Mapping of QTLs Associated with Carbon Isotope Discrimination in Soybean Using a Recombinant Inbred Population.

Drought stress is a major constraint for soybean production. Water use efficiency (WUE) is considered as an important physiological trait associated with drought tolerance. The carbon isotope discrimination (Δ¹³C) provides an integrated measurement of WUE in C₃ plants due to its substantial genetic variance, high heritability, and small genotype by environment interaction (G×E). The objective of this study was to identify quantitative trait loci (QTLs) associated with the Δ¹³C using a recombinant inbred line (RIL) population derived from a cross between KS4895 × Jackson. The experiment was conducted in three environments to evaluate the Δ¹³C under water-stressed and well-watered conditions. Analysis of variance of Δ¹³C showed significant effects of genotype, environment, treatment, and genotype x treatment interaction. Averaged values of Δ¹³C across all environments identified three QTLs on chromosomes Gm05, Gm06, and Gm20 at LOD scores of 3.2 or above using composite interval mapping (CIM) analysis, which collectively accounted for 79% of the phenotypic variation with individual R² values ranging from 8 to 25.1%. Multiple interval mapping (MIM) analysis identified two QTLs on chromosomes Gm06 and Gm20, which explained phenotypic variation from 17.2 to 22.2%. The QTLs on Gm06 and Gm20 also showed the additive × additive epistasis interaction that contributed approx. 4.2% to total phenotypic variation. The favorable allele for the QTL on Gm20 was from Jackson, while QTL on Gm06 received favorable allele from KS4895. These two QTLs were also observed in all single environments and QTL on Gm20 was reported in previous genome wide association studies of carbon isotope ratio. Identified genomic regions can be important resource in soybean breeding program to improve tolerance to drought stress.