Effects of Elevated CO2 and O3 on Phenolic Compounds in Spring Wheat and Maize Leaves

C₃ crops are generally considered more sensitive than C₄ crops to the elevated CO₂ and O₃, but it is unclear whether the concentrations of phenolic compounds in them are affected. In this paper, an enrichment experiment with open-top chamber was conducted to examine the effects of elevated CO₂, O₃, and their combination on the contents of total phenolic compounds and flavone in the leaves of spring wheat (C₃ crop) and maize (C₄ crop). The results showed for spring wheat, the total phenolic contents in its leaves at jointing stage was significantly higher under elevated CO₂ and/or O₃, with the sequence of CO₂ plus O₃ > O₃ > CO₂ > ambient, while at grain-filling stage, the total phenolic content was lower under CO₂ plus O₃ than under CO₂, O₃, and ambient. The total phenolic content in maize leaves at jointing stage had the similar variation trend with that for wheat, but at grain-filling stage, the total phenolic content was slightly affected by elevated CO₂ and/or O₃. The flavone content in spring wheat leaves was significantly lower under CO₂ and/or O₃ stress at jointing stage, but had lesser difference at grain-filling stage under the stress. The same variation trend was observed in the flavone content in maize leaves at jointing and grainfilling stages, i.e., CO₂ plus O₃ > CO₂ > ambient > O₃. C₃ plant was more sensitive than C₄ plant to the CO₂ and/or O₃ stress. Foundation item: The National Natural Science Foundation of China (No. 30570348 and 30500069).