Evaluation of morphological characteristics, glandular trichomes structure and their relationship with essential oil content in four species of Thyme (Thymus spp.)

The leaves of thyme are the most important organs producing essential oils, and the glandular trichomes provide a unique capacity for the synthesis and storage of active ingredients. In this study, the morphological characteristics and types of trichomes' structure were measured in four thyme species including: T. transcaucasicus, T. carmanicus, T. daenensis and T. armeniacus. The results of variance analysis indicated a significant difference between species in terms of stem length, leaf length and number, plant fresh and dry weight, leaf and stem dry weight, number of glandular trichomes on the adaxial and abaxial side of the leaf, the trichome length, and essential oil yield and percentage. T. armeniacus showed the highest trichome number; while, the highest trichome length was related to T. transcaucasicus. The highest essential oil percentage and yield were observed in T. carmanicus and T. daenensis respectively, which could be due to the greater number of glandular trichomes and leaves in these species. The results of scanning electron microscope (SEM) revealed that among various types of glandular trichomes, peltate glandular trichome existed in the leaf of four species examined. Positive and significant correlation coefficients between the number of glandular trichomes on the adaxial and abaxial side of the leaves with the essential oil percentage and yield indicate the role of this feature in the production of essential oil. The stepwise regression analysis, implemented for dry weight, indicated that the highest regression positive coefficient was observed for the leaf dry weight, explaining 97% of the trait variation. When the essential oil yield was assumed as a dependent variable, only the trichome length entered the model and explained 49% of the trait variation.