In this study, an experiment was carried out to study the process of stress adaptation in Lemna gibba L. grown under cadmium stress (0-20 mg Cd L-1). The level of photosynthetic pigments and soluble proteins decreased only upon exposure to high Cd concentrations (for pigments 5 mg Cd L-1; for soluble proteins 10 mg Cd L-1). At the same time, the level of malondialdehyde (MDA) increased with increasing Cd concentration. These results suggested an alleviation of stress that was presumably the result of antioxidants such as superoxide dismutase (SOD), catalase (CAT), glutathione reductase (GR) and glutathione S-transferase (GST) as well as ascorbate peroxidase (APX), which increased linearly with increasing Cd levels. In addition, the praline content in L. gibba increased with increasing cadmium levels. These findings suggest that Lemna is equipped with an efficient antioxidant mechanism against Cd induced oxidative stress which protects the plant's photosynthetic machinery from damage. We also found that moderate Cd treatment (0.05-5 mg L-1 Cd) alleviated oxidative stress in plants, while the addition of higher amounts of Cd (10-20 mg L-1) could cause an increasing generation of ROS, which was effectively scavenged by the antioxidative system. (c) 2013 Elsevier Inc. All rights reserved.