Effect of Plant growth promoting rhizobacteria (PGPR) and mycorrhizal fungi inoculations on essential oil in Melissa officinalis L. under drought stress

Melissa officinalis L. (Lemon balm) is one of the most important medicinal plants in the Lamiaceae family, whose essential oil compounds are affected by the inoculation of beneficial microorganisms and different irrigation regimes. In the present study, the effect of inoculation of plant growth-promoting rhizobacteria (PGPR), arbuscular mycorrhiza fungi (AMF) and PGPR + AMF (co-microbial) on growth, photosynthetic pigments, relative water content (RWC), proline and essential oil of lemon balm subjected to 100 (control) and 50% (drought stress) of field capacity (FC) in greenhouse conditions was investigated. The results showed that 50% FC irrigation increased proline content (2.9-fold) and declined chlorophyll a (40.4%), b (40.2%), carotenoids (44.6%) and RWC (17.8%) compared to the well-watered plants. Plants watered with 50% FC showed a decrease in height, dry weight of roots and shoots by 28.9, 24.1 and 25.6%, respectively, over the control. However, inoculation of PGPR, AMF and co-microbial by improving photosynthetic pigments, proline content and RWC increased plant tolerance and thus restored the growth and biomass of lemon balm under drought stress. Co-microbial inoculation also altered the chemical composition of secondary metabolites and increased the essential oil yield of lemon balm leaves. PGPR + AMF inoculation in lemon balm plants is recommended to increase plant tolerance to limited water conditions and also to improve the yield of essential oils for medical purposes.