Nitrogen Fixation of Soybean Genotypes with Contrasting Alleles for Shoot Ureide Concentration.

In soybean, nitrogen fixation is more sensitive to drought than other physiological process like transpiration. Therefore, nitrogen availability limits yield under water stress. Previous research has found that drought sensitivity of nitrogen fixation is associated with high shoot nitrogen and ureide concentrations.  Shoot ureide concentration is a quantitative trait controlled by several genes.  Genotypes with low shoot ureide concentration may be able to prolong nitrogen fixation more, under water stress, than genotypes with higher shoot ureide concentration.  By hybridizing KS4895 (a high shoot ureide concentration cultivar) and Jackson (a low shoot ureide concentration cultivar) and deriving lines at F₅, a mapping population of 97 recombinant inbreed lines (RILs) was created. Using 170 polymorphic simple sequence markers (SSRs) and 494 single nucleotide polymorphic markers (SNPs) a high density linkage map (<20 cM) was constructed. Phenotypic information for shoot ureide concentration measured from R4 to early R5, was collected under irrigated conditions in 3-year field trails in Fayetteville, AR. By composite interval mapping (CIM) 5 quantitative trait loci (QTL) were identified on 4 linkage groups. Based on the alleles in every QTL, 2 groups of 9 RILs each, with expected high or low shoot ureide concentration (based on genotypic data) were created. We are currently conducting field experiments in Fayetteville, AR and Keiser, AR, under well watered and drought conditions in which we compare ureide concentration and nitrogen fixation between the two groups of RILs. If the group with expected low ureide concentration shows improved nitrogen fixation under drought, these QTLs will be helpful in selecting genotypes with drought tolerant nitrogen fixation.