Introduction
Humic acid (HA) is a principal component of humic substances known for enhancing cell membrane permeability, increasing photosynthesis and respiration rates, improving mineral uptake, and promoting protein synthesis. Its effects on plants under drought stress have been demonstrated in several studies, showing positive impacts on growth parameters, osmotic regulation, antioxidant activities (e.g., superoxide dismutase, peroxidase, and catalase) and photosynthetic efficiency. Drought stress, as a major abiotic stress, negatively affects crop growth and productivity by disrupting water relations, reducing leaf size, and limiting root development. Plants respond to drought stress through various physiological and biochemical mechanisms. Research suggests that optimal concentrations of humic acid can effectively alleviate drought-included damage.
 Materials and Methods
This study investigated the effects of drought stress and humic acid on Dragon’s head plant. Two experiments were conducted: In the germination experiment, germination characteristics were evaluated under five drought stress levels (0, -0.4, -0.8, -1.2, and -1.6 MPa) in a completely randomized design with four replications at the Faculty of Agriculture, Shahrekord University in 2017. In the field experiment, a split-plot design with randomized complete blocks and three replications was used to assess humic acid and water deficit effects. Main treatments included four irrigation levels (50, 70, 90 and 110 mm evaporation from the pan surface), while sub-treatments comprised four humic acid concentrations (0, 5, 10, and 15 l ha^-1). And various indicators were measured at the end of the experiment.
  Results and Discussion
Drought stress reduced germination percentage (16%), germination speed index (42%), and seedling fresh weight, but increased root length. Also, osmotic stress caused by PEG, increased root length (40%) and decreased the shoot length (38%). Field results showed significant effects of both drought stress and humic acid on flower count, inflorescences, and seeds per plant, seed yield, and biological yield. Drought stress reduced these parameters, while humic acid application enhanced them. Drought stress (110 mm Evaporation) increased mucilage percentage (13%) but decreased essential oil yield, whereas humic acid treatments proved essential oil yield. HA treatment (15 l ha^-1) increased biomass (36%), dry weight of plant (32%), number of seeds and number of flowers per plant (17%) compared to the control. HA treatment (15 l ha^-1) was particularly effective in enhancing seed yield and medicinal properties. Drought stress is one of the most important abiotic stresses that adversely influences the growth and production of crops citation. As a consequence of drought stress, both stomatal and non-stomatal limitations lead to a decline in photosynthesis, growth and yield. Furthermore, showed that the use of HA improves the photosynthetic rate of plants under drought stress via increasing the gas exchange rate and electron transport flux. HA increases the chlorophyll content and photosynthetic rate, and decreases the transpiration rate of plants under drought stress.
Conclusion
Drought stress, decreased seed germination indicators such as percentage and germination rate of plantlet. Also, decreased the biological yield, dry weight, seed yield, harvest index and essential oil yield and increased in the mucilage percentage. HA treatment (15 l ha-1) improved many indicators including biomass, dry weight of plant, number of seeds and number of flowers per plant and essential oil yield. Therefore, HA was determined in this experiment to be an auxiliary factor in improving drought stress tolerance. Humic acids are organic molecules that play essential roles in improving soil properties, plant growth, and agronomic parameters. HA significantly promoted the growth of Dragon’s head seedlings under drought stress. HA is a principal component of humic substances, it can have various biochemical effects on plants, such as increasing cell membrane permeability, increasing photosynthesis and respiration rates, enhancing mineral uptake and enhancing protein synthesis and hormone-like activity.