Effect of high night temperature on leaf senescence, physiological activity, and growth in soybean

Physiological processes that cause a negative growth response to high night temperatures (HNTs) were investigated in soybean. An HNT (28​°C) was established in a phytotron during the first (HL) and second (LH) halves of the reproductive stage and the entire (HH) reproductive stage, along with a continuously low (LL) night temperature (22​°C). Plant biomass was lower in HH than that in LL, whereas the harvest index was relatively stable. N accumulation in the plants tended to decline with HNT. The leaf area and leaf N concentration tended to be lower in HL and HH than in LL, suggesting an earlier onset of leaf senescence under HNT in the early reproductive stage. The light-saturated leaf photosynthetic rate was lower in HH than in LL during the late reproductive stage, associated with a low stomatal conductance and leaf photosynthetic rate per intercellular CO₂ concentration. A slower rise in the leaf photosynthetic rate in the morning occurred likely because of the HNT, coinciding with the low leaf water potential at predawn and low leaf photosynthetic rate to intercellular CO₂ concentration ratio. A moderately increasing gradient of night temperature from 21.7 to 23.9​°C was created in a mini-field in another experiment using a temperature gradient chamber, and in this experiment, crop growth and N dynamics were similar to those obtained in the phytotron study. These results suggest that the biomass growth of soybean is affected by HNT, particularly during the early reproductive stage.