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 (H2O2) 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 H2O2 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.