The lag phase of seed development plays an important role in determining the maximum potential final seed weight in soybean (Glycine max L.)
Soybean (Glycine max L.) cultivars exhibit substantial variation in seed weight; however, the developmental and physiological mechanisms contributing to this variation remain incompletely characterized. Here, we investigated the relationship between early seed developmental dynamics and final seed weight by comparing two large-seeded and two small-seeded cultivars under control and depodding conditions. Depodding, achieved by retaining a single pod per node, minimized assimilate competition. Final seed weight was positively correlated with cotyledon cell number and duration of the lag phase, which is a key early stage of seed development. Large-seeded cultivars exhibited significantly longer lag phases and higher cotyledon pavement cell numbers than small-seeded cultivars, suggesting that extended lag phases promote enhanced cell proliferation, contributing to increased seed weight. Depodding further increased the cotyledon cell number; however, this response was associated with accelerated embryo development rather than an extension of the lag phase. These findings indicate that both the duration of the lag phase and the rate of early embryo development influence cotyledon cell proliferation and ultimately seed weight. Moreover, genetic factors and assimilate availability regulate developmental processes through distinct pathways. Together, our results highlight the importance of early seed developmental timing in determining final seed weight and provide new insights into the developmental basis of yield-related traits in soybean.