Potato is one of the most importantant crops in the world that ranks as the fourth major crop after wheat, rise and maize economically. Water deficit is the first limiting factor for crop productivity in the majority of the world agricultural areas. The production of resistant potato varieties to water deficit is considered one of the objectives of plant breeding in areas where exprience limiting resources of water. Therefore, due to the importance of potato cultivation in Iran and the limitation of water resources, this research was conducted to investigate the responses of ten cultivars of the most commonly cultivated potato cultivars in Iran to water stress in order to determine the relationship between the measured traits and the resistance of cultivars for better understanding of physiological and biochemical basis of tolerance to water deficit. This research was conducted in the form of a field experiment and two pot experiments. In the field experiment and the first pot experiment, ten potato cultivars (Agria, Arinda, Marfona, Banba, Born, Sante, Milva, Satina, Jelly and Spirit) were evaluated under two irrigation levels (30 and 60% depletion of soil water in the field experiment and 30 and 80% depletion of soil water in the pot experiment). The field experiment was conducted in a randomized complete block design with three replications and the first pot experiment was conducted in greenhouse as a factorial based on a randomized complete block design with four replications. Water deficit treatments were applied in the field experiment from 50% flowering satge untill tuber harvesting and in the first pot experiment during the vegetative growth stage. In these two experiments, the amount of photosynthesis, stomatal conductance, photosynthetic water-use efficiency, quantum efficiency of photosystem II, chlorophyll content, plant dry weight and tuber yield (measured only in the field experiment) were decreased but ion leakage, the activities of antioxidant enzymes and proline content were increased under water stress. Among ten tested cultivars, due to water deficit, the amount of reductions in tuber yield in the field experiment ranged from 27 to 81 percent and in shoot dry weight ranged from 30 to 86 percent in the first pot experiment. According to stress tolerance index (STI) calculated based on the reduction in plant dry weight due to water deficit applied at vegetative growth stage in the first pot experiment, Agria was the most tolerant cultivar. However, according to STI calculated based on the reduction in tuber yield due to water deficit induced at 50% flowering satge untill tuber harvesting in the field Arinda ranked first in terms of tolerance to water stress. Antioxidant enzymes of peroxidase and superoxide dismutase had important roles in induction of resistence to water stress under field condition. Also, ion leakage was highly correlated with the degree of water-stress tolerance of potato cultivars. The second pot experiment was conducted under greenhouse environment, as factorial in a completely randomized design with three replications, to investigate the effect of two irrigation levels (30 and 60% water depletion) and three concentrations of salicylic acid (0, 0.5 and 1 mM) and glycine betaine (0, 1 and 2 mM) on physiological characteristics, dry mater accumulation and tuber yields of four potato cultivars (two resistant and two susceptible cultivars to water deficit) selected based on the results of the field experiment. Water deficit treatments in this experiment were applied after plant establishment during vegetative and reproductive growth stages untill tuber harvesting. Foliar application of salicylic acid and glycine betaine positively effected all growth parameters of potato plants including increase in photosynthesis, stomatal conductance, photosynthetic efficiency of water use, chlorophyll content, plant dry weight and the reduction of ion leakage and also significantly affected the activities of antioxidant enzyme, proline content and tuber yield, depending on cultivar, irrigation level and the applied concentration of these growth regulators. The results of this study showed that there was significant variation among potato cultivars in terms of the response to water deficit and foliar application of salicylic acid and glycine betaine improved not only the growth characteristics of potato cultivars but also their ability to withstand water deficit. Key Words: Potato, water deficit, antioxidant enzymes, salicylic acid, glycine betaine