (2021–2023) with two wheat cultivars (Zhongmai36 and Jimai22), we investigated yield components, PTDR-PTPA coordination, and grain-filling dynamics under HT, DS, and combined drought-heat stress (DHS). The results demonstrated that stress treatments significantly reduced yield by 14.1–33.3% (Zhongmai36) and 14.9–29.5% (Jimai22), driven by declines in grain numbers per spike (7.0–18.4%) and 1000-grains weight (5.0–17.5%). Random Forest analysis revealed the contribution of 1000-grains weight to the yield variation under all stresses amounts to 55.1–59.1%, while grain numbers per spike dominated 56.1–70.3% for Jimai22 under DS and DHS. Additionally, pre-flowering total dry matter remobilization (PTDR) increased by 18.6–21.5% under DHS due to enhanced contributions from the stem-sheath and leaf, partially mitigating yield losses. However, post-flowering photosynthetic accumulation (PTPA) decreased by 7.5–50.8%, with the most severe reductions under DHS. Meanwhile, the PTDR/PTPA ratio rose significantly under DHS, highlighting compensatory remobilization under combined stresses. The timing and contribution of dry matter remobilization varied across organs and stress conditions. The stem-sheath contributions peaked at 10–20 days after stress (DAS) under HT and DS, while leaf remobilization occurred earlier (0–10 DAS). Cultivar-specific responses were evident. The Zhongmai36 showed greater drought tolerance but higher HT sensitivity, whereas Jimai22 prioritized maintaining grains per spike (GNS) under DHS, reflecting divergent adaptation strategies to concurrent stresses. These findings underscore the importance of breeding strategies targeting organ-specific remobilization efficiency and photosynthetic retention during grain-filling are critical for climate-resilient wheat production.