The perennial Medicago sativa cv. Gabs is widely grown on saline soils in Tunisian oases. The mechanisms by which this NaCl-tolerant cultivar maintains a positive growth balance were analyzed. In this plant of considerable agronomic interest, biochemical analyses were conducted in order to study the effects of salinity on mature leaves. Free-radical detoxification mechanisms and changes induced by the accumulation of reactive oxygen species (ROS) in response to the NaCl stress were compared between the upper (young) and lower (old) carbohydrate source leaves. Long-term NaCl (150 mM) treatment significantly reduced the size of source leaves supporting growth. Salinity damage was greater in the lower than in the upper leaves. This damage was associated with a high Na + : K + ratio and a decrease in the activity of H 2 O 2 -scavenging enzymes, leading to lipid peroxidation. In lower source leaves that were mainly affected by ionic stress, superoxide dismutase (SOD) was overexpressed and guaiacol peroxidase (GPX) activity increased. In contrast, in upper source leaves that were mainly exposed to water deficit, catalase and ascorbate peroxidase (APX) activities increased whereas GPX activity was unchanged. The upper source leaves maintained adequate ionic and water status and an efficient ROS detoxification, allowing sinks to be supplied with photoassimilates and maintaining a positive growth balance in this cultivar of alfalfa.