Introduction

The world population is increasing rapidly, and it is expected that over the next 30 years, the population will grow by another 20 billion people (Keshavarz 2020). The growing global population and the associated increase in fossil fuel consumption will further impact on the rate of global warming. One of the most dramatic effects of global warming is the increased occurrence and intensity of abiotic stress events such as salinity and drought stress that will lead to reductions in crop productivity in arid and semiarid areas. Iran is a producer of various medicinal plants (Keshavarz Mirzamohammadi et al. 2021a). The quality and quantity of medicinal plants is severely decreased under unfavorable growth conditions due to changes in several morphophysiological traits. In peppermint, essential oils (EO) are stored in glandular trichomes. This limits toxicity of the EO to the plant itself, as many terpenes are potentially toxic to plant tissues (Zhang et al. 2008). The productivity of peppermint is attributed to its production of EO, which is the result of the activity of secondary metabolism and mobilization of EO to trichomes. All of these processes can be affected stress conditions (Khalvandi et al. 2019).

EO from peppermint plants have been used as anticonvulsants, stimulants, invigorants, gastric secretions reductants, and pain relievers (Zhang et al. 2022). Salinity stress has been shown to affect the plant’s photosynthetic system, resulting in increased production of reactive oxygen species (ROS). This causes oxidative stress damage such as chlorophyll decomposition, lipid peroxidation, and structural modification of proteins (Keshavarz Mirzamohammadi et al. 2021b). ROS also cause mutations in the DNA and damage to nucleic acids, and lead to disruption of the plant’s natural metabolism. This ultimately affects the yield and quality of plant products such as EO. Plants can mount a defense mechanism against ROS by activating the ascorbate-glutathione cycle to scavenge hydrogen peroxide (H2O2), which is formed by photorespiration and the Mehler reaction. Plants also increase various enzymatic and non-enzymatic antioxidant levels to reduce and scavenge oxidative damage. These defense mechanisms are also important for medicinal and aromatic plants, to maintain the activity of secondary metabolism and EO production. To neutralize the damage caused by abiotic stress such as salinity, the plant will need to increase the activity of antioxidant enzymes that neutralize ROS, such as superoxide dismutase (SOD), catalase (CAT), peroxidase (POX), polyphenol oxidase (PPO), glutathione S-transferases (GSTs), and ascorbate peroxidase (APX), as well as increasing levels of stress-protective metabolites such as proline (Keshavarz and Khodabin 2019). Proline accumulation in stressed plants is achieved through increased proline synthesis and inactivation of its degradation (Keshavarz et al. 2016; Rah Khosravani et al. 2017).

Plants constantly modify the activity and spectrum of physiological processes and developmental aspects in response to changes in environmental conditions such as biotic and abiotic stress (e.g., drought, heat, cold, chilling, UV). Multiple hormonal signals are involved in adjusting developmental and biological processes of plants in response to environmental cues (Karami et al. 2016). Major plant growth regulators such as cytokinins, auxins, gibberellins, abscisic acid, and ethylene are known to play a role in adjusting plant growth to environmental change. Furthermore, salicylic acid (SA), jasmonic acid (JA), and brassinosteroids (BR) also play vital roles in a wide range of cellular processes and developmental aspects (Keshavarz and Sadegh Ghol Moghadam 2017). SA is a plant phenolic compound that controls a wide variety of processes in plants, including seed germination, stomatal closure, photosynthesis, and transpiration rates, as well as signaling pathways involved in the antioxidant response of plants under unfavorable conditions (Keshavarz et al. 2016). Former reports have shown that SA stimulates the H+-ATPase activity of the tonoplast in soybean (Glycine max L.) under osmotic stress conditions (Ghassemi-Golezani and Farhangi-Abriz 2018).

