Soybean Genotypes with Elevated Thermal Demand and Photoperiod Sensitivity Show Adaptive Flowering Across Solstice Phases in Lower Latitudes

Soybean flowering at low latitudes is accelerated by short photoperiods and high temperatures, reducing vegetative growth and yield. Developing climate-resilient types requires genotypes that delay flowering through higher thermal (GDD) demand and sensitivity to photoperiodic cues. In this study, ten soybean genotypes including the long juvenile (AGS 25 & SL 525) and photoinsensitive (JS 95 − 60 & MAUS 32) were evaluated through year-round monthly sowings. Flowering responses were analyzed using both temperature, absolute photoperiod and dynamic seasonal cues such as the rate of change in daylength (Δdaylength) within a solstice-based framework. AGS 25 consistently accumulated the highest GDD before flowering and responded strongly to both thermal efficiency and Δdaylength, exhibiting not only stability across seasons but also strong adaptability to varying environments. Standardized regression and solstice-wise models confirmed that thermal time accelerated development while Δdaylength modulated seasonal induction especially during transitional phases. Stability analyses (GGE, AMMI, Finlay–Wilkinson) further identified AGS 25 as broadly adapted and phenologically stable. In contrast, photoperiod-insensitive lines flowered prematurely under short days. These findings highlight that combining high GDD requirement with photoperiod responsiveness provides adaptability and resilience under tropical and subtropical conditions supporting breeding of long juvenile, thermos responsive genotypes for expanding cultivation at lower latitudes