The Changes and Degradation of Tobacco Root Exudates in Tobacco Field with Continuous Cropping

We analyzed contents of tobacco root exudates in 0-20 cm and 20-40 cm soil where tobacco had grown for 1 year, 2 years, and 3 years in order to investigate variation of exudates in tobacco field and identify allelochemicals. The results showed that the content of benzoic acid, 4-hydroxyphenylacetic acid, 3-oxygen-4-hydroxyphenylacetic acid and phthalic acid dioctyl ester in tobacco field were high, of which 4-hydroxyphenylacetic acid was the highest, and the content of 4-hydroxybutyric acid and glycerin were low. Hydroxybutyric acid, 3-methyl-2-hydroxy butyric acid, 4-hydroxybutyric acid, cinnamic acid, oleic acid, stearic acid amide and nicotine had no direct allelopathic influence to tobacco. The decomposition rates of cinnamic acid, 3-methoxy-4-hydroxyphenyl acetic acid, phthalic acid, 3-hydroxy benzoic acid, dioctyl phthalate, 4-hydroxyphenyl acetic acid and myristic acid were less than 50%. We proposed that the following acids were allelochemicals, including benzoic acid, 3-hydroxybenzoic acid, 4-hydroxybenzoic acid, lauric acid, oterephthalic acid, 3-methoxy-4-hydroxyphenyl acetic acid, 3,4-dihydroxybenzoic acid, myristic acid, palmitic acid, 9,12-octadecadienoic acid, acrylic acid, scopoletin, cinnamic acid, phthalic acid dioctyl phthalate and 4- hydroxyphenylacetic acid.