Clerodendrum indicum (L.) Kuntze (Lamiaceae), an annual shrub, is renowned for being used in folk medicine in South Asian countries. Several pharmacological properties and many bioactive secondary metabolites from C. indicum have been well documented. However, the phytotoxic activities and the related phytotoxic substances with the allelopathic activity of C. indicum have not yet been reported. Thus, we explored the phytotoxic activity of C. indicum and identified its phytotoxic substance. In the experiment, the dry C. indicum leaves were extracted with aqueous methanol, and then the filtrate of the extracts was concentrated using a rotary evaporator to obtain crude extracts. The C. indicum crude extracts significantly inhibited the shoots and roots of six target species: alfalfa, cress, lettuce, barnyard grass, Italian ryegrass, and timothy. The inhibition increased when the extract concentration was increased. The crude extracts of C. indicum were separated in several chromatography steps, and a phytotoxic substance was isolated and characterized using spectroscopy as p-coumaric acid. p-Coumaric acid significantly suppressed the growth of lettuce and timothy seedlings at concentrations greater than 0.3 and 1 mM, respectively. The concentrations of p-coumaric acid required for 50% inhibition (I50) of the shoots and roots of lettuce and timothy were 0.65 and 0.17 mM, and 0.81 and 0.67 mM, respectively. This study is the first report on isolating p-coumaric acid in C. indicum. The results, therefore, suggest that p-coumaric acid may partly contribute to the phytotoxic properties of C. indicum.
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