Effect of salinity stress on the physiological characteristics, phenolic compounds and antioxidant activity of Thymus vulgaris L. and Thymus daenensis Celak

Salinity stress is one of the most important challenges in agriculture systems. The present work was aimed to evaluated the effects of saline irrigation, using different NaCl concentrations (0, 30, 60, and 90 mM), on growth, physiological characteristics, phenolic compounds, and antioxidant activity of Thymus vulgaris and Thymus daenensis, that are two of the most important medicinal and aromatic plants in Iran. An analytical method based on HPLC-DAD has been developed to identify the main metabolites (mainly phenolic compounds) in thyme extracts. The results indicated that concentrations of 60 and 90 mM NaCl significantly decreased the plant dry matter production by about 28 and 40% in T. vulgaris and 34 and 39% in T. daenensis, respectively, compared with untreated plants. Application of NaCl also induced an increase in Na+content in shoots and leaves while the K+ and Ca2+contents decreased with the salinity stress. Total phenolic content increased by around 20% after the application of 60 mM NaCl, compared with control plants. Also, an increase in leaf flavonoid content by 38.6% and 36.6% was observed in plants grown under salt stress conditions after the application of 60 and 90 mM NaCl, respectively. Both investigated species were rich in phenolic acids, flavonoids, and phenolic monoterpenes. Cinnamic acid was the major constituent in both species. This compound increased by 31.4% in T. vulgaris treated with 60 mM NaCl compared with the control. Concentrations of 60 and 90 mM NaCl increased significantly the amount of gallic and rosmarinic acids in T. vulgaris (25 and 31.6%, respectively) and T. daenensis (20.4 and 27.6%, respectively). On the other hand, the mentioned NaCl concentrations did not significantly change the content of caffeic, syringic, and vanillic acids in T. vulgaris. Interestingly, the higher amount of chlorogenic acid (increase of 4.4%) in T. vulgaris was found under control condition, whereas in T. daenensis this compound increased when salinity stress intensified. The main outcome of the present study was the enhancement of thyme phenolic content under salt stress treatments. Furthermore, salinity stress significantly influenced the antioxidant activities of plant extracts and may be recommended as an appropriate approach to enhance the antioxidant capacity of medicinal plants. In conclusion, the obtained data demonstrated that salinity stress decreased significantly the yield of both species, whereas improved the phenolic contents and antioxidant capacity of thyme plants.