Lower photosynthetic rate and photosynthetic nitrogen use efficiency in northern Japanese soybean cultivars than Midwestern U.S. cultivars

Abstract

In major soybean‐producing regions such as the United States, farm yield has increased in recent years. This gain is accompanied by genetic improvements in net photosynthetic rate (PN). However, farm yields in Japan remain low and unchanged, suggesting a lack of progress in improving yield and the underlying PN through recent breeding. It is unclear how much Japanese breeders have improved PN, or whether the PN of Japanese cultivars is inferior to cultivars from the United States and, if so, which physiological traits should be improved to increase PN in Japanese cultivars. To answer these questions, we compared PN and related physiological traits of cultivars released during the past 60 yr in the Tohoku region of northern Japan (JP group) with those of cultivars from the Midwestern United States (U.S. group). In the U.S. group, PN increased with year of release at both flowering and the beginning of seed filling (respectively, R2 and R5) stages, and was accompanied by an increase in leaf N content by area (Narea). This demonstrates improved PN resulting from recent breeding. In contrast, there was no difference in PN between old and new cultivars in the JP group, indicating that PN was not improved by breeding. PN at R5 in the JP group was lower than in the U.S. group and was associated with lower stomatal conductance in the JP group. Narea at R5 was higher in the JP group than in the U.S. group and was associated with lower photosynthetic N use efficiency in the JP group.