Linking volatile profile, glandular trichomes and pollinators in Salvia verticillata L. (Lamiaceae)

For plants, due to their immobility, the release of volatile organic compounds (VOCs) provides a way to interact with organisms across distances, providing a detailed language for communication. In this context, and as part of a four-year research project (IRIS identification - project 2014-PDF-0363) focused on a selection of Salvia species (Lamiaceae) preserved at the Ghirardi Botanic Garden (Toscolano Maderno, Bs), we addressed our attention on Salvia verticillata L. (lilac, whorled sage). This is a herbaceous perennial species native to central and eastern Europe and western Asia (1), widely cultivated for ornamental purposes. It is mainly pollinated by bees and flies (2). The primary goal of this work is gaining the basic information to tentatively decipher the content and meaning of the chemical messages. Therefore, we specifically analysed the phytochemistry of the VOCs spontaneously emitted by leaves and flowers. Since in the Lamiaceae family the biosynthesis of volatiles occurs in glandular trichomes, we also combined a detailed micromorphological investigation on their distribution and histochemistry. In addition, focal plant observations on the pollinators spectrum were performed. Vegetative and reproductive organs of S. verticillata are thickly covered by glandular hairs (3), belonging to two main morphotypes: large peltate and small capitate trichomes. The former are uniformly arranged on the abaxial sides of leaves, sepals and petals, with the latter scattered among them. The capitate types are abundant on the leaf adaxial surface, whereas calyx and corolla adaxial ones are hairless. The histochemical investigation revealed the presence of heterogeneous chemical composition within the storing chambers: both hair types are terpene producers, jointly with muco-polysaccharides in peltate and flavonoids in capitate trichomes. On the leaf, peltate trichome productivity is limited to the early developmental stage of the organ, whereas on the flower, their secretory activity last up to the maturity of the calyx and the corolla. The small capitate trichomes are active during all the stages of development of the organ bearing them. The VOC emission profiles of leaves and flowers are greatly variable. The flower emission showed a higher complexity, with more constituents than the leaves. Leaf profile exhibits 20 exclusive VOCs, dominated by germacrene D (38.39%). Flower profile is characterized by 37 exclusive VOCs, among which 1,8-cineole (10.39%) is the most abundant one. Sesquiterpene hydrocarbons are the major chemical class in both profiles (83.31% in leaves and 73.66% in flowers) and 19 common VOCs were detected: β-caryophyllene, bicyclogermacrene, δ-elemene, β-bourbonene and γ-muurolene are the dominant constituents, with the former occurring in significant amounts (21.33% in leaves, 15.48% in flowers) and the others in divergent relative abundances, that differ of about 6.0 up to 15.0%. The target species attracted mostly bees belonging to two functional groups: medium-size bees (mainly honeybees) and large bees (mainly bumblebees). Medium-size bees were three times more frequent than large bees. Pollinators visited on average 29.1 flowers, out of the 56.5 on anthesis (i.e. available to pollination), the collected resource being always nectar. The peltate trichomes are widespread on the whole plant surface and display similar histochemical features regardless of their distribution pattern; however the different duration of their secretory activity on the vegetative and reproductive organs suggest diverse ecological roles: they might function in repelling predators and pathogens on young leaves and in alluring specific pollinators on mature flowers. The co-occurrence of the small capitate trichomes, their predominance on leaves and the productivity in flavonoids support the hypothesis of a synergistic defensive role. The VOC profiles can be associated to literature data concerning chemical compounds and biotic responses: germacrene D, detected only in the leaves, and β-caryophyllene are common deterrents (4). Conversely, an attractive role towards bees is documented only for 1,8-cineole (4), exclusive of the flowers. Plant strong attractiveness towards Hymenoptera is certainly confirmed. Experiments are underway to understand if and to what extent the different floral whorls and the floral resources play a role in the strategies of pollinators attraction.