Effects of CO₂ concentration enrichment on the grain quality of different rice varieties

Increasing atmospheric carbon dioxide (CO₂) concentration leads to global warming and has a profound effect on the growth and development of crops. As a substrate for plant photosynthesis, high CO₂ concentration can increase rice yields, but the effect on rice quality is still unclear. The genotypic variation in rice quality in response to high CO₂ concentration is less studied than rice yield. In this study, eight rice varieties were grown under ambient and elevated CO₂ concentrations (200 μmol·mol^-1 higher than ambient) in a free-air CO₂ enrichment (FACE) platform. The effects of elevated CO₂ concentrations on rice processing quality, appearance quality, eating quality, and nutritional quality were studied, and grain quality differences among rice varieties in response to elevated CO₂ concentration were also investigated. All of the quality traits varied significantly among the tested varieties. Compared with rice plants grown under ambient CO₂ concentration, plants grown under FACE treatment tended to have decreased brown rice percentage, white rice percentage, and head rice percentage, but had significantly increased yields of brown rice, white rice, and head rice (by 23.7%, 23.5%, and 20.9%, respectively). FACE treatment had little effect on the head rice length, head rice width, and the ratio of head rice length to width, but significantly increased the chalky rice rate and chalkiness degree. Averaged across all rice varieties, the elevated CO₂ concentration increased the chalky rice rate and chalkiness degree by 18.6% and 31.8%, respectively. FACE treatment reduced the amylose concentration and gel consistency by an average of 6.5% and 3.1%, respectively, but the reduction was not significant. The response in the rapid visco analyzer (RVA) profile of rice starch to elevated CO₂ concentration was also studied. FACE treatment increased the mean value of peak viscosity and breakdown by 1.3% and 6.9%, respectively, for all varieties, but decreased the mean value of hot viscosity, cold viscosity, and setback by 2.2%, 5.1%, and 65.6%, respectively. However, only the reduction in setback was statistically significant. The phytic acid concentration of the rice grains significantly increased by 5.3% on average, whereas the protein content significantly decreased by 9.9% under FACE conditions. The response of the rice quality traits to high CO₂ concentration varied in direction and magnitude among different varieties. An interaction between CO₂ treatment and rice variety was found for head rice length, chalky rice rate, chalkiness degree, peak viscosity, hot viscosity, and cold viscosity. The above data suggest that increased atmospheric CO₂ concentration significantly increased grain yield, but tended to diminish the processing quality, appearance quality, and nutritional quality of the rice grains while improving the palatability of cooked rice. The response of rice grain quality to elevated CO₂ concentration varied across rice varieties.