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Abstract The initial aim of this study was to evaluate an effect of elevated CO 2 concentration and air temperature (future climate) and O 3 pollution on mono- and mixed-culture grown summer rape (Brassica napus L.) and wild mustard (Sinapis arvensis L.). The second task was to reveal the mechanisms of the shift in plants' competitiveness in response to single and combined environmental changes. Plants were grown in mono- and mixed-cultures under current climate (CC) (400 μmol mol−1 of CO 2 , 21/14 °C day/night temperature) or future climate (FC) conditions (800 μmol mol−1 of CO 2 , 25/18 °C day/night temperature) with and without O 3 treatment (180 μg m−3). Competition had relatively little effect on growth of both species at current climate, independent of O 3 treatment. In contrast, competitive effect of both plant species considerably increased under FC, and especially FC + O 3 conditions, when growth of mixed-culture rape reduced up to 48% and that of wild mustard up to 80%. The mechanisms of elevated competitiveness of rape under the future climate consisted of better antioxidative protection, particularly elevated total antioxidative capacity and activities of peroxidase and ascorbate peroxidase. Whereas stronger oxidative damage, disproportionally high activities of H 2 O 2 scavenging enzymes and lower pool of soluble sugars in mixed-culture wild mustard reduced its competitiveness under FC + O 3 conditions. In conclusion it must be pointed out, that regardless improved competitive abilities of rape under FC and FC + O 3 conditions, competition with wild mustard reduced growth, indicating increased weed-induced yield losses in the future climate, especially with concomitant intensification of O 3 pollution. Highlights • Competition had little effect on summer rape and wild mustard at current climate. • Competitive effect of both plant species increased under the future climate (FC). • FC increased rape's competitiveness via better antioxidative protection. • Redox imbalance and lowered TSS reduced weed competitiveness at FC + O3 conditions. [ABSTRACT FROM AUTHOR]