Elevated CO2 could reduce spikelet fertility and grain appearance quality of rice (Oryza sativa) grown under high-temperature conditions

· It is projected that atmospheric CO2 concentrations will increase because of industrial activities. Elevated CO2 reduces transpiration from leaves via stomatal closure, which increases plant temperature; therefore, higher levels of CO2 could adversely affect crop production via increasing plant temperature, particularly for plants that are susceptible to heat stress, such as rice (Oryza sativa). · Two Japanese rice cultivars with different heat-tolerance, Hinohikari and Nikomaru, were grown in pots inside open-top chambers and exposed to ambient CO2 (400 µmol mol–1) or elevated CO2 (550 µmol mol–1) from the beginning of the tillering stage to maturity. The study was conducted in Nagasaki, in the Kyushu region of Japan, where heat stress on rice has become increasingly evident.· During the flowering period, the air temperature was sufficiently high to induce heat stress effects in rice, such as low spikelet fertility, even under ambient CO2 conditions for both cultivars. Although elevated CO2 significantly improved the net photosynthesis and whole-plant growth of the cultivars, there were no significant effects on grain yield, which in turn reduced harvest index. In both cultivars, adverse effects on other processes occurred with elevated CO2, such as reductions in spikelet fertility and grain appearance quality, which are typical manifestations of heat stress in rice. The elevated CO2 treatment showed significant reductions in stomatal conductance and transpiration rate of flag leaves, which could increase plant temperature.· Under high-temperature condition, therefore, elevated CO2 could induce manifestations of heat stress in Japanese rice via reduction in transpiration due to stomatal closure.