Ornamental potential and freezing tolerance of six Thymus spp. species as ground-covering plants in the landscape

Abstract

© 2018, Lithuanian Institute of Agriculture. All rights reserved. The current study looks at the possibility of using three native (Thymus kotschyanus Boiss. and Hohen, T. fedtschenkoi Ronniger and T. pubescence Boiss & Kotschy ex Celak), one endemic (T. daenensis Jalas) and two common (T. serpyllum L. and T. vulgaris L.) thyme (Thymus spp.) species as ground-covering plants in landscaping. The ornamental potential and freezing tolerance of the species were examined over a period of two years from 2014 to 2016. T. kotschyanus indicated some important ornamental characteristics, including early flowering habit, long flowering period, more inflorescences per plant, larger flowers, relatively high area coverage and a higher ranking in visual assessment. In contrast, T. serpyllum was observed to have more flowers per inflorescence, more and long inflorescences, high visual assessment ranking and area coverage. The survival rates of the plants put under freezing chamber conditions at -10°C and -30°C were 100% and 0%, respectively, while at -20°C the highest survival rates were observed only in T. vulgaris and T. serpyllum. Moreover, all species had 100% survival rate in harsh winter conditions of Ardabil Province, Iran. The results of electrolyte leakage measurements demonstrated that among all species, T. serpyllum, T. kotschyanus and T. vulgaris had the highest level of hardiness against the cold. The highest proline contents were also observed in T. vulgaris and T. kotschyanus, respectively. The activity of peroxidase antioxidant enzyme in T. vulgaris and T. serpyllum did not change in response to freezing stress. However, it decreased in T. kotschyanus by increasing the stress severity. No significant difference was observed in the superoxide dismutase and ascorbate peroxidase enzyme activities of the species. Overall, all the examined species showed valuable ornamental characteristics and freezing stress tolerance. However, only T. kotschyanus, T. serpyllum and T. vulgaris were suggested to be used for landscaping programs.