Mentha pulegium L. is a medicinal and aromatic plant belonging to the Labiatae family present in the humid to the arid bioclimatic regions of Tunisia. We studied the effect of different salt concentrations on plant growth, mineral composition and antioxidant responses. Physiological and biochemical parameters were assessed in the plant organs after 2 weeks of salt treatment with 25, 50, 75 and 100 mM NaCl. Results showed that, growth was reduced even by 25 mM, and salt effect was more pronounced in shoots (leaves and stems) than in roots. This growth decrease was accompanied by a restriction in tissue hydration and K ? uptake, as well as an increase in Na ? levels in all organs. Considering the response of antioxidant enzymes to salt, leaves and roots reacted differently to saline conditions. Leaf and root guaiacol peroxidase activity showed an increase by different concentration of NaCl, but superoxide dismutase activity in the same organs showed a slight modification in NaCl-treated leaves and roots. Moreover, polyphenol contents and antioxidant activity were analysed in M. pulegium leaves and roots under salt constraint. The analysis showed an increase of total polyphenol content (2.41-8.17 mg gallic acid equivalent g -1 dry weight) in leaves. However, methanol extract of leaves at 100 mM NaCl displayed the highest DPPHÁ scavenging ability with the lowest IC 50 value (0.27 lg ml -1 ) in comparison with control which exhibited IC 50 equal to 0.79 lg ml -1 .

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Arabidopsis thaliana plants (wild-type accessions Col and N1438) were submitted to a prolonged, mild salt stress using two types of protocols. These protocols allowed salt-treated plants to absorb nutrients either through a part of their root system maintained in control medium (split-rooted plants) or during episodes on control medium alternating with salt application (salt alternation experiment). Full-salt treatments (salt applied continuously to whole root system) resulted in severe (but non-lethal) growth inhibition. This effect was partly alleviated in split-rooted plants on mixed salt-control medium and in plants submitted to salt-control medium alternation. The activity of the various isoforms of superoxide dismutases (SODs) did not appreciably change with the treatments. The abundance of the mRNAs of the seven SOD genes present in Arabidopsis genome was determined using real-time polymerase chain reaction. The two protocols gave qualitatively identical results. The expression level was increased by full-salt treatments for some genes and diminished for other genes. However, the nature of these genes differed according to the accessions: the responses to salt of FSD1 and MSD were opposite in Col and N1438. In Col, salt treatments inhibited the expression of FSD1 and strongly stimulated that of CSD1 and MSD. In N1438, the stimulation by salt concerned FSD1 and CSD1 and MSD expression being inhibited. In both accessions, the expression of CSD2 and CSD3 was lowered by salt. For all genes, the treatments that mitigated stress partially restored SOD expression to control level. Thus, the changes in SOD transcript abundance accurately reflected the severity of the salt stress.