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Many adverse effects of tropospheric O3 on C3 crop plants are ameliorated by elevated concentrations of atmospheric CO2, but the extent of the interaction can vary, depending on the species, gas concentrations, and other experimental conditions. A 2-yr open-top field chamber experiment was conducted to examine this interaction in peanut (Arachis hypogaea L.) by testing the effects of O3 and CO2 mixtures on yield and seed quality. Treatments were ambient CO2 (375 µmol mol–1) and CO2 additions of approximately 173 and 355 µmol mol–1 in combination with charcoal-filtered (CF) air (22 nmol O3 mol–1), nonfiltered (NF) air (46 nmol O3 mol–1), and NF air plus O3 (75 nmol O3 mol–1). At ambient CO2, pod number was suppressed 16% in NF air and 44% in elevated O3. Pod and seed mass were not significantly affected in NF air but were lowered 33 to 37% in elevated O3. Elevated CO2 increased yield parameters 7 to 17% for plants grown in CF air and restored yield in NF air and elevated O3 treatments to control or higher levels. Gas treatment effects on peanut market grade characteristics were small. No treatment effects were observed on the protein and oil contents of seeds, but there were changes in fatty acid composition. Overall results indicate that increasing concentrations of tropospheric O3 will suppress yield of O3–sensitive peanut cultivars, while elevated CO2 will moderate this response. Elevated O3 and CO2 are not expected to have major effects on peanut seed composition and quality.

Abbreviations: AA, ambient air • CF, charcoal-filtered air • ELK, extra large kernels • NF, nonfiltered air • OZ, 1.56 x ambient O3 • TSMK, total sound mature kernels