Date of Graduation


Document Type


Degree Name

Doctor of Philosophy in Crop, Soil & Environmental Sciences (PhD)

Degree Level



Crop, Soil & Environmental Sciences


Larry C. Purcell

Committee Member

Jeffery D. Ray

Second Committee Member

Richard E. Mason

Third Committee Member

Mary C. Savin

Fourth Committee Member

Qingyang Zhang


Drought, Genomic, GWAS, QTL Mapping, SNP, Soybean


Drought stress is a major global constraint for crop production, and improving crop tolerance to drought is of critical importance. Direct selection of drought tolerance among genotypes for yield is limited because of low heritability, polygenic control, epistasis effects, and genotype by environment interactions. Crop physiology can play a major role for improving drought tolerance through the identification of traits associated with drought tolerance that can be used as indirect selection criteria in a breeding program. Carbon isotope ratio (δ13C, associated with water use efficiency), oxygen isotope ratio (δ18O, associated with transpiration), canopy temperature (CT), canopy wilting, and canopy coverage (CC) are promising physiological traits associated with improvement of drought tolerance. Genome-wide association studies (GWAS) are one of the genomic approaches to provide a high mapping resolution for complex trait variation such as those related to drought tolerance. The objectives of this research were to identify genomic regions and favorable alleles that contribute to drought-tolerant traits. A diverse panel consisting of 373 maturity group (MG) IV soybean accessions was evaluated for δ13C, δ18O, canopy wilting, canopy coverage, and canopy temperature in multiple environments. A set of 31,260 polymorphic SNPs with a minor allele frequency (MAF) ≥ 5% was used for association mapping of CT using the FarmCPU model. Association mapping identified 54 significant SNPs associated with δ13C, 47 significant SNPs associated with δ18O, 61 significant SNPs associated canopy wilting, 41 and 56 significant SNPs associated with CC for first and second measurements dates, respectively, and 52 significant SNPs associated with CT. Several genes were identified using these significant SNPs, and those genes had reported functions related to transpiration, water transport, growth, developmental, root development, response to abscisic acid stimulus, and stomatal complex morphogenesis. Favorable alleles from significant SNPs may be an important resource for pyramiding genes to improve drought tolerance and for identifying parental genotypes for use in breeding programs.