## 4 Discussion
It should be emphasized that in stressed plants in the present study, we observed an increase in EOs but not in VOC emissions. Studies carried out on other plant species have demonstrated comparable variations in volatile emissions vs. internal oil [56; 57]. In aromatic plants, healthy plants maintain a basal emission of volatiles, which are released from the glandular trichomes where they are accumulated [4]. Initially, it was believed that the volatiles simply diffuse through cell membranes because of the lipophilic properties of these metabolites. However, current studies indicate that for several plant volatiles compounds, the internal concentration required for them to passively diffuse is very high, suggesting that this amount could be toxic within the membranes themselves [58; 59; 30]. Just exactly how these metabolites are released into the atmosphere is not known, but several hypotheses have been developed, among which the presence of an ATP-driven membrane transporter has been suggested, whose function is to expel toxic compounds from the plant itself, thereby avoiding their accumulation [58; 59; 60]. Active transport proteins could also allow strict controls on VOC emission rates, which might have allowed plants to send volatile-encoded messages with a higher accuracy [58]. Currently, there is brief evidence available on the consequences of drought stress on aromatic plants inoculated with PGPR. In a previous report, we described that under similar conditions inoculation with these strains alleviated the adverse effects of water stress, resulting in an increase of shoot weight, leaf area and significantly less reduction in plant growth. Additionally, drought-stressed plants inoculated with PGPR had a remarkably higher total phenolic content and enzymatic activity than water-stressed plants without PGPR inoculation. The peroxidation of membrane lipid was also reduced in PGPR treated plants subjected to water stress [14]. Is well known that phytohormones have an important function in plant growth, immunity and the accumulation of secondary metabolites. In particular, JA is implicated in stimulating trichomes formation, as well as the biosynthesis of EOs. However, it does not act alone but instead interacts with SA, thus co-regulating the synthesis of secondary metabolites [56; 20]. Regarding the effect observed in the current study, the increase in PT and EO yield might be related to the increase in JA and SA recorded in plants subjected to stress and PGPR inoculation (Figure 5). In fact, the levels of JA and SA increased in inoculated plants subjected to water stress at similar levels, as happened in plants only inoculated. It was reported previously that JA and SA are implicated in the ISR signaling pathways triggered by PGPR [12; 20]. ABA also showed the same tendency, with, as expected, levels increasing in relation to regularly watered plants, since ABA controls the expression of several genes that help the plant to survive under drought stress [61]. However, the levels found in plants stressed and inoculated were similar to those in plants subjected to water stress. In the present study, as shown in the scheme (Figure 6), there was a rise in endogenous phytohormone levels (SA, JA, ABA), as well as an increase of EO yield and PT density, in peppermint treated with PGPR and subjected to water stress compared to regularly watered plants. In contrast, VOC emissions were reduced. The same positive effect was observed in trichome density, EO main components and total EO yield in inoculated PGPR stressed plants, and also in plants only inoculated or MS stressed alone, as can be clearly seen in the PCA graphic (green ellipse) (Figure 5). Summing up, it has been shown that drought stress or inoculation per se are capable of inducing an increase in EO yield and PT density, suggesting that these treatments when used individually seem to reach a limit, in relation to the kind of plant defense response, and with no synergism effects being observed with the application of both treatments together.