European Journal of Soil Biology 100 (2020) 103217 Available online 31 August 2020 1164-5563/© 2020 Elsevier Masson SAS. All rights reserved. Arbuscular mycorrhizal fungi inoculation improve essential oil and physiological parameters and nutritional values of Thymus daenensis Celak and Thymus vulgaris L. under normal and drought stress conditions Ali Abdollahi Arpanahi a, * , Mohammad Feizian a , Ghazaleh Mehdipourian b , Davood Namdar Khojasteh c a Soil Sciences Department, Agriculture Faculty, Lorestan University, Khurramabad, Iran b Soil Sciences Department, Agriculture Faculty, Isfahan University of Technology, Isfahan, Iran c Soil Sciences Department, Agriculture Faculty, Shahed University, P.O. Box 18151-159, Tehran, Iran A R T I C L E INFO Handling editor: Christoph Tebbe Keywords: Arbuscular mycorrhizal fungi Drought stress Essential oil Thyme ABSTRACT Purpose: The objective of this study was (a) to investigate the effects of arbuscular mycorrhizal fungi (AMF) and drought stress on Thymus daenensis Celak and Thymus vulgaris L., and (b) to examine the ability of AMF to improve drought stress tolerance in Thymus species. Methods: The experiment was conducted as a factorial pattern in a randomized complete design with three replications in 2017. Factors are four irrigation regimes, consisting of well-watered irrigation after depletion of 20–25% of feld capacity (FC), irrigation after depletion of the 35–40% of FC, irrigation after depletion of the 55–60% of FC and two mycorrhizal fungus treatments: non-inoculation with AMF and inoculation with my- corrhiza (F. mosseae, R. intraradices). Results: The results show that drought stress reduces root and shoot dry weight, relative water content, Photo- synthetic pigments, gas change parameters, and nutritional values of two Thymus species, but AMF inoculation increases them. Proline, malondialdehyde, electrolyte leakage, and stomatal resistance increases with increasing water stress, but AMF inoculation ameliorates these increases in corresponding treatments. AMF inoculation increases essential oil production, but water stress in Thymus daenensis decreases it, and in Thymus vulgaris in- crease, it then decreases it in severe water stress. The results show that root colonization and soil spore density in AMF inoculated treatments increases in low drought stress but decreases with increasing water stress severity. Conclusions: The results suggest that AMF inoculation could be an excellent strategy to alleviate the adverse effects of water stress in two Thymus species cultivation in drought stress conditions. 1. Introduction One of the key predictions of climate change is that, in many eco- systems, environmental stresses such as drought and extreme tempera- ture are likely to increase in frequency and severity [1,2]. Drought stress is one of the most common adverse factors that limit crop production in arid and semiarid regions of the world [3]. Drought stress affects morphological, physiological, and biochemical characteristics of plants [4]. Drought stress induces various physiological and metabolic re- sponses such as decreases in plant height, total fresh and dry weight, stomatal closure, a decline in growth rate, and photosynthesis [5]. One of the main practices in sustainable agriculture is the application of biologic and organic fertilizers. An alternative strategy to enhance plant drought tolerance and reduce the need for chemical fertilizers is arbuscular mycorrhizal fungi (AMF) symbiosis. Improving plant growth by AMF has previously been reported [6,7]. The positive effects of AMF have been ascribed to the improvement of water status in plants [8]. Improving physiological parameters in mycorrhiza inoculated plant under drought stress conditions has been previously reported [9,10]. AMF by increasing water and nutrients uptake, decreasing leaf malon- dialdehyde and hydrogen peroxide content, increase growth and phys- iological parameters of plants (e.g., stomatal conductance, transpiration rate, leaf water potential) under drought stress conditions [11,12]. Pirzad and Mohammadzadeh [13] showed that inoculation with fungi is * Corresponding author. E-mail address: abdollahi.al@fa.lu.ac.ir (A.A. Arpanahi). Contents lists available at ScienceDirect European Journal of Soil Biology journal homepage: www.elsevier.com/locate/ejsobi https://doi.org/10.1016/j.ejsobi.2020.103217 Received 15 February 2020; Received in revised form 10 June 2020; Accepted 24 June 2020