Evaluation of the growth, essential oil composition and antioxidant activity of Dracocephalum moldavica under water deficit stress and symbiosis with Claroideoglomus etunicatum and Micrococcus yunnanensis

In recent years, the application of certain microorganisms such as Claroideoglomus etunicatum, an arbuscular mycorrhiza fungus (AMF), and Micrococcus yunnanensis, a plant growth promoting bacteria (PGPB), has been considered to be an effective and sustainable strategy for improving crop yield and quality under various environmental conditions. In the present study, a completely randomized factorial design was used to investigate the effects of AMF and PGPB inoculation under various irrigation regimes (40, 70 and 100% Field Capacity) on the growth parameters, antioxidant activity and essential oil (EO) composition of Dracocephalum moldavica, as a drought-sensitive plant. Although the results indicate that water deficit stress reduces plant growth, AMF or PGPB alone or in combination significantly mitigated the adverse effects of the stress. The improved number of branches per plant and plant height were partly responsible for the increase in fresh and dry weights upon symbiotic associations. Also, both water deficit stress and inoculations elevate the activities of antioxidant enzyme ascorbate peroxidase, guaiacol peroxidase and superoxide dismutase. Water deficit stress resulted in 1.3-3.5-fold rise in the H2O2 and malondialdehyde contents, but inoculations significantly prevented elevation in the concentrations of these compounds. With respect to EO, volumes of EO extracted from plants under water deficit stress significantly increased upon symbiotic association with AMF and/or PGPB. The main EO constituents were geranial, neral, geranylacetate, and geraniol. Although the percentage of each constituent and the composition of the EO showed some variation under water deficit stress and symbiotic associations, no major change in these parameters was observed under the prevailing conditions. The results strongly suggest that AMF or PGPB inoculation alone or in combination can be employed to mitigate the harmful effects of drought stress, and, in general, there are no significant differences between the inoculation treatments with respect to their effects on the parameters investigated. Higher antioxidant enzyme activities and lower H2O2 content could be part of the mechanisms which confer partial resistance of this sensitive plant to water deficit stress.