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We have examined the influence of sewage sludge application on the capacity for acclimation to drought of N-2-fixing alfalfa plants (Medicago sativa L. cv. Aragon). Plants were grown in pots with a mixture of perlite and vermiculite (2:1, v/v) and sludge, which was incorporated into substrate at rate of 10% (w/w). Pots without sludge but with inoculated plants were used as control treatment for comparison. Two levels of irrigation were imposed: (I) well-watered and (2) drought stress during three cycles of moisture stress and recovery. Sludge application improved net photosynthesis resulting in higher plant growth and photosynthetic products (soluble sugars) required for nodule metabolism. Drought did not alter nodule soluble proteins or leghemoglobin concentration but nodule dry mass was strongly decreased. Under drought, nodules of sludge-treated plants had higher N-compounds as proline, antioxidant metabolites as ascorbate and some antioxidant enzyme activities as catalase than untreated plants. All these changes might have contributed to protect nodules from oxidative stress as indicated by low products of lipid peroxidation. This study provides evidence for a beneficial effect of sewage sludge application for nodulated alfalfa plants subjected to cyclic drought. (C) 2009 Elsevier B.V. All rights reserved.