Evaluation of Freezing Tolerance of Some Thyme (Thymus spp.) Species in Green Space

Only one-third of the total land area on earth is free of ice and 42% of land experiences temperatures below −20°C. Plants may or may not establish in extreme cold or hot temperatures, prolonged dry or wet soil conditions. For this reason, there is a growing demand for native plants in the landscape. Factors influencing plant selection include assessments of hardiness and aesthetics, ease of propagation and culture, naturalization potential. There is a high genetic variation in the flora of Iran, about 1810 of 8000 recorded taxons in Iran are native. So there is a great potential for using native plant in regions with harsh climates. The genus Thymus L. belongs to the Lamiaceae family, consist of about 215 species of herbaceous perennials and small shrubs in the world. They originated from Mediterranean region. This genus is presented in Iranian flora by 14 species .Overall, cold acclimation results in protection and stabilization of the integrity of cellular membranes, enhancement of the antioxidative mechanisms, increased intercellular sugar levels as well as accumulation of other cryoprotectants including polyamines that protect the intracellular proteins by inducing the genes encoding molecular chaperones. So, the aim of this study was to assess the freezing stress tolerance of some thyme species as ground covering species in landscaping projects Seeds of Theme species were sown in containers filled with sand, manure, field soil mixture and maintained in glasshouse. Then, the plants transferred to outdoor condition until the late May. At 24 hours before the end of the acclimation period, the plants irrigated and transferred in their containers to a freezing chamber with 3°C for subjection to freezing temperatures (-10, -20 and -30oC). The temperature reduced at the rate of 2°C per h, and after 1 hour at exposure to the freezing temperature, the containers with plants immediately transferred to growth chamber at 4oC for 24 hours to reduce the speed of ice melting and then returned to the glass. Survival rates, proline accumulation, electrolyte leakage, antioxidant enzymes activity were used to evaluate the freezing tolerance of species. For field experiment germinated seeds in container maintained in greenhouse until the environmental conditions were not limiting factor. After that, seedling planted in outdoor condition and subjected to Ardabil city freezing weather in 2 years. In early spring survival rate recorded. Moreover physiological and biochemical and physical responses of species were evaluated in 3 different times (June, December, February).  Results showed that freezing stress adversely affects growth of theme species. There were 100% survival in all species under Ardebil outdoor condition (during two years), but under freezing chamber condition plant treated with -10oC and -30oC showed 100% and 0% survival, respectively. While in -20oC the highest survival rate were shown in T. vulgaris and T. sepyllum, but T. daenensis had the lowest. The lowest values of electrolyte leakage were related to T. sepyllum, T. kotschyanus, and T. vulgaris. The highest chlorophyll and proline content achieved in T. vulgaris and T. kotschyanus, respectively. Peroxidase antioxidant enzyme activity in T. kotschyanus and T. sepyllum were higher than other species. While there were no significant differences among the species in superoxide dismutase and ascorbate peroxidase enzyme activity. According to the results, it can be deduced that T. kotschyanus, T. vulgaris, and T. sepyllum are more frost tolerant than others. However, because of high survival percent in Ardabil climate and under -20oC, all species can be used as frost tolerant ground cover in landscapes. According to the results all of the studied thyme species are frost tolerant but T. sepyllum, T. kotschyanus, and T. vulgaris have the ability to withstand the harsh weather condition without a considerable loss in their ornamental potential.