Jasmonates are fatty acids (α-linolenic acid and hexadecatrienoic acid); derivatives of jasmonates, such as jasmonic acid (JA), methyl jasmonate, and jasmonate-isoleucine are synthesized in the plant’s cytoplasm (Qiu et al. 2014). JA is a stress-related hormone in higher plants and is involved in the regulation of several aspects of plant growth and developmental processes (Ali and Baek 2020). JA- and SA-mediated signaling pathways are involved in mounting resistance to biotic and abiotic stresses, including activation of the antioxidant defense system (Wang et al. 2021). BR are a group of steroidal hormones which are found in free form and/or as conjugates with fatty acids and sugars. BR are involved in regulation of plant growth, including cell elongation and cell division in stems and roots, reproductive development, photomorphogenesis, and abiotic stress responses.

Various stress factors profoundly affect plant growth and development, as well as several productivity and yield factors. It is therefore essential to improve our understanding of how the environment influences critical yield components, in order to identify strategies to genetically improve abiotic stress tolerance and maintain productivity under unfavorable growth conditions. This study aimed to investigate the effect of salinity stress on the activity of antioxidant enzymes, malondialdehyde (MDA) content, hydrogen peroxide, electrolyte leakage (EL), proline synthesis, and EO content of different species of mint under hydroponic conditions.

Materials and Methods

This research was conducted at a greenhouse in Varamin city, Iran, in two consecutive years (2017 and 2018). Seeds of mint were obtained from the Institute of Forests and Rangelands, Iran. First, the seedlings were planted in boxes (600 mL) containing a perlite:sand mix (3:1) as a factorial experiment in a randomized complete block design with three replications in the greenhouse (temperature 28 °C; light and dark periods of 16 and 8 h, respectively; relative humidity of 60%; Yi and Wetzstein 2010). Salinity treatments (two rhizomes per pot, four pots for each treatment) used in this experiment were carried out using three NaCl levels: 0 mM (control), 30 mM (mild), and 60 mM (severe) NaCl stress. Four hormone treatments were carried out on a duplicate set of salt treatments: no treatment (control), 100 mg L−1 SA, 60 mM JA, and 1 mM BR. The hormone concentrations used in this experiment were based on those previously described by Keshavarz et al. 2016 and Złotek et al. 2016. The hormone solutions were applied as a spray to the leaves of the plants. The hydroponics setup used Hoagland nutrient solution to feed the plants and the solution was replaced every 7 days. Salinity stress was applied from the 15th day after planting. After 3 weeks of growth, plants were sprayed with plant hormones once a day at dusk for 4 days.

The physiological traits and antioxidant enzyme activity of peppermint leaves were measured in the two center rows of each subplot. Samples were frozen in liquid nitrogen and stored at −80 °C until used for analyses. Leaf samples (~ 0.4 g) were extracted with 50 mM potassium phosphate buffer at pH 7.0 and then centrifuged at 13,000 rpm for 20 min, whereafter 0.2 mL of supernatant was used for antioxidant measurement by spectrophotometer (Varian Cary Win UV 6000i, USA). SOD activity was estimated based on the method described by Ahanger et al. (2020) and absorbance was read at 560 nm. The method described by Nakano and Asada (1981) was used to measure APX enzyme activity by measuring the absorbance of the assay solution comprising ascorbic acid and H2O2 at 290 nm using a spectrophotometer. CAT activity was measured using the method described by Dhir et al. (2009) and is based on monitoring the rate of reduction of H2O2 at 240 nm in the reaction mixture. PPO and POX enzyme activities were measured using the Ghanati et al. (2002) method. Glutathione S-transferase (GST) enzyme activity was measured using the Zinovieva et al. (2021) method and absorbance was monitored at 340 nm. The supernatants of the leaf samples were measured by a spectrophotometer (SPEKOL 1300, Analytic Jena, Germany) and the results were expressed as units (U) mg−1 protein fresh weight (FW; Keshavarz and Khodabin 2019). Proline and protein content were measured following the bovine serum albumin and acidic ninhydrin method (Bates et al. 1973). The method of Zhang et al. (2008) was used to test the contents of malondialdehyde (MDA) in peppermint leaves and the absorbance of the assay mixture at 532 and 600 nm was measured (extinction coefficient 155 mM−1 cm−1). Determination of hydrogen peroxide (H2O2) activity was performed using the method described by Alisofi et al. (2020). To measure electrolyte leakage (EL), fresh leaf samples were placed in 20 ml of distilled water. After 24 h, the electrical conductivity of each sample was measured. To measure the amount of total EL due to cell death, the tubes containing the leaves were placed in a hot (boiling) water bath for 30 min. After cooling, the electrical conductivity of the samples was measured again and, finally, EL was calculated (Shi et al. 2006).

