Variations in the accumulation of three secondary metabolites in Euclea undulata Thunb. var. myrtina as a function of seasonal changes

Abstract

Euclea undulata Thunb. var. myrtina is a widely distributed shrub in South Africa. The roots are used by traditional healers in the treatment of diabetes. Research indicated that the roots of this plant contain epicatechin and α-amyrin-3O-β-(5-hydroxy) ferulic acid, two secondary metabolites that show potential to treat diabetes. It was found that α-amyrin-3O-β-(5-hydroxy) ferulic acid inhibits α-glucosidase, while epicatechin lowers blood glucose levels. However, existing literature also indicated the presence of the naphthoquinone 7-methyl-juglone in the roots and due to its cytotoxic nature, 7-methyl-juglone poses a potential threat when E. undulata is used in the treatment of diabetes.In order to assist in the effective and safe use of this plant in the treatment of diabetes, this project aimed to determine whether the presence of these metabolites is seasonal. It further aimed to contribute to more sustainable harvesting methods by investigating stems and leaves in addition to roots for the presence of these secondary metabolites.Leaves, stems and roots were harvested from E. undulata plants in three different localities with different climatic conditions, with a specific focus on winter and summer rainfall regions. The plant material was air dried at room temperature and ground to a powder, which was subjected to NMR metabolomic analysis.Multivariate data analysis resulted in clustering of samples collected in the rainy and dry seasons for both winter and summer rainfall regions, as well as delineations between roots, stems and leaves due to their secondary metabolite contents. Clustering of samples in the rainy and dry seasons of the same plants was also observed with the OPLS-DA plots indicated clustering of samples harvested during the dry and rainy seasons.Results for the summer rainfall area indicate seasonal fluctuations in the levels of α-amyrin-3O-β-(5-hydroxy) ferulic acid in roots. It is also noteworthy that the roots always contain 7-methyl-juglone and only found in the leaves on the rainy season of the summer rainfall area. The stems can be harvested for α-amyrin-3O-β-(5-hydroxy) ferulic acid and the leaves for epicatechin in both seasons in the summer rainfall area. Results from the winter rainfall area indicate far less seasonal fluctuation. Most metabolites appear to be either present or absent in the respective organs regardless of season with the exception of epicatechin that is absent in the roots in the rainy season. Fluctuations were, however, detected in epicatechin levels in stems and 7-methyl-juglone levels in roots. From the results obtained it is evident that α-amyrin-3O-β-(5-hydroxy) ferulic acid occurs in roots, stems and leaves in the winter rainfall area and is independent of temperature whereas 7-methyl-juglone is present only in the roots. Leaves can replace the use of roots when harvested in a winter rainfall area irrespective of season as α-amyrin-3O-β-(5-hydroxy) ferulic acid and epicatechin are present and 7-methyl-juglone are absent.