Melon (Cucumis melo L.) is one of the most important horticultural crops in Iran often cultivated in arid and semiarid regions of the country with salinity problems. The objective of this work was to better understand the mechanisms of physiological and biochemical responses to salinity stress of five Iranian melon landraces “Samsuri”, “Kashan”, “Khatouni”, “Suski-e-Sabz”, and “Ghobadlu” from different geographical origins, and “Galia” F1 cultivar. Plants were grown under greenhouse conditions and irrigated with half-strength Hoagland solution containing 0, 30, 60, or 90 mM NaCl for 60 days. Increase in the external salt concentration was accompanied by an obvious depression in leaf relative water content, membrane stability index, chlorophyll a and b and carotenoid contents, stomata and trichome density, leaf area, specific leaf area, biomass, leaf and stem Kþ concentrations as well as leaf and stem Kþ/Naþ ratios in all landraces studied. In contrast, hydrogen peroxide, lipid peroxidation, proline and soluble carbohydrate contents, activity of antioxidant enzymes as well as leaf and stem Naþ and Cl concentrations, all increased significantly with increasing stress over all plants. Moreover, carbon isotope discrimination (D13C), determined on leaf organic matter, was found to be associated with evaluated traits. For example, a highly positive correlation between D13C and both biomass production and salt tolerance index was notable when all saline treatments were averaged (r ¼ 0.998 and 0.998, respectively). Also, scatter plot and clustering analysis showed that “Suski-e-Sabz” and “Ghobadlu” were placed close to “Galia” F1, a salt tolerant cultivar, indicating that their similar behavior under salinity. Overall, the present results indicated a significant genetic variability for most of the traits studied, suggesting that “Suski-e-Sabz” and “Ghobadlu” could be introduced as the superior landraces and the most promising tolerant parents in the future melon breeding p
