Evaluation of phytotoxic activity of leaf and stem extracts and identification of a phytotoxic substance from Caesalpinia mimosoides Lamk.

Caesalpinia mimosoides (Fabaceae) has been used as a folk medicine and reported to have pharmacological properties. However, the phytotoxicity of C. mimosoides has not yet been studied. Therefore, the phytotoxic activity of C. mimosoides leaf and stem extracts was evaluated against the growth of dicotyledonous and monocotyledonous plants. Aqueous methanol extracts of C. mimosoides leaves and stems exhibited inhibitory effects on the growth of all the test plants. The growth of the test plants decreased with the increase of concentrations of the extracts. Comparison of the concentrations required for 50% growth inhibition (I₅₀ values) indicates that the leaf extracts had more inhibitory effects than the stem extracts. Thus, the active substances in the leaf extracts were purified using column chromatographic separations and a growth inhibitory substance was isolated and identified as methyl gallate by using spectral analysis. This study reports the first isolation of methyl gallate in C. mimosoides leaves. Methyl gallate at 10 mM completely inhibited the growth of cress shoots and inhibited its roots by 4.7% of control growth. At the same concentration, the shoots and roots of barnyard grass were inhibited by 84.2 and 1.7% of control growth, respectively. I₅₀ values of methyl gallate for the growth of cress and barnyard grass ranged from 2.3 to 2.9 and 0.3 to 30.5 mM, respectively. The results suggest that methyl gallate may be responsible for the phytotoxic effects of C. mimosoides.