The Leaf Capitate Trichomes of Leonotis leonurus: Histochemistry, Ultrastructure and Secretion

The leaf capitate trichomes of Leonotis leonurus (Lamiaceae) were investigated with scanning and transmission electron microscopy and their secreted material characterized by histochemical reactions. The trichomes, consisting of a four-celled globular head and a stalk of variable length and morphology, were classified into two categories: capitate trichomes of Type I with a short and cylindrical stalk and Type II with a long and conical stalk, supported by a cellular pedestal. According to histochemical tests they secreted, in addition to polysaccharides and proteins, small amounts of essential oils and flavonoids. During the secretory phase, the ultrastructure of the head cells of both trichomes was characterized by a highly developed endoplasmic reticulum (ER) and numerous active Golgi stacks, cellular compartments involved in the synthesis and transport of a mucilaginous secretion. Unlike peltate trichomes, the glandular cells of capitate trichomes did not exhibit the typical ultrastructural features of cells actively involved in the secretion of terpenoids. It is suggested that the protein-polysaccharide component of the total secretion reaches the plasma membrane via Golgi-derived vesicles, while lipophilic constituents may be transported directly by ER. After exocytosis, the secretion stored temporarily in the periplasmic space, passes through the wall to accumulate in a small subcuticular space formed by cuticle detachment. In the absence of cuticular rupture, the release of secretion probably occurs through cuticular micropores.