Mitigation of water deficit stress in Dracocephalum moldavica by symbiotic association with soil microorganisms

Symbiotic association with soil microorganisms (plant growth-promoting rhizobacteria (PGPR) and/or arbuscular mycorrhizal fungus (AM)) can help plants to cope with drought stress. The goal of the present study was to consider the effects of Micrococcus yunnanensis (PGPR) and Claroideoglomus etunicatum (AM) under drought stress on growth, mineral nutrients uptake, and secondary metabolites of D. moldavica, as a sensitive plant to water deficit. Experiments were carried out as factorial completely randomized design consisting of three factors, PGPR, AM, and irrigation levels (100, 85, 70, 55, 40, and 25 % field capacity). The results showed that PGPR and AM stimulated plant growth significantly and increased photosynthetic pigment contents of leaves. Higher uptake of nutrient elements partly improved plant growth and dry matter production upon symbiotic associations. Despite a significant increase in the soluble sugars and proline contents under water deficit stress, symbiosis significantly decreased the amounts of those compounds. Drought stress alone elevated plant antioxidant capacity and total phenolic content, but they were decreased in plants inoculated with PGPR and AM. Also, coinoculation increased the contents of hesperetin, eugenol, p-coumaric acid, and these effects were more pronounced under moderate drought stress than other conditions for hesperetin and p-coumaric acid accumulation. Significantly, higher content of rosmarinic acid was found under drought stress, but inoculations significantly decreased the concentrations of this compound. Overall, these results showed a successful coexistence of soil microorganisms (PGPR and AM) with D. moldavica alleviates the harmful effects of drought stress through higher mineral nutrient uptake as well as secondary metabolites and photosynthetic pigments contents.