Wounding-induced xylem occlusion in stems of cut chrysanthemum flowers : roles of peroxidase

A wounding-induced xylem occlusion, resulting in severe leaf wilting, occurs in stems of cut chrysanthemum flowers (Dendranthema grandiflora), cv. Vyking. The blockage develops after about 1 h in flowers held in air at 20 °C. It is initially located in the lowermost 2 cm of the stem and upon prolonged exposure to air it is also found above 2 cm. We tested the possible role of peroxidase (EC 1.11.1.7) and phenoloxidases in the blockage. Some peroxidase inhibitors (copper ions and 3-amino-1,2,4-triazole) delayed the occlusion, and treatment with compounds that inhibit peroxidase but stimulate phenoloxidase (catechol, hydroquinone, p-phenylene diamine) had the same effect. Some inhibitors of phenoloxidase (p-nitrophenol, p-chlorophenol, p-nitrocatechol, and sodium metabisulfite) also delayed the occlusion. Phenoloxidase activity in plants comprises catechol oxidase (EC 1.10.3.1) and laccase (EC 1.10.3.2). The blockage was considerably delayed by catechol oxidase inhibitors (tropolone and 4-hexylresorcinol), even more so than with general phenoloxidase inhibitors. The results indicate that the occlusion is mainly due to a physiological (oxidative) process, requiring both peroxidase and catechol oxidase activity