Hydrolytic Activity and Ultrastructural Changes in Fruit Skins from Two Prickly Pear (Opuntiasp.) Varieties during Storage

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Abstract
The activity of four cell wall hydrolases, pectinmethylesterase (PME), polygalacturonase (PG), cellulase, and β-galactosidase (β-Gal), was measured in fruit skins of two prickly pear varieties, Naranjona and Charola, during storage at 18 °C and 85−95% relative humidity (RH). In Naranjona (Opuntia ficus indica), of short postharvest life (ca. 2 weeks), PG, cellulase, and β-Gal increased their activity more than twice, whereas PME activity tended to increase only slightly during storage. In Charola (Opuntia sp.), of long postharvest life (ca. 2 months), only β-Gal increased its activity (77%), showing a high PG activity from the beginning of storage. Transmission electron microscopy observations showed middle lamella dissolution at the end of storage for both varieties. Naranjona showed a higher cell wall enzymatic activity than Charola, in agreement with their storability differences. Our results suggest that PG and cellulase in Naranjona and PG and β-Gal in Charola are the main enzymes responsible for cell wall hydrolytic and ultrastructural changes in skins of stored prickly pears. Keywords: Opuntia sp.; cell wall; polygalacturonase; pectinmethylesterase; cellulase; β-galactosidase; nonclimacteric