Drought stress is one of the most significant abiotic stresses, adversely affecting plant growth and yield. The application of growth regulators such as salicylic acid (SA) can effectively enhance plant resistance to both biotic and abiotic stresses. This study aimed to investigate the effect of salicylic acid on the physiological responses and grain yield of Dragon’s head (Lallemantia iberica) under water deficit conditions. The experiment was conducted using a split-plot design based on a completely randomized block with three replicates in 2017 and 2018 at the Agricultural and Natural Resources Research Center of Zanjan Province. Water deficit was the main factor at three levels: full irrigation (control), irrigation cut-off from the beginning of flowering to the end of grain filling, and irrigation cut-off from the beginning of branching (appearance of the sixth leaf pair on the main stem) to the end of grain filling. The sub-factor was salicylic acid foliar application at concentrations of 1, 2, and 3 mM, with a control (foliar spray with distilled water). Drought stress, particularly when irrigation ceased at the branching stage, reduced leaf relative water content (RWC) (by up to 15% compared to full irrigation), soluble protein concentration, and grain yield (by up to 74%). It also increased hydrogen peroxide (H₂O₂), malondialdehyde (MDA), proline, and soluble carbohydrate contents, along with antioxidant enzyme activities. SA foliar spray improved these traits, with the 2 mM concentration being most effective, increasing grain yield by 24.3% and mitigating oxidative damage. Among the investigated traits, the 2 mM concentration of salicylic acid had the most favorable effect in mitigating the negative impacts of irrigation cut-of and enhancing grain yield compared to the 1 and 3 mM concentrations.
