Investigation of defensive responses of Thymus vulgaris L. seedlings to drought stress under in vitro conditions

Thymus vulgaris is a plant of the Lamiaceae family, with numerous medicinal, antioxidant and nutritional properties. Growth and crop productivity and antioxidant activities of the plant are affected by many environmental factors such as drought. Therefore, a controlled experiment under in vitro conditions was conducted to evaluate the effects of drought stress on growth, accumulation of compatible metabolites and antioxidant activity of thyme seedlings. Seedlings of thyme were cultured in MS medium containing 0, 2 and 4% sorbitol and mannitol for 14 days. Both sorbitol and mannitol were found to reduce the growth parameters, photosynthetic pigments but increased the proteine, secondary metabolites and catalase and ascorbate peroxidase activity. Compared to the control, both treatments led to a significant decrease in superoxide dismutase activity at 4% while no significant differences were observed between control and mild stress (2%). Moreover, the results indicated that the accumulation of compatible metabolites, proline and the reducing sugar contents in leaves and roots, increased significantly with increasing drought concentration in the culture media. Increased compatible metabolites, secondary metabolites and antioxidant enzymesactivities could consider as a defensive reaction of thyme against drought stress.