Inoculating moldavian balm ( Dracocephalum moldavica L.) with mycorrhizal fungi and bacteria may mitigate the adverse effects of water stress

Plant growth-promoting bacteria (PGPBs) play a crucial role in mitigating the oxidative damage caused by water stress in different plant species. The aim of this study was to determine the effects of PGPBs and mycorrhiza-like fungi ( Piriformospora indica ) on improving drought tolerance in moldavian balm ( Dracocephalum moldavica L.), a medicinal and aromatic plant. For this purpose, a greenhouse study was conducted in a factorial experiment based on a randomized complete design with three replications. Results indicate that water stress reduces the membrane stability index (MSI), total chlorophyll content (Chlt), carotenoids, and maximum photochemical efficiency of photosystem II (Fv′/Fm′) in moldavian balm plants, while increasing superoxide dismutase (SOD) activity, malondialdehyde (MDA) content, and hydrogen peroxide (H 2 O 2 ) content compared to the control (no water stress). Inoculation with PGPBs and Piriformospora indica helped alleviate the negative effects of water stress. The highest MSI (48%) and Fv′/Fm′ value (0.82) were observed when inoculated with Enterobacter and Piriformospora , respectively, under non-water-stressed conditions. Inoculation with Agrobacterium , Piriformospora , and Enterobacter improved the Chlt and leaf proline contents, as well as the SOD activity under high water stress, compared to the non-inoculated control values. Furthermore, inoculation with Pseudomonas under high water deficit stress levels increased the MDA content (0.51 mmol g −1 FW) and H 2 O 2 levels (0.40 mmol g −1 FW). The highest yield of flowering branches (2.414 g pot −1 ) in moldavian balm was obtained with Enterobacter . Based on the enhanced physiological and biochemical responses, as well as increased antioxidant enzyme activity that improve water tolerance in this plant, it is recommended to use PGPBs and Piriformospora indica fertilization.