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IntroductionE. purpurea is a plant that blooms in North America during the summer. It is essential for the pharmaceutical industry because it boosts the immune system and can be used for various external and internal ailments. Abiotic stresses like drought are significant problems for plant productivity. Many studies have been done on how these stresses affect agricultural plants because they cause economic losses. Drought affects plant growth and development by reducing crop growth rate, biomass accumulation, cell division and expansion, leaf size, stem elongation, root proliferation, and stomata oscillations. Although the effect of drought stress on various characteristics of coneflower has been studied in many research, few researchers have investigated the changes in the essential oil content of this important medicinal plant under drought stress. Due to a water shortage in Iran and the growing use of herbal medicines, studying how drought stress affects the composition of coneflower essential oil is crucial.Materials and methodsThis research was carried out in 2021 in the greenhouse of the Department of Horticultural Sciences, Faculty of Agriculture, Bu-Ali Sina University, Hamedan, Iran. The study was done as a factorial based on a completely random design with two factors of drought stress (80, 60, and 40% FC) and growth stage (10 leaves, pre-flowering, and flowering stage) in three replications. Took coneflower seeds were planted in cultivation trays. After 60 days, the seeds germinated, and the seedlings were prepared and transferred to pots. The weight method was used to determine soil moisture content. The pots were weighed daily and watered to maintain the intended level of irrigation. After the drought stress period ended, the plants with their roots were transferred to the lab, and some growth, biochemical, and phytochemical characteristics were measured.Results and discussionThe results of the analysis of variance showed that the interaction effect of irrigation levels and phenological stages was significant for the characteristics of leaf area, stem length, flower diameter, flower dry weight, chlorophyll a, chlorophyll b, carotenoid and total phenol and flavonoid of the flower and root. The results showed that drought stress caused a significant decrease in the growth characteristics, chlorophyll a, carotenoid, and increased chlorophyll b content of coneflower in different stages of growth. The concentration of coneflower's total phenol and flavonoid under severe drought stress increased by 37.58 and 25.67, respectively. Also, the antioxidant capacity of the coneflower flower increased under severe drought stress, but drought stress did not affect the antioxidant capacity of the roots. In addition, the results showed that more than 70% of the essential oil components at the time of applying the control in the 10-leaf and pre-flowering stage, and severe stress in all three stages of growth consisted of four compounds: Germacrene D, n-Dodecane, n-Tridecane, and n-Undecane. Compared to the control, most of the essential compounds of coneflower under severe drought stress decreased in the 10-leaf growth stages and pre-flowering but increased in the flowering stage. Severe drought stress caused a decrease in Germacrene D in the 10-leaf and flowering stages (41.6 and 41.3%, respectively) and increased it in the pre-flowering stage (77.2%). During drought conditions, plants reduce the number and area of their leaves as an adaptation strategy and first defense mechanism. Plants exposed to environmental stress can increase their stress tolerance by producing more phenol, flavonoid, and antioxidants. These non-enzymatic antioxidants help reduce the adverse effects of stress on plants. Based on the findings of various research, it can be said that Germacrene D, Spathulenol, β-Caryophyllene, and α-Humulene are the main components of the essential oil of coneflower. In agreement with our results, the research findings showed that severe drought stress (40% FC) caused a significant decrease in the phytochemical compounds of coneflower. Secondary metabolites are influenced not only by genetics but also by changing environmental patterns.ConclusionWith increasing drought stress levels, growth characteristics, chlorophyll a, carotenoid, and root total phenolic content of coneflower decreased significantly. Also, drought stress increased stem diameter, chlorophyll b, total phenol content, total flavonoid, and antioxidant capacity of flowers. Under severe drought stress, four compounds of Germacern D, N-dodecane, N-tridecane, and N-one-decane, which are the dominant components of the root essential oil of this plant in the present study, decreased in the 10-leaf and flowering stages but increased in the pre-flowering stage. In general, it can be concluded from the current research results that applying drought stress can help improve coneflower root essential oil composition.