Cell wall peroxidases in cucumber roots induced by phenolic allelochemicals

Seven-day-old seedlings of the cucumber (Cucumis sativus L.) cv. 'Dar' were treated with 0.5 mM solutions ferulic and p-coumaric acids. Peroxidase located in the cell walls Of cucumber roots showed affinity to all four applied substrates. The degree of affinity to particular substrates followed the order: (a) covalently bound peroxidases : syringaldazine > guaiacol > ferulic acid > p-coumaric acid and (b) for ionically bound peroxidases: guaiacol > syringaldazine > ferulic acid > p-coumaric acid. The exposure of cucumber roots to ferulic and p-coumaric acids increased the hydrogen peroxide, which induced the activity of peroxidases, covalently bound to cell walls, but did not influence the activity of ionically bound peroxidase. Ferulic acid showed a stronger effect than p-coumaric acid. The results indicate that exogenous hydroxycinnamic acids (ferulic and p-coumaric) after penetrating a plant, may be subjected to peroxidative oxidation in the root cell walls.