Contrasting grain-quality and starch structure-function responses of double-cropping rice to elevated CO₂ and warming

Rice starch functionality under concurrent elevated CO₂ and warming remains unclear in double-cropping systems. We conducted a two-year field experiment to assess the effects of elevated CO₂ (ECO₂, +110 μmol mol^-1) and warming (+1.5 °C) on yield and grain quality. Detailed analyses of starch molecular order, relative crystallinity, thermal properties, and pasting properties were conducted using 2019 samples. The relative importance of the two climate drivers differed between the two cultivar-season systems. In early-season rice, warming was the dominant factor, enhancing molecular order and relative crystallinity; gelatinization enthalpy and setback increased by 39.7% and 40.0%, respectively, while starch swelling was restricted. In late-season rice, ECO₂ was the main driver, increasing gelatinization enthalpy and setback by 11.0% and 23.2%, respectively. These coordinated responses were associated with greater chalkiness and less favorable pasting functionality, indicating a partial decoupling between productivity gains and preservation of starch-based grain quality under experimentally elevated CO₂ and warming.