Asian Journal of Research in Biology

An increase in the concentration of contaminants such as heavy metals and herbicides in aquatic systems through runoff disturbs aquatic ecosystems. The present in vitro study investigates oxidative stress and antioxidant responses in Lemna minor after 96 h of exposure to graded concentrations of cadmium (Cd) (0, 0.1, 0.5, 1, 1.5 and 2 mg/L) and paraquat dichloride (0, 0.5, 1, 1.5, 2 and 2.5 mg/L). Lipid peroxidation, expressed as malondialdehyde (MDA) equivalents, increased under both stressors in a dose-dependent manner (cadmium: +42% to +121%; paraquat: +28% to +200%), indicating increased production of reactive oxygen species (ROS). Activities of major antioxidant enzymes, including superoxide dismutase (SOD), peroxidase (POD), ascorbate peroxidase (APX) and glutathione reductase (GR), generally increased with pollutant concentration, indicating an induced enzymatic defense response. SOD activity was elevated under both cadmium and paraquat exposure, while APX and GR activities gradually increased during treatment. CAT activity increased at low to moderate cadmium concentrations and decreased at the highest cadmium concentration (2 mg/L), which may indicate enzyme inhibition or an overloaded scavenging capacity under severe stress. Paraquat caused a consistent increase in CAT activity. Overall, the results show that L. minor initiates a coordinated antioxidant response to diminish ROS generated by cadmium and paraquat exposure, but very high metal stress may impair specific defenses. These results suggest the potential of L. minor as a bioindicator of pollutant-induced oxidative stress. Furthermore, the antioxidant responses to Cd and paraquat dichloride may inform its use in phytoremediation studies.