Effect of net shading and season on plant growth, productivity and quality of bush tea (Athrixia phylicoides DC.)

Abstract

Bush tea (Athrixia phylicoides DC.) is a South African native plant that grows naturally in different parts of South Africa that are characterized by different climatic conditions. The effects of agricultural practices on plant growth, yield and chemical composition of bush tea have been studied on pots experiment under controlled environment. However, the effect of shade net and season on field grown bush tea and its quality have not yet investigated. Hence, its quality is determined by the accumulation, partitioning and distribution of secondary metabolites which are affected by major abiotic and biotic factors which include environmental conditions, UV light, temperature, water availability, type and composition of soil. Currently, there is no recommended best production method and season for bush tea crop in Southern Africa that maximizes plant growth, yield, accumulation, partitioning and distribution of secondary metabolites. Therefore, this thesis evaluated the effect of the application of shade nets and season on plant growth, development and quality of bush tea. Net shade application reduced bush tea plant growth and yield when compared to plants exposed to direct sunlight during the summer season. Hydroxycinnamic acid recorded higher in 80% white shade net plots than the plants exposed to direct sunlight. The study also evaluated the effect of UV-induced geometrical isomerization of hydroxyl-cinnamic acid-containing molecules of bush tea using ultra high-performance liquid chromatogram quadrupole time-of-flight mass spectrometry (UHPLC-QTOF-MS). The study highlighted that light impacted the chemistry of plants which results in the formation of newly formed metabolites which are not naturally part of the plant. Secondary metabolites which have been responded to photo isomerization were discovered to be structurally related by the formation of a very tight molecular family using when molecular networking algorithm. This study also investigated the metabolite diversification of bush tea through glucaric acid conjugation by cinnamic acid derivatives using UHPLC-QTOF-MS. The findings revealed that hydroxyl-cinnamic acids (HCAs) derivatives undergo photo-isomerization during post ultra-violet (UV) light exposure, evidenced by the emergence of photo-isomers. The study indicated the ability of conducive environment to promote plant growth, development, yield as well as enhancing quality of bush tea. Future prospects will include investigation of the effect of different colours of ultraviolet light on bush tea extracts and response of molecular network of bush tea exposed to different types of shade nets using UHPLC-QTOF-MS.