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Soybean [Glycine max(L.) Merr.] yields will be protected by introgressing pathogen resistance, an objective of soybean breeding for pathogen resistance at Iowa State University (ISU). Germplasms AR4SCN (Reg. No. GP‐410, PI 684651), AR5SCN (Reg. No. GP‐411, PI 684652), AR6SCN (Reg. No. GP‐412, PI 684653), AR7SCN (Reg. No. GP‐413, PI 684654), and AR8SCN (Reg. No. GP‐414, PI 684655), are resistant to soybean cyst nematode (SCN, Heterodera glycines[HG] Ichinohe) HG type 0 (Race 3) and HG type 1.3.6.7 (Race 14). In 2007, ISU Projects 3722 of the Agronomy Department and Project 3384 of the Plant Pathology Department, the Iowa Agricultural and Home Economics Experiment Station, and the ISU Research Foundation released the lines. Each line is an F12:13bulk of 60 agronomically uniform plants from the cross of the genetically diverse SCN parents PI 88788 and ‘Columbia’. The cross made at the Isabela Substation, Puerto Rico, underwent generation advances in Puerto Rico and Iowa. Phenotyping/molecular characterizations of lines at Arelli's, Cianzio's and Diers’ labs confirmed the presence of high copy numbers of the full‐length sequence repeat of the highly important major PI 88788‐type rhg1‐bgene, conferring high levels of SCN resistance on each germplasm. In Iowa, average seed yield of the maturity group (MG) II line AR4SCN was 57% of the check ‘Dwight’. For MG III lines (AR5SCN, AR6SCN, AR7SCN, AR8SCN), yield was 77% of ‘Macon’. The germplasms will serve in the development of SCN‐resistant genotypes adapted to northern US regions where SCN HG type 0 and/or 1.3.6.7 may be prevalent. Lines may also serve as research genetic stocks.

Alleles that control four to sevenfold increases in the content of stearic acid in soybean [Glycine max(L.) Merr.] oil have been obtained by artificial mutagenesis. The objective of this study was to evaluate the association of three alleles for high stearic acid with agronomic and chemical characters of soybean. Three mutant lines, A6 with 282 g kg−1stearic acid, A9 with 163 g kg−1stearic acid, and A10 with 146 g kg−1stearic acid, each with a different allele at a single locus controlling stearic acid content in the seed oil, were crossed to one of three high‐yielding genotypes with stearic acid content of about 40 g kg−1. The F1plants of each mating were backcrossed without selection to the high‐yielding parent. Pairs of highand low‐stearic‐acid lines were selected from BC1F2‐derived lines segregating for a high‐stearic‐acid allele. The pairs of lines were evaluated in a replicated test at three Iowa locations. There were no significant differences in the average yield of the high‐ and the low‐stearic‐acid lines from the A9 and A10 crosses. In the A6 cross, the low‐stearic‐acid lines had significantly greater seed yield than high‐stearic‐acid lines by an average of 7.7%. For the other characters measured, the maximum differences between the mean performance of high‐ and low‐stearic‐acid lines for any of the three crosses were 2.9 days for time of maturity, 4 cm for plant height, 0.3 units for lodging score, 8 mg seed−1for seed weight, 13 g kg−1for seed protein, and 1 g kg−1for seed oil. The stearic acid contents of the BC1F3‐derived lines generally were less than those of their donor parent, which indicated that modifying genes may influence this character.

Breeding for seed protein and oil content of soybean [Glycine max(L.) Merr.) genotypes adapted to temperate climates would require less time if selection could be practiced in tropical locations during the winter. The primary objective of the study was to determine the effectiveness of selection in tropical environments for seed composition of genotypes adapted to the northern United States. Twenty random F4‐derived lines from each of two crosses were evaluated in six plantings at Isabela, PR, and in one planting at Ames, IA, during each of 2 years. Protein, oil, and protein + oil percentage were determined for individual plants and for plots harvested in bulk. The genotype ✕ planting interactions were not significant for any of the traits, except for oil percentage in one cross. Phenotypic and rank correlations for performance of the genotypes in Iowa and Puerto Rico ranged from 0.75 to 0.92 and were highly significant. for each of the three traits, ranking of entries among the Puerto Rico environments was similar to the ranking between the Iowa environments. The results indicated that plantings in tropical locations can be used effectively to select for protein, oil, and protein + oil percentage among soybean lines adapted to temperate environments.

Selection based on genotypes at marker loci may increase the efficiency of plant improvement. Two interspecific crosses of soybean [Glycine max(L.) Merr ✕ G. soja Sieb. & Zucc.] were used to examine the relationships between isozyme marker loci and agronomic traits. The parents had different alleles at six isozyme loci for Cross 1 (A80‐244036 ✕ PI 326581) and eight isozyme loci for Cross 2 (A81‐157007 ✕ PI 342618A). Approximately 4000 BC2F4‐derived lines were evaluated in two replications at two locations for maturity (MAT), height (HT), lodging (LDG), plant type (PLT) and vining (VNG). Lines were identified that were homozygous for G. sojaalleles at different numbers of marker loci, from 0 to 5. Within each locus class, there were a number of specific enzyme genotypes, and each enzyme genotype was represented by a number of lines from different BC2F1families. Associations were observed for specific enzyme genotypes and every quantitative trait in both crosses, but the particular isozyme genotype/quantitative trait relationships were population specific. No complete or very tight linkages between marker loci and loci affecting quantitative traits were identified in this study, but such linkages would be useful in the identification and isolation of genes affecting the expression of quantitative traits. For every trait except MAT, an increase in the numbers of isozyme marker loci that were homozygous for alleles from G. sojaresulted in a change in the phenotype toward the G. sojaparent in both crosses. The relationship between numbers of homozygous marker loci and parental phenotype could increase the efficiency of an introgression program by hastening recovery of the recurrent parent phenotype while maintaining specific desired alleles from the donor parent.

