Responses of carbon and oxygen stable isotopes in rice grain (Oryza sativa L.) to an increase in air temperature during grain filling in the Japanese archipelago

Stable isotopic compositions of carbon (δ13C) and oxygen (δ18O) in plants reflect growth conditions. Therefore, these isotopes might be good indicators of changes in environmental factors, such as variations in air temperature caused by climate change. It is predicted that climate change will lead to a greater increase in minimum air temperatures (primarily during the night) than in maximum air temperatures (primarily during the day) in many parts of Japan. In the present study, we investigated whether the δ13C and δ18O of the rice grain Koshihikari (Oryza sativa L.) from the northern latitudes (30.49°–37.14°) of Japan reflect variations in air temperature during grain filling and are related to the yield and proportion of first-grade rice (<15 % transparency, roundness, and cracking) as an indicator of quality. We revealed that rice δ13C was not correlated with mean maximum or minimum air temperatures for each prefecture. By contrast, rice δ18O was positively correlated with mean minimum air temperature, suggesting that rice δ18O reflects changes in night air temperature. We further showed that an increase in the mean minimum air temperature during grain filling had a negative effect on rice yield and quality. Our findings indicate that the δ18O of rice grain may be a good indicator of physiological changes in response to minimum air temperatures during grain filling.