Three screening techniques for characterizing drought tolerance were evaluated using 20 soybean [Glycine max (L.) Merr.] genotypes. These techniques involved: 1) germinating seed in polyethylene glycol‐600 (PEG) at −0.6 MPa osmotic pressure, 2) subjecting seedlings to PEG‐600 at −0.6 MPa osmotic pressure in hydroponic solution for 14 days, and 3) a heat tolerance test based on the cellular membrane thermostability. Genotypic variability associated with drought tolerance was demonstrated by the evaluation techniques. Based on these procedures, 2 of the 20 cultivars tested were relatively drought tolerant, 2 were relatively susceptible, and the remaining cultivars fell into an intermediate group. Highly significant correlations were found between the hydroponic seedling and heat tolerance tests, indicating that both drought and heat tolerance were identified in the same cultivars under the conditions of this study. It is doubtful that the seed germination test could be used as a reliable procedure for identifying drought tolerant cultivars because it tended to reflect differences in seed quality and had no relation to field performance. Although reproducibility and consistency of genotypic differences did exist using the heat tolerance tests, comparisons with more yield data are needed to evaluate this technique. Of the three procedures evaluated in this study, the hydroponic seedling test seemed to be the most reliable and potentially useful as a means for screening for drought tolerance in soybeans.

Soybean improvement via plant breeding has been critical for the success of the crop. The objective of this study was to quantify genetic change in yield and other traits that occurred during the past 80 yr of North American soybean breeding in Maturity Groups (MGs) II, III, and IV. Historic sets of 60 MG II, 59 MG III, and 49 MG IV soybean cultivars, released from 1923 to 2008, were evaluated in field trials conducted in 17 U.S. states and one Canadian province during 2010 to 2011. Averaged over 27 MG II and MG IV and 26 MG III site-years of data, the estimated rates of yield improvement during the 80 yr were 23 kg ha -1 yr -1 for MGs II and III, and 20 kg ha -1 yr -1 for MG IV cultivars. However, a two-segment linear regression model provided a better fit to the data and indicated that the average current rate of genetic yield gain across MGs is 29 kg ha -1 yr -1 . Modern cultivars yielded more than old cultivars in all environments, but particularly in high-yielding environments. New cultivars in the historic sets used in this study are shorter in height, mature later, lodge less, and have seeds with less protein and greater oil concentration. Given that on-farm soybean yields in the United States are also increasing at a rate of 29 kg ha -1 yr -1 , it can be inferred that continual release of greater-yielding cultivars has been a substantive driver of the U.S. onfarm realized yield increases.

Soybean is the world’s leading source of vegetable protein and demand for its seed continues to grow. Breeders have successfully increased soybean yield, but the genetic architecture of yield and key agronomic traits is poorly understood. We developed a 40-mating soybean nested association mapping (NAM) population of 5,600 inbred lines that were characterized by single nucleotide polymorphism (SNP) markers and six agronomic traits in field trials in 22 environments. Analysis of the yield, agronomic, and SNP data revealed 23 significant marker-trait associations for yield, 19 for maturity, 15 for plant height, 17 for plant lodging, and 29 for seed mass. A higher frequency of estimated positive yield alleles was evident from elite founder parents than from exotic founders, although unique desirable alleles from the exotic group were identified, demonstrating the value of expanding the genetic base of US soybean breeding.

Genome-wide association analysis identified 61 SNP markers for canopy wilting, which likely tagged 51 different loci. Based on the allelic effects of the significant SNPs, the slowest and fastest wilting genotypes were identified. Drought stress is a major global constraint for crop production, and slow canopy wilting is a promising trait for improving drought tolerance. The objective of this study was to identify genetic loci associated with canopy wilting and to confirm those loci with previously reported canopy wilting QTLs. A panel of 373 maturity group (MG) IV soybean genotypes was grown in four environments to evaluate canopy wilting. Statistical analysis of phenotype indicated wide variation for the trait, with significant effects of genotype (G), environment (E), and G × E interaction. Over 42,000 SNP markers were obtained from the Illumina Infinium SoySNP50K iSelect SNP Beadchip. After filtration for quality control, 31,260 SNPs with a minor allele frequency (MAF) ≥5% were used for association mapping using the Fixed and random model Circulating Probability Unification (FarmCPU) model. There were 61 environment-specific significant SNP-canopy wilting associations, and 21 SNPs that associated with canopy wilting in more than one environment. There were 34 significant SNPs associated with canopy wilting when averaged across environments. Together, these SNPs tagged 23 putative loci associated with canopy wilting. Six of the putative loci were located within previously reported chromosomal regions that were associated with canopy wilting through bi-parental mapping. Several significant SNPs were located within a gene or very close to genes that had a reported biological connection to transpiration or water transport. Favorable alleles from significant SNPs may be an important resource for pyramiding genes to improve drought tolerance and for identifying parental genotypes for use in breeding programs.

Soybean [Glycine max (L.) Merr.] is often exposed to high daytime and nighttime temperatures during critical growth stages. Threshold mean daily temperature for photosynthesis, respiration, and reproductive process in soybean is ≥26°C. In future, the magnitude of increase in nighttime temperatures will be greater than in daytime temperatures. The objectives were to determine effects of high daytime or nighttime temperatures on (i) leaf photosynthetic and respiration rates; (ii) pollen germination, pod‐set, and seed weight; and (iii) pollen phospholipids profile. Soybean plants were exposed to high daytime temperature (39/20°C), high nighttime temperatures (30/23°C, 30/26°C, and 30/29°C), or optimum temperature (30/20°C) for 10 d at flowering stage. High daytime temperature (39/20°C) or nighttime temperatures (30/29°C) increased leaf respiration rates and decreased leaf chlorophyll content, photosynthetic rate, photochemical quenching, and electron transport rate compared to optimum temperature. Likewise, high temperature decreased pollen viability and germination. Lower pollen germination at high temperature may be due to decreased levels of saturated phospholipids and phosphatidic acid in pollen grains compared with optimum temperature. Pod‐set and seed weight were decreased by high daytime or nighttime temperature. In conclusion, high daytime (39/20°C) or nighttime (30/29°C) temperature decreased leaf photosynthetic rate and pollen germination, leading to lower pod‐set and seed weight.