Main Conclusion: The drought conditions and the application of ABA reduce the photosynthetic activity, and the processes related to the transpiration of Dracocephalum moldavica L. At the same time, the plant increases the production of phenolic compounds and essential oil as a response to stress conditions. In the semi-arid regions, drought stress is the most important environmental limitations for crop production. Abscisic acid (ABA) plays a crucial role in the reactions of plants towards environmental stress such as drought. Field experiments for two consecutive years in 2016 and 2017 were conducted to evaluate the effect of three watering regimes (well-watered, moderate and severe drought) and five exogenous ABA concentrations (0, 5, 10, 20 and 40 μM) on growth, photosynthesis, total phenolic and essential oil content of Dracocephalum moldavica L. Without ABA application, the highest photosynthetic rate (6.1 μmol CO₂ m⁻² s⁻¹) was obtained under well-watered condition and, moderate and severe drought stress decreased photosynthesis rate by 26.39% and 34.43%, respectively. Some growth parameters such as stem height, leaf area, leaf dry weight and biological yield were also reduced by drought stress. ABA application showed a decreasing trend in photosynthesis rate and mentioned plant growth parameters under all moisture regimes. The highest seed yield (1243.56 kg ha⁻¹) was obtained under well-watered condition without ABA application. Increasing ABA concentration decreased seed yield in all moisture regimes. The highest total phenolic content (8.9 mg g⁻¹ FW) and essential oil yield (20.58 kg ha⁻¹) were obtained from 20 and 5 μM ABA concentration, respectively, under moderate drought stress.

To investigate how and in what amounts biochar and methyl jasmonate can improve drought tolerance of barley. A two-year experimental study was conducted in a factorial randomized complete block design (n = 5) in the research greenhouse of Zanjan University, Iran, to investigate the possible effects of biochar and methyl jasmonate on some traits of winter barley under drought conditions. Two irrigation regimes, D₀ (full irrigation in soil field capacity as control) and D₁ (withholding irrigation immediately after flowering stage), three methyl jasmonate spray densities [0 (M₀), 50 (M₅₀), and 100 (M₁₀₀) μM] and four levels of biochar in soil [0% (B₀), 0.25% (B_0.25), 0.5% (B_0.5), 1% (B₁) per soil weight] were used in this experiment. In this study, drought reduced two-year average leaf area (LA) by 96%, stomatal conductance (gs) by 84%, and photosynthetic water use efficiency (PWUE) by 64%. In addition, drought reduced chlorophyll-b by 1.5% and 81% and transpiration rate (Tr) by 2.5% and 78% in the first and second years, respectively. However, the application of biochar and methyl jasmonate improved all the traits studied in both D0 and D1 drought-treated plants. For most of the parameters studied, the optimal combination of biochar and methyl jasmonate that optimized water use efficiency and alleviated drought was 0.25% and 50 μM, respectively. The synergistic action of biochar and jasmonates improved the tolerance of barley to water stress.