Identifying Drought Tolerance in Soybean: From the Growth Chamber to the Field and Back Again.

Nitrogen fixation in legumes provides important economic advantages for crop production by eliminating the cost of nitrogen fertilizer, but the extreme sensitivity of N₂ fixation to drought in soybean makes the crop particularly vulnerable to drought. Over 20 years ago, the genotype Jackson was identified as having prolonged N₂ fixation during the early stages of drought. This early research from growth chamber and field experiments determined that during drought, ureides accumulate throughout the plant. The genotype Jackson, however, had considerably lower concentrations of ureides under drought than did drought-sensitive genotypes. From a recombinant inbred population developed from a cross between KS4895 and Jackson, we identified quantitative trait loci (QTLs) for ureide and N concentration. Lines from this population with extremes for ureide and N concentration were evaluated in field experiments for their response to drought. Lines with low ureide or N concentrations under well-watered conditions had higher rates of N₂ fixation and higher yield when exposed to a severe drought than those lines with high ureide or N concentrations. Under well watered conditions, however, N₂ fixation and yield increased as the ureide or N concentrations increased. In this population it appears that low shoot N concentration may be beneficial in prolonging N₂ fixation under drought conditions but that under water-replete conditions, lines with low shoot N concentration may have lower N₂ fixation rates and yield. A key question is whether the benefits of low shoot N concentration can be separated from the adverse effects under water-replete conditions. Several of the QTLs for ureide concentration were confirmed in a genome-wide association study (GWAS) on a set of 373 diverse, maturity group IV accessions that were phenotyped in four environments and genotyped with approximately 12,000 single nucleotide polymorphisms. Results from the evaluation of accessions for shoot ureide N concentrations were taken back to the growth chamber. Twelve maturity group IV accessions that were extremes for shoot ureide and N concentration were evaluated for their ability to prolong N₂ fixation during drought. As hypothesized, there was a strong association between genotypic sensitivity of N₂ fixation to drought and shoot N concentration. Two of the accessions with low shoot N concentration had substantial ability to prolong N₂ fixation during drought. Research from physiological studies, molecular mapping, and phenotyping have provided tools that are now being utilized in a targeted breeding program for increasing drought tolerance of N₂ fixation.