Drought stress is one of the major abiotic stresses, which adversely affects crop growth and yield. drought is the main environment factor limiting wheat productivity Iran. An experiments was carried out in 2007-2008 on Mahidasht Agricultural Research Center in order to study the effects of drought stress on yield and some morphological traits of wheat cultivars under drought stress conditions. The experiments conducted to base on split plot in a randomized complete block design with three replications. Main plots included drought stress treatments at 4 levels: I1- drought stress at the start of stem elongation stage trough the ripening stage with irrigating after 80% depletion of soil moisture ;I2 -drought stress at the start of boot stage through the repining stage with irrigating after 80% depletion of soil misture;I3- drought stress at the start of grain – filling stage trough the ripening with irrigating after 80%depletion of soil moisture; and I4- full irrigation (irrigating during growth period after 40% depletion of soil moisture). And subplots included cultivars treatments at 3 levels: Chamran (C1), Marvdasht(C2), and Shahriar (C3). A significant difference was observed between drought stress treatments so that the highest grain yield was obtained for control treatment (I4)and the lowest for drought stress treatment (I1).Compared to control treatment(I4), treatments (I1), (I2), and (I3) exhibited 81% , 53% , and 40% yield decreases ,respectively. Results showed that in comparison with other drought stress treatments, imposing drought stress at the start of stem elongation stage through the ripening stage (I1) had the most impact on reducing the yield of wheat cultivars. In different growth stages, different cultivars response to moisture stress and irrigation discontinuance differently. In response to moisture stress during different growth stages, Shahriar cv (C3) was damaged more severely than Chamran cv (C1) was, the latter enjoyed more yield stability under such conditions.
