Wheat Source–Sink Metabolic Network Dynamics during Grain Filling under Drought-Heat Combined Stress

Abiotic stresses such as drought (DS), heat (HS), and drought-heat combined stress (HD) frequently occur during wheat grain filling, significantly impacting yield and quality. This study investigated the response of a widely cultivated high-quality, strong-gluten wheat variety (Xinong 979) to DS, HS, and HD at the morphological, physiological, biochemical, and molecular levels during grain filling. Results indicated that abscisic acid (ABA) accumulation ability and proline accumulation were significantly enhanced under HD, with significant reductions in photosynthetic capacity, grain dimensions, and thousand kernel weight (TKW). Starch granule formation morphology was notably inhibited under stress. However, starch accumulation was promoted at 10 DAP under HS and HD, particularly under HS. The relative contents of grain protein and glutenin macropolymer increased, leading to significant changes in grain quality. Combined stress more severely impacted yield and quality formation than individual stress, with source–sink metabolism showing significant tissue specificity and a correlation to diverse stresses. This study constructed dynamic changes of source and sink tissues at the biological level and gene expression levels under various stress conditions during grain filling, revealing key regulatory processes in yield and nutritional quality formation under HD, thereby providing a vital basis for breeding stress-resistant wheat.