To investigate the effect of salicylic acid on some morphological traits and yield of dragonhead under water deficit conditions, a field experiment was done at Department of Natural Resource and Watershed of Piranshahr, Iran, in 2012. The experiment was arranged as a split plot, based on randomized complete block design with three replications. The experimental treatments consisted of three levels of irrigation after 40, 80 and 120 mm evaporation from pan as the main plots and three levels of the plant foliar application with salicylic acid as the subplots (0, 0.5 and 1 mM). The results showed that the irrigation treatments had significant effects on the plant height, the main stem diameter, number of lateral branches, floral branches length, number of leaves and biological yield. The maximum and minimum value of the mentioned traits respectively was obtained in the treatments of 40 and 120 mm. Salicylic acid effect on the biological yield was significant, so that the maximum biological yield (638 g.m-2) belonged to the 1 mM salicylic acid application, while the control had 24.8 percent increase and had no significant difference with the 0.5 mM salicylic treatment. Interaction of the water deficit stress and salicylic acid on the number and length of floral branches in the plant was significant. The highest number of floral branches (mean 3.73 per plant) was observed in 40 mm evaporation from the pan with foliar application of 0, 0.5 and 1 mM salicylic acid and its lowest value was obtained with two levels, 80 and 120 mm, of evaporation from the pan without salicylic acid application.
