Plant growth-promoting rhizobacteria enhance essential oil production and antioxidant activity of Mentha piperita under water deficit stress

In the present study, the effect of plant growth-promoting rhizobacteria (PGPR) was investigated on the essential oil (EO) yield, EO compositions, and antioxidant activities of peppermint plants grown under water deficit stress. PGPR-treated and untreated plants were subjected to four irrigation regimes based on field capacity (FC): well-watered (control, 100 % FC), low drought stress (LDS, 75 % FC), moderate drought stress (MDS, 50 % FC), and severe drought stress (SDS, 25 % FC). Antioxidant activity of EOs was investigated using three different methods, namely radical scavenging (DPPH), metal ion reducing (CUPRAC), and metal chelation assays. The current study results indicated that LDS and MDS significantly increased the EO yield compared to the control, while SDS remarkably decreased it. PGPR mitigated the negative effects of drought stress (DS) in peppermint and significantly enhanced the EO yield at all stress levels. For example, under LDS and MDS conditions, the EO yield was 1.62 % and 1.59 % in untreated plants, respectively, whereas these values increased to 1.73 % and 1.84 % in PGPR-treated plants. The most significant result was observed in samples under SDS. Accordingly, under SDS conditions, the EO yield decreased to 1.34 % in untreated plants, whereas it increased significantly to 1.77 % in PGPR-treated plants. Furthermore, DS and PGPR significantly influenced antioxidant activity and the main components of EOs. Among these, menthol content increased from 28.45 % in the control to 34.75 % under SDS, whereas menthofuran decreased from 35.03 % to 32.84 %. The results show that PGPRs not only increased tolerance to DS but also improved EO yield and antioxidant activity. • Water deficit stress triggers secondary metabolite biosynthesis in peppermint. • EO yield increased under LDS and MDS, but decreased under SDS conditions. • PGPR improved water use efficiency and enhanced high value metabolites. • The study demonstraes an eco-friendly approach to water-saving peppermint cultivation. • Results suggest practical strategies for sustainable peppermint production under drought.