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.