Document Type : Research Paper

Authors

• Maryam Alipour
• Maryam Haghighi
• Mehdi Rahimmalek

Department of Horticulture, College of Agriculture, Isfahan University of Technology, Isfahan, Iran

Abstract

     Background and objectives: Thyme (Thymus spp.) is an important medicinal and aromatic plant belonging to the Lamiaceae family. It is highly valued for its phenolic compounds and essential oils, which are widely used in the pharmaceutical, food, and cosmetic industries. The genus comprises more than 200 species and exhibits considerable genetic diversity in Iran, providing substantial potential for breeding programs. Drought is a major constraint on thyme production in semi-arid regions and can influence plant growth and essential oil composition. Quantitative indices for assessing drought tolerance, including the stress tolerance index (STI), geometric mean productivity (GMP), mean productivity (MP), harmonic mean (HM), and yield stability index (YSI), can be used to identify drought-tolerant genotypes. Previous studies on different Thymus species demonstrated that GMP and STI exhibit stronger correlations with biomass and essential oil yield than some other indices. Therefore, evaluating different populations under both drought-stress and non-stress conditions is essential for identifying superior genotypes with desirable agronomic and phytochemical characteristics.
Methodology: This study evaluated ten thyme (Thymus spp.) populations, comprising nine native Iranian populations and Th. vulgaris from Europe as a reference population. The experiment was conducted in pots under greenhouse conditions with natural light and a relative humidity of 65%. A two-factor factorial experiment was arranged in a completely randomized design (CRD) with three replications, each containing five pots per treatment. The potting substrate consisted of a mixture of field soil, sand, and well-rotted manure in a 1:2:3 ratio, with a total substrate weight of 5 kg per pot. Irrigation was managed according to soil field capacity and adjusted daily by weighing the pots. Two irrigation treatments were applied: 100% of field capacity as the control and 40% of field capacity as the drought-stress treatment. Aboveground dry biomass was harvested 50 days after the application of drought stress and subsequently dried. Essential oils were extracted from 100 g of dried plant material using hydrodistillation, and essential oil content was determined as a volumetric percentage. Quantitative drought tolerance indices, including STI, GMP, MP, HM, and the stress susceptibility index (SSI), were calculated. Analysis of variance and mean comparisons using the least significant difference (LSD) test were performed in SAS software. Correlation coefficients, cluster analysis using Ward’s method, and biplot diagrams were also employed to assess relationships among the measured traits and classify the populations according to their responses to drought stress.
Results: The results indicated that population, irrigation regime, and their interaction significantly affected aboveground dry biomass, essential oil content, and essential oil yield at the 5% significance level. Drought stress significantly reduced dry biomass in all populations, although the magnitude of the reduction varied among them. Th. pubescens, Th. kotschyanus-2, and Th. vulgaris exhibited the smallest reductions in biomass under drought stress. In contrast, essential oil content increased in most populations under drought conditions, with the highest values recorded in Th. fedtschenkoi and Th. kotschyanus-2. This increase may be attributable to the concentration of secondary metabolites associated with reduced vegetative growth. Analysis of quantitative drought tolerance indices revealed significant differences in drought stress among populations. Based on STI, GMP, HM, the yield index (YI), and the drought resistance index (DI), Th. kotschyanus-2 and Th. fedtschenkoi were identified as the most drought-tolerant populations, whereas Th. daenensis-1 and Th. daenensis-2 were the most sensitive. Principal component analysis showed that the first two principal components explained 57.98% of the total variation among the populations. The first component was primarily associated with drought tolerance indices, whereas the second was associated with sensitivity indices. The biplot and cluster analysis jointly classified Th. kotschyanus-2, Th. pubescens, and Th. vulgaris as high-performing, drought-tolerant populations. In contrast, Th. daenensis-1, Th. daenensis-2, Th. serpyllum, and Th. migricus were classified as drought-sensitive populations with relatively low performance.
Conclusion: Overall, the findings indicated that thyme populations responded differently to drought stress, with some populations maintaining relatively high performance or increasing essential oil content despite reduced vegetative growth. The STI, GMP, and HM indices were identified as the most reliable criteria for selecting drought-tolerant populations. Based on these indices, Th. kotschyanus-2 and Th. fedtschenkoi were identified as promising candidates for cultivation under drought conditions. These populations may therefore be valuable genetic resources for breeding programs aimed at improving drought tolerance and supporting the sustainable cultivation of thyme in arid and semi-arid regions

Keywords

• Thyme
• drought stress
• drought tolerance indices
• essential oil yield
• genetic diversity

Main Subjects

• Identification, introduction, ecology and ethnobotany