Response of rice grain quality to elevated atmospheric CO2 concentration: A meta-analysis of 20-year FACE studies

Abstract

Compared with growth and yield, far less attention has been devoted to the impact of elevated CO2 on rice grain quality. Exposure to elevated CO2 induces numerous physiological changes in plants that can alter the chemical composition and thus the quality of rice grains. This meta-analysis was conducted to synthesize the effect of free air CO2 enrichment (FACE, approximately 550 μmol mol−1) on grain quality of rice grown under open field conditions. Factors that could modify the CO2 effects were also investigated. They included cultivars, nitrogen applications, environmental temperatures and grain types. On average, elevated CO2 decreased head rice percentage by 8%, which led to no increase in head rice yield, despite substantial increases of brown rice yield and white rice yield that were obtained under FACE conditions. Elevated [CO2] increased chalky grain percentage by 26% and chalkiness degree by 30%, which significantly impaired rice appearance. However, the cooking and eating quality was improved by elevated [CO2], as indicated by the changes in starch RVA profiles and the palatability of cooked rice. The nutritional value of FACE rice declined as shown by the 2–9% decreases in the concentrations of nitrogen, phosphorus, magnesium, sulfur, zinc, iron, copper, manganese and amino acids; meanwhile the anti-nutrient phytate and the molar ratio of phytate to zinc were increased. High nitrogen application alleviated negative effects of elevated [CO2] on milling quality and rice appearance but further decreased the bioavailability of essential nutrients. The CO2-induced decreases in element concentrations of white rice were generally higher than those in brown rice. In general, CO2-induced changes on grain quality were seldom modified by rice cultivars, temperatures or experimental locations.