A greenhouse experiment was conducted to investigate the oxidative stress produced by sewage sludge addition on nodulated alfalfa (Medicago sativa L. cv. Aragón) plants. Two types of sludge were incorporated into substrate: anaerobic mesophilic digested (AM) and autothermal thermophilic aerobic digested (ATAD) sludge. Pots without sludge but with inoculated plants were used as control treatment for comparison. Results showed that sludge amended plants had increased tissue accumulation of heavy metals that induced oxidative stress. This is characterized by induction of the antioxidant enzymatic activities and alterations in the redox state of ascorbate. ATAD sludge application produced a reduction in nodulation, increased nodule antioxidant enzyme activities and decreased ascorbate/dehydroascorbate ratio. As a consequence, nodules of ATAD treatment suffered from oxidative damages as evidenced by high malondialdehyde levels. By contrast, AM application enhanced plant growth and no deleterious effects on nodulation were found. Nodules developed in AM sludge had increased antioxidant enzyme activities, ascorbate/dehydroascorbate ratio and improved capacity for thiol synthesis. Results clearly showed that nodulated alfalfa performed better in AM than in ATAD sludge and suggest that differential response appears to be mediated by plant ability to thiol synthesis and to maintenance of a more equilibrated antioxidant status.
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