Microscopic and histochemical characterization of leaves of the medicinal plant Ocimum obovatum

Abstract

Ocimum obovatum E. Mey. subsp. obovatum var. obovatum has been valued for its hair restorative properties for decades on the African continent. The member of the Lamiaceae is also traditionally prescribed as a remedy for infantile abdominal cramps and a hot water extract of the leaves is used to treat epigastric conditions in children. Commonly known as 'cat's whiskers', the aromatic plant can be seen growing along the KwaZulu-Natal coastline and the Western Cape of southern Africa. Ocimum obovatum is also common in Zimbabwe and Swaziland as well as northern and west Africa [1]. The medicinal properties of the plant are attributed to the essential oils supposedly produced and secreted by appendages on the foliar surfaces referred to as trichomes [2]. Traditional light and electron microscopy studies revealed the presence of two types of glandular trichomes and one type of non-glandular trichome across all stages of leaf development. The glandular trichomes were classified as large, four-celled peltate trichomes and smaller capitate trichomes. The latter were further classified into two subtypes; Type I capitate trichomes with a single basal cell and two head cells and Type II capitate trichomes with a single basal and stalk cell and an ovoid head cell. Histochemical and phytochemical studies showed that essential oils of a terpenoid nature were present in the head cells of glandular trichomes. Flavonoids, triterpenoids, tannins, saponins, fixed oils and fats, phenolics and cardiac glycosides were also detected in a crude ethanolic extract of the leaves using phytochemical test methods.