Rice grain yield and quality responses to free‐air CO 2 enrichment combined with soil and water warming

Rising air temperatures are projected to reduce rice yield and quality, whereas increasing atmospheric CO 2 concentrations ([CO 2 ]) can increase grain yield. For irrigated rice, ponded water is an important temperature environment, but few open‐field evaluations are available on the combined effects of temperature and [CO 2 ], which limits our ability to predict future rice production. We conducted free‐air CO 2 enrichment and soil and water warming experiments, for three growing seasons to determine the yield and quality response to elevated [CO 2 ] (+200 μmol mol −1 , E‐[CO 2 ]) and soil and water temperatures (+2 °C, E‐T). E‐[CO 2 ] significantly increased biomass and grain yield by approximately 14% averaged over 3 years, mainly because of increased panicle and spikelet density. E‐T significantly increased biomass but had no significant effect on the grain yield. E‐T decreased days from transplanting to heading by approximately 1%, but days to the maximum tiller number (MTN) stage were reduced by approximately 8%, which limited the panicle density and therefore sink capacity. On the other hand, E‐[CO 2 ] increased days to the MTN stage by approximately 4%, leading to a greater number of tillers. Grain appearance quality was decreased by both treatments, but E‐[CO 2 ] showed a much larger effect than did E‐T. The significant decrease in undamaged grains (UDG) by E‐[CO 2 ] was mainly the result of an increased percentage of white‐base grains (WBSG), which were negatively correlated with grain protein content. A significant decrease in grain protein content by E‐[CO 2 ] accounted in part for the increased WBSG. The dependence of WBSG on grain protein content, however, was different among years; the slope and intercept of the relationship were positively correlated with a heat dose above 26 °C. Year‐to‐year variation in the response of grain appearance quality demonstrated that E‐[CO 2 ] and rising air temperatures synergistically reduce grain appearance quality of rice.