Mechanical Stress, Storage Time, and Temperature Influence Cell Wall-degrading Enzymes, Firmness, and Ethylene Production by Cucumbers

Abstract

Abstract Cucumber (Cucumis sativus L.) fruit (‘Heinz 3534’) subjected to mechanical stress followed by storage for 48 hr exhibited visible degeneration of the mesocarp and endocarp, which was accompanied by several-fold increases in the activity of the enzymes pectin methylesterase, peroxidase, polygalacturonase, and xylanase. The activity of all these enzymes increased in the endocarp, whereas only pectin methylesterase and polygalacturonase increased in the mesocarp, and pectin methylesterase, peroxidase, and polygalacturonase increased in the exocarp. Further, the increase in the activity of pectin methylesterase, peroxidase, and polygalacturonase was less when cucumbers were stored at 0° or 10°C vs. 25° or 38° after mechanical stress. Cucumbers stored for only 8 hr after mechanical stress, or not stressed, and stored for 8 or 48 hr showed no consistent significant increases in enzyme activity. Endocarp firmness of fruit stored at 25° or 38° for 8 hr after mechanical stress was lower than that of unstressed fruit, but this decrease was not evident after 48 hr of storage, and mesocarp firmness was not affected by mechanical stress regardless of storage temperature or time. Ethylene production was stimulated significantly by 8-hr storage at 0°, following mechanical stress, and by 48-hr storage at 0°, but was unaffected by all other treatment and storage regimes. These data indicate that mechanical stress induces biochemical and morphological changes in the major tissues of cucumber fruit, but tissue firmness and/or ethylene production will not serve as indicators of these changes. Moreover, the effects of mechanical stress do not appear to be mediated through the action of ethylene.