Nitrogen Utilization in the Supernodulating Soybean Variety “Sakukei 4” and Its Parental Varieties, “Enrei” and “Tamahomare”

The supernodulating variety "Sakukei 4", which has improved growth and yield, was recently developed. To evaluate its physiological traits related to the high productivity, we compared dry matter production, nitrogen (N) accumulation and N utilization in Sakukei 4 with those in the parental normally nodulating varieties, Enrei and Tamahomare, and a nonnodulating line, En1282, in pot experiments. The seed yield of Sakukei 4 was similar to that of Enrei and Tamahomare. Leaf area and relative ureide abundance in xylem sap were maintained for longer growth period in Sakukei 4 and Tamahomare than in Enrei. The total amount of nodule N2 fixation at maturity was also larger in Sakukei 4 and Tamahomare than in Enrei and En1282. In En1282 and Enrei, a larger part of N accumulated in leaves and stems before seed maturation was translocated to seeds during seed filling. However, Sakukei 4 and Tamahomare, more N tended to remain in leaves and stems at maturity. These observations suggested that the physiological traits of dry-matter accumulation and N utilization in Sakukei 4 were quite similar to those in Tamahomare, despite the difference in nodulation. N use efficiency for dry matter production (NUED) was lower in Sakukei 4 than in Enrei and Tamahomare, probably due to the energy cost for sustaining a greater number and mass of nodules. This observation indicated that the low NUED of Sakukei 4 impairs its productivity and seed yield, and therefore Sakukei 4 could not exceed Enrei or Tamahomare in seed productivity.