To investigate the effects of post-anthesis weak light stress on dry matter accumulation and translocation, grain yield, and starch quality in soft wheat, and to provide insights for optimizing high-yield, high-quality cultivation techniques in the Jianghuai region, a field experiment was conducted from 2022 to 2024 using two soft wheat cultivars, Quanmai 725 (QM725) and Yangmai 15 (YM15). Three shading treatments were applied during the grain-filling stage (7-35 days post-anthesis): S1 (10% shading), S2 (20% shading), and S3 (30% shading), with natural light conditions serving as the control (CK). The results showed that, compared to the control, post-anthesis weak light stress significantly increased the translocation amount, translocation rate, and contribution rate of pre-anthesis assimilates to the grain, while decreasing the accumulation and grain contribution rate of post-anthesis photosynthetic assimilates. Weak light stress also led to a significant reduction in both the number of grains per spike and the 1000-grain weight, ultimately decreasing grain yield. Despite an increase in grain protein content under all shading treatments, starch content, as well as the accumulation of both grain protein and starch, was significantly reduced. Furthermore, weak light stress after anthesis markedly decreased the volume, surface area, and numerical proportion of B-type starch granules (particle size ≤10 μm) in soft wheat grains, while increasing the volume and surface area proportion of A-type starch granules (particle size >10 μm). However, the numerical proportion of A-type starch granules remained largely unaffected, with significant variation observed between years. Among B-type starch granules, weak light stress had a greater impact on those with a particle size of 0.1-2.8 μm than on those ranging from 2.8-10.0 μm. Similarly, among A-type starch granules, the impact on granules larger than 22.0 μm was more pronounced than on those between 10.0 and 22.0 μm. In addition, weak light stress significantly reduced key pasting properties of wheat starch, including peak viscosity, trough viscosity, and final viscosity. Although starch enthalpy parameters improved, the onset, peak, and conclusion temperatures of gelatinization were significantly reduced. Overall, post-anthesis weak light stress strongly influenced dry matter accumulation, translocation, and its contribution to grain formation, leading to a reduction in grain number per spike, 1000-grain weight, and overall grain yield. While protein content increased, starch content significantly declined, thereby negatively affecting yield formation. As light intensity decreased after anthesis, B-type starch granules were more affected than A-type granules, with reductions in their volume, surface area, and numerical proportion, while A-type granules exhibited an increase in volume and surface area. Additionally, weakened gelatinization characteristics, including reduced peak viscosity, onset temperature, peak temperature, and conclusion temperature, ultimately impaired wheat grain quality.
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