The effect of various selected shade nets on field-grown bush tea’s physicochemical and antioxidant properties (Athrixia phylicoides DC.)

Abstract

Bush tea (Athrixia phylicoides DC.) is an indigenous herbal tea plant in South Africa with medicinal potential. Various parts of this plant are used to make herbal infusions and have been used traditionally for different purposes, including treating other diseases owing to the presence of pharmacologically active metabolites and antioxidants that are believed to be the active ingredient in the plant. However, knowledge of the manipulation of agronomic practice to enhance the accumulation of physicochemical antioxidant properties is not yet present. Therefore, this research was aimed at evaluating the effects of various shade nets colours and light intensities on the physicochemical, polyphenols and antioxidant activities of field grown bush tea leaves. The field experiment was conducted during the 2018 and 2019 autumn seasons at the University of Venda, School of Agriculture. Three replicates of the experiment were conducted using a Randomized Complete Block Design (RCBD) with three distinct degrees of light intensity (i.e., 40%, 50%, and 80%), three shade nets (specifically, black, green, and white) and full sunshine (control, 100%). The individual plots were each 4.8 m x 4.8 m in size. There were 24 plants per plot, with each plot having four plant rows spaced 1.2 m apart and 0.75 m apart within each row. When necessary, a drip watering system was applied to irrigate the plots except during rainy periods. When necessary, weeding was done manually using a hand hoe throughout the cropping seasons. To protect the plants from termites, methamidophos was sprayed at a concentration of 10 mL per 20 L immediately following each weeding session. At the end of the season, the leaves were harvested by cutting the base of the stem and placed inside brown paper bags and stored in the shade at room temperature to allow shade drying process as it is the best method for retaining bioactive compounds and phytochemicals. The leaves were detached from the stems after drying, by the process of threshing and the leaves were ground using the Retsch miller machine. The parameters such as moisture, ash, dry matter, vitamin C, and colour attributes (a*, b*, c*, H° and ΔE) were analysed. In addition, parameters such as chlorophyll content a, chlorophyll content b, total chlorophyll content (TCC), total flavonoid content (TFC), total phenolic content (TPC), as well as antioxidant activities following ABTS and FRAP methods were analysed using a spectrophotometer. For each treatment, the samples were analysed three times. Analysis of variance was conducted in all the collected data using the statistical package SAS version 9.4 statistical software and student’s t-LSDs (Least Significant Differences) were computed at 5%, comparing the means of substantial source effects at a considerable level. The analysis of Pearson's Correlation Coefficient was used to establish the association between the climatic factors and nutritional compositions, polyphenols, antioxidants activities, chlorophyll contents and colour attributes of bush tea. The results revealed that different shade nets colours and light intensities significantly (p <0.0001) influenced the nutritional compositions (moisture, ash, dry matter, vitamin C), polyphenols (TPC and TFC), antioxidants activities (ABTS and FRAP) and physicochemical properties (colour attributes, chlorophyll a, chlorophyll b, and TCC) of field grown bush tea in both years 2018 and 2019. Black shade net colour had a noteworthy influence on total chlorophyll content, chlorophyll a content, and chlorophyll b content. In contrast, green shade net influenced on H° values and the white shade net on TFC. In contrast, the use of shade nets colours had no effects on dry matter (92.86%), vitamin C (3.03 mg AA/100g), or colour attributes [b* (20.65) and c* (20.65)] values of bush tea as they were highly significant at control. The use of 80% light intensity, significantly influences the moisture content, chlorophyll a, chlorophyll b and total chlorophyll content and H°. However, control had a higher influence on dry matter, vitamin C, b*, c* and FRAP results. A very strong, strong, and moderate positive correlation was observed between the climatic factors and nutritional compositions, polyphenols, antioxidants, and chlorophyll contents of field-grown bush tea, under the influence of shade nets colours and light intensities during autumn 2018 and 2019. However, colour attributes: a*, b* and H° resulted in weak/ no correlations. This study is the first to demonstrate different shade nets colours and light intensities as the key factor of accumulating of chlorophyll contents and antioxidants in field-grown bush tea plants. The use of a black shade net together with 80% light intensity resulted in being the best treatment to promote the accumulation of antioxidants compared to other treatments. And it is evident that black shade net together with 80% light intensity was of significant influence after control (100% light intensity). Future studies will be to determine the regulatory effects of shade treatments on the biosynthetic pathway of flavonoids and the accumulation pathways of chlorophyll contents under the shade nets treatment.