The effect of various zinc concentrations in sewage sludge on alleviating oxidative damage induced by cadmium toxicity in lettuce

A major concern on the application of sewage sludge in agricultural lands is relatively high cadmium (Cd) concentrations in these wastes which might be toxic to the plant and causes oxidative damage. Much higher Zn concentration in sewage sludge in comparison with cadmium may affect oxidative damage induced by cadmium. The aim of the present study was to investigate the effects of various concentrations of zinc (30, 60, 90, 120 and 150 mg kg-1 soil) on oxidative damage caused by sewage sludge containing different levels of cadmium (0.75, 1.5 and 3 mg kg-1 of soil) on lettuce. The results showed that increasing the concentration of cadmium, in addition to increasing malondialdehyde concentration (MDA), generally increased the activity of superoxide dismutase (SOD), catalase (CAT), ascorbate peroxidase (APX) and peroxidase (POX). Declining malondialdehyde concentration by increasing zinc concentration in sewage sludge indicates the role of this element in reducing oxidative damage caused by cadmium toxicity in the plant. Generally, by increasing zinc concentration in the sewage, the activity of SOD, CAT, APX and POX decreased. According to the results, it seems that lower activity of antioxidant enzymes the plant treated with high Zn sewage sludge is at least in part due to reduction of oxidative damage caused by cadmium in the plant.