For the purpose of studying the effects of water deficit stress on yield and water use efficiency of three wheat cultivars under field condition, an experiment was initiated in Research Farm of Islamic Azad University located in Kermanshah/Iran during 2010-2011. The experimental treatments were arranged as split plots based on a randomized complete block design with three replications. The main plots were allocated to four different irrigation regimes : I1- full irrigation (irrigation during growth period after 40% depletion of soil moisture) I2 - drought stress at the start of anthesis stage ( 56 Zadoks) through the grain filling stage(70 Zadoks) with irrigation after 80% depletion of soil moisture; I3 - drought stress at the start of anthesis stage(56 Zadoks) through the ripening stage with irrigation after 80% depletion of soil moisture; and I4 - drought stress at the start of grain filling stage ( 70 Zadoks ) through the ripening with irrigation after 80% depletion of soil moistur. The subplots were allocated to three Cultivars treatments of C1 (Roshan- Back Cross), C2 (Karaj 1) and C3 (Marvdasht). The results showed that with increasing drought stress grain yield, biological yield, harvest index and water use efficiency decreased but evapotranspiration efficiency (ETE) was increased. Under control (I1) conditions, WUE mean was 1.27 kg.m-3 for all cultivars, but equal to 1.11, 0.91 and 0.73 kg.m-3 for stress treatments I2, I3, and I4, respectively. The highest and lowest harvest index and water use efficiency were observed for Roshan-Back Cross cultivar and Mrvdasht cultivar under well- watering, respectively. The Roshan-Back Cross cultivar had higher yield stability than the others and its yield reduction under stress conditions was lower than others.
