Ultrastructural insight into terpene-producing trichomes and essential oil profile in Salvia greggii A. Gray

Micromorphological investigations on the terpene-producing trichomes at the active secretory stage were performed on leaves and flowers of the Mexican Salvia greggii A. Gray (Lamiaceae), cultivated in Italy. Two types of glandular trichomes are described and compared: peltate, widespread on both the vegetative and the reproductive organs, and medium-stalked capitate, typical of the calyx abaxial side. The histochemical evidences indicate an exclusive terpene production for both morphotypes. TEM survey highlighted several common ultrastructural features that validated the light microscope observations, however a great variability was detected at the plastidome level. Different plastid populations characterized each trichome type, also in relation to the different distribution pattern on the plant epidermis. In the peltates plastids invariably displayed a well-developed internal membrane system and various types of osmiophilic inclusions: on the leaf and the calyx plastoglobuli were also observed, whereas on the corolla abundant starch grains. True leucoplast are only detected in the calyx capitate trichomes. 49 compounds were detected in the essential oil (EO) obtained from the aerial parts, with terpenes representing 98.76% of the total EO, besides negligible amounts of non-terpene derivatives and apocarotenoids. Sesquiterpenes and monoterpenes occurred in similar relative abundances: 52.7% versus 46.1%, respectively. Among sesquiterpenes, γ-muurolene (11.48%) and guaiol (6.15%) dominated for hydrocarbons and oxygenated compounds, respectively. β-pinene (20.24%) prevailed among the monoterpene hydrocarbons, resulting also as the most abundant compound in the essential oil. 1,8-cineole (11.00%) is the dominant compound of the oxygenated monoterpenes. The ultrastructural appearance of the plastidome was linked with the various terpene classes present in the EO profile. We argued that monoterpenes were probably synthetized in the calyx capitate trichomes and the sesquiterpenes in the peltates.