Secretory structures of croton gratissimus Burch. Var. gratissimus (Euphorbiaceae): Micromorphology and Histophytochemistry.

Abstract

Croton gratissimus Burch. variety (var.) gratissimus (Euphorbiaceae) has a widespread distribution in tropical Africa and is frequently used in African traditional medicine to treat various ailments. In South Africa, dried leaves of C. gratissimus are smoked to treat influenza, colds and fever. Due to its extensive use in traditional medicine, research on the phytochemical composition of C. gratissimus has been documented. According to literature, these phytochemicals are possibly secreted or accumulated in secretory structures. However, little or no research is available on the structures involved in the production and/or accumulation of phytochemicals in C. gratissimus. Therefore, this study aimed to describe the micromorphology of trichomes and laticifers from the leaves and stems of C. gratissimus as well as to identify the possible site of synthesis of phytochemicals. Furthermore, the chemical composition and antibacterial properties of phytochemicals in the leaves and stems were also determined. In addition, the antibacterial activity of biosynthesised silver nanoparticles (AgNPs) from leaf and stem crude extracts was also investigated. Microscopic investigations revealed the presence of lepidote and glandular trichomes, and non-articulated unbranched laticifers on/in the leaves and stems of C. gratissimus. The lepidote trichomes formed a dense indumentum over the abaxial surface of leaves throughout all developmental stages, canopying the underlying glandular trichomes. Laticifers were present in the leaves and stems and were predominantly associated with the vascular tissue in both organs. All structures stained positive for alkaloids, phenolic compounds and lipids with histochemical tests. Phytochemical analyses of the leaves and stems revealed alkaloids, amino acids, phenolic compounds, flavonoids, carbohydrates, terpenoids, saponins and fixed oils and fats in both leaf and stem extracts. The methanolic leaf and stem extracts demonstrated weak to strong activities against various bacteria strains, which are attributed to the several bioactive compounds identified from Gas Chromatography-Mass Spectrometry (GC-MS) analyses. In addition, AgNPs were successfully biosynthesised from the methanolic leaf and stem extracts. Particles synthesised from both extracts were spherical in shape, but their size distribution differed between organs. Antibacterial assays demonstrated a stronger activity of particles from leaf extracts compared to those from stems. These findings corroborate the use of C. gratissimus in traditional medicine and indicate that various structures are involved in the production of bioactive compounds which contribute to the medicinal properties of this plant. Furthermore, the antibacterial activities exhibited by the extracts and AgNPs suggest that C. gratissimus is a potential source of antibacterial agents.