Negative Synergistic Effects of Drought and Heat During Flowering and Seed Setting in Soybean

Rising intense heat waves combined with lower precipitations are the new norms of current global scenarios. These altered climatic conditions negatively impact soybean yield potential and quality. Ten soybean cultivars were subjected to four different growing conditions: control, drought, heat, and combined drought and heat to understand the physiological, yield, and molecular changes. Stomatal conductance was reduced by 62% and 10% under drought and heat, respectively. This reduction was further exacerbated to 93% when exposed to combined stress. The highest canopy temperature was recorded at +8oC with combinatorial treatment, whereas drought and heat exhibited +5.4oC and +2oC, respectively. Furthermore, combined stress displayed a more pronounced negative impact on greenness-associated vegetative index; the gene expression analysis further corroborated these findings. Each one degree Celsius increase in temperature during flowering seed-filling reduced seed weight by ~7% and ~4% with and without drought, respectively. The seed protein increased under drought, whereas the oil showed a converse trend under drought and combined stresses. Most physiological and yield traits showed no significant correlations between control and individual or combined stress. This suggests that selecting crops for combinatorial stress tolerance may not be appropriate based on nonstress or individual stress performance. Thus, incorporating combined stress-resilient traits into elite soybean cultivars could significantly boost soybean production under hot and dry climatic conditions.