Plants were harvested 70 days after planting and EO was extracted from powdered dried leaves (20 g) of different mint species using an all-glass Clevenger-type apparatus with 500 mL water for 150 min. Data were expressed as percentage EO per gram dry weight.

The experiment adopted a randomized complete block design. The data obtained from the experiment were analyzed and the results compared by the least significant difference test (SAS version 9.1; SAS institute, Cary, NC, USA). XLSTAT software (2018 version; New York, NY, USA) was used to perform principal component analysis (PCA). Varimax rotation was used to optimize the PCA.

Results

PCA Results

The results of PCA in the absence of salinity stress showed that JA foliar application was strongly associated with total protein levels, EO content, and GST levels. On the other hand, no foliar application of JA was highly correlated with proline, CAT, and POX. Under mild salinity stress (30 mM NaCl), BR and SA foliar applications were strongly associated with H2O2, POX, and EL, but JA foliar application was highly correlated with CAT and total protein content. No foliar application of plant hormones was associated with SOD, proline, and MDA (Fig. 1). The PCA at severe salinity (60 mM NaCl) showed that SA treatment was strongly correlated with GST, SOD, PPO, EL, proline, and total protein (Fig. 1).

Fig. 1
figure 1

Results of biplot of the first (F1) and second (F2) components based on principal component analysis

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Activity of SOD

The results displayed in boxplots show that the amount of SOD was significantly increased (Fig. 2). Interaction between salinity and hormone treatments had a significant effect on SOD activity (Table 1) and the highest activity of SOD was observed for severe salinity stress (Fig. 1). The highest and lowest SOD activity was 50.25 and 10.9 U mg−1 protein, respectively; these figures were observed for severe salt stress combined with JA application and no salinity conditions without any hormone application, respectively (Fig. 1). The activity of SOD in mild and severe salinity conditions increased by 53.29 and 73.19%, respectively. Under control conditions (no salinity), there was no significant difference between JA, SA, and BR treatments in terms of their effects on SOD activity. Under severe salinity conditions, the SA and BR treatments both had a significant quantitative and similar effect on SOD activity (Fig. 1).

Table 1 Analysis of variance (mean squares) for the effects of different treatments (salinity, plant hormones, and their interaction) on the physiological traits of peppermint
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Fig. 2
figure 2

Boxplot based on the comparison between different levels of salinity stress in different hormone applications related to superoxide dismutase (SOD), polyphenol oxidase (PPO), catalase (CAT), peroxidase (POX), ascorbate peroxidase (APX), and glutathione S‑transferase (GSTs) characteristics

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Activity of APX

The results shown in boxplots indicate that APX activity was increased (Fig. 2). APX activity was affected by salinity stress (Table 1). APX activity was lower under control conditions compared to mild and severe salinity stress, and APX activity levels under mild and severe salinity were similar (Table 1).

