Identification of allelopathic compounds from rice (Oryza sativa L.) straw and their biological activity

Experiments were conducted to identify allelochemicals from rice (Oryza sativa L.) straw extracts of four rice cultivars (Gin shun, Kasawala mundara, Philippine 2 and Juma 10), and to test their biological activity on barnyard grass (Echinochloa crus-galli P. Beauv. var. oryzicola Ohwi). High performance liquid chromatography (HPLC) analysis showed that the concentration and composition of allelopathic compounds depended on the cultivar. Among the compounds identified were p-hydroxybenzoic acid at 6.87 mg g(-1) in Gin shun, p-coumaric acid at 0.34 mg g(-1) in Kasawala mundara, ferulic acid at 0.05 mg g(-1) in Philippine 2, and p-hydroxybenzoic acid at 6.34 mg g(-1) in Juma 10. Preliminary identification by HPLC analysis resulted in peaks with retention times near those of standards, including p-hydroxybenzoic acid (m/z = 138). This was confirmed with electron impact/mass spectra. In a bioassay with nine known allelochemicals and their mixtures, p-hydroxybenzoic acid (10(-3) M) showed the greatest inhibitory effect on barnyard grass seed germination, seedling length, and dry weight. This suggests that this compound may be a key factor in rice allelopathy on barnyard grass.