Allelopathic plants: 20. Hairy Beggarticks (Bidens pilosa L.)

Bidens pilosa L. is an annual major weed in warm areas of the world and is serious weed in many crops. In the bioassays and root exudates experiments, it significantly suppressed the growth of test plants. In field experiments, application of its shoots biomass at 2 t ha(-1) drastically suppressed (80%) the weed density and weed dry weight than hand-weeding and herbicide treatments and completely controlled the emergence of Rotala indica, Commelina diffusa, Jussiaea decurrens. Moreover, rice tiller numbers, panicles and grain were stimulated and yield was enhanced by 20%. Many secondary metabolites (phenolics, polyacetylenes and triterpens) involved in allelopathic action were found in this weed, of these phenylheptatriyne (PHT) and its derivatives amounts were highest in B. pilosa oil. Total 23 phenolic compounds [including salicylic acid, vanillin, p-hydroxybenzoic acid, p-coumaric acid, ferulic acid] were identified and isolated in shoots and roots. Caffeic acid was in highest amount (117.4, 298.7, and 350.3 mu g g(-1) in leaves, stems and roots) followed by pyrocatechin (responsible for phytotoxicity). However, the fate and actual modes of action of these compounds are not well understood, whether these compounds can inhibit emergence of paddy weeds and enhanced the rice yields as well as contributing to its strong invasiveness in nature need to be further studied.