Activity of CAT

The boxplots show that the amount of CAT was increased in plants treated with salinity (Fig. 2). The combined analysis of variance (ANOVA) results of CAT activity showed that the simple effects of salinity and hormones and the interaction between salinity stress and plant hormone treatment were significant at the 1% level (Table 1). Both mild and severe salinity stress significantly increased CAT activity compared to the control treatment (Fig. 2). The results also indicate that application of JA caused a significant increase in CAT activity, in contrast to SA and BR treatments. Based on dual interactions, JA under severe salinity stress had the highest CAT activity (0.61 U mg−1 protein), while the lowest CAT activity was observed in the absence of plant hormones under control conditions (0.26 U mg−1 protein).

Activity of PPO

The results shown in boxplots demonstrate that the amount of PPO was increased by salt treatment (Fig. 2). The ANOVA of experimental data for PPO activity show that the simple effects of salinity levels and plant hormone treatments and their interaction were significant (Table 1). A comparison of the two-way interaction (Table 1) revealed that BR application under mild salinity stress (0.30 U mg−1 protein) had the highest PPO activity. In addition, plants treated with JA under control conditions (0.1 U mg−1 protein) had the lowest PPO activity, with levels below untreated controls.

Activity of POX

The boxplots show that the amount of POX was increased by salinity treatment (Fig. 2). The main effects of salinity and plant hormones on POX enzymes activity were significant at the 1% probability level (Table 1). According to results of the ANOVA analysis, POX activity was highest under severe salinity stress (1.11 U mg−1 protein) compared to control and mild salinity stress (40.54 and 7.20%, respectively; Table 1). The data showed that JA treatment caused the highest POX activity rate (average of 1.01 U mg−1 protein). However, all plant hormone applications were at the same statistical levels.

Activity of GSTs

The boxplots show that the amount of GST activity was increased (Fig. 2). The ANOVA analysis shows that the main effects of salinity and plant hormone treatments had a significant effect at the 1% probability level on the GST enzymes activity (Table 1). Results of salinity levels showed that the highest GST activity was obtained under severe salinity stress (Table 1). The lowest GST activity (0.18 U mg−1 protein) was recorded from plants grown in soil with the lowest rate of salinity. Increasing salinity levels progressively increased the rate of GST activity. Plant hormones also had a significant effect on GST activity (Table 1), but all plant hormone applications had a similar effect.

Protein Content

The boxplots show that the total protein content of leaf tissues was decreased (Fig. 3). Analysis of variance indicated that salinity and the dual interaction of salinity and plant hormone treatments had considerable effects on the protein content of peppermint leaves (Table 1). Comparison of the mean values showed that the protein content of leaves remained stable in non-saline conditions (Table 1). The protein content was greatest (2.04 mg g−1 FW) and lowest (0.68 mg g−1 FW) in non-saline-treated plants without hormone application and plants under severe salinity treatment and without hormone application, respectively.

Fig. 3
figure 3

Boxplot based on comparison between different levels of salinity stress in different hormone applications related to protein, proline, malondialdehyde (MDA), electrolyte leakage (EL), and H2O2 characteristics

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Proline Content

The boxplots show that the amount of proline was increased (Fig. 3). Measurement of proline content after NaCl treatment showed a remarkable increase in proline content. The data indicate that proline content was significantly affected by salinity stress, hormone application, and the interaction between both treatments (Table 1). Plants experiencing severe salinity stress showed, on average, a higher proline content (77.26 μg g−1 FW) in the absence of hormone applications (Table 1). For all salinity levels, no significant effect of SA and BR treatment was observed on proline content.

Malondialdehyde

The boxplots show that the amount of MDA was increased in salinity treatments (Fig. 3). MDA levels of peppermint leaves were significantly (P < 0.01) influenced by salinity, plant hormone treatments, and the interaction between the two (Table 1). The maximum MDA content of peppermint leaves (Table 2) was recorded for severe salinity stress and no hormone application (137.5 nmol g−1 FW), followed by SA (89.66 nmol g−1 FW) and JA treatments (88.33 nmol g−1 FW). The lowest MDA levels were observed in peppermint treated by 0 mM NaCl and in the presence of hormone applications (Table 1).

