Production of hydrogen peroxide (H 2 O 2 ) by the lignifying xylem of several vascular plants has been studied using a new histochemical method based on the H 2 O 2 -dependent oxidation of 3,5,3′5′-tetramethylbenzidine (TMB) catalysed by cell wall peroxidases. This method allows H 2 O 2 to be determined in the range of 5–100 μ m , where other methods, such as the KI/starch reagent, fail. With this method, it has been possible to determine H 2 O 2 production in the lignifying xylem of a wide range of vascular plants (gymnosperms and angiosperms). The capability of xylem tissues of sustaining H 2 O 2 production lends support to the hypothesis that cinnamyl alcohol polymerization in xylem vessels is caused by an H 2 O 2 -dependent oxidative coupling process.
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A. Ros Barceló (1998) studied this question.