The seeds were germinated in seedling plastic tray filled up with a mixture of peat moss and coco peat (1:1). The trays were placed in a glass house at the University of Massachusetts, Amherst, USA, in September 2015. After two weeks, seedlings were transplanted into plastic pots (30 cm diameter and 30 cm height, three seedlings per pot). The pots growth media consisted of 2:1:1 ratio of sieved field soil, sand, and leaf mold, respectively. The medium in each pot was supplemented with 5.8 mg P, 3.3 mg N, and 13.8 mg K to warrant the plant growth; the electrical conductivity was 0.3 mS/cm (Beckman EC meter instrument. cedar grove, New jersey, USA); the pH was 8.04. Plants were maintained at the temperature of 18-25 ℃ ; glass house conditions were set as follows: photoperiod, 16 h; relative humidity, 60-70%; light intensity, 180 µmol m 2 s^-1. The source of light was a high pressure sodium lamp in addition to the day light in order to attain 16 h of light per day. Fourty days after planting (establishment period), the uniform sized plants were treated with four different salinity levels including: 0, 30, 60, and 90 mM NaCl. The irrigation with saline solution (250 mL in each pots) was performed every two days . Total amount of NaCl per pot during experiment was 8.75 L. To avoid osmotic shock the salinity treatment started with 15 mM NaCl, and was progressively increased (every two days) to reach the maximum salinity level in each treatment. To prevent water and nutrient leaching, a plastic dish was inserted under each pot and the leached water was given back to pots.

Salinity has negative impact on plant production and stimulates several physiological and biochemical modifications in thyme species. Here, a major decline in dry matter, relative water content (RWC), photosynthetic pigment contents, K⁺, Ca⁺ as well as increase in Na⁺ and EL was observed. Our work showed that T. daenensis was similar to T.vulgaris in terms of tolerance to high levels of salinity stress and both species were moderately tolerant to severe salt stress. It should be noted that T. daenensis, as an Iranian endemic species, competes with commercial species like T. vulgaris as a rich source of phenolic compounds. Furthermore, the relative salt tolerance of both species could be related to the exclusion of Na⁺ from the vascular system in order to protect tissues from salt toxicity and also to the increase of phenolic content and radical scavenging activity. Thus, the plant behavior under salinity stress may be used to boost the production of bioactive compounds to be used on an industrial level for the manufacture of nutraceuticals, functional foods and cosmetics.