## 4 Discussion  
In the present study, we have validated that under regular irrigation GB03 and WCS417r strains enhance trichome density and EO yield on peppermint plants, as earlier reported by [PERSON] et al., [17]. In previous studies it was observed that peppermint plants inoculated with GB03 or WCS417r and grown under drought stress showed a significant increase in shoot fresh weight with respect to MS and SS plants, which was approximately 15% higher than the respective control (MS or SS), with the same tendency being observed for leaf area and number of leaves. MS-inoculated plants showed an increase in shoot fresh weight compared to non-inoculated plants [14]. In the current research, when plants were grown under water stress, the EO production showed an increase, at similar levels as in inoculated plants, but the inoculation of stressed plants did not modify this parameter. As expected, PT revealed the same tendency as EO yield. In concordance with our results, [PERSON] et al., [28] observed that the number of glandular trichomes were increased under salt stress in _M. piperita_. In addition, there was  
Figure 5: PCA plot for the response of peppermint plants subjected to drought stress inoculated with PGPR strains. Green ellipse, drawn manually, shows a strong effect of MS as well as inoculation treatment (GB03 or WCS417r) or their combination (MS + PGPR) with the variable PT, EO and its emission as VOCs. In contrast, control and SS treatments, alone or combined with GB03 or WCS417r, were located far from the variable assessed (yellow ellipse), representing a minor effect. EO: Essential Oil, VOC: Volatile Organic Compounds, PT: Pelitate Trichome; JA: Jasmonic Acid; SA: Salicylic Acid and ABA: Abscisic acid; MS: moderate stress; SS: severe stress. a strong correlation previously reported between EO production and the total number of glandular trichomes [29; 30]. It is worth mentioning that when aromatic plants are cultivated in semi-arid environments they are mostly more \"fragrant\" than when cultivated in moderate climates. In addition, under stressed environments, the amounts of EO are increased [1]. According to these considerations, the more severe the stressed the condition, the more severely it will influence the metabolism, having an effect on the biosynthetic pathways involved in the synthesis of secondary metabolites [1; 31]. It has been suggested that the induction of EO yield under drought stress may be because when plants grow under stress conditions, they only allocate low amounts of carbohydrates from photosynthesis to plant development, and instead use these for the synthesis of secondary and reserve metabolites, thus generating a balance between growth and defense [1; 32; 33]. The increase in EO in this case could be working as a mechanism to dissipate \"extra\" energy [1; 34]. Indeed, in favorable conditions, most of the plant's energy is directed to primary metabolism and only a small portion to the secondary metabolism [35; 36]. Although the system achieves a proper function under normal environmental conditions, the presence of water stress causes an imbalance, and when water is restricted, the stomata are closed. Consequently, significantly fewer reducing equivalents (NADPH + H+) are consumed to fix CO\({}_{2}\) by the Calvin cycle. As a consequence, only minor amounts of oxidized reduction equivalents (NADP+) are accessible as electron acceptors. Generating an accumulation of a large number of NADPH + H+ produces a strong over-reduced state in the chloroplasts, and consequently, the photosystems will favor the production of oxygen, generating superoxidized radicals that must be eliminated by the antioxidants enzymes. Furthermore, due to the abundance of reduced coenzymes, all reactions that use NADPH + H+ will be favored, for example the synthesis of highly reduced secondary metabolites [1; 37], including EOs, as observed in the present study.