Elevated CO2 Could Induce Heat Stress in Two Japanese Cultivars of Rice (Oryza Sativa L.) via Reduction in Transpiration Under High Air Temperature Conditions

Two Japanese cultivars of rice (Oryza sativa L.), Hinohikari and Nikomaru, were planted using potting soil on June 13, 2018, and were exposed to elevated CO2 from June 26 to October 9 using open-top chambers. The study was conducted in Nagasaki, in the Kyushu region of Japan, where the air temperature is relatively high. There were two treatments: ambient CO2 treatment with approximately 400 µmol mol–1 (ppm) CO2, and elevated CO2 treatment with approximately 550 ppm CO2. The elevated CO2 treatment significantly increased the net photosynthetic rate and whole-plant dry mass of the two rice cultivars. However, this treatment did not produce significant effects on grain yield and adversely affected grain appearance quality of both cultivars. Among the yield components, spikelet fertility was significantly reduced by exposure to elevated CO2. These adverse effects were typical manifestations of heat stress in rice. Even under ambient CO2 treatment, there was relatively low spikelet fertility and grain appearance quality, because air temperature during the cultivation period was higher than the standard climatological normal of air temperature. Furthermore, under elevated CO2 treatment, significant reductions in transpiration rate of flag leaves were observed during the flowering period. This may cause an increase in temperature of the canopy, including the panicle, and induce heat stress. These results suggest that elevated CO2 could induce heat stress in rice via reduction in transpiration under high air temperature conditions.