Soil salinity is one of the major environmental constraint for agricultural crops in 20% of total arable land and nearly 50% of irrigated lands worldwide. Salinity may affect plant growth, development and many physiological functions due to both harmful ion effects and water deficit stress caused by a reduced osmotic potential in the soil solution. Saline and sodic soils are expanding especially in arid and semiarid regions of the world, saline and alkaline soils account for 12.5% of agricultural lands in Iran. In order to evaluate the effect of salt stress on net photosynthetic rate (A), stomatal conductance to the CO 2 (gs), substomatal CO2 concentration (Ci), leaves chlorophyll content, quantum efficiency of PS II (Fv/Fm), antioxidant enzymes activity, lipid peroxidation (LP), leaves proline content and cation concentration in shoots of sesame genotypes, an experiment was conducted under greenhouse conditions using a hydroponic nutrient solution at the College of Agriculture, Isfahan University of Technology, Isfahan-Iran, during April-Aguest, 2010. The experiment was arranged as a factorial based on a randomized complete block design with three replications. Seven sesame ( Sesamum indicum L.) genotypes (Naz Takshakhe, Naz Chandshakhe, Ardestan, Varamin, Yekta, Darab and Oltan) were treated by three levels of salinity including 0 (control), 30 and 60 mM NaCl during the vegetative stage. All measurements were made at week 5 after application of NaCl to the hydroponic medium. Catalase (CAT), Ascorbate Peroxidase (APX), Glutathione Reductase (GR) and Superoxide Dismutase (SOD) activities increased under 60 mM level of salinity by 31.5, 56, 75.5 and 37.8% , respectively, compared to the control treatment. Leaf proline content of sesame genotypes increased with salinity, the highest increase (95.5%) was observed in 60 mM level of the salinity. Severe salinity led to significant decreases in A (64.24%), gs (63.2%), total chlorophyll content (30.6%) and Fv/Fm (11.8%). While under severe salinity significant decreases in potassium (47.8%), calcium (44.2%) and magnesium (42.06%) contents were observed, a 6.5 fold increase in sodium content in shoots of sesame genotypes was detected compared to the control . Furthermore a 15.3 fold increase in
