Essential oils, amino acids and polyphenols changes in salt-stressed Thymus vulgaris exposed to open–field and shade enclosure

• Different microclimates such as greenhouse or open-field affect the yield of thyme.
• The highest production of thymol was obtained with highest level of salt and under open-field.
• Both high light and salinity influence the content of EOs, phenolics and amino acid of thyme leaves.

The aim of this research was to investigate the effect of two production systems, open-field and shade-enclosure, used together with two NaCl doses to find the best environmental option with optimum composition of some valuable compounds in thyme plant (Thymus vulgaris). The plants were evaluated after two and four weeks of treatments for growth, essential oils (EOs) composition as well as for the amino acids, total polyphenol and flavonoids contents. The results showed that thyme plants grow better under shade-enclosure conditions. The application of 50 mM NaCl in the medium did not affect the thyme growth under both production conditions. However, with 150 mM NaCl the highest decrease in the plant growth was shown under shade enclosure conditions. Thymol, β-cymene, myrcene, linalool and camphene were the dominant EOs of thyme leaves. According to our results, β-cymene and α-thujene was three times more in plants cultivated under open-field conditions when compared to shade enclosure conditions after two weeks. In contrast, myrcene was more than four times higher in open field conditions. Furthermore, for thyme plants cultivated under open field conditions, the increase in the medium salinity have increased considerably thymol, the main EO, to reach 49% of the total in the plants treated with 150 mM NaCl for four weeks. However, the linalool and β-cymene percentages have decreased with increasing salinity but only under shade enclosure conditions. Still under open-field the β-cymene increased considerably with increasing the salinity of the medium. Besides, the applied light intensity in combination with NaCl treatments had also a pronounced effect on the amino acids, the polyphenols and flavonoids contents in thyme leaves. It was concluded that different production systems used together with NaCl salinity may be carefully considered in the cultivation of thyme species to get the desired concentrations of some primary and secondary metabolites.