Soybean genotypes with elevated thermal demand and photoperiod sensitivity show adaptive flowering in lower latitudes

Soybean flowering at lower latitudes is often accelerated by short days and high temperatures, which can shorten vegetative growth and reduce yield. Improving adaptation under such conditions requires genotypes that delay flowering by maintaining adequate thermal requirement while remaining responsive to day length. In this study, ten soybean genotypes, including long juvenile types (AGS 25, SL 525, JS 93-05) and photoperiod-insensitive lines (JS 95-60, MAUS 32), were evaluated through year-round monthly sowings. Flowering time (DTR1) was analysed in relation to growing degree days (GDD) and day length using both pooled annual data and season-wise partitioning into four seasonal phases (winter, vernal, summer, and autumn). Flowering varied widely across sowing months, and environmental effects explained the largest share of variation. Across the full year, both GDD and day length significantly influenced flowering. Seasonal analyses improved the fit of the regression and revealed phase-dependent effects: temperature had a consistent influence across all phases, whereas day length showed a significant effect only in the winter and vernal phases. Genotype-wise analyses confirmed independent thermal and photoperiod effects. Stability analyses distinguished predictable genotypes (MACS 450, JS 97-52) from more responsive genotypes such as AGS 25, which required higher thermal accumulation before flowering. Allelic validation indicated that the delayed flowering and elevated thermal requirement of AGS 25 were associated with its E1e9 background, whereas photoperiod-insensitive lines flowered earlier under short-day conditions.