The experimental design was a split

The experimental design was a split plot in a randomized complete block design with four replications. Each replication was divided into three main plots, which differed in severity of imposed water shortage. The four sunflower hybrids (Azargol, Euroflor, Allestar and Karaj) were randomly distributed within the sub-plots in each of the drought stress treatments (main plots). The water deficit treatments were applied by changing in irrigation intervals. Irrigations were carried out when an amount of evaporated water (from Class ‘‘A pan’’ evaporation) reached to 60 (I ;well-irrigated), 120 (I2; moderate water deficit) and 180 (I;extreme water deficit) mm, respectively. Amount of irrigation applied identical for all WD treatments from the beginning of planting time until the complete establishment of sunflower plants (eight-leaf (V) stage). After this stage, the plots were irrigated according to their prescribed treatment. The Azargol, Euroflor, Allestar and Karaj hybrids are the current sunflower hybrids cultivated in many arid and semi-arid regions of Iran. The sunflowers seeds were hand-sown on 7 m × 3.6 m plots (6 rows) on 22 May 2008 and 23 May 2009, respectively. In order to prevent the lateral spread of water, plots were surrounded with dykes, and a 3-m wide strip was left bare between main plots. Seeds were sown at 3 cm depth in the middle of rows, with 0.6 m between rows and 0.20 m between seed groups. This yielded a population density of about 8.3 plants m -2 8. Before planting, the soil surface of the cultivated area was thoroughly irrigated using a solid-set movable sprinkler system. Triple super phosphate fertilizer was applied broadcast before sowing at a rate of 100 kg ha. The N fertilizer (150 kg ha–1), in the form of urea, was applied at planting (one-third of the application) and sidedressed at the R growth stage (two-thirds of the application). At the 2-3 leaf stage, plants were selected for uniformity and thinned out to the recommended plant density. Weed control was performed manually without any chemical additive. 4 1 3 –1