The micromorphological and essential oil status of the foliar secretory structures of Ocimum obovatum E. Mey. ex Benth. subsp. obovatum (Lamiaceae)

Abstract

Ocimum obovatum E. Mey ex Benth. var. obovatum is a traditionally used medicinal plant that grows along the KwaZulu-Natal coast and the western Cape of South Africa. The plant is noted for its hair restoration properties, remedy for infantile abdominal pain and cramps and its use as an enema to treat epigastric conditions in children. The aims of this study were to document the micromorphology and ultrastructure of the foliar secretory structures responsible for the production and secretion of the essential oils and chemical composition of the secretion, which gives the plant a distinct aroma. It is believed that these oils contain the active ingredients that contribute to the medicinal properties of the plant. A variety of microscopic methods and histochemical and phytochemical tests were used to achieve this. Leaves in all stages of development were pubescent and gland dots, characteristic of plants in the genus, were found on both adaxial and abaxial surfaces. Three types of trichomes were found on both leaf surfaces across all stages of development; non-glandular trichomes and two types of glandular trichomes. Non-glandular trichomes are single, multicellular and uniseriate with microornamentation and a supportive cellular pedestal. The glandular trichomes consisted of peltate and capitate trichomes. Peltate trichomes are made up of four head cells and a very small basal cell that gives the glands the appearance of being sessile. The capitate trichomes were further divided into two types based on the morphology of the glands. Type I capitate trichomes are smaller than the larger peltate trichomes and are composed of one basal cell and a head consisting of two broad head cells. Type II capitate trichomes consisted of one basal cell, one stalk cell and a single oval head cell. Histochemical tests showed that peltate and Type I capitate trichomes have cutinized or suberized walls in the stalk cell to prevent the apoplastic flow of secretory material into neighbouring mesophyll tissue. The histochemical stains also showed that the secretory material present in the glandular trichomes are lipid in nature and essential oils are present. Ultrastructural studies showed polymorphic leucoplasts, few Golgi bodies, numerous vesicles and mini vacuoles, mitochondria and short profiles of endoplasmic reticulum cisternae. Phytochemical tests revealed the presence of essential oils that are terpene-rich. Flavonoids, tannins, saponins, terpenoids, fixed oils and fat, phenolics and cardiac glycosides were also detected in a crude ethanolic extract of the leaves. These chemical compounds appear to be responsible for the medicinal properties for which the plant is traditionally exploited.