An effective breeding strategy is needed for the development of high‐protein, high‐yielding soybean [Glycine max(L.) Merr.] cultivars. This study was conducted to evaluate a backcross strategy for transferring genes for high seed protein from a low‐yielding plant introduction to high‐yielding lines with a average protein content. Pando, a plant introduction with low yield and about 480 g kg−1of seed protein was crossed to three high‐yielding lines with an average protein content about 400 g kg−1. Selection for high protein was practiced between two generations of backcrossing to the high‐yielding parent. After two backcrosses, 95 random BC2F2‐derived lines from each population were evaluated for seed yield, protein, and oil content in Iowa. None of the BC2F2‐derived lines equaled the protein content of Pando, but an average of 72% of the lines from the three populations had significantly higher protein than the recurrent parent. On average, about 19% of the BC2F2‐derived lines with a protein content significantly different (α = 0.05) from the recurrent parent in seed yield. The results indicated that when a low‐yielding, high‐protein donor parent is utilized. selection for high protein between two backcross generations would effectively increase the seed protein in the backcross progeny above that of the high‐yielding recurrent parent and still result in some progeny having a seed yield not significantly different from that parent.

Efficient indirect selection for seed weight in a soybean [Glycine max(L.) Merrill] breeding program on the basis of the width or length of green pods requires a rapid technique for determining maximum dimensions of the pods. Our objective was to determine if the ratio of seed width (SW) to pod width (PW) could be used to establish when green pods have attained maximum width and length. Seed and pod development of nine soybean lines differing in seed weight were evaluated in Ames, Iowa, and in two environments at Isabela, Puerto Rico. Width of the seeds and pods were measured from suture to suture and pod length (PL) was measured from the base of the sepals to the tip of the pod. Differences among lines and environments were statistically significant in the combined analysis of variance for the SW/PW ratio at the earliest time that maximum PW and PL occurred. Among lines and environments, seeds were 28 to 50% as wide as the pods when maximum PW and PL first occurred. Data combined over environments indicated that all lines attained a SW/PW ratio of 50% within 4 days after the earliest time that maximum PW or PL occurred. The results indicated that 50% would be a conservative SW/PW ratio for assuring that a green pod had reached its maximum width and length. Use of the SW/PW ratio is an efficient method of determining the earliest time that green pods can be measured for indirect selection of seed weight in soybeans.

Soybean [Glycine max(L.) Merr.] genotypes with an exceptionally high level of resistance to iron‐deficiency chlorosis on calcareous soil have been developed by recurrent selection. The objective of this study was to compare alternative strategies for the use of the highly resistant genotypes as parents in developing populations from which high‐yielding cultivars with chlorosis resistance could be selected. Single‐cross populations were developed by mating two highly resistant genotypes with three high‐yielding, chlorosis‐susceptible lines. The F2‐derived lines from each single cross were evaluated for chlorosis resistance on calcareous soil in the field, and the superior ones from each population were backcrossed to the high‐yielding parent. The F4‐derived lines from the single‐cross populations and BC1F3‐derived lines from the backcross populations were evaluated for seed yield on noncalcareous soils and for chlorosis resistance on calcareous soil. The backcross populations had a higher mean yield and a lower level of chlorosis resistance than did the single‐cross populations. Seven percent of the lines in the single‐cross populations and 1% of the lines in the backcross populations were equal to the highly resistant parents for chlorosis resistance. An average of 36% of the lines in the single‐cross populations and 43% of the lines in the backcross populations had a seed yield not significantly different from the recurrent parent and chlorosis resistance not significantly less than a check cultivar with acceptable resistance. Phenotypic correlations between the yield of lines on noncalcareous soil and their chlorosis resistance on calcareous soil were not significant in half of the populations and were negatively associated (p<0.01) in the other populations. The negative correlations indicated either that there was no physiological association between the traits or that the association was favorable for the selection of high‐yielding lines with chlorosis resistance.

Soybean [Glycine max(L.) Merr.] cultivars with a seed weight of <100 mg seed−1are preferred for some food products. The wild species Glycine sojaSieb. & Zucc. has a seed weight of <30 mg seed−1, but undesirable agronomic characteristics prevent its direct use in commercial production. The feasibility of transferring the genes for low seed weight from G. sojainto high‐yielding cultivars of G. maxthrough backcrossing was studied. For three G. max✕ G. sojacrosses, selection between backcross generations for low seed weight was compared with no selection during backcrossing. For each cross, 40 F2‐derived lines from each backcross generation from the BC0to BC3were evaluated in two replications at two locations for seed weight, seed coat color, lodging, petiole retention, and maturity. Selection resulted in backcross progeny with smaller seed weights than those obtained from backcrossing without selection. Of the BC3F2‐derived lines, 37% had seed weights of ≤100 mg seed−1in the populations obtained by selection, compared with 18% of those obtained without selection. From each cross, BC3F2‐derived lines were evaluated for yield. An average positive correlation of 0.44 was found between seed weight and yield among selected populations and 0.54 among unselected populations. Up to 89% of the yield of the recurrent parent was exhibited by lines with seed weights of 100 mg seed−1or less.