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One of the symptoms of senescence in harvested horticultural crops is the loss of greenness that comes with the degradation of chlorophyll (Chl). With senescence, peroxidase, which is involved in Chl degradation, increased greatly in stored horticultural crops. C132-hydroxychlorophyll a, an oxidized form of Chl a, is formed in vitro through Chl oxidation by peroxidase. Peroxidase mediates Chl degradation in the presence of phenolic compounds such as p-coumaric acid and apigenin, which have a hydroxyl group at the p-position. Apparently, not all phenolic compounds are able to degrade Chl in this system, and their effectiveness appears to depend on their molecular configuration. In peroxidase-mediated Chl degradation, peroxidase oxidizes the phenolic compounds with hydrogen peroxide and forms phenoxy radical; then, the phenoxy radical oxidizes Chl and its derivatives to colorless low molecular weight compounds through the formation of C132-hydroxychlorophyll a,a fluorescent Chl catabolite and a bilirubin-like compound as an intermediate. In addition to the phenoxy radical, superoxide anion, which is formed in the peroxidase-catalyzed reaction, might be involved in Chl oxidation. Moreover, Chl degradation by peroxidase seems to occur in the chloroplast and/or the vacuole. The involvement of peroxidase in Chl degradation in senescing horticultural crops is also discussed.