Congruence of Phytochemical and Morphological Profiles along an Altitudinal Gradient in Origanum vulgare ssp vulgare from Venetian Region (NE Italy)

Plants of Origanum vulgare L. ssp. vulgare from the Veneto region (NE Italy) were selected to study the variability of the essential-oil composition from leaves and inflorescences throughout an elevation gradient. We investigated also the morphology of non-glandular and glandular trichomes, their distribution on the vegetative and reproductive organs, as well as the histochemistry of the secreted products, with special focus on the terpenoidic fraction. Since glandular trichomes are prerequisite for the essential-oil synthesis, the second objective was to establish whether its production is related to glandular hair number, and density. Essential-oil contents decline with increasing altitude, and the micromorphological observations revealed a decrease in trichome density along the same direction. Moreover, GC/MS analysis together with principal component analysis (PCA) showed that the three investigated populations were significantly different in chemical composition. Therefore, an important interpopulation variability for low-, mid-, and high-altitude sites was established, suggesting the likely occurrence of different biotypes associated with altitudinal levels. Hence, the involvement of abiotic factors such as temperature and drought in the chemical polymorphism of O. vulgare associated with elevation is briefly discussed.