Identification of New Germplasm Sources and Traits for Breeding Heat-Tolerant Soybean Varieties

The heat-tolerant soybean breeding is constrained by limited genetic diversity for heat tolerance, lack of efficient selection criteria, and incomplete knowledge of heat tolerance mechanisms. The objectives of this study were to characterize the heat tolerance of a soybean recombinant inbred line (RIL) population based on traits defining leaf function, pollen viability and yield, and identify genotypes and traits that can be included in breeding programs for heat tolerance. Field trials were conducted at Pendleton, SC in 2022 and 2023 to test the heat stress responses of 192 RILs [derived from DS 25-1 (heat tolerant) x DT97-4290 (heat-susceptible)], parental lines, and 12 check varieties. Plants were grown at ambient temperatures until the onset of flowering; thereafter, a heat stress treatment (38-42°C at least for 4 hrs during the daytime) was established for 14 days using heat tents. The RILs 22, 26, 38, 54, 79, and 116 were identified as the most heat-tolerant and the RILs 177, 185, and 195 as the most heat-sensitive based on leaf physiology (chlorophyll index, chlorophyll fluorescence, lipid peroxidation, and photosynthesis), pollen viability, and yield (aboveground biomass, seed yield, and hundred seed weight). Seed yield was correlated with chlorophyll index, photosynthesis, and aboveground biomass under heat stress. Aboveground biomass was associated with the highest heritability (H2 = 0.48), reinforcing its significance as a key selection criterion for heat tolerance in soybean. The new germplasm sources for heat-tolerant variety development and the traits identified for improving the selection efficiency provide valuable resources to soybean improvement programs.