Plant growth promoting rhizobacteria (PGPR) confer drought resistance and stimulate biosynthesis of secondary metabolites in pennyroyal (Mentha pulegium L.) under water shortage condition

Water deficit is one of the main devastating environmental factors limiting crops productivity. Plant growth promoting rhizobacteria (PGPR) can improve plant growth in stress conditions and enhance plant tolerance to drought stress. The aim of this study was to investigate the role of PGPR to protect pennyroyal (Mentha pulegium L.) as an important industrial and functional plant against drought damages. The greenhouse study was conducted as a factorial experiment in a completely randomized design. The first factor was PGPR inoculation (Azotobacter chroococcum (Ac), Azospirillum brasilense (Ab), Ac + Ab and control (without PGPR)). The second factor was irrigation regime that applied at three levels: field capacity (FC), 0.7 FC and 0.4 FC. Drought stress significantly decreased chlorophyll fluorescence (Fv/Fm) and relative water content (RWC) of pennyroyal. Conversely, antioxidant enzymes activity, osmoprotectants accumulation, membrane lipid peroxidation, abscisic acid (ABA) and secondary metabolites including phenolic, flavonoid and essential oil contents and DPPH radical scavenging activity of the plant extract increased due to water deficit. PGPR inoculation significantly reduced adverse effects of water stress on physio- biochemical characteristics and secondary metabolites production in pennyroyal. The results indicated that the bacteria were different in their ability to enhance plant investigated characteristics, and co-inoculation of the bacteria was more effective on improving of pennyroyal physiological and phytochemical parameters. The highest ABA, proteins and soluble sugars, phenolic, flavonoid and oxygenated monoterpenes contents as well as DPPH radical scavenging activity were observed in the plants treated with Ac + Ab under severe drought stress.