Soybean is a primary source of plant-based oil and protein for human diets. Seed size and weight are important agronomic traits that significantly influence soybean yield. Despite their importance, the genetic mechanisms underlying soybean seed size and weight remain to be fully elucidated. In order to identify additional, major quantitative trait loci (QTL) associated with seed size and weight, we developed segregating populations by crossing a large-seeded soybean variety “Kebaliang” with a small-seeded soybean variety “SUZUMARU”. We evaluated seed length, width, thickness, and hundred-seed weight across two generations, F₄ and F₅, in 2022 and 2023. Employing bulked segregate analysis with whole-genome resequencing (BSA-seq), we detected 18 QTLs in the F₄ population and 12 QTLs in the F₅ population. Notably, six QTLs showed high stability between the two generations, with five derived from two pleiotropic loci (qSS4-1 and qSS20-1) and one specific to seed width (qSW14-1). Further validation and refinement of these loci were carried out through linkage mapping using molecular markers in the F₅ population. Additionally, we identified 18 candidate genes within these stable loci and analyzed their sequence variations and expression profiles. Together, our findings offered a foundational reference for further soybean seed size research and unveiled novel genetic loci and candidate genes that could be harnessed for the genetic enhancement of soybean production.

Stem strength is a key factor influencing lodging resistance in soybeans and other crops. To identify quantitative trait loci (QTLs) associated with stem strength in soybean, we assessed the peak forces required to break a 20 cm stem base segment for each individual within a collection of 2138 plants from eight F₂ and F₃ segregating populations in 2023 and 2024. These populations were derived from four crosses between soybean varieties with contrasting stem strength. Most populations exhibited an approximately normal distribution of stem strength. Using BSA-seq, we identified 17 QTLs associated with stem strength from four populations. Among these, one QTL overlapped with a previously reported locus, while the remaining 16 represented novel loci. Notably, nine loci overlapped with known lodging QTLs, suggesting a genetic relationship between stem strength and lodging. Three QTLs were repeatedly detected in multiple populations, indicating their stability. Further linkage mapping with molecular markers confirmed these three stable QTLs. Among them, qSS10 and qSS19-2 were identified as major QTLs, refined to 1.06 Mb and 1.54 Mb intervals, with phenotypic variation explained (PVE) 23.31–25.15% and 14.21–19.93%, respectively. Within these stable QTL regions, we identified 13 candidate genes and analyzed their sequence variation and expression profiles. Collectively, our findings provide a valuable foundation for future research on stem strength in soybeans and reveal novel genetic loci and candidate genes that may be utilized for the genetic improvement of soybean lodging resistance and yield stability.