Effects of nitrogen fertilization on tannin content of bush tea (Athrixia phylicoides L.) grown in different seasons

Abstract

Bush tea, Athrixia phyliciodes L. (Asteraceae family) is a popular plant in South Africa used predominantly as an herbal tea and for medicinal purposes, including the cleansing or purifying blood and the treating of boils (carbuncles) and coughs. In some parts of South Africa, bush tea is used as an aphrodisiac. In this study, bush tea was grown under varying nitrogen (N) levels in four experiments that collectively covered a whole year to determine the seasonal nutrient requirements for improvement of quality. The treatments consisted of 0, 100, 200, 300, 400 and 500 kg·ha-1 N in a randomized complete block design, replicated four times (autumn, winter, spring, summer) under 50% shade nets. The tannins were extracted with acetone, mixed with reagent (butanol-HCL for condensed tannins and potassium iodate for hydrolysable tannins), and levels measured spectrophotometrically. The results of this study demonstrated that both condensed and hydrolysable tannins increased quadratically in response to N nutrition reaching a maximum at 300 kg·ha-1 N. Maximum content of condensed tannins was 4.5% which was achieved when growing the plants in during autumn and winter, whereas maximum content of hydrolysable tannins was 0.1%, achieved during spring and summer. Therefore, for improved condensed tannin content in bush tea leaves growing during the autumn (4.5%) or winter (3.7%) and hydrolysable tannins during spring and summer (0.1%), 300 kg·ha-1N is recommended.