It is predicted that atmospheric carbon dioxide (CO2) level will double by the end of the current century. Although the fertilization effect of CO2 on plant growth is well documented, studies that investigate plant nutritional requirements under elevated CO2 are scarce. Potassium (K) is an essential plant nutrient with prominent roles in key physiological processes. Aim of this work, was to determine the effects of K deficiency on plant growth as affected by elevated CO2 and how antioxidant defense systems (activities of SOD, CAT, POX, APX, GR, MDHAR, DHAR, lipid peroxidation and total antioxidant activity) respond to K deficiency under ambient (a-CO2: 400 ppm) or elevated (e-CO2: 900 ppm) atmospheric CO2 conditions in durum (Tritium durum cv. Saricanak-98) and bread wheat (Triticurn aestivum cv. Adana-99). Plants were grown in hydroponics with sufficient (1850 mu M), low (60 mu M)or deficient (20 mu M) K and under a-CO2 or e-CO2. As expected, e-CO2 promoted biomass production with adequate K supply, however in low and deficient K plants biomass was either not affected or even decreased by e-CO2. It was observed that low or deficient supply of K induced oxidative stress, but e-CO2 had no significant impact on antioxidative response of plants and thus could not alleviate detrimental effects of K deficiency. Under K deficiency, CAT activity decreased in both species but this decrease was accompanied with increases in POX and APX which may be for adapting to the changing environment. In general, responses in antioxidant defense enzymes were linked to K nutritional status of plants rather than e-CO2 conditions. (C) 2017 Elsevier B.V. All rights reserved.