The aim of the bachelor thesis was to determine the effect of oxidative stress caused by excessive irradiation, on two variants of Acer platanoides, wild type and Crimson King. The experiment was carried out on 8 individuals that were over-irradiated. Immediately after high light treatment, the assimilation organs were collected. By assessing the concentration of assimilation pigments, we determined the contents of chlorophyll a (chl a), chlorophyll b (chl b) and carotenoids (car). We further assessed markers of oxidative stress using hydrogen peroxide H2O2 and malonyl aldehyde (MDA) concentrations. The last method was slow kinetics of chlorophyll a fluorescence. From the results, we observed a higher content of pigments (chl b, car) and a higher PSII efficiency in Crimson King; however, a decrease in H2O2 and an increase in MDA were also observed in this variant. Wild type responded to irradiance with an increase in H2O2 and a decrease in MDA. However, the decrease in H2O2 concentration and the increase in MDA antioxidant in the Crimson King cultivar is probably not due to a lower level of oxidative stress, but on the contrary, due to a lower efficiency of the antioxidant system and the binding of H2O2 to components of cell membrane, which subsequently causes its peroxidation. For the chl a fluorescence point of view, the photochemistry yield was higher in Crimson King, whereas non-photochemical quenching (NPQ) values were observed to be higher in the wild type compared to Crimson King.