Allelopathic potential of Macaranga tanarius (L.) Muell.-Arg.
Macaranga tanarius is widely distributed in the abandoned lowlands of Taiwan where substantial amounts of leaves accumulate on the ground. A unique pattern of weed exclusion underneath trees is often found and thought to result from allelopathic interactions. Density-dependent phytotoxicity analysis of Lactuca sativa L. (lettuce) growing in soil mixed with the powder of M. tanarius leaves showed a significant deviation from the expected yield-density relationship. Lettuce growth was most suppressed in the low seed density experiment suggesting that the phytotoxins produced during leaf decomposition inhibit the growth of lettuce seedlings. Bidens pilosa and Leucaena leucocephala, growing in soil mixed with the leaf powder of M. tanarius were also suppressed. Aqueous leaf extracts were bioassayed against lettuce and B. pilosa, and exhibited a significant suppression in radicle growth. Compounds identified from leaves included nymphaeol-A (1), nymphaeol-B (2), nymphaeol-C (3), quercetin (4), abscisic acid (ABA) (5), blumenol A (6), blumenol B (7), roseoside II (8), tanariflavanone A (9), and tanariflavanone B (10). ABA was the major growth inhibitor. At concentrations of 20 ppm, ABA suppressed lettuce germination, while at 120 ppm it inhibited the growth of Miscanthus floridulus, Chloris barbata, and Bidens pilosa. At 600 ppm, quercetin, blumenol A, and blumenol B, caused 20-25% inhibition of radicle and shoot growth of M. floridulus. The amount of ABA in M. tanarius leaves was approximately 3-5 µg g-1 dry weight, significantly higher than previously reported. We conclude that the pattern of weed exclusion underneath stands of M. tanarius and its invasion into its adjacent grassland vegetation results from allelopathic interactions.