Genetic analysis of some quantitative traits in Thymus daenensis Celak under salinity stress using Polycross test

Cultivation development of adaptable Thymus daenensis Celak to specific environmental conditions through population improvement requires the creation of a genetic pool rich of superior genotypes and exploitation of this diversity to identify genotypes with high combinability. In this research, to study the genetic diversity and combinability of different T. daenensis ecotypes under salinity, a factorial greenhouse experiment was conducted in a randomized complete block design with three replications. The factors included salinity stress (0 and 90 mM NaCl) and ecotype (12 half-sib families derived from a polycross between 12 T. daenensis ecotypes). Salinity decreased all the measured traits (except for proline and carotenoids content) significantly compared to the control. The ecotypes differed significantly from each other in terms of length and leaf width, length of lateral branches, aerial parts dry weight, essential oil content, and proline content. Essential oil content and leaf width obtained the highest phenotypic and genotypic coefficient of variation. General heritability of the studied traits varied from 4.61% (leaf length) to 81.07% (leaf width). The highest general heritability was assigned to leaf width, essential oil content, and aerial parts dry weight. Regarding the higher genotypic and phenotypic coefficient of variation, heritability, and genetic advance of the traits including essential oil content and aerial parts dry weight, these traits could be considered as the most important criteria for the parents selection in the breeding programs of T. daenensis. The results showed that the ecotypes Malayer2, Zagheh, Arak, and Ilam had high combinability in terms of essential oil content. Considering the sufficient genetic diversity observed between the studied ecotypes along with their adequate combinability, it could be recommended to breed T. daenensis through the strategy of improving the population by producing synthetic cultivars for normal (non-stress) and salinity conditions.