The process of stress adaption was studied in Groenlandia densa (opposite-leaved pondweed) grown in the presence of different Ni concentrations (0-20 mg Ni L(-1)). The results showed that Ni concentrations in plants increased with the increasing Ni supply levels and reached a maximum of 47.57mg kg(-1) DW at 5mg L(-1) Ni treatments. The level of photosynthetic pigments (Chl a, Chl b and Chl total) decreased only upon exposure to high Ni concentrations. However, total soluble proteins increased with increasing nickel supply levels. At the same time, the level of malondialdehyde (MDA) increased with increasing Ni concentration. Significant increases in antioxidant activities of studied enzymes such as superoxide dismutase (SOD), catalase (CAT), glutathione reductase (GR), glutathione S-transferase (GST) and ascorbate peroxidase (APX) were recorded in this plant subjected to increasing Ni levels. Cellular antioxidant levels showed a decline suggesting a defensive mechanism to protect against oxidative stress caused by nickel. In addition, the proline content in G. densa increased with increasing nickel levels. These findings suggest that G. densa is equipped with efficient antioxidant mechanism against Ni-induced oxidative stress which protects photosynthetic machinery from damage.