**1. Introduction**

Many aromatic plants, such as *Mentha piperita* L. (peppermint), are important sources of essential oil (EO) production. The EOs are generated and stored in glandular trichomes, where they form complex mixtures of secondary metabolites (SM) mainly composed of the volatile mono- and sesquiterpenes responsible for the characteristic aromas of various plant species [1,2]. Therefore, the quality of aromatic plants is recognized by the composition and concentration of these components for each species. Furthermore, the quantity and quality of SM is determined by environmental factors including temperature, soil quality, light intensity, and/or water availability [3].

Biotic and abiotic stresses are major constraints on crop yield, with environmental stress representing a strong restriction on increasing crop productivity as well as affecting the use of natural resources. A soil is considered to be saline when the ion concentration reaches an electrical conductivity of >4 dS m<sup>−</sup>1, measured on a saturated soil at 25 ◦C, and consequently interferes with the growth of species of agricultural interest [4]. Salinity impacts agricultural production in most crops by affecting the physical-chemical properties of the soil and the ecological balance of the cultivated area [5]. As salinity affects many aspects of the physiology and metabolism of the plants, the presence of soluble salts in general has a negative consequence for the plant's growth by decreasing the water potential and thus restricting the absorption of water by the roots (osmotic effect). In addition, the absorption of specific saline ions leads to their accumulation in tissues in concentrations at which they can become toxic and induce physiological disorders (ionic toxicity) in the plant, with high concentrations of saline ions being able to modify the absorption of essential nutrients and leading to nutritional imbalances (nutritional effect) [6]. These effects are reflected by a decrease in germination, vegetative growth, and reproductive development [4,7].

Plant tolerance to salt stress is linked to the use of different strategies, including osmotic adjustment, the exclusion of toxic ions from the aerial part, translocation of photoassimilates to underground organs, an increased growth of the root system, and ensuring the availability of water and nutrients, among others. Furthermore, salinity can produce an accumulation of reactive oxygen species (ROS) [6], which may lead to a deterioration of photosynthetic pigments, lipid peroxidation, alterations in the selective permeability of the cell membranes, protein denaturation, and DNA mutations [8–10]. Damage of the cell membrane produces small hydrocarbons such as malondialdehyde (MDA), which is a sign of membrane cellular damage. Plants have well-described protection and repair systems that mitigate ROS damage. In addition, certain species have developed protective mechanisms that include enzymatic and non-enzymatic components [11,12].

Plant growth promoting rhizobacteria (PGPR) are beneficial microorganisms capable of colonizing the rhizosphere of plants and benefiting them both directly and indirectly [13]. It is well known that PGPR functions in different ways: synthesizing specific compounds for the plants, helping the uptake of nutrients, and protecting the plants from diseases [14–16]. In general, it has been observed that the negative effects that salinity produces in plant development can be mitigated by the use of microorganisms as inoculants, which is an alternative technology to improve the abiotic stress tolerance capacity of plants [17–21]. In this regard, considerable attention has been focused on understanding the molecular, physiological, and morphological mechanisms underlying rhizobacterial-mediated stress tolerance. In fact, the mechanisms by which these bacteria mediate abiotic stress tolerance continue to be widely studied, largely because they are difficult to elucidate [22,23].

Advances in research have revealed that certain PGPR strains are capable of emitting microbial volatile organic compounds (mVOCs) [24–28]. These compounds mainly consist of an abundant and very complex mixture of compounds, including alcohols, alkanes, alkenes, esters, ketones, sulfur, and terpenoids, characterized by their low molecular weight and high vapor pressure under normal conditions, which can vaporize significantly and enter the atmosphere. The analysis of mVOCs is a developing research area that has an effect on the applied agricultural, medical, and biotechnical applications, with a related interesting mVOC database containing available information regarding microbial volatiles having been published [29]. Recent studies have also provided new insights into the participation of mVOCs in inter- and intra-specific communication [30]. These compounds have been observed to have the ability to promote plant growth and induce systemic resistance (ISR) against pathogenic organisms, thereby improving the well-being of crops [24,27,28,31,32]. VOCs from *Paraburkholderia phytofirmans* have been shown to increase plant growth rate and tolerance to salinity, reproducing the effects of direct bacterial inoculation of roots [32]. Thus, the emission of mVOCs is currently recognized as being a very relevant aspect in microorganism–plant interactions [17,21,28,33,34].

We have previously demonstrated that both the direct inoculation of PGPR and exposure to VOCs emitted by these rhizobacteria stimulate the biosynthesis of SM and increase the biomass production in different aromatic plants [25,26,35–39]. Although there are few reports about the effects of mVOCs emitted by rhizobacteria on the SM yield of aromatic plants under conditions of abiotic stress, studies related to the emission of volatile organic compounds with biological activity by rhizobacteria is a novel area attracting increasing interest.

It should also be noted that it is necessary to examine the use of fertilizers and chemical synthesis pesticides related to the concentration of salts in the soil in order to develop sustainable agriculture, as this is key to assessing the proposal of alternative and complementary strategies. Taking this into consideration, among the possible alternatives, the use of microbial inoculants, considered to be a clean technology aligned with the principles of sustainable agriculture, becomes more relevant. Thus, the present study was founded on the hypothesis that the investigation of mVOCs with respect to the description of their biological functions and ecological roles is crucial for elucidating the mechanisms related to the control of critical biological processes in plant health and that this could also offer useful benefits to confront agronomic and environmental complications. In this present study, the aim was to explore the potential of mVOCs in ameliorating salinity effects in *M. piperita,* with an important objective of the study being to evaluate the role of mVOCs in EOs and the phenolic compound levels, as well as their function in the antioxidant status of plants grown under salt stress conditions.
