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

A bioactivity-guided isolation method was developed with the objective of isolating the allelochemicals in rice (Oryza sativa L.), Roots of the allelopathic rice cultivar Taichung Native 1, grown hydroponically, were extracted 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 [Echinochloa crusgalli (L.) Beauv,] at the lower concentration at which they were tested, Identified compounds were azelaic acid; p-coumaric acid; 1H-indole-3-carboxaldehyde; 1H-indole-3-carboxylic acid; 1H-indole-5-carboxylic acid; and 1,2-benzenedicarboxylic acid bis(2-ethylhexyl)ester. rho -Coumaric acid, a known allelochemical, inhibited the germination of lettuce (Lactuca sativa L.) seedlings at 1 mM, However, rho -coumaric acid was active against barnyardgrass only at concentrations higher than 3 mM, The two most active fractions obtained from the bioassay-guided isolation were still a mixture of compounds as analyzed by gas chromatography-mass spectrometry (GC-MS), Further fractionation is being done to isolate and identify the allelochemical(s) in these active fractions. This work has demonstrated the use of bioassay-guided isolation in identifying allelochemicals in rice and has correlated observed field activity with laboratory experiments.