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Levels and histochemical localization of peroxidase and polyphenol oxidase, and levels of anthocyanins and (+)-catechin, were studied in fruit of two strawberry ( Fragaria × ananassa Duch.) cultivars ('Oso Grande' and 'Chandler'), which show different degrees of susceptibility to enzymatic browning after processing. Although the levels of anthocyanins at the processing-ripe stage may be important in determining pigment stability, and therefore market suitability, the color stability of 'Chandler' is apparently determined by the lower endogenous levels of peroxidase and polyphenol oxidase in the processing-ripe stage, which are also accompanied by a lower (+)-catechin content. Polyphenol oxidase was localized almost exclusively in the cortex and to a lesser extent in the pith, showing a complementary pattern to that shown by peroxidase, which was localized in the vascular bundles. Since peroxidase and polyphenol oxidase showed a complementary localization pattern in the fruit, these results strongly suggest a synergic role for these two oxidative enzymes in pigment decay and the associated browning reaction, which occurs in processed strawberry fruit and their derived foods. under mildly oxidizing conditions, it was found that phenolic metabolism in strawberries may be either oxidative (H 2O2 inde- pendent) or peroxidative (H 2O2 dependent), and that treating strawberry slices with H 2O2 stimulates oxidative phenomena that take place in the absence of H 2O2, such as anthocyanin and (+)- catechin degradation, and brown polymer formation (Lopez- Serrano and Ros Barcelo, 1999). These results suggested a synergic role for both peroxidase and polyphenol oxidase in (+)- catechin and anthocyanin degradation, and in brown polymer formation, during the aging of strawberry fruit under mildly oxidizing conditions. The color stability of processed strawberry fruit is known to differ according to the cultivar (Lopez-Serrano and Ros Barcelo, 1995b, 1996). For example, 'Oso Grande' and 'Chandler', two widely used strawberry cultivars in the food industry, both show distinctive features. The former lacks anthocyanins in its heart and is therefore considered a white heart cultivar, while 'Chan- dler', which shows a high heart anthocyanin level, is considered as a red heart cultivar. The color (pigment) stability of processed 'Chandler' is greater than that of 'Oso Grande' (Lopez-Serrano and Ros Barcelo, 1995b), and is therefore more suitable for processing, although differences in anthocyanin content are slight. In this report, we studied the levels and histochemical localization of both peroxidase and polyphenol oxidase, the two enzymes involved mainly in enzymatic browning in processed strawber- ries (Lopez-Serrano and Ros Barcelo, 1999; Spayd and Morris, 1981) during fruit ripening, in order to explain differences in color stability between the two strawberry cultivars. In addition, levels of anthocyanins at the processing-ripe stage, which deter- mine strawberry color, and of (+)-catechin, a pro-oxidant com- pound (Richard-Forget and Gauillard, 1997; Subramanian et al., 1999), were also studied.

Histochemical Localization and Developmental Expression of Peroxidase and Polyphenol Oxidase in Strawberries