Mohsen Ashrafi - Department of Agronomy and Plant Breeding, Faculty of Agriculture, University of Zanjan, Zanjan, Iran.
Mohammad Reza Azimi Moqadam - Department of Agronomy and Plant Breeding, Faculty of Agriculture, University of Zanjan, Zanjan, Iran.
Parviz Moradi - Research Division of Natural Resources, Zanjan Agricultural and Natural Resources Research and Education Centre, AREEO, Zanjan, Iran.
Farid Shekari - Department of Agronomy and Plant Breeding, Faculty of Agriculture, University of Zanjan, Zanjan, Iran.

Drought as the most important abiotic stress has deleterious effects on plants. Developing drought tolerant varieties can help produce plants in a sustainable way. This study was conducted to identify drought tolerant and drought sensitive thyme species including Thymus vulgaris, T. vulgaris (origin: Spain), T. carmanicus, T. daenensis and T. kotschyanus and to study the mechanism used by them to cope with drought stress. For this purpose, relative water content, water use efficiency, soil water depilation rate, root:shoot ratio, drought resistance index and a new criterion FC ceased growth were used. T. carmanicus and T. daenensis had the lowest and the highest reduction on relative water content, respectively. In terms of water use efficiency and soil water depletion curve, the highest and the lowest values were detected for T. daenensis and T. carmanicus, respectively. The most and the least root:shoot ratios were recorded for T. daenensis and T. vulgaris (origin: Spain), respectively. Analyses by drought resistance index and PCA revealed that T. carmanicus is drought susceptible, T. kotschyanus and T. vulgaris are semi-drought susceptible, and T. daenensis and T. vulgaris (origin: Spain) are semi-drought tolerant species. FC ceased growth analysis showed that T. carmanicus stopped its growth at higher FC, while T. kotschyanus stopped it at lower FC. Therefore, based on this criterion and considering the sustainability of growth under drought condition, T. carmanicus and T. kotschyanus are the least and the most drought tolerant Thymus species.

Thyme, Breeding, water deficiency, agriculture, Screening, drought tolerance