Evaluation of Thymus daenensis subsp. daenensis ecotypes using drought tolerance indices

Thymus daenensis subsp.daenensis is a well –known medicinal plant which is endemic in Iran and grows in many regions of Iran. Thymol and carvacrol are the main components of many Thymus species and thymol content in Thymus daenensis subsp.daenensis is about 70%. One of the major abiotic stresses influencing crop productivity is water deficit. Evaluation of genotypic variability under different moisture stress conditions is an essential step for a successful breeding program focused on drought tolerance. Different strategies have been suggested for selecting resistant genotypes to drought stress. Drought indices based on loss of yield under stress conditions compared to normal state have been used for screening drought resistant genotype. Many indices of drought resistance have previously been proposed for use in selection of drought resistant genotypes in various crops. Thus, the objectives of the present study were to determine the most suitable quantitative drought resistance indices and identify drought- resistant ecotypes in Thymus daenensis subsp.daenensis.

In order to determine the most suitable quantitative drought resistance indices and identify drought resistant Thymus daenensis subsp. daenensis ecotypes, two experiments under non-stress and drought stress conditions were carried out using randomized complete block design with four replications in Abarkouh at 2017-2018 cropping season. The seeds of 12 ecotypes that used in this study collected from different provinces of Iran such as Isfahan (Isfahan, Faridan, Fereydunshahr and Semirom), Markazi (Arak1, Arak2 and Shazand), Lorestan (Khoram abad1, Khoram abad2 and Aligudarz) and Hamadan (Hamadan and Malayer). In this research, drought resistance indices such as tolerance index (TOL), stress susceptibility index (SSI), mean productivity (MP), geometric mean productivity (GMP), harmonic mean (HM), stress tolerance index (STI), yield index (YI) and yield stability index (YSI) were calculated using dry yield under non-stress (Yp) and drought stress (Ys) conditions. Combined analysis of variance for dry yield was performed using the SAS software (version 9). Analysis of variance for dry yield under non-stress and stress conditions and drought resistance indices and comparison of their means using by Duncan’s test were done by the SAS software (version 9). Correlations between dry yield under non-stress and stress conditions and drought resistance indices, principle component and biplot analyses, classification of Thymus daenensis subsp..daenensis ecotypes using by ward method based on Euclidean distance were done using MINITAB (version 18) and SPSS (version 23) softwares.

The results of combined analysis of variance revealed that the differences between studied ecotypes and between two stress conditions were significant (p < 0.01) in dry yield. Analysis of variance indicated that there were significant differences (p < 0.01) between the ecotypes for Yp, Ys, and MP, GMP, HM, STI and YI indices. Correlation analysis showed that there are significant correlation (p < 0.01) between MP, GMP, HM, STI and YI (indices that were significant in analysis of variance) and dry yield under non-stress and drought stress conditions and so these indices were suggested as the most suitable indices for screening the ecotypes. Principle component analysis showed that two first components explain 99.46% of variation that exist between data. First component explained 79.42% of data variation and named yield potential and drought resistance component. Second component explained 20.04% of data variation and named drought susceptibility component. Distribution of ecotypes in biplot graph represented the genetic diversity among ecotypes for selecting them based on drought resistance. Cluster analysis using by ward method based on Euclidean distances clustered studied Thymus daenensis subsp. daenensis ecotypes in three groups and this clustering was approved by discriminant analysis. Ecotypes in cluster 1 and 3 were recognized as susceptible and resistant ecotypes, respectively.

Multivariate biplot graph indicated that Khoramabad1, Arak1, Khoramabad2, Fereydunshahr and Hamadan ecotypes were located next to the vectors of drought resistance indices, as: MP, GMP, HM, STI and YI and so these ecotypes were suggested as resistant ones. Cluster analysis showed that Khoramabad1, Arak1, Khoramabad2, Fereydunshahr and Hamadan ecotypes could be suggested as drought resistant and Arak2 and Shazand ecotypes as susceptible ones to drought stress and cross between these ecotypes can produce the maximum genetic variation and heterosis.