Protein Composition and Starch Structural Differentiation During Wheat Grain Filling Are Associated with Mature Grain Quality

Wheat grain quality is progressively established through the accumulation and structural organization of storage reserves during grain filling, yet the relationships between developmental traits and mature-grain quality remain incompletely understood. This study aimed to characterize changes in grain developmental traits during grain filling and to clarify their relationships with mature-grain quality, with particular attention to quality attributes relevant to the use of wheat grain as a raw material for food production. In this study, four wheat cultivars contrasting in mature-grain quality were evaluated with mature-grain quality assessed over two harvest years and grain-filling dynamics examined during the 2024–2025 growing season. Grain growth was characterized at 10, 16, 23, and 29 days after anthesis (DAA), while protein and starch accumulation and protein fractions were examined during grain development and at maturity. Starch granule-size distribution, relative crystallinity, and thermal properties were further assessed at 23 and 29 DAA and at maturity, together with mature-flour pasting properties and correlations between developmental and mature-quality traits. All cultivars showed similar basic patterns of grain weight gain, dehydration, and progressive protein and starch accumulation, whereas cultivar-specific differences in protein fractions, amylose content, starch granule characteristics, and starch structure became increasingly evident during development. Although differences in total starch content among some cultivars narrowed at maturity, amylose content, granule size, B-type granule proportion, relative crystallinity, and thermal properties remained distinct and were associated with variation in gluten-related and pasting properties. Within the growing season examined, developmental traits at both 23 and 29 DAA were associated with mature-grain quality, with more significant associations detected at 29 DAA than at 23 DAA. These season-specific associations suggest that changes in protein composition and starch structure during mid-to-late grain filling may be related to mature-grain quality. These results help clarify how grain-filling characteristics are associated with final wheat grain quality.