Physiological mechanisms of slow canopy wilting in early maturity group soybeans were identified and the underlying QTLs were mapped and confirmed to protect soybean yield under drought in the field.
BackgroundDrought stress is a major limiting factor of soybean [Glycine max (L.) Merr.] production around the world. Soybean plants can ameliorate this stress with improved water-saving, sustained N2 fixation during water deficits, and/or limited leaf hydraulic conductance. In this study, carbon isotope composition (δ13C), which can relate to variation in water-saving capability, was measured. Additionally, nitrogen isotope composition (δ15N) and nitrogen concentration that relate to nitrogen fixation were evaluated. Decrease in transpiration rate (DTR) of de-rooted soybean shoots in a silver nitrate (AgNO3) solution compared to deionized water under high vapor pressure deficit (VPD) conditions was used as a surrogate measurement for limited leaf hydraulic conductance. A panel of over 200 genetically diverse soybean accessions genotyped with the SoySNP50K iSelect BeadChips was evaluated for the carbon and nitrogen related traits in two field environments (Athens, GA in 2015 and 2016) and for transpiration response to AgNO3 in a growth chamber. A multiple loci linear mixed model was implemented in FarmCPU to perform genome-wide association analyses for these traits.ResultsThirty two, 23, 26, and nine loci for δ13C, δ15N, nitrogen concentration, and transpiration response to AgNO3, respectively, were significantly associated with these traits. Candidate genes that relate to drought stress tolerance enhancement or response were identified near certain loci that could be targets for improving and understanding these traits. Soybean accessions with favorable breeding values were also identified. Low correlations were observed between many of the traits and the genetic loci associated with each trait were largely unique, indicating that these drought tolerance related traits are governed by different genetic loci.ConclusionsThe genomic regions and germplasm identified in this study can be used by breeders to understand the genetic architecture for these traits and to improve soybean drought tolerance. Phenotyping resources needed, trait heritability, and relationship to the target environment should be considered before deciding which of these traits to ultimately employ in a specific breeding program. Potential marker-assisted selection efforts could focus on loci which explain the greatest amount of phenotypic variation for each trait, but may be challenging due to the quantitative nature of these traits.
Nitrogen fixation of soybean [Glycine max (L.) Merr.] is particularly vulnerable to drought, since, in most genotypes, N2 fixation activity decreases very early in the soil drying cycle. Although a few soybean genotypes, including ‘PI 471938’, have been identified that express N2 fixation tolerance of drought, it is unknown how readily this trait is transferred to progeny. Unfortunately, the techniques used to phenotype for the tolerance trait are either too crude or too labor‐intensive to allow the acquisition of data required for a classical inheritance assessment. In this study, a heterogeneous inbred family (HIF) population derived from PI 471938 × ‘Hutcheson’ was studied for its N2 fixation drought tolerance to obtain a preliminary indication of the expression of this trait in progeny genotypes. An in situ flow‐through acetylene reduction assay was used to track the N2 fixation rates of 13 HIFs over dry‐down periods lasting about 2 wk. There was a distinct segregation among the HIFs, with nine exhibiting tolerance equivalent to PI 471938 and four exhibiting sensitivity equal to or greater than Hutcheson. These results indicate that N2 fixation drought tolerance might be transferred to progeny lines fairly readily, or at least retained in a selected population such as these HIFs.
Cowpea (Vigna unguiculata L. Walp.) is often considered a crop species appropriate for drier environments. However, little or no information exists on two key physiological traits for drought conditions: early decrease in transpiration rate in the soil drying cycle and sustained N fixation activity under low soil‐water conditions. In this study, the responses of these two traits to soil drying were compared among 10 genotypes. The fraction of transpirable soil water at which transpiration rate began to decline varied from 0.59 to 0.24. The genotypes with the higher thresholds for the transpiration decrease offer a conservative water use strategy during soil drying and, hence, may be especially appropriate for very dry areas. The fraction of transpirable soil water at which N fixation rate began to decline ranged from 0.33 in one line to another line in which there was no decline in N fixation rate. Five lines had especially low thresholds for the decline in N fixation rate, which would be highly desirable in arid areas. In fact, N fixation tolerance to drought in these five lines is superior to any responses observed in other grain legumes. There was no correlation between the thresholds for decline in transpiration and N fixation.
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