Date of Graduation

7-2020

Document Type

Dissertation

Degree Name

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

Degree Level

Graduate

Department

Crop, Soil & Environmental Sciences

Advisor

Larry C. Purcell

Committee Member

Jeffery D. Ray

Second Committee Member

Mary C. Savin

Third Committee Member

Richard E. Mason

Fourth Committee Member

Ken Korth

Keywords

Canopy Temperature, Carbon Isotope Ratio, Drought, Nitrogen Isotope Ratio, QTL Mapping, Soybean

Abstract

Soybean (Glycine max L.) is one of the major row crops in the United States, and its production is often limited by drought stress. Physiological traits from exotic germplasm that confer drought tolerance may be useful in improving commercial soybean production. For example, carbon isotope ratio (δ13C) is positively correlated with water use efficiency (WUE), and nitrogen isotope ratio (δ15N) is negatively correlated with N2 fixation; canopy temperature (CT) is an indicator for genetic variation in transpiration and stomatal conductance. Therefore, the objectives of this research were to identify the genomic regions associated with: (1) δ13C and δ15N using a population of 196 F6-derived recombinant inbred lines (RIL) from PI 416997 × PI 567201D that was phenotyped in four environments, (2) CT and δ13C using a population of 168 F5-derived RILs from KS4895 × Jackson that was phenotyped in multiple environments and irrigation treatments. In the PI 416997 × PI 567201D population, δ13C and δ15N had a wide phenotypic range in all environments, and PI 416997 had higher δ13C and lower δ15N values than PI 567201D. δ13C had high heritability (90%) whereas the heritability of δ15N was relatively lower (35%), indicating that δ15N was more affected by the environment. QTL mapping identified eight loci on seven chromosomes associated with δ13C, and these loci explained between 2.5 to 30% of the phenotypic variation. There were 13 loci on 10 chromosomes associated with δ15N, explaining 1.7 to 14.4% of the phenotypic variation. There were strong interactions between QTLs and environments for δ15N. In the KS4895 × Jackson RIL population, Jackson had a cooler canopy than KS4895, and the heritability of CT had low heritability (31%) across environments. There were 11 loci present on eight chromosomes associated with CT that individually explained 4.6 to 12.3% of the phenotypic variation. The heritability of δ13C in KS4895 × Jackson RIL population heritability was 83% when estimated over environments and over irrigation treatments. A total of 24 QTLs associated with δ13C were identified and clustered in nine genomic loci on seven chromosomes. The identified QTLs for δ13C, δ15N, and CT were co-localized with genomic regions associated with drought tolerance-related traits from previous studies. These genomic regions may be important resources in soybean breeding programs to improve tolerance to drought. Further research is needed to fine map the identified QTLs and validate markers linked with these regions.

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