Objective: Drought stress is one of the most serious consequences of climate change and represents a major limitation to plant growth and productivity, particularly in arid and semi-arid regions. The valuable medicinal plant Dragonhead is highly sensitive to severe drought stress, which leads to reductions in yield. The use of environmentally friendly approaches, such as biofertilizers and organic amendments, may contribute to improving photosynthesis, enhancing yield, and increasing drought tolerance. This study aimed to evaluate the effects of drought stress and the application of Pseudomonas fluorescens strain P-169, arbuscular mycorrhizal fungi, and biochar on morphological traits, chlorophyll content and biomass yield of Dragonhead.

Method: The experiment was conducted at a research farm in Shiraz using a split-plot arrangement in a randomized complete block design with three replications during the 2023 and 2024 growing season. Irrigation treatments were assigned to main plots at three levels: irrigation after 25% (control), 50%, and 75% soil moisture depletion (I1, I2, and I3, respectively). Fertilizer treatments were allocated to subplots at eight levels: F1 (control), F2 (mycorrhiza), F3 (Pseudomonas), F4 (biochar), F5 (mycorrhiza + Pseudomonas), F6 (mycorrhiza + biochar), F7 (Pseudomonas + biochar), and F8 (mycorrhiza + Pseudomonas + biochar). After collecting the data and ensuring its normality and the homogeneity of experimental years, a combined analysis of variance was performed using SAS software v.9.2, and the means were compared with the help of Duncan's test at a five percent significance level.

Results: The results showed that drought stress significantly affected all measured traits. The highest plant height, number of lateral branches, number of flowering branches, chlorophyll a and b contents were obtained under the I1 treatment, whereas drought stress caused a significant decline in these traits. Among fertilizer treatments, F8 produced the highest values for most characteristics. The I1F8 combination resulted in the maximum stem diameter (0.85 cm), total chlorophyll content (24.3 mg g⁻¹), total biomass yield (2348 kg ha⁻¹), and leaf and flower yield (1792 kg ha⁻¹), while the I3F1 treatment exhibited the lowest values, with reductions of 34%, 74%, 72.3%, and 66.7%, respectively.

Conclusion:

Overall, the results indicate that drought stress, by disrupting nutrient absorption and chlorophyll synthesis, leads to a reduction in the vegetative characteristics and yield of this medicinal plant and the combined application of Pseudomonas, mycorrhiza, and biochar can serve as an effective and sustainable strategy for mitigating drought stress by improving soil properties, strengthening the plant photosynthetic system, and alleviating the negative impacts of drought, ultimately leading to a significant increase in plant productivity.