Effect of salinity stress on some morpho-physiological traits and quantity and quality of essential oils in Peppermint (Mentha piperita L.)

Introduction Salinity stress is one of the main environmental stress limiting growth, development and economical yield of plants in different areas of the world, especially in the countries which are located in the arid and semi-arid regions. The extent of damage to the plant and reduction in yield heavily depends on the level of salinity in soil or irrigation water. Reducing growth and morphological development at the initial stages of the stress and accelerate tissue senescence at later stages will result in reduced agricultural productivity. Plant species and cultivars vary in their tolerance to salinity and ability to maintain physiological functions and yield production under stress. Increasing the concentration of salts in the soil solution usually leads to osmotic stress, specific ion toxicity and ionic imbalances and consequently decreases in plant growth. Peppermint (Mentha piperita L.) is one of the most important medicinal plants and its essential oils are widely used in pharmaceutical, food, cosmetics and hygienic products. This plant is a popular herb that can be used in different forms such as oil, dried powder and leaf extract. Peppermint is comprised of a high amount of natural essential oils, as well as menthol and Menthone. It is known for its refreshing, cooling, bactericidal and anti-irritant properties. The concentrations of various secondary metabolites in different plant species are strongly depended on the growth conditions, especially stress conditions. Although the negative effect of salinity stress on essential oil yield is well documented but the effects of salinity stress on chemical composition of essential oil is not predictable and constant. Therefore in the present study, we have evaluated the effects of the different levels of salinity stress (0, 2, 4 and 6 dS/m) on morpho-physiological traits and also essential oil components of Peppermint. Materials and methods In order to evaluate the effects of salinity stress on morpho-physiological parameters and essential oil of peppermint an experiment was conducted based on completely randomized design with five replications and four treatments. The treatments were included different levels of salinity (i.e. 0, 2, 4 and 6 dS/m). Studied traits were relative water content, leaf area, the photosynthetic pigments and dry weight (root, shoot and leaf). Essential oil component were determined with gas chromatography-mass spectrometry (GC-MS). Analysis of variance (ANOVA) was done by SAS 9.4 software and mean comparisons were done by Duncan’s multiple range test (DMRT) at 1% probability. Results According to the results, different levels of salinity stress had significant effects on the studied traits at a level of 1% probability. The highest amount of the relative water content (67.6%), plant leaf area (1609 mm), root dry weight (12.4 g), shoot dry weight (17.1 g), leaf dry weight (17.4 g), Chlorophyll a (2.84 mg/g FW), Chlorophyll b (1.62 mg/g FW), total Chlorophyll (4.46 mg/g FW), carotenoid (1.04 mg/g FW) and essential oil (0.23%) were obtained from control while the lowest amount of the relative water content (59.1), leaf area (1076 mm), root dry weight (7.34 g), shoot dry weight (8.72 g), leaf dry weight (9.18 g), Chlorophyll a (1.82 mg/g FW), Chlorophyll b (0.94 mg/g FW), total Chlorophyll (2.77 mg/g FW), carotenoid (0.74 mg/g FW) and essential oil (0.13%) were obtained from 6 ds/m2. The results showed that with increasing salinity stress, the concentration of most of the essential oil components such as Menthol, D-Limonen, α-pinene, Sabinene, Menthofuran, 1,8-Cineole were reduced but Menthone was an exception where its content increased under salinity stress.. Conclusion Increasing the salinity stress significantly impressed all of the studied morpho-physiological traits in Peppermint and reduced them. Although most of the essential oil components were decreased under salinity stress but some components of essential oil such as Menthon were increased under salinity stress conditions. Reducing biological yield and also essential oil content will cause significant decrease of essential oil yield per unite area.