Allelopathic potential and chemical composition of rhinacanthus nasutus extracts

Aqueous methanol extracts of Rhinacanthus nasutus (L.) Kurz inhibited the shoot and root growth of cress (Lepidum sativum L.), lettuce (Lactuca sativa L.), alfalfa (Medicago sativa L.), timothy (Phleum pratense L.), Italian ryegrass (Lolium multiflorum Lam) and crabgrass (Digitaria sanguinalis L.) at concentrations > 0.1 g dry weight equivalent extract/mL and the inhibition increased with increasing extract concentration. These results suggest that the extract of R. nasutus may possess allelopathic activity. An active substance was then isolated and determined by ¹H-, ¹³C-NMR and MS as 4-hydroxy-3-methoxybenzoic acid (vanillic acid). This is the first report of finding the vanillic acid in R. nasutus. The concentrations required for 50% growth inhibition of cress and lettuce were about 470 - 800 μM and the concentrations required for 50% growth inhibition of timothy and Italian ryegrass were 0.17 - 8.2 μM, which suggest that effectiveness of vanillic acid on timothy and Italian ryegrass was much greater than on cress and lettuce. The inhibitory activity and the occurrence of vanillic acid in R. nasutus suggest that vanillic acid may play an important role in the allelopathic properties of R. nasutus. The information obtained could be utilized in the development of bioherbicide for future weed management.