Table 2 Principal essential oil composition of peppermint as influenced by salinity levels and plant hormone treatments
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H2O2 Concentration

The boxplots show that the amount of H2O2 was increased by salinity treatment (Fig. 3). Plant hormones and salinity treatments affected H2O2 levels in leaves (Table 2). Treatment of peppermint plants with 30 and 60 mM NaCl caused increments in H2O2 concentration of 48.4 and 38.8% as compared to control plants, respectively. In comparison to control plants, treatments of peppermint plants with plant hormones (JA, SA, and BR) resulted in a considerable reduction in H2O2 concentration (Table 1). The highest levels of H2O2 were found in control plants and plants grown under mild and severe salinity stress.

EL Percentage

The boxplots show that the amount of EL was increased by salinity stress (Fig. 3). Peppermint plants irrigated with 30 and 60 mM NaCl showed an increase in EL of 16.7 and 41.5% compared to control plants, respectively. The application of plant hormones caused a reduction in EL percentage (Table 1). No hormone application (control) under severe salinity stress had the highest EL (77.26%), followed by JA (63.66%), SA (62.56%), and BR (58.36%).

EO Content and Oil Profile

Salinity, plant hormone treatments, and their interaction significantly affected the EO content as compared to control conditions (Table 1). The maximum EO content (3.75%) was observed in 30 mM NaCl and JA foliar application, while the minimum level (2.20%) was observed in plants treated by 60 mM NaCl and without hormone application (Table 1). Based on our results, mild salinity stress (30 mM NaCl) increased the EO content compared to 0 mM NaCl, but increasing the severity of salinity to 60 mM NaCl decreased the EO content. For all salinity levels, there was no significant difference between SA and BR application (Table 1). Gas chromatography mass analysis found 18 EO ingredients to be the major components of EO: terpinolene, menthone, isomenthone, menthofuran, menthol, piperitone, and menthyl acetate. The results showed that peppermint treated with JA produced the highest levels of menthol in non-salinity and 30 mM NaCl conditions (55.43 and 54.39%, respectively), but plants treated with SA under 60 mM NaCl yielded the highest levels of menthol (52.26%). The different hormone treatments resulted in the lowest content of menthofuran, while the highest menthofuran levels were found for the non-hormone applications under all salinity levels.

Discussion

Previous studies conducted on peppermint (Mentha piperita) and oregano (Origanum onites L.) have reported increased levels of antioxidant enzyme activity under salinity stress (Khalvandi et al. 2019; Hancioglu et al. 2019). SOD, APX, POX, PPO, and CAT are the main enzymes of the ROS protection system in plants. Increased activity of these enzymes is therefore likely to improve stress tolerance, including salinity stress (Keshavarz and Sadegh Ghol Moghadam 2017). The increase in SOD activity following hormone application during mild and severe salt stress might be due to the combined effect of a salt toxicity response causing generation of ROS and the induction of a ROS defense response that is triggered by plant hormones. SOD is one of the key enzymes of the antioxidant defense system in plants and is the first step in the ROS elimination process. With the activity of SOD, O2− is converted to H2O2. Zhang et al. (2022) reported that SOD regulates the intercellular concentration of O2− in the plant under stress conditions. Our results showed that SOD activity was increased by hormone applications. JA in particular has the capability to induce antioxidant synthesis, thereby boosting ROS scavenging and salinity stress tolerance (Myers et al. 2023).

APX also plays a vital role in plant defense against oxidative stress by scavenging H2O2 in the chloroplasts, cytosol, mitochondria, and peroxisomes of plant cells. In addition, APX plays a vital role in the glutathione-ascorbate cycle, as well as in the Mahler and xanthophyll cycles. APX, like glutathione peroxidase, converts H2O2 to H2O (Zhang et al. 2008). Under mild and severe salinity, the APX activity increased, which might be due to the partitioning of O2− molecular oxygen into H2O2 or oxygen (O2) through the action of the APX enzymes. The extra content of H2O2 can act as a signaling molecule to induce the antioxidant response systemically in all plant parts to increase the plant’s defense against unfavorable conditions.

