Modulation of antioxidant responses of Medicago sativa under cadmium and copper stress

Alfalfa (Medicago sativa) plants exposed for 8 weeks in pots to increasing concentrations of cadmium (Cd; 0.35 and 7 mg·kg-1dry soil) and copper (Cu; 3.5 and 70 mg.kg-1dry soil) were tested for metal accumulation, growth inhibition and oxidative stress response in roots and leaves. The investigated parameters were biometric measurements (root and shoot length), malondialdehyde accumulation (MDA; index of lipid peroxidation), catalase (CAT), glutathione reductase (GR) and superoxide dismutase (SOD) activities, and reduced glutathione (GSH) content. Analysis of Cd uptake showed that roots accumulated almost 40 fold more Cd than above-ground parts of the plant. Cu was more mobile, as it exhibited only 2 to 6-fold higher concentration in roots than in leaves. Both metals rendered a pronounced MDA accumulation in roots, especially at the highest doses and in parallel a decrease in the activities of the antioxidant enzymes CAT, GR and SOD, as well as a reduction in GSH content. The different stress signature observed in Cd- and Cu-treated plants could help to understand the mechanisms of toxicity triggered by each metal.   Key words: Alfalfa, heavy metals, growth, oxidative stress.