Exploring the phytochemical variation of the “Pepper-bark” tree (Warburgia salutaris) using HPTLC and UPLC-MS

Warburgia salutaris, commonly known as the “pepper-bark” tree, is an evergreen tree found in the southern parts of Africa and widely used for its antimicrobial properties. The bark is the most often used part and destructive practices such as “ring-barking” and excessive harvesting has resulted in Warburgia salutaris being classified as an endangered species. In this study, various methods were used to explore the phytochemical variation between wild-type and cultivated bark, root and leaf specimens from various geographical locations. Ultra performance liquid chromatography coupled to mass spectrometry (UPLC-MS) was used to generate data and subsequently analysed using multivariate data analysis methods to determine the variation, similarities and trends of the data. High performance thin layer chromatography (HPTLC) was used to develop chemical fingerprints to visually distinguish plant material (e.g. roots and leaves). Chemometric analysis of the chromatography data indicated that most of the variation could be attributed to seasonal variation, age of the plant the plant part and not whether the specimen originated from a wild or cultivated source. Bioactivity has mainly been attributed to the drimane sesquiterpenes, especially polygodial. In these specimens, the polygodial content ranged from 0.1 to 67.8 µg/g in wild bark and 0 to 0.6 µg/g in wild leaf. The cultivated bark specimens contained higher ranges of polygodial ranging from 11.0 to 174.0 µg/g while low values were documented for the cultivated leaf specimens (0 to 0.9 µg/g). The HPTLC chemical fingerprint with distinguishing bands allowed for visual discrimination between plant parts. However, within geographical location groups, the phytochemical profiles for all specimens were conserved. The HPTLC method developed can easily be implemented as a screening tool to discriminate between large batches of bark and leaf material.