Shortage of water resources imposes serious decreases in crop productivity annually in Iran . Therefore this research was aimed at studying the effect of irrigation regimes on antioxidant enzymes, Membrane Stability Index (MSI), Relative Water Content (RWC) in leaves, yield and yield components of safflower genotypes. A 3-replicate split plot RCBD field experiment was conducted at the Lavark Research Farm, College of Agriculture , Isfahan University of Technology, Isfahan-Iran, in 2009. Three levels of irrigation based on evaporation from a standard after 75 mm, I 2 as moderate drought=irrigation after 115 mm evaporation and I 3 as severe drought=irrigation after 155 mm evaporation served as main plots. Six safflower genotypes consisting Arak 2811, Kooseh, Nebraska-10, C 111 , S 149 and K 12 were considered as sub plots. Leaf Area Index (LAI), antioxidant enzymes, MSI and RWC at flowering stage and yield attributes at two weeks after physiological maturity were measured. Catalase (CAT) and Ascorbate Peroxidase (APX) activities increased under I 2 irrigation level by 19 and 8.5% and decreased under I 3 level by 33.8 and 45% compared to I 1 respectively. Severe drought led to significant decreases in LAI (64.9%), MSI (33.8%) and RWC (34.9%). While under I 3 level of irrigation decreases in head/plant (27%), seed/head (15.6%), grain yield (35.6%), biological yield (33.9%) and oil percntage (11.4%) were observed, an increase in protein percntage (17%) was detected compared to I 1 . While severe drought inhanced the level of antioxidant enzymes in more resistant genotypes, it decreased these enzymes in more sucesptible genotypes. The greatest decreases in MSI, LAI and RWC along with the decreases in CAT and APX activities under water deficit was observed with S 149 , Kooseh, Nebraska and Arak, leading to a lower resistance to water deficit stress with the latter genotypes. In K 12 and C 111 genotypes increases in CAT and APX along with maintenance of MSI and RWC led to more resistance to the drought stress. Therefore, it can be concluded that activation of antioxidant defence in safflower has a key role in resistance to the stress but, significant interaction between genotypes and irrigation levels for antioxidant enzymes, LAI, MSI and RWC indicates the quantity of these antioxidant enzymes is not a sole determinant of resistance to water deficit stress at least with genotypes used in our study. Instead plant capabilities in maintaining its RWC, MSI and LAI under water deficit
