The secretory apparatus of Ceratotheca triloba (Pedaliaceae) : morphology and chemical composition of the secretion.

Abstract

Ceratotheca triloba (Bernh.) E. Mey. ex Hook. f., commonly known as the wild foxglove, is a fast growing annual that is indigenous to southern Africa. The surface of the plant is covered with fine, hair-like trichomes, which exude sticky, aromatic substances. The plant is traditionally used to treat various abdominal ailments, insect infestation of the skin and leaf extracts are administered to induce abortion. In order to provide a scientific basis for the medicinal properties of C. triloba, this study was initiated to characterise the morphology and ultrastructure of the foliar trichomes and to determine the chemical composition of the secretion. Phytochemical tests and a range of microscopic techniques including stereo microscopy, scanning electron microscopy, transmission electron microscopy, histochemistry and fluorescence microscopy were used. Leaf surface imaging with stereo microscopy and SEM indicated the presence of two morphologically distinct glandular trichomes, capitate and peltate. The capitate trichomes are tall, consisting of a single basal cell, 2-4 stalk cells, a neck cell and a head that is made up of four secretory cells. Capitate trichomes of up to 14 stalk cells were observed on the abaxial side of flower petals. The peltate trichomes consisted of a basal cell, a single stalk cell and a multicellular head. The peltate trichome head generally consisted of four cells, but occasionally were seven or eight-celled. Fully developed trichomes were found to be concentrated on emergent leaves and probably serve protective function. The density of trichomes decreased as the leaf expanded. TEM showed that the secretory cells of peltate trichomes contained centralised nuclei and numerous peripheral vacuoles. Numerous mitochondria and ER cisternea were found throughout the cytoplasm. Osmiophilic plastids which were found near nuclei and golgi apparatus were in close proximity to the peripheral vacuoles. Similar observations were made for the stalk cells of both trichomes. The head cell of capitate trichomes however, contained large amounts of osmiophilic substances, ER cisternae and mitochondria. The head cells of peltate trichomes store the secretory product in vacuoles and secrete them upon cell rupture or cell dissolution. The secretory product of capitate trichomes is secreted through the cell membrane via an eccrine mode of secretion. Peltate and capitate trichomes appear to be secreting similar compounds, but the composition of each compound in the secretory material may vary between the trichomes. Histochemical and phytochemical tests reveal that the secretions are comprised of mucilage, phenolic compounds, lipids, flavonoids, tannins, saponins and fixed oils which may contribute to the medicinal properties of C. triloba. The observations made in this study provide useful information for additional research in the Pedaliaceae, and specifically in C. triloba. Future studies should isolate the active compounds for antimicrobial and antioxidant testing. Cytotoxicity testing should also be undertaken to test safety and efficacy of the active compounds.