Dissolved organic carbon (DOC) levels in aquatic systems are influenced by various factors ranging from anthropogenic perturbations (e.g. land-use systems) to natural processes related to weather phenomena. It is estimated that up to 30 % of the total DOC in streams is contributed by terrestrial leaf litter alone. In the present study, leaf litter-derived DOC from terrestrial (Quercus robur) and aquatic (Phragmites australis) plants was used to study the impact of leaf-litter decomposition products on the aquatic macrophytes Ceratophyllum demersum and Lemna minor. Photosynthetic oxygen release, marker enzymes of the antioxidative system, glutathione redox dynamics and growth rate were used as testing parameters. The results indicate that leaf-litter DOC has the potential to impact the tested aquatic macrophytes by imposing oxidative stress, slowing down photosynthesis and reducing growth rate. However, elevated enzyme activities and overexpressed glutathione reductase gene provided protection against lipid peroxidation. Species-specific phenotypic plasiticity was manisfested by both macrophytes, with a potential by C. demersum to acclimate, depending on the source of DOC. Given the magnitude of the effects observed on the macrophytes at environmentally realistic DOC concentrations, especially on growth rate reduction, it is suggested that both allochthonous and autochthonous leaf litter has the potential to structure aquatic ecosystems.