2.1.4. Treatments

MS media (plant growth media) and Hoagland media (bacterial growth media) were supplemented with different salt concentrations: 0, 75, and 100 mM NaCl. For each experimental set, both the plant and bacteria were grown under the same concentration of NaCl but without contact with each other. Salt level concentrations were selected based on previous observations: at lower concentrations (25 and

50 mM), plant growth was not affected, and at higher levels (125 and 150 mM), the rooting capacity decreased significantly. Experiments were repeated three times (10 jars per treatment; 1 plant/jar).

#### *2.2. Essential Oil Extraction and Analysis*

Shoot samples were individually weighed and subjected to hydrodistillation in a Clevenger-like apparatus for 40 min. The volatile fraction was collected in dichloromethane, and β-pinene (1 μL in 50 μL ethanol) was added as an internal standard (as it was previously reported, β-pinene is not present in peppermint plants [37]). The major *M. piperita* EO components, which comprise ~60% of the total oil volume, are limonene, linalool, (−) menthone, (−) menthol, and (+) pulegone. These compounds were quantified in relation to the standard added during the distillation procedure described above. The flame ionization detector (FID) response factors for each compound generated essentially equivalent areas (differences *p* < 0.05).

Chemical analyses were performed using a Perkin-Elmer Q-700 gas chromatograph (GC), equipped with a CBP−1 capillary column (30 m × 0.25 mm, film thickness 0.25 μm) and a mass selective detector. Analytical conditions were as follows: injector temperature 250 ◦C; detector temperature 270 ◦C; oven temperature programmed from 60 ◦C (3 min) to 240 ◦C at 4◦/min; carrier gas = helium at a constant flow rate of 0.9 mL/min; source 70 eV. The oil components ((−) menthone, (−) menthol, and (+) pulegone) were established by comparison of the diagnostic ions (NIST 2014 library) and GC retention times with those of the respective authentic standard compounds purchased from Sigma-Aldrich [34]. GC analysis was performed using a Shimadzu GC-RIA gas chromatograph fitted with a 30 m × 0.25 mm fused silica capillary column coated with Supelcowax 10 (film thickness 0.25 μm). The GC operating conditions were as follows: injector and detector temperatures 250 ◦C; oven temperature programmed from 60 ◦C (3 min) to 240 ◦C at 4◦/min; detector = FID; carrier gas = nitrogen at a constant flow rate of 0.9 mL/min.

#### *2.3. Total Phenolic Content (TPC) Determination*

The total phenolic content of the extract was determined by the Folin–Ciocalteu method, as previously described by Cappellari et al. [41]. The TPC were expressed in terms of μg gallic acid (a common reference compound) equivalent per g plant fresh weight using the standard curve.

### *2.4. Antioxidant Activity*

The capacity of radical scavenging in extracts against stable DPPH• (2,2-diphenyl−1-picrylhydrazyl) was determined by the Brand-Williams et al. method [42] with minor modifications, as previously described by Chiappero et al. [43]. A calibration curve was obtained using ascorbic acid, and the scavenging capacity of the plant extracts was expressed as mM ascorbic acid equivalents (AAE) per g fresh weight (mM AEE/g FW). All experiments were performed in triplicate for each experimental unit.

#### *2.5. Lipid Peroxidation*

Lipid peroxidation was measured by quantifying the malondialdehyde (MDA) production using the thiobarbituric acid reaction. The MDA content was measured following the method of Heath and Packer [44], with some modifications, as reported by Chiappero et al. [43]. The amount of MDA was determined by its molar extinction coefficient (155 mM−<sup>1</sup> cm<sup>−</sup>1), which was expressed as μmol MDA/g FW (grams of fresh weight). The experiments were performed in triplicate for each experimental unit.

#### *2.6. Statistical Analysis*

Data were subjected to a two-way analysis of variance (ANOVA) (mVOcs × salt stress), followed by a comparison of multiple treatment levels with those of the control, using the post hoc Fisher LSD test. Infostat software version 2018 (Group Infostat, Universidad Nacional de Córdoba, Argentina) was used for the statistical analysis. Principal component analysis (PCA) using Infostat statistical package was conducted. The analysis of extracts shows the relationships among the treatments (mVOCs exposure and salt stress conditions) and the different variables measured (EO, TPC, lipid peroxidation (MDA), and antioxidant capacity (AAE)). At least 15 observations were used for each treatment in the multivariate dataset.

### **3. Results**
