A reappraisal of the content and the differences of phenolic acids between allelopathic and non-allelopathic rice accessions

To clarify the confusion about the contents of phenolic acids in allelopathic and non-allelopathic rice accessions, various methods [4-Aminoantipyrine (4-AAP) method, Folin-Ciocalteu (FC) method, and Solid-Phase Extraction/High Performance Liquid Chromatography (SPE-HPLC)] were used. In 4-AAP method, three benzoic acid derivatives (p-hydroxybenzoic, vanillic, and syringic acid) had 29.3% - 51.2% recoveries and two were below 5% recoveries. Three cinnamic acid derivatives (caffeic, ferulic, and p-coumaric acids) gave < 4% recoveries. The recoveries of these phenolic compounds by FC method varied from 43.3% to 168.1%. The total contents of phenolic compounds at 3- 7 leaf stages were always significantly higher in the culture solution of allelopathic rice PI312777 than non-allelopathic rice Lemont. The maximum content was 710 µg/plant in the culture solution of PI312777 at the 6-leaf stage, significantly higher than 10 µg/plant/day, reported earlier by 4-AAP method. Besides, at 3- 7 leaf stages, the total contents of 7 phenolic acids exuded were significantly higher by PI312777 than Lemont. These results supported our suspicion that both benzoic acid and cinnamic acid derivatives cannot be determined accurately by the 4-AAP method. We suggested that the phenolic acids are the principal components in rice allelopathy.