averaged, on a 130 g kg Ϫ1 seed moisture basis, a seed yield of 2401 kg ha Ϫ1 , a seed protein content of 354.1 g Soybean [Glycine max (L.) Merr.] seed protein is negatively correkg Ϫ1 , and a seed oil content of 185.6g kg Ϫ1 (Hurburgh, lated with seed oil and often with yield. Our goal was to examine the 2001). These statistics indicated that 67.25 kg of the crop basis for these correlations at a quantitative genetic locus (QTL) level.could be processed into 19.7 kg of 48% protein meal Seventy-six F 5 -derived recombinant inbred lines (RILs) from the matproduct and 4.90 kg of oil product.ing of the high-protein (480 g protein per kg seed) G. max accession PI 437088A with the high-yield cultivar Asgrow A3733 (420 g kg Ϫ1 Soybean protein and oil contents in various regions seed protein content) were evaluated in six irrigation treatments (i.e., of the USA can deviate significantly from the foregoing 100, 80, 60, 40, 20, and 0% replacement of weekly evapotranspiration national averages. Some of that geographic variability loss) of a two-replicate experiment conducted for 2 yr. The RILs were arises from meteorological events that, in any given genotyped with 329 random amplified polymorphic DNAs (RAPDs), season, randomly affect some regions but not others. simple sequence repeats (SSRs), and four other markers, creatingFor example, high temperatures during soybean seed a 2943-centimorgan (cM) genetic map of 35 linkage group (LGs) that, development can elevate seed oil (Howell and Cartter, on the basis of SSR homology, aligned with the 20 known soybean 1958), while severe drought can depress seed protein LGs. The phenotypic regression of RIL protein and oil on yield re-(Specht et al., 2001). Other geographic variability arises vealed respective linear coefficients of Ϫ2.6 and ϩ1.6 percentage from region-specific climatic parameters. Notably, seed points per kg ha Ϫ1 (a protein/oil exchange ratio of Ϫ1.6). A seed protein is typically lower in the northwestern than in protein, oil, and yield QTL mapped close to RAPD marker OPAW13a southeastern soybean-growing states. In 2001, that rein a small LG-I interval that was flanked by the SSR markers Satt496 gional difference spanned 3 percentage points, the and Satt239. The additive effects of the PI 437088A allele on seed protein, oil, and yield were a respective ϩ1.0 and Ϫ0.6 percentage largest ever observed in 17 yr of survey data (Hurburgh, points (a protein/oil ratio of Ϫ1.6) and Ϫ154 kg ha Ϫ1 . Given that the 2001). Processors may not be able to derive the valuable genetic-based protein/oil exchange ratio of 1.6 is smaller than the 2.0 48% soybean meal if the soybean seed has too low of a calorific-based oil/protein ratio, one might expect the remaining 0.4 protein content. To offset this geographical disadvantage, units of carbon and/or energy to be made available for other seed breeders developing high yielding cultivars adapted to dry matter. However, yield almost invariably falls when seed protein northern and western production regions must practice is genetically ...

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.

A set of nested association mapping (NAM) families was developed by crossing 40 diverse soybean [ (L.) Merr.] genotypes to the common cultivar. The 41 parents were deeply sequenced for SNP discovery. Based on the polymorphism of the single-nucleotide polymorphisms (SNPs) and other selection criteria, a set of SNPs was selected to be included in the SoyNAM6K BeadChip for genotyping the parents and 5600 RILs from the 40 families. Analysis of the SNP profiles of the RILs showed a low average recombination rate. We constructed genetic linkage maps for each family and a composite linkage map based on recombinant inbred lines (RILs) across the families and identified and annotated 525,772 high confidence SNPs that were used to impute the SNP alleles in the RILs. The segregation distortion in most families significantly favored the alleles from the female parent, and there was no significant difference of residual heterozygosity in the euchromatic vs. heterochromatic regions. The genotypic datasets for the RILs and parents are publicly available and are anticipated to be useful to map quantitative trait loci (QTL) controlling important traits in soybean.

The effect of temperature on yield of soybeans (Glycine max (L.) Merr.), is often underestimated despite reports of a significant relationship between yield and growing season temperatures. The identification of genotypes having heat tolerance appears to be warranted, but a simple, rapid technique for measuring tolerance to high temperature is needed. A technique previously used for assessing genotypic differences in membrane thermostability (heat tolerance) in sorghum (Sorghum bicolor (L.) Moench) was evaluated for use in soybeans. The technique involves the measurement by electrical conductance of the amount of electrolyte leakage from heat‐damaged leaf tissue cells after exposure to elevated temperatures. The relationship between the degree of injury and the temperature at which that injury was induced was observed to be a sigmoidal response. Genotypic differences in heat tolerance were associated with differences in the relative position of the response curve with respect to the treatment temperature. Greatest sensitivity in detecting genotypic differences occurred at temperatures inducing about 50% injury. Genotypic differences were greatest in newly developed leaf tissue. Consequently, only the most recently developed leaves should be used in the assay. Plant‐to‐plant variation was appreciable and necessitated the use of bulked leaflets from several plants as samples. Genotypic differences were consistent across sampling dates, indicating that the assay can be conducted during any phase of vegetative growth. Results obtained from cultivar trials over several years show significant differences among genotypes and consistent relative ranking of genotypes in different environments. Although requiring replication to achieve a sufficiently small standard error, the technique shows promise as a screening method.

The magnitude of a seed yield increase that occurs when soybeans [Glycine max (L.) Merr.] are irrigated depends upon the phenologic timing of the irrigation in relation to the temporal sequence with which the components of seed yield are established and fixed. Eight soybean cultivars, varying in stem growth habit and maturity, were irrigated according to a factorial treatment design in which either no irrigation or one irrigation was applied at three reproductive stages: R1 to R2 flowering (F), R3 to R4 pod elongation (P), or R5 to R6 enlargement (S). An F irrigation increased the numbers of pods/plant (+ 2.8) and seeds/plant (+ 5.8), but an offsetting decrease occurred 100‐seed weight (−0.9 g), resulting in little change in seed yield. P irrigation had no effect on 100‐seed weight, but increased the numbers of pods/plant (+3.4) and seeds/plant (+7.9) resulting in a large increase in seed yield. An S irrigation resulted in only slight increases in the numbers of pods/plant (+1.0) and seeds/plant (+3.3), but greatly increased 100‐seed weight (+1.4 g), again leading to a large seed yield increase. These observations suggested that irrigation early in reproductive ontogeny greatly reduced flower and pod abortion, whereas irrigation later in ontogeny reduced ovule abortion within developing pods. The cultivar ‘Harcor’ possessed unusually high numbers of oneand two‐seed pods, while ‘Elf’ had substantially fewer numbers of three‐seed pods, and ‘Woodworth’ possessed larger numbers of fourseed pods. Irrigation timing differentially influenced the frequencies of the various pod classes relative to their contribution to the increase in total pods/plant, primarily because of effects on ovule abortion within developing pods. The effects of irrigation timing on number of seeds/plant and 100‐seed weight were thus consistent with the effects on seed yield reported earlier.