## 4 Discussion
This increment in EO yield was connected with a rise in PT density, where the EOs are synthesized and stored and by the upregulation observed in the expression of \(Pr\) in plants (inoculated or not) grown under drought stress. Although we do not observe any up regulation of \(Lim\)\(s\), it has been reported that in peppermint, the most important enzymes involved in the monoterense synthesis, were controlled at the gene expression level [38]. The difference shown between substance produced and transcript levels may be explained by either post-transcriptional modification, protein stability or an increased flux in the metabolic pathway [39]. It is important to highlight the inconsistencies found in the bibliography in relation to water stress and EO yield, even in plants of the same species. In agreement with our observations, [PERSON] et al. [40] registered two fold more EO in peppermint grown under water stress. In concordance, in _S. officinalis_, MS generated a 4.5-fold increment in EO content and a 2.6-fold increment in plants subjected to SS [32]. In penpyroyal plants grown under water stress, a 40% increase in EO content was also shown [28], with comparable responses having been observed for other Lamiaceae species grown under water stress, such as _Lavandula angustifolia_[41], _Petrselinum crispum_[42] and _Thymus vulgaris_[43]. However, in a one-year field trial, [PERSON] et al. [44] observed that different Lamiaceae subjected to drought stress did not present variations in the amount of EOs, while in _Lavandula latifolia_ and _Salvia sclarea_, the EO content actually decreased. Other authors have also indicated negative effects on EO content due to water stress in _Menhta arensis_[45], ref. [46]_Rosmarinus officinalis_ and _Salvia officinalis_[47]. In relation to the main monoterenes (menthone, menthol and pulegone), water stress also induced changes in EO concentrations. In agreement with our observations, [PERSON] et al. [40] recorded a significant increase in menthol content in _M. piperita_ plants subject to water stress, with similar results being obtained in M. arvensis grown under water stress [45]. Related to this, it was reported that oxygenated monoterenes, for example menthone, menthol and pulegone, could be reason of the EOs antioxidant activity [48]. Many of the differences observed in the bibliography related to the EO content may be due to variations in the experiment design. In general, there are difference in the substrate used and in the length of the assays. For example, [PERSON] et al., [40] used a mixture of soil, peat and coco peat, while [PERSON] et al. [49] used enriched soil and [PERSON] et al., [28] worked with sandy soil. In contrast, our assay was carried out in vermiculite, irrigated with [PERSON]'s solution in order to control the supply of nutrients. Another distinctive methodology difference is concerning the generation of water stress and the irrigation method applied. [PERSON] et al. [40] used drinking water and the stressed plants were rehydrated, since they suspended irrigation for 2 rounds and then resumed it again. Regarding VOCs, the results obtained in the current study showed that water stress reduced the emissions, which were lower for higher applied stresses. Comparable results were recorded in _Salvia dolomitica_[50] and in woody plants [51]. In contrast, in other studies, drought stress did not influence the emission of VOCs, such as in _Zataria multifora_[52]. Nevertheless, it was reported that water stress stimulated emission in thyme [53]. The diversity of results about the consequence of water stress on plant VOC emissions could also be due to differences in the degree of stress or damage produced to the plant by drought. Therefore, a high level of drought could reduce emissions, whereas a minor drought stress could actually increment emissions [54]. Nevertheless, a rise in the emissions of different terpen can in some cases be a result of an increase in trichome density [55].