CAT is one of the best-known antioxidant enzymes, playing a critical role in neutralizing H2O2 and reducing its destructive effects on peroxisomes, glyoxysomes, and mitochondria. CAT can also capture H2O2 produced by SOD activity, converting H2O2 into water and oxygen (Keshavarz et al. 2021). CAT has a very high affinity for H2O2 and does not reach saturation at any concentration of H2O2. In addition, CAT receives hydrogen from other electron donors such as methanol, ethanol, and formic acid and converts H2O2 into water (Wang et al. 2021). PPO are antioxidants that accelerate the oxidation of phenols to O‑quinones. O‑quinones are highly reactive molecules that can nonenzymatically change secondary molecules to form complex brown melanin polymers (Zhang et al. 2022). GSTs are part of the antioxidant defense systems of plants and microbes and also participate in ROS scavenging (Dresler et al. 2014).

The effect of the plant hormones used in this study on the activity of the tested ROS response enzymes indicates that these hormones play a role in the signaling pathways that naturally regulate the activity of these enzymes. However, the results indicate that different plant hormones affect the activity of different enzymes. In addition, the effect of the hormones is in some cases only present under salt stress, suggesting that ROS produced by stress also play a role in activating the response. The variation in defense mechanisms is different in other plant species and may be due to differences in the rate of stomatal closure and, consequently, in the rate of carbon dioxide fixation. Treatment of plants with specific hormones can facilitate induction of physiological activities of ROS response enzymes, which can provide the plants with a faster and stronger reaction to stress conditions (Keshavarz et al. 2016). The increase in CAT activity in plants treated with JA (under salinity stress) may be due to improved oxygen absorption and improved nutrient uptake by the cotyledons or embryos.

Based on the current findings, proline content was increased by salinity stress, but hormone application decreased the proline content of stressed plants. This may indicate that hormone treatment alleviates the stress response. Qiu et al. (2014) showed that accumulation of proline decreased in wheat (Triticum aestivum L.) under osmotic stress, while higher pyrroline-5-carboxylate synthase (P5CS) activity in stressed peppermint plants correlated with higher proline content (Hosseini et al. 2023). Other studies have reported that JA and abscisic acid coordinate to regulate each other’s responses to stress conditions (Moradi-Ghahderijani et al. 2017; Hancioglu et al. 2019; Sabourifard et al. 2023).

The present results document that stressed plants show a significant decrease in protein content, especially under severe salt-stress conditions, compared to non-stressed plants, while treatment of plants with JA caused a remarkable enhancement of protein content compared to untreated plants. The reduction in protein content in stressed plants may be due to the building of amino acids, especially tryptophan, which is vital pathway for indole-3-acetic acid (IAA) biosynthesis (Wang et al. 2021).

Based on the current results, protein content increased with JA application. This might be attributed to the role of JA in stimulating cytokinin, amino acid, and protein biosynthesis. JA was also shown to induce expression of proteinase inhibitor genes in tomato plants (Lycopersicon esculentum L.) and genes encoding vegetative storage proteins in bitter melon (Momordica charantia) seedlings under salt stress (Alisofi et al. 2020; Zinovieva et al. 2021). JA stimulates and promotes the antioxidant defense system and other specific ROS protection mechanisms to protect plant cells from the adverse effect of salt stress. Application of exogenous JA in peppermint significantly increased the content of MDA and H2O2, indicating that JA promotes salt tolerance in plants by activation of related gene sets and enzymes by increasing the endogenous JA content and thereby triggering associated response pathways (Wang et al. 2021; Keshavarz et al. 2016) and leading to a higher content of antioxidant metabolites in peppermint.

