A greenhouse experiment was performed in order to investigate the effects of different levels of water stress on leaf water potential(Ψ_w),stomatal resistance(r_s),protein content and chlorophyll(ChI) content of tomato plants(Lycopersicon esculentum Mill.cv.Nikita).Water stress was induced by adding polyethylene glycol(PEG 6 000) to the nutrient solution to reduce the osmotic potential(Ψ_s).We investigated the behavior of anti-oxidant enzymes,such as catalase(CAT) and superoxide dismutase(SOD),during the development of water stress.Moderate and severe water stress(i.e.Ψ_s= -0.51 and -1.22 MPa,respectively) caused a decrease inΨ_w for all treated(water-stressed) plants compared with control plants,with the reduction being more pronounced for severely stressed plants.In addition,r_s was significantly affected by the induced water stress and a decrease in leaf soluble proteins and ChI content was observed.Whereas CAT activity remained constant,SOD activity was increased in water-stressed plants compared with unstressed plants.These results indicate the possible role of SOD as an anti-oxidant protector system for plants under water stress conditions.Moreover,it suggests the possibility of using this enzyme as an additional screening criterion for detecting water stress in plants. A greenhouse experiment was performed in order to investigate the effects of different levels of water stress on leaf water potential (Ψ_w), stomatal resistance (r_s), protein content and chlorophyll (ChI) content of tomato plants (Lycopersicon esculentum Mill.cv.Nikita ) .Water stress was induced by adding polyethylene glycol (PEG 6 000) to the nutrient solution to reduce the osmotic potential (Ψ_s) .We investigated the behavior of anti-oxidant enzymes, such as catalase (CAT) and superoxide dismutase (SOD) , during the development of water stress. Moderate and severe water stress (ieΨ_s = -0.51 and -1.22 MPa, respectively) caused a decrease inΨ_w for all treated (water-stressed) plants compared with control plants, with the reduction being more pronounced for severely stressed plants. In addition, r_s was significantly affected by the induced water stress and a decrease in leaf soluble proteins and ChI content was observed. Thereas CAT activity was constant, SOD activity was increased in water-stres sed plants compared with unstressed plants. these results indicating the possible role of SOD as an anti-oxidant protector system for plants under water stress conditions. Moreover, it suggests the possibility of using this enzyme as an additional screening criterion for detecting water stress in plants .