Post‐Flowering High Temperature and Drought Stress Impair Wheat Yield via Physiological and Molecular Disruption of Source‐Sink Dynamics and Starch Metabolism

Enhancing wheat resilience to concurrent high-temperature (HT) and drought stress (DS) is critical for sustainable agriculture under climate change. However, the physiological and molecular mechanisms underlying their combined effects (DHS) remain poorly understood. Here, a 3-year field study evaluated post-flowering HT, DS and DHS impacts on two wheat cultivars, Zhongmai 36 (ZM36) and Jimai 22 (JM22). Yield losses under HT, DS and DHS averaged 16.9%, 15.3% and 31.7%, respectively, with DHS exhibiting supra-additive effects. Stressors reduced net photosynthetic rate (Pn), PSII efficiency (Fv/Fm) and proton conductivity (gH+/vH+) in flag leaves, leading to 18.1% lower sucrose and 42.4% reduced post-flowering photosynthetic accumulation under DHS. Furthermore, vascular bundle number and area in rachis decreased by 23.8% and 12.9% (ZM36) and 14.3% and 20.3% (JM22) under DHS, impairing sucrose transport. Transcriptomic analysis revealed downregulation of starch biosynthesis genes (AGPase, SS and GBSS) and upregulation of starch degradation genes (ISA3, BAMY2) under DHS, creating a metabolic 'sink trap'. Collectively, these multi-level findings elucidate the hierarchical disruption of source-sink coordination under combined stresses and provide actionable targets for breeding climate-resilient wheat. SUMMARY STATEMENT: Field study revealed post-flowering combined heat and drought stress caused supra-additive yield loss in wheat by impairing source-sink coordination and altering starch metabolism via downregulation of biosynthesis genes (AGPase, SS and GBSS) and upregulation of degradation genes (ISA3 and BAMY2).