Date of Graduation

12-2016

Document Type

Thesis

Degree Name

Master of Science in Crop, Soil & Environmental Sciences (MS)

Degree Level

Graduate

Department

Crop, Soil & Environmental Sciences

Advisor

Pengyin Chen

Committee Member

Ken Korth

Second Committee Member

Esten Mason

Third Committee Member

David Miller

Abstract

Soybean is a major cash crop used as a source of high–quality protein and oil. Salt stress is one of the main abiotic stresses causing significant yield losses in soybean, which is considered a moderately salt–sensitive crop. Breeding selection is a promising strategy to improve salt tolerance as soybean germplasm display wide variation in response to salinity stress. However, the physiological and genetic mechanisms for salt tolerance are not quite clear. The discovery of novel QTL/genes associated with salt tolerance facilitates the development of tolerant cultivars through marker-assisted selection (MAS). The objectives of this study were: 1) identify/confirm QTL associated with salt tolerance, and 2) evaluate progressive shoot ion accumulation in sensitive/tolerant genotypes and leaf physiological changes induced by salt stress during early growth stage. For the first objective, QTL mapping was performed using an F2:3 population from Jake (tolerant) x Ozark (sensitive). A major QTL was found on chromosome 3 linked to four SNP loci in the same genomic region previously reported, explaining 37% to 49% of the phenotypic variation in LSS, PDP, leaf chlorophyll and leaf chloride content. Additionally, a new minor QTL linked with two SNP markers was identified on chromosome 19 explaining 5% of leaf chlorophyll variation. These QTL and linked SNP markers will be useful in MAS for salt tolerance. For the second objective, two sensitive (Desha, Ozark) and two tolerant (Jake, Lee) cultivars were treated with NaCl and KCl at 80 mM and 120 mM from stage V1 to V5. The most adverse effects on tolerant and susceptible varieties, was caused by KCl compared to NaCl stress. Under KCl treatment, the tolerance capacity of the excluders was severely inhibited causing early death, while under NaCl stress, tolerant varieties were able to accumulate up to 2.3 and 3.8 times less leaf Cl- and leaf Na+, respectively, than the sensitive ones. Plant death occurred when shoot ion concentration reached 80,000 mg/kg and 18,000 mg/kg of Cl- and Na+, respectively, under 120 mM NaCl. Under 120 mM KCl, plants died when leaf Cl- content reached 120,000 mg/kg and leaf K+ content was over 100,000 mg/kg.

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