Variation in morphological traits among Thymus mastichina (L.) L. populations

Abstract

Thymus mastichina (L.) L. is an endemic species of the Iberian Peninsula and one of the most collected of wild plants in Spain. Despite the huge amount of possibilities for the use of T. mastichina subsp. mastichina as an industrial crop, cultivation is almost absent and no selection has been made. To identify the most useful and discriminating traits and to study the morphological variability of Spanish marjoram, a total of 16 populations were studied during 2011 and 2012 and 20 morphological traits (15 quantitative and 5 qualitative) were used for characterization. Analysis of variance, principal component analysis, correlations among traits and cluster analysis have been carried out for data processing. Intra and inter-population variability were highly statistically significant for all the quantitative traits, revealing the polymorphism of the species, although variability among populations was greater than variability within populations showing that populations are morphologically distinguished from each other. Number of flowers per flower head, number of flower heads per branch, length/width of the bract, and bract width were the most variable traits. The most valuable traits for characterization were related to the bract form and size, leaves and capitula. The ranges of morphological variation for the cultivated specimens compared with references to wild specimens in the bibliography, showed slightly larger sizes of leaves, bracts and inflorescences and smaller sizes of calyces and corollas. Cluster analysis classified the populations into four groups. Populations from group three (CLM4, TM23 and TM43) were highlighted by the biggest flower heads and they may be more productive for essential oil extraction.