Secretory structures in the leaves of Hibiscus sabdariffa L.

Hibiscus sabdariffa (Roselle) is an economically important crop cultivated in parts of Asia and Africa. The leaves are consumed as a vegetable, whereas the calyces are used in a variety of different beverages. Leaves of H. sabdariffa have been studied for their anticancer and cholesterol-lowering effects, though very little is known with regard to the compounds produced in the leaves and how these are synthesized. Here, we provide an anatomical overview of the foliar secretory structures present and discuss some of the cellular mechanics involved in the secretion of secondary metabolites. The micromorphology of the external leaf surfaces and internal secretory structures were examined. The leaves of H. sabdariffa, though glabrous to the naked eye, are covered with glandular capitate trichomes. Using ultrastructural techniques, a subcuticular space was identified on the lower surface of the trichome, which compartmentalized and stored secreted substances until release onto the leaf surface. The function of the secretions observed are still yet unknown, however due to the presence of phenolics and alkaloids within trichomes determined using histochemical methods, they may serve as herbivore deterrents. Mucilage accumulating cells, also known as mucilage idioblasts were also encountered during the anatomical investigations and were characterized using light and electron microscopy. These findings have implications in the taxonomical classification of Hibiscus, and furthers research on this valued medicinal plant and the potential sources of useful phytometabolites within it. (C) 2018 Published by Elsevier B.V. on behalf of SAAB.