Stability of phenolic acids and the effect on weed control activity

Phenolic acid is a very important class of allelochemicals with allelopathic weed control activity. In this study, three benzoic acid derivatives (syringic, 4-hydroxybenzoic, and vanillic acids), three cinnamic acid derivatives (cinnamic, 4-hydroxycinnamic, and ferulic acids) were tested, and high-performance liquid chromatography was used to conduct a dynamic analysis on the changes in the concentration of phenolic acids in a bioassay based on the initial concentration and test time. The results showed that the concentration of individual phenolic acids and a solution of mixed phenolic acids decreased to a certain extent irrespective of environment, i.e., bioassay (4–7 days) or a rice-growing environment, and a significant decrease in concentration was measured after 48 h. Based on the above results, the laboratory bioassay was conducted using a fresh solution of phenolic acids every 48 h. The results showed that the instability of phenolic acid could affect its weed control activity, and this effect was more significant for high concentrations of phenolic acids. On the other hand, changing the solution did not have a significant impact on the weed control activity of phenolic acids in the natural environment (pH 6.50), in which allelopathic rice release phenolic acids. These results reveal the instability of phenolic acids could significantly reduce the inhibition rate on the growth index for receptor plants in an indoor bioassay.