On the basis of previous studies, an antioxidant enriched high yield potential genotype (Accession VA13) was selected for this investigation. This genotype was grown in pots of a rain shelter open field of Bangabandhu Sheikh Mujibur Rahman Agricultural University, Bangladesh (AEZ-28, 24° 23′ north latitude, 90° 08′ east longitude, 8.4 m.s.l.). The seeds were sown in plastic pots (15 cm in height and 40 cm length and 30 cm width) in a randomized complete block design (RCBD) with three replications. N: P₂O₅:K₂O were applied @92:48:60 kg/ha as a split dose. First, in pot soil, @46:48:60 kg ha 1 N: P₂O₅:K₂O and second, at 7 days after sowing (DAS) @46:0:0 kg/ha N: P₂O₅:K₂O. The genotype was grouped into three sets and subjected to four salinity stress treatments that are, 100 mM NaCl, 50 mM NaCl, 25 mM NaCl, and control or no saline water (NS). Pots were well irrigated with fresh water every day up to 10 days after sowing (DAS) of seeds for proper establishment and vigorous growth of seedlings. Imposition of salinity stress treatment was started at 11 DAS and continued up to 40 DAS (edible stage). Saline water (100 mM NaCl, 50 mM NaCl and 25 mM NaCl) and fresh water were applied to respective pots once a day. At 40 DAS the leaves of Amaranthus tricolor were harvested. All the parameters were measured in six samples.

At Moderate salinity stress (MSS) and Severe salinity stress (SSS) conditions, leaf color parameters and pigments, vitamins, phenolic acids, flavonoids and antioxidant capacity of A. tricolor leaves were very high compared to control condition. Hence, salt-stressed A. tricolor leaves had a good source of natural antioxidants compared to plant grown in normal cultivation practices.

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.

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.