Yield and related traits for a soybean breeding line ‘Tokei 1122’ with QTLs for long terminal racemes under high planting density conditions

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.