عنوان مقاله [English]
Introduction: Thyme is a perennial plant belongs to Lamiaceae. Thymus is a well–known medicinal plant, native to Southern Europe and its essential oil is manufactured commercially for use in cough drops, mouth-washes, liniments, toothpastes, detergents and perfumes. The herb is approved by Commission E in the treatment of bronchitis, whooping cough and upper respiratory inflammation. The Thymus genus comprises over 300 species of which, 18 are found in Iran, which grow wild in many regions and four of them are endemic. There are serious difficulties in the taxonomical interpretation of the taxa belonging to the genus Thymus owing to the high variability of populations with respect to many morphological and micro-morphological traits. The morphology and different components of essential oils in different species of Thymus are variable due to hybridization and polyploidization, despite its rare self-pollination. In general, intraspecific hybrids of the genus Thymus seem to possess intermediate morphological characteristics and composition of essential oil in comparison with the relevant characteristics of the parent plants. The accumulation of the essential oil takes place in gland hairs, which are distributed on the surface of the epidermis of the aerial parts of the plant. The content of the essential oil can depend on origin, climate, harvest, as well as drying and storage conditions. By carefully examining the structural, phenotypic, chemical and genetic aspects of the natural populations of a plant, it is possible to select either them or samples from them as an important step in the process of breeding the desired medicinal plant.
Material and Methods: In order to have a comparison microscopically and morphologically characteristics of Thymus vulgaris, two landraces of Thymus transcaspicus were studied in completely randomized design with four replications at the faculty of Agriculture in Ferdowsi University of Mashhad. Inflorescence length, number of flowers per inflorescence, shoot height, leaf, bud and flower length, leaf width, the largest diameter of canopy, weight, fresh and dry herb yield, percent of essential oil, distinct and type of trichomes, were measured. For glands morphological and structure analysis leaves were processed according to conventional techniques for scanning electron microscopy (SEM). In order to obtain comparable results the leaves from the third node of the apex were used for SEM analyses. Leaf samples were fixed in glutaraldehyde and paraformaldehyde. After being dehydrated, they were coated with gold in the vacuum evaporator. Finally, the preparations were observed and photographed with a KYKY-2800B SEM.
Results and Discussion: The ANOVA of the data showed that there was considerable variation for all studied characters. Results of mean comparisons showed that the highest height, the largest diameter of canopy and percent of essential oil were for T. vulgaris. Maximum inflorescence length, number of flowers per inflorescence, leaf, bud and flower length, leaf width, was also recorded for T. transcaspicus. Results of essential oil (EOs) analysis by GC/MS showed that thymol and carvacrol were the major compounds of all EOs samples, with highest and lowest content observed thymol and carvacrol in T. vulgaris (64.75 and 6.78 %). T. vulgaris produced more essential oil as compared to another thyme species. Two types of glandular trichomes are distributed on the outside surface of leaf, namely, peltate ones (PGTs) and capitate ones (CGTs). The glandular hairs of Lamiaceae likely release various secretions, such as the essential oil and polysaccharide. When trichomes got matured, essential oil could be found on all of the glandular trichomes on aerial organs of Thymus. Besides essential oil, what else trichomes could secrete needs further study.
Conclusion: The essential oil is highly complex chemical compounds. The content and composition of it vary depending on season, growing conditions, and plant species. High essential oil content is related to a much secretion of glandular trichomes. An application for variety protection was filed for ‘T. vulgaris’, now recommended to producers in Mashhad interested in a thymol-rich thyme. We can carry out a selection program to optimize quality and yield of thyme (T. transcaspicus L.) landraces, with used of T. vulgaris and crossing with clones of T. transcaspicus.