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

Experiments were conducted to identify allelochemicals from rice (Oryza sat iva L.) straw extracts of four rice cultivars (Gin shun, Kasawala mundara, Philippine 2 and Juma 10), and to test their biological activity on barnyar d grass (Echinochloa crus-galli P. Beauv. var. oryzicola Ohwi). High perfor mance liquid chromatography (HPLC) analysis showed that the concentration a nd composition of allelopathic compounds depended on the cultivar. Among th e compounds identified were p-hydroxybenzoic acid at 6.87 mg g(-1) in Gin s hun, 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) i n Juma 10. Preliminary identification by HPLC analysis resulted in peaks wi th retention times near those of standards, including p-hydroxybenzoic acid (m/z = 138). This was confirmed with electron impact/mass spectra. In a bi oassay with nine known allelochemicals and their mixtures, p-hydroxybenzoic acid (10(-3) M) showed the greatest inhibitory effect on barnyard grass se ed germination, seedling length, and dry weight. This suggests that this co mpound may be a key factor in rice allelopathy on barnyard grass.