Water-soluble phenolic acids and flavonoids involved in the bioherbicidal potential of Ulex europaeus and Cytisus scoparius

Abstract The use of allelopathic foliage incorporated into the soil as a bioherbicidal amendment is a promising weed management strategy which allows combining a cocktail of allelochemicals at low effective individual concentrations. In previous studies, the herbicidal potential of Ulex europaeus and Cytisus scoparius has been argued to be mediated by volatile organic compounds (VOCs). The present work undertakes for the first time the potential involvement of phenolic acids and flavonoids in the herbicidal bioactivity of both shrub species. Such compounds are those most likely to be released to soil water when the allelopathic foliage is incorporated into the soil. By dose-response in vitro bioassays of aqueous extracts, the flowering foliage of both shrub species was shown to release water-soluble compounds capable of inhibiting the germination and/or early growth of two agricultural weeds: Amaranthus retroflexus and Digitaria sanguinalis. Far from expected, such phytotoxicity was enhanced when the aqueous extracts were applied to fresh soil. Via HPLC-DAD, 11 and 17 phenolic compounds were identified from U. europaeus and C. scoparius aqueous extracts, respectively, some of them cited for the first time in both species. The phenolic acids trans-cinnamic, p-coumaric, and ferulic, when were separately assayed in vitro at 1 mM, resulted phytotoxic to one or other target weed and physiological processes. However, except for the caffeic acid and vanillin, the phenolic acids lost effectiveness when applied at lower concentrations (0.1 or 0.01 mM). Otherwise, it was noteworthy that the flavonoids luteolin, ellagic acid, kaempferol, and naringenin were only phytotoxic when applied at low concentrations (≤0.1 mM). The proven herbicidal efficiency of the shrub foliage was argued to derive from the phytotoxicity of the water-soluble compounds assayed, and maybe from the combined or synergistic action with the VOCs emitted from the fresh plant material.