Title : Secretory structures of the leaves of Hibiscus sabdariffa Linn. (Malvaceae)
Abstract:
Plant secretory structures are known for their role as phytochemical factories, and this character has been exploited by mankind for many centuries. More recently, however, medicinal plants have received considerable scientific interest, for the investigation of chemical principles in the drug discovery process. Hibiscus sabdariffa is an ancient medicinal herb of African origin that was used by the Chinese for the treatment of various cardiovascular ailments. Leaves of this species have recently been investigated for their anticancer and antihyperlipidemic properties. The present research employed various advanced microscopy techniques to determine and characterise the sites of phytochemical synthesis as well as to evaluate the main compound classes found within the leaves. Leaves appeared dark green with deep red margins and possessed glandular trichomes on both the abaxial and adaxial regions. The glandular capitate trichomes appeared to be clavate comprising of a multicellular club-shaped head and a 2-3 celled stalk. The frequency of trichomes differed with varying stages of leaf maturity with older leaves showing fewer numbers per unit area than emergent leaves. However, the abaxial surface was more abundantly covered than the adaxial surface. Histochemical evaluations on fresh leaf sections of H. sabdariffa showed the presence of numerous chemical compounds within trichome head cells. These included phenolics, alkaloids, acidic polysaccharides, acidic lipids and lipid oils which were prevalent in different regions of the trichome. Whereas phenolic and alkaloid compounds, as well as the flavone and flavanone compound groups were detected in methanol, hexane and chloroform crude extracts. Trichome ultrastructure revealed the existence of a subcuticular chamber on the ventral surface of the trichome, different to those observed in similar capitate trichomes. Secretion produced in the head cells were seen being deposited into the subcuticular chamber after traversing the periplasmic space and the cell wall. They appeared as electron-dense amorphous deposits. A second type of secretory structure was observed within the mesophyll and epidermis of H. sabdariffa leaves. Mucilage idioblasts were seen frequently among the adaxial epidermal cells. Their external appearance was characterised by a thickened cuticle surrounded by radially arranged epidermal cells. Mucilage idioblasts and mucilage secreting ducts are a distinguishing characteristic of the genus, and are described here in detail. Antibiotic resistance and the evolution of proliferative diseases necessitates the exploration for alternative and novel drug therapies. Research on the synthesis and activities of pharmacologically relevant compounds are important in furthering the advancement of drug development.
