The leaf secretory apparatus of Hibiscus surattensis and Hibiscus sabdariffa (Malvaceae) : micromorphology, histo-phytochemistry and ultrastructure.

Abstract

The research presented here forms the basis of the ethnobotanical and ethnopharmacological evaluation of Hibiscus surattensis, which is a widely distributed vegetable and medicinal shrub used by African and Indian traditional practitioners. Using light microscopy together with advanced electron microscopy techniques, the leaf topography and internal structure was examined. A closely related species, Hibiscus sabdariffa was also investigated for foliar and histophytochemical comparisons. Hibiscus sabdariffa is a widely consumed medicinal species with well-known biological activity and known chemical principles. Both species belong to the section Fucaria within the genus Hibiscus. Analyses of foliar secretory tissues showed that both H. surattensis and H. sabdariffa were characterised by capitate trichomes as well as mucilage-producing ducts and idioblasts. Head cells of capitate trichomes were between 5 and 7, while lignified stalk cells occurred in numbers of 2 or 3. A basal cell supporting the trichome was implanted in the epidermis comparative study between the two species showed some variation of chemical composition in trichome head cells. Polysaccharidic, alkaloids, phenolic and acidic lipid components were identified for both H. surattensis and H. sabdariffa whereas H. sabdariffa showed an intense staining of proteinaceous substances. Densities of capitate trichomes varied from emergent to mature developmental stages, with the highest number occurring on the emergent abaxial surface for both H. surattensis and H. sabdariffa. However, a distinct trend was observed for H. surattensis, where a decrease in capitate trichomes with progressive development was associated with the proliferation of mucilage idioblasts, particularly on the mature adaxial surface. Mucilage producing tissues are assumed to be associated with a protective role against dehydration in a number of plant families. Idioblasts observed in H. surattensis were visible leaf surface structures, embedded in the epidermal tissues. They contained considerable amounts of acidic polysaccharides and acidic lipids, and appeared to be implicated in reducing evaporative water loss in fully expanded leaves. Mucilage ducts were identified in vascular tissue within leaf veins. They occurred parallel with the conducting tissue and comprised of an epithelial layer of cells which seemed to be secreting a mucilage into an extracellular lumen. Crystal idioblasts were also identified in tissues of both H. surattensis and H. sabdariffa. The sequestration of calcium oxalate is assumed to be governed and regulated by specialised mesophyllous idioblasts that in turn appeared to be triggered by excess calcium within the plant body to differentiate into crystal forming cells. The supposed function of calcium oxalate crystals includes mechanical support and herbivory avoidance. TEM revealed the cellular processes which involved crystal idioblast development in H. surattensis. This appeared to demonstrate that degeneration of organelles which are assumed to occur when crystals reach maximal proportions. The non-glandular trichome component of each Hibiscus species was diverse in trichome type and might have accounted for differences in leaf texture and the leaf indumentum. The prickly texture of Hibiscus surattensis was attributed to rigid stellate and falcate trichomes as well as restrorse prickles, whereas the leaves of H. sabdariffa were glabrous with few falcate, bi- and trifurcate trichomes found mainly at the base or along the midvein. Preliminary phytochemical experiments which involved methanolic, chloroform and hexane extracts, yielded favourable results, which showed that leaves of H. surattensis and H. sabdariffa were chemically similar, in this regard, the author recommended further investigation into the phytochemical nature of H. surattensis.