Morphology, histochemistry, and ultrastructure of foliar mucilage-producing trichomes of Harpagophytum procumbens (Pedaliaceae)

The morphology, histochemistry, and ultrastructure of foliar mucilage-producing trichomes of Harpagophytum procumbens are investigated using a combination of light and electron microscopy. The leaves of H. procumbens bear short and long glandular trichomes comprising 1 or 2 basal epidermal cells, a stalk of 1-3 cells, a short neck cell, and a head of 2-5 cells. These trichomes are distributed on both leaf sides with a greater abundance on the abaxial side. Both types of trichomes secrete copious amounts of secretion as droplets onto the leaf surface, and the release occurs through micropores in the head cuticles. The secreted material is mainly constituted of mucilaginous polysaccharides, in addition to phenolic compounds and total lipids. The stalk surfaces of long trichomes are densely covered by numerous micropapillae helping the mucilage droplets to slide onto the leaf surface. The short and long trichomes have a similar ultrastructure: the secretory head cells are cytoplasmically dense due to the abundance of mitochondria, ribosomes, small vacuoles, plastids, Golgi bodies, and elements of endoplasmic reticulum. Golgi-derived vesicles are developed in the head cells, and each plastid contains large starch grains.