Ultrastructural evaluation of the glandular indumentum in selected Lamiaceae species

This work is part of a two-year research project (IRIS identification - project 2014-PDF-0363) focused on the plant heritage preserved at the Ghirardi Botanic Garden of Toscolano Maderno (Bs), which have a long tradition in the study and conservation of medicinal species. We presented the results of an extensive ultrastructural investigation on the glandular indumentum of both the vegetative and reproductive organs in selected target-species belonging to the Lamiaceae family: Ballota acetabulosa Benth., Ballota rupestris Vis., Lavandula angustifolia Mill., Lavandula dentata L., Salvia greggii Grey, Salvia uliginosa Benth., Salvia verticillata Benth., Scutellaria sieberi Benth., Scutellaria altissima L. and Scutellaria caucasica A.Ham. The investigation showed the occurrence of different types of glandular trichomes: (i) peltate trichome, present on leaves and inflorescences of all of the species. It is constituted by a basal epidermal cell, a neck-cell and by a multicellular glandular head surrounded by a large subcuticular space in which the secretion is stored. The histochemical tests proved the production of terpenoidic, polysaccharidic and flavonoidic compounds. The presence of both lipophilic and hydrophilic components appears as osmiophilic droplets immersed in an abundant granular matrix. The secreted material is released after cuticle rupture. (ii) short capitate hair widespread on both the vegetative and the reproductive organs of all the examined species. It is constituted by a basal epidermal cell, a neck-stalk cell and by a glandular head of 2-4 cells surrounded by a thin subcuticular space. The secreted material exhibits a strong positive response only to the hydrophilic dyes. The most striking ultrastructural feature is the presence of abundant Golgi bodies, sites of the synthesis and storage of polysaccharides. (iii) long capitate trichomes observed only on Salvia species. It is composed by 1-2 epidermal cells, 2 stalk cells, 1 neck cell and by a globose head of 1-2 secretory cells surrounded by a storing chamber. The secretion is exclusively constituted by terpenes. The cytoplasm of the secreting cells is rich in plastids containing starch granules and a well-developed smooth endoplasmic reticulum, cellular compartments typical of a terpenoidic secretion. (iv) long capitate trichomes observed only on the inflorescences of Scutellaria species. It is composed by 2 epidermal cells, 2-4 stalk cells, 1 neck cell and by 8 up to 18-20 secretory cells. Each secretory cells presents a small subcuticular space at the apex. The secretion is characterized by a complex composition, positive to lipophilic and hydrophilic dyes. Therefore, the secretory products are constituted of polysaccharides, terpenes and polyphenols. On the apex of each secreting cell a small portion of the cuticle layer raises, originating a small chamber in which electrondense fibrillar or granular material is stored. Occasionally some prearranged openings for the release of the secretion were observed, but most part of the secreting material seems to be extruded through the outer periclinal wall. These highly-specialized secretory structures are characterized by a wide morphological and chemical polymorphism and by different mode of release of the secretory products. This extensive range of variability is probably crucial in determining different ecological rules.