Salinity imposed upon peppermint plants causes increased H2O2 production and this may trigger a ROS signaling response, while at the same time negatively affecting plant cells (Keshavarz et al. 2021). In this study, salinity caused lipid peroxidation and increased MDA production, which in turn disturbs the fluidity and integrity of cell membranes (Qiu et al. 2014). SA and JA are known to cause decreased accumulation of MDA and H2O2 by increasing antioxidant capacity to scavenge ROS (Keshavarz et al. 2016). In the current study, SA and JA decreased EL in stressed plants, suggesting that these hormones induced the production of organic acids such as malate, citrate, or thiol-containing compounds. This was also observed in studies involving other stresses (Dresler et al. 2014).

The data in Table 2 show that compared with 0 mM NaCl (control), mild salinity stress significantly increased the EO content in peppermint leaves. This might be due to variation in the density of glands per unit leaf area. Keshavarz et al. (2018) reported that stress conditions decreased the amount and proportion of leaves distributed on the plant, while significantly increasing monoterpenes and/or sesquiterpene biosynthesis as a result of increased gland density per leaf area. While unfavorable conditions can decrease plant growth and biomass, the increase in biosynthesis of plant secondary compounds can compensate to some extent for the loss in biomass (Keshavarz Mirzamohammadi et al. 2021b). Some studies described both positive or negative effects of plant growth regulators on plant growth and yield (Khalvandi et al. 2019). Khalvandi et al. (2019) concluded that the EO content of peppermint was improved by application of JA under saline conditions. Foliar application of JA to thyme plants (Thymus daenensis) under water-deficit conditions also caused an increase in EO content (Alavi-Samani et al. 2015). The higher EO content caused by application of JA in peppermint at all salinity levels might be attributed to a combination of increased nutrient uptake, plant biomass production, increased leaf oil gland density, and higher monoterpene biosynthesis (Hancioglu et al. 2019).

Some studies have indicated that the EO composition of peppermint varies depending on agronomic and genetic factors (Keshavarz Mirzamohammadi et al. 2021b). Additionally, water availability, temperature, biotic, and abiotic stressors can stimulate synthesis of secondary metabolites (Keshavarz Mirzamohammadi et al. 2021a). Beside these factors, it has been shown that plant hormones can change the biosynthesis of some plant bioactive components (Karami et al. 2016). The different plant hormones tested in the current study had slightly different effects on the EO compounds. Menthol is a major component of peppermint EO. Menthol is synthesized in the chloroplast and its levels may depend on the activity of carbon fixation (Loreto et al. 1996). Jasmonates are thought to induce various plant defense responses as well as affecting secondary metabolism to produce compounds that can act as antioxidant scavengers in plants (Zinovieva et al. 2021).

Accumulation of EO was correlated with the maximum percentage of volatile compounds which have a phenolic ring with an OH group (Keshavarz Mirzamohammadi et al. 2021b). According to Khalvandi et al. (2019), foliar application of JA at 50 µL caused a significant increase in some major essential oil components. The exogenous application of JA may stimulate production of some biologically active compounds. The study conducted by Zhang et al. (2022) indicated that the synthesis of some bioactive compounds such as carotene, anthocyanins, flavonoids, or vitamin C can be induced by exogenous application of JA. It is possible that these compounds are the main contributors to the antioxidant capacity.

Conclusion

The results of the current experiment show that the applied salinity stress affected all of the studied traits, and that these changes were more evident in plants with hormone application than in controls. The data suggest that application of mild and severe salinity stress induced a remarkable improvement in antioxidant activity and in proline, H2O2, and MDA contents and EL, as well as a decline in the protein content. The essential oil percentage was affected by salinity stress. In addition, a high proline content under salinity stress may be a positive method for protecting against high salinity stress and help to alleviate oxidative damage. The hormone applications showed that biochemical characteristics are most significantly influenced by SA and JA treatment, indicating that these hormone treatments could affect salinity tolerance in peppermint.