When watermelon seeds were cultured in a Petri dish together with amaranth, barnyard grass, cockscomb, lettuce or tomato seeds, the shoot growth of amaranth and cockscomb was markedly promoted, whereas the shoot growth of lettuce and tomato was inhibited. The shoot growth of barnyard grass was not affected. These results suggest that plant-selective allelopathic substance(s) affecting the shoot growth of other plant seedlings were exuded from watermelon seeds. An allelopathic substance was isolated from the exudates of germinating watermelon seeds and identified as vanillic acid by its spectral analysis and Rf value on TLC. Vanillic acid promoted the shoot growth of cockscomb at the concentrations of 300 to 10 mg/l and that of amaranth at the concentrations of 30 to 3 mg/l, although the shoot growth of amaranth was inhibited by 300 mg/l of vanillic acid. The shoot growth of lettuce and tomato was inhibited at the concentrations higher than 30 mg/l by vanillic acid. However, the shoot growth of barnyard grass was not affected at the concentrations used. All these results suggest that vanillic acid may play as a major component of allelopathic substance(s), which shows plant-selective activity, in the exudates of germinating watermelon seeds.
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