Peltate Glandular Trichomes of Leonotis leonurus Leaves: Ultrastructure and Histochemical Characterization of Secretions

Abstract

The histochemical characterization of the oleoresin produced by peltate trichomes of Leonotis leonurus revealed terpenoids and flavonoid aglycones. At the onset of secretion, glandular cells were more densely cytoplasmic than the other trichome cells. The lateral stalk wall underwent cutinization, and the cuticle over the glandular cells became thicker. During the active secretory stage, the most striking ultrastructural features of glandular cells were the hypertrophy of the leucoplastidome and the extensive proliferation of ER. The high development of these two cellular compartments was related with the biosynthesis and transport of the secretory product. A granulocrine secretion mechanism may operate alone or concurrently with an eccrine process. In the glandular head, the loosening of the outer wall fibrillar matrix, and the accumulation of secretion in the newly formed interfibrillar spaces led to a secretory cavity development by detachment of the cuticle and the outermost pectic layer of cell wall. Successive accumulation of secretion in the secretory cavity conferred the spherical shape, characteristic of a peltate gland, to the trichome. The interfibrillar spaces, initially small and elongated, enlarged, became roundish, and appeared as vesicles delimited by an electron-dense layer. These vesicle-like structures are interpreted as lipophilic secretion globules in a hydrophilic phase. It is suggested that pectic polysaccharide wall constituents may be the main components of this hydrophilic matrix. The dense layer around the vesicles may represent an interphase between lipophilic and hydrophilic compounds. The secretion seemed to remain trapped in the secretory cavity, since no cuticular disruption was observed.