Combined Drought and Heat Stress Reduce Soybean Yield by Limiting the Number of Nodes Per Branch in a Temperature Gradient Chamber

Climate‐induced increases in temperature and water scarcity threaten global soybean production. While the individual effects of drought or heat stress on yield are well documented, their combined impact on specific yield components remains poorly defined. To address this gap, we conducted a two‐year study in Japan using a temperature gradient chamber and a single soybean cultivar. The study applied four temperature gradients under well‐watered and drought stress conditions to examine seed yield and its components. Combined drought and heat stress reduced total yield more than either stress alone. In 2024, average yield under combined stress declined by 19.4% compared to 2023, coinciding with an increase of 1.5 kPa in maximum vapour pressure deficit and 2.5°C in maximum temperature. This yield reduction was primarily attributed to lower seed weight on branches rather than on the main stem. Pearson correlation and multiple regression analyses identified the number of nodes per branch as the key yield determinant under stress conditions. We further found that the transition from vegetative growth to beginning bloom was the most stress‐sensitive stage. During this period, maximum temperature and vapour pressure deficit reached 53.3°C and 10.7 kPa, significantly inhibiting node formation on branches and limiting subsequent pod and seed set. These findings advance understanding of how concurrent drought and heat stress constrain soybean yield through specific developmental pathways, and underscore the need to incorporate branch‐level traits into stress‐resilient breeding strategies.