The morphology and chemical composition of the trichomes of Withania somnifera (Solanaceae)

Abstract

For centuries, plants have been used in the cosmetic, culinary and medicinal industries. Recently however, the use of plants in the medicinal industry has increased due to the widespread awareness of the harmful effects of synthetic drugs on humans. Withania somnifera (Dunal.) is an evergreen perennial shrub found in the drier parts of Africa, particularly South Africa and Asia. Since the phytochemical compounds within the extracts of W. somnifera act upon both the nervous and reproductive systems, it is used to treat a wide variety of ailments such as arthritis, stress, ulcers, and tremors. This species has therefore been cultivated to extract the phytochemicals produced. The aim of this study was to characterise the micromorphology of the foliar trichomes of W. somnifera as well as to elucidate the location and composition of the secretory products. Stereomicroscopy and scanning electron microscopy (SEM) were used to characterise the foliar trichomes. A series of histochemical and phytochemical tests were performed to determine the location and composition of the compounds that are responsible for the healing properties of the extracts of W. somnifera. Trichome density and length was also determined in three developmental stages of the leaves. Histochemically stained leaf sections and SEM showed the presence of four morphologically distinct trichome types: glandular capitate, non-glandular dendritic, non-glandular bicellular and non-glandular multicellular. Uniseriate, glandular capitate trichomes consisted of a six-celled secretory head, single-celled stalk and a single basal cell. Secretions from the glandular heads of capitate trichomes were visible on the leaf surface during ESEM and histochemical staining. Non-glandular dendritic trichomes, which appeared to emanate from single basal cells, consisted of 2-4 celled stalks and varying branch numbers. These dendritic trichomes exhibited cuticular warts which are involved in the “Lotus-Effect”. Uniseriate, non-glandular bicellular and multicellular (3-6 cells) trichomes also appeared to emanate from single basal cells. Glandular capitate and non-glandular dendritic trichomes were aggregated on the mid-vein of young and mature leaves, possibly to protect underlying vasculature. Histochemical staining and phytochemical testing revealed the presence of two major phytochemical compounds of medicinal importance, i.e. alkaloids and phenolic compounds. These compounds are used to treat a wide variety of ailments, such as dysentery, TB, paralysis, asthma and inflammation, and also act as chemical deterrents in plants. The results of this study explain possible roles of glandular capitate, non-glandular dendritic, non-glandular bicellular and non-glandular multicellular trichomes based on their morphology and foliar distribution. Future studies should aim at determining the biosynthetic pathways, as well as the modes of secretion of alkaloids and phenolic compounds.