Searching for rice allelochemicals: An example of bioassay-guided isolation

A bioactivity-guided isolation method was developed with the objective of i solating the allelochemicals in rice (Oryza sativa L.), Roots of the allelo pathic rice cultivar Taichung Native 1, grown hydroponically, were extracte d and fractionated, with the activity of the fractions followed using a 24- well culture plate microbioassay, Some of the fractions obtained consisted of pure compounds, but none inhibited the growth of barnyardgrass [Echinoch loa crusgalli (L.) Beauv,] at the lower concentration at which they were te sted, Identified compounds were azelaic acid; p-coumaric acid; 1H-indole-3- carboxaldehyde; 1H-indole-3-carboxylic acid; 1H-indole-5-carboxylic acid; a nd 1,2-benzenedicarboxylic acid bis(2-ethylhexyl)ester. rho -Coumaric acid, a known allelochemical, inhibited the germination of lettuce (Lactuca sati va L.) seedlings at 1 mM, However, rho -coumaric acid was active against ba rnyardgrass only at concentrations higher than 3 mM, The two most active fr actions obtained from the bioassay-guided isolation were still a mixture of compounds as analyzed by gas chromatography-mass spectrometry (GC-MS), Fur ther fractionation is being done to isolate and identify the allelochemical (s) in these active fractions. This work has demonstrated the use of bioass ay-guided isolation in identifying allelochemicals in rice and has correlat ed observed field activity with laboratory experiments.