Background: Agastache foeniculum is one of the perennial aromatic plants of the Lamiaceae family. The essential oil (EO) of this plant has potent antibacterial and antifungal properties, and hence it is extensively used in the pharmaceutical and food industries. Water-deficit stress occurring at any plant growth stage changes the morphological, phytochemical, and physiological processes. Melatonin is considered as a new kind of plant growth regulator which increases plant tolerance to environmental stresses. Hypotheses: The response to water deficit in A. foeniculum is Mel-dependent and affects their physiological and phytochemical characteristics Studied species: Agastache foeniculum Study site and dates: Tabriz, Iran, 2019. Methods: For this purpose, plants were subjected to water-deficit stress, which was followed by foliar application of melatonin as biostimulants on dry matter and secondary metabolites of A. foeniculum. Results: Results indicated that water-deficit stress (at 60 % of Field capacity) increased shoot dry weight, total phenol, anthocyanins and flavonoids content as well as EO content. Application of various melatonin concentrations improved the content of the mentioned traits. According to GC-MS analysis, ten chemical compounds were recognized in the EO composition of A. foeniculum. The most important EO compound (%) in the studied treatments was methyl chavicol, under severe water-deficit stress conditions with Mel at concentration of 100 μM. Conclusions: The results indicated that the application of Mel (at 100 µM) as a bio-inducer can alleviate the detrimental effects of water-deficit stress (at 60 and 30 % field capacity) and improve the quality of EO
