Effect of elevated carbon dioxide (CO2) on phenolic acids, flavonoids, tocopherols, tocotrienols, γ-oryzanol and antioxidant capacities of rice (Oryza sativa L.)

• The rice grain (endosperm, bran, husk, whole grain) is rich in natural antioxidants.
• Elevated CO2 reduces the content of γ-oryzanol in all rice milling fractions.
• Elevated CO2 reduces the contents of all phenolic acids and flavonoids in rice.
• Elevated CO2 reduces all vitamin E isomers in the endosperm and whole grain.
• Overall, the in vitro antioxidant capacity of rice decreases under elevated CO2.

There have been no studies conducted with the objective of investigating the effect of elevated CO2 concentrations ([CO2]) on antioxidants in grains. Therefore, a two-year field experiment was conducted using open-top chambers with two levels of atmospheric CO2 (375 and 550 μmol/mol) to evaluate their effects on rice grain antioxidants. Following exposure to high [CO2], the total phenolic content of all rice milling fractions decreased (3%–18%), with the highest reduction in the brown rice for sinapic acid (167%), and in the white rice for p-hydroxybenzoic acid (100%). The total flavonoid content also decreased under elevated [CO2] in all rice milling fractions (8%–14%), with apigenin (25%) being highly affected in the white rice, and tricin (12%) in the bran. The same trend was found for γ-oryzanol, with decreases of 35%, 32%, 25%, and 2% in the white rice, brown rice, husk, and bran, respectively. In the white and brown rices, tocopherols and tocotrienols were all lower under elevated [CO2], with reductions larger for α-tocotrienol (69%), γ-tocotrienol (46%), and α-tocopherol (38%). Good correlations between antioxidant contents and DPPH radical scavenging capacities indicated that these decreases may be meaningful in the preventive ability of rice against free radical-mediated degenerative diseases.