Lodging tolerance is an important trait of soybean [Glycine max (L.) Merr.] that affects yield stability and combine‐harvesting efficiency. Previously, a quantitative trait locus analysis of lodging scores was performed using recombinant inbred lines developed by crossing the lodging‐susceptible cultivar Toyomusume with the lodging‐tolerant cultivar Toyoharuka, and the major quantitative trait locus qLS19‐1 was identified. However, the morphological traits associated with qLS19‐1 remain unclear. To develop high‐precision DNA markers for qLS19‐1, it is necessary to identify its associated lodging tolerance‐related traits. Here, we studied the effects of qLS19‐1 on lodging‐related traits, primarily pushing resistance, using the Toyomusume‐backcrossed line and near isogenic lines (NILs). The Toyoharuka allele at qLS19‐1 may increase the number of primary lateral roots and cause a high pushing resistance, resulting in a lower ratio of the pushing resistance moment. The negative correlation between the number of primary lateral roots and the lodging scores in 2 yr suggests that this trait's influence on lodging score is stable. The numbers of primary lateral roots of six recombinant NILs were evaluated for the fine mapping of qLS19‐1. Sat_099 and BARCSOYSSR_19_1255 were the molecular markers that correlated with changes in the number of primary lateral roots among the recombinant NILs. We believe that the gene responsible for controlling primary lateral root number is located within the associated 356‐kb region. Thus, the investigation of the number of primary lateral roots and/or the marker‐assisted selection for qLS19‐1 may improve lodging tolerance in breeding programs.

Low-branching soybean cultivars have few nodes per plant, and there have been many cases in northern Japan where growing these cultivars at high plant densities did not improve yield. Soybean cultivar 'Tokei 1122ʹ (T22), which has a long terminal raceme, was bred to improve the yield of low-branching cultivars under dense planting conditions. To elucidate the effects of long racemes on the suitability of T22 for dense planting, we compared the yield and yield components of T22 and 'Toyoharuka' (TH), which is a low-branching cultivar, at various planting densities. There was no significant difference in the seed yield of these cultivars at low planting densities, whereas the seed yield of T22 was significantly greater than that of TH under dense planting conditions. The increase in the number of seeds per unit area was greater for T22 than for TH. An analysis of variance revealed a significant interaction between cultivar and planting density for seed yield, number of pods, and number of pods per node. Moreover, the low-branching cultivar 'Tokei 1122ʹ with long terminal racemes produced a higher yield under non-lodging conditions than the conventional low-branching cultivar at planting densities of 33 plants m −2 or higher. The greater yield of T22 is likely because of its long terminal raceme, which increases the number of pods per node and the sink capacity (number of seeds) at high planting densities.

To elucidate the effects of maturity and the stem growth habit on the planting density-dependent branching plasticity of soybean cultivars, we studied the branch traits of 12 cultivars or lines planted at different densities (8.3, 16.7, and 22.2 plants m −2 ) in Sapporo (2012) and Ebetsu (2013). The 12 cultivars and lines consisted of three determinate cultivars from Hokkaido, three indeterminate cultivars from the northern US, and near-isogenic lines with the backgrounds of Canadian, US, and Japanese cultivars exhibiting diverse stem growth habits. We investigated the relationship between the maturity or stem growth habit and branching plasticity, which was calculated based on the ratios of different branch traits under sparse and dense planting conditions. The use of the ratios of the total branch length and the number of nodes per branch under sparse and dense planting conditions as a measure of branching plasticity revealed varietal differences across years. For the determinate and indeterminate cultivars in both years, branching plasticity was positively correlated with the number of days until stage R5 (onset of seed filling), which is when branches cease to elongate. Comparisons of Japanese and US cultivars and near-isogenic lines for the Dt1 gene (mediating the stem growth habit) indicated that the branching plasticity of indeterminate cultivars and lines is greater than that of determinate cultivars, with a large variation among backgrounds and cultivars. The results of this study imply that branching plasticity is greater in late-maturing soybean cultivars. Moreover, the indeterminate growth habit substantially enhances branching plasticity.

Core Ideas Five Canadian soybean cultivars achieved high yields in Hokkaido, northern Japan, in screening tests. OAC Dorado soybean produced a significantly greater yield than a Hokkaido's leading cultivar. OAC Dorado soybean had six characteristics that were distinct from the Hokkaido cultivars. OAC Dorado soybean is an important germplasm for high‐yield breeding. The broadening of genetic diversity is essential to improving soybean [Glycine max (L.) Merr.] seed yields, and exotic germplasms can be a source of new alleles that improve yield. The stem termination habit is an important trait affecting seed yield in soybean, and this habit can be used to classify soybean into indeterminate, semi‐determinate, and determinate phenotypes. The genetic background and environment determine whether indeterminate phenotypes have a higher yield than determinate phenotypes. Most soybean cultivars developed in high latitude countries such as Canada, Switzerland, and Poland have an indeterminate growth habit, but this is not found in any of the commercial Japanese cultivars. This study investigated high‐yielding Canadian soybean cultivars growing in Hokkaido, northern Japan. Five Canadian cultivars, Haroson, RCAT Angora, Block, RCAT Alliance, and OAC Dorado, produced significantly greater than Hokkaido's leading cultivar Yukihomare in the preliminary screening tests. OAC Dorado also produced a significantly greater yield than Yukihomare in the trials (116%). OAC Dorado had six characteristics that were distinct from the Hokkaido cultivars: an indeterminate growth habit; high pod number, especially on branches; high seed number per pod; long reproductive period; low protein content; and short lower internodes. Our findings indicate that OAC Dorado is an important germplasm for high‐yield breeding and suggest that it may be possible to breed a high‐yielding cultivar with an indeterminate growth habit in Hokkaido.