Micromorphological and phytochemical survey on Ballota acetabulosa (L.) Benth

Within the Open Science project entitled "Botanic Garden, factory of molecules", a multidisciplinary study approach was applied on Ballota acetabulosa (L.) Benth., preserved at the Ghirardi Botanic Garden (Toscolano Maderno, BS, Italy). Micromorphological, histochemical, and phytochemical investigations were conducted on the vegetative and reproductive organs. For the first time, this survey reported the histochemical assays and the ultrastructural observations on the secreting structures, as well as the characterization of the volatiles spontaneously emitted from leaves and flowers. Four trichome morphotypes were identified: peltate and short-stalked, medium-stalked, and long-stalked capitate trichomes, each with a peculiar distribution pattern. The histochemical analysis was confirmed by the ultrastructural observations, with the peltates and long-stalked capitates as the main sites responsible for terpene production. The head-space characterization revealed that sesquiterpene hydrocarbons prevailed both in leaves and flowers, with γ-muurolene, β-caryophyllene, and (E)-nerolidol as the most abundant compounds. Moreover, a comparison with literature data concerning the ecological roles of the main compounds suggested a dominant defensive action both at the leaf and flower level. Therefore, we correlated the trichome morphotypes with the production of secondary metabolites, in the attempt to link these data to their potential ecological meanings. Finally, we made the obtained scientific knowledge available to the visitors of the Botanic Garden through the realization of a new labelling dedicated to B. acetabulosa that highlights the "invisible", microscopic features of the plant.