Soybean [Glycine max (L.) Merr.] is primarily grown as a source of protein and oil. A quantitative trait locus (QTL) controlling seed protein concentration was previously mapped to linkage group (LG) I of soybean. The objectives of this study were to fine map the QTL and to determine if additional recombination could reduce the inverse phenotypic relationship between seed protein concentration and yield and oil concentration. The fine mapping was done with two sets of backcross populations that were tested in the field and with genetic markers. These populations were developed by the introgression of a high protein allele from the Glycine soja Sieb and Zucc. plant introduction (PI) 468916 into the genetic background of the breeding line A81–356022. The first set (Set 1) included three populations of backcross‐four (BC4) lines, and the second set (Set 2) included four populations of BC5 lines. The populations segregated for different segments of the genomic region where the QTL maps. Tests of the two sets of populations resulted in the localization of the QTL for protein and oil to a 3‐cM interval between the simple sequence repeat (SSR) marker Satt239 and the amplified fragment length polymorphism (AFLP) marker ACG9b. The results from the agronomic trait evaluations were inconsistent, making it difficult to definitively conclude whether the protein QTL controls these other traits through pleiotropy.

Further enhancement of maize (Zea mays L.) N‐use efficiency (NUE) will benefit from a thorough understanding of how genetic improvement has shaped N use parameters. Since selection for grain yield has occurred at high N fertilizer rates, our hypothesis was that modern hybrids would have a greater response to supplemental N than hybrids from earlier eras. In 2009 and 2010, 21 single‐cross maize hybrids released between 1967 and 2006 were characterized for grain yield and N use traits. While the ability to acquire mineralized soil N did not change over era, the utilization increased with decade of introduction (0.24 kg kg–1 of plant N [kgplantN–1] yr–1; R2 = 0.37). Increases of grain yield at high N (86 kg ha–1 yr–1; R2 = 0.68) over era were accompanied by increases at low N of 56 kg ha–1 yr–1 (R2 = 0.69). Grain yield improvements at all levels of N were associated with decreased barrenness and increased kernel number expressed on a per‐plant and per‐area basis. Fertilizer N response, NUE, increased at a rate of 0.16 kg kg–1 of fertilizer N (kgN–1) yr–1 (R2 = 0.40). Increased NUE was positively correlated with improved N‐uptake efficiency (r = 0.76, P ≤ 0.001), due to the greater postflowering N uptake of more recent hybrids. The response of grain yield to fertilizer N in current hybrids is more dependent on uptake of fertilizer N than the efficiency of fertilizer N utilization, and approximately two‐thirds of genetic gain for grain yield at high N can be explained by improvements in grain yield at low N.

The narrow genetic base of elite soybean, Glycine max (L.) Merr., germplasm may impede further attempts to improve grain yield and other important agronomic characters. Germplasm collections of wild soybean, Glycine soja Siebold & Zucc., are a source of genetic variability for soybean breeding programs. The objectives of this research were to use genetic markers to characterize diversity among 60 G. soja accessions collected in China and to compare this diversity with 18 U.S. ancestral soybean genotypes, 12 Chinese G. max plant introductions (PIs), and 47 elite soybean lines from the northern USA. These accessions were genotyped with a set of 72 simple sequence repeat markers. The G. soja accessions were found to contain more alleles per locus (17) than the U.S. ancestral genotypes (5.8), the Chinese PIs (5.5), or the elite lines (4.5). Multivariate analyses were able to separate the G. max lines from the G. soja accessions and identify the most diverse subset of G. soja accessions. Multidimensional scaling separated G. soja accessions from high and low latitudes, while Ward's clustering method separated the G. soja accessions into distinct clusters that tended to include accessions from similar geographical regions. These data will be useful to breeders selecting G. soja accessions as parents in a breeding program and for establishing a core collection of G. soja to be used in future research.

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