LAUSR

ABSTRACT To evaluate soybean performance in response to high night temperature (HNT), a soybean cultivar, Fukuyutaka, was grown in 2 years under different night temperatures on a mini field in a temperature gradient chamber (TGC) at Kyoto, Japan. The average night temperature ranged from 21.7°C to 23.9°C and from 20.3°C to 22.8°C in 2017 and 2018, respectively. An additional experiment was conducted using a phytotron observing the same cv. under NT as high as 28°C (extreme HNT) and the control at 22 C from a R1 to R6.5 period. In the TGC yield tended to decline by 4.6% per °C when the results from the 2 years were combined. The responses of yield components tended to offset and the harvest index did not respond to varying NT. And thus the decrease in yield by HNT resulted from the change in biomass. The reduction of biomass was attributable more to branch growth than to main stem and more to leaf growth thant to other organs. Leaf area and leaf weight showed responses to NT as convex curves, suggesting that HNT may inhibit leaf growth. Reduction of biomass was more evident by extreme HNT in the phytotron. Pn was not significantly affected by HNT in TGC. The estimated maintenance respiration (Rm) increased with HNT but it seemed to explain the biomass response to HNT only partially. The results suggested that the growth and yield of soybean are potentially affected by increased NT through a negative impact on biomass production caused by multiple factors. GRAPHICAL ABSTRACT