Discovery of coumarin as the predominant allelochemical in Gliricidia sepium

A crude methanol extract of Gliricidia sepium (Fabaceae) leaves inhibited the growth of lettuce (Lactuca sativa) radicles. To isolate and characterise the inhibitory compound, the extract was fractionated based on the total activity on lettuce radicle elongation. The n-hexane-soluble fraction obtained by liquid-liquid partitioning of the crude methanol extract showed strong inhibitory activity. A compound corresponding to the major peak in high performance liquid chromatography was isolated from the fraction and identified as coumarin. The EC50 of coumarin for the growth of lettuce radicles was 23.3 µmol L-1. On the basis of the coumarin content (11.6 mmol kg-1 fresh weight) and the total activity of coumarin (500) in G. sepium, we concluded that the inhibitory activity of G. sepium was primarily due to coumarin.