Drought stress negatively affects plants’ growth and productivity. Some plant growth regulators like brassinosteroids may alleviate water scarcity effects. Therefore, a two-year field experiment (2017 and 2018) was conducted to determine the effects of three different irrigation regimes and four concentrations of 24-epibrassinolide (a brassinosteroid) on the physiological and biochemical properties and yield of Lallemantia iberica as a medicinal plant. The experiment was conducted in three replications as a split-plot based on a completely randomized block design. Irrigation regimes including full watering, watering until flowering, and watering until branching were considered as the main plots, and 0, 0.5, 1, and 1.5 μM concentrations of 24-epibrassinolide were applied as subplots. The results showed that water scarcity decreased relative water content (RWC), soluble proteins, photosynthetic pigment content, photosynthesis-related traits, grain yield, and grain protein. Supplementary irrigation had positive effects on the mentioned traits. Applying the concentration to 1 μM 24-epibrassinolide reduced the drought stress adverse effects on total chlorophyll, soluble sugars, protein content, photosynthesis rate, transpiration rate, stomatal conductance, intercellular CO2 concentration, intrinsic water use efficiency, and grain protein percentage. The highest PhR (13.12 μMol CO2 m−2 s−1) and TR (0.419 mMol H2O m−2 s−1) were observed during the second year in plants under non-stress conditions, which were sprayed with a solution containing 1.0 μM 24-epibrassinolide. Our results indicate that suitable concentrations of 24-epibrassinolide can be used as a strategy to reduce the harmful effects of drought stress on the growth and yield of plants by improving the biochemical and photosynthesis traits.
