Postharvest ethylene production and ACC levels were determined in netted muskmelon fruits (Cucumis melo L. var. reticulatus `Magnum 45') exposed to temperature extremes by heating for 3 hr at 45C and/or storage at 4C. The possibility of using seal-packaging to protect the fruit against temperature-induced changes in ethylene production was examined by wrapping melons before treatment with a high-density polyethylene (HDPE) shrink-film. Ethylene production measured in fruit immediately after heating or removal from refrigeration was only 30% of the level determined before treatment, and continued to decline during refrigerated storage. However, the concentration of ACC in these same tissues remained constant or even increased slightly during storage. Wrapping fruit in HDPE film had no effect on the tissue concentrations of ACC or capacity for ethylene synthesis. In contrast to initial measurements, heated or refrigerated fruit held at room temperature (25C) for 24 hr produced ethylene at rates that equalled or exceeded the levels for freshly harvested fruit. These results strongly suggest that temperature-imposed restrictions on ethylene synthesis by netted muskmelon fruit are reversible and occur at the step responsible for converting ACC to ethylene via EFE rather than in the synthesis of ACC. Chemical names used: 1-aminocyclopropane-1-carboxylic acid (ACC).
Postharvest sugar content and activities of four enzymes of sucrose metabolism were followed in the edible mesocarp tissue of the netted muskmelon (Cucumis melo L., var. reticulatus, cv Magnum 45). Melons harvested at full-slip were shrink film-wrapped to inhibit water loss and/or heated to 45°C for 3 hr before storage at 4°C for up to 18 days. Sucrose content of edible mesocarp remained constant between harvest and 12 days of storage. The sucrose content of nonheated fruit declined between 12 and 18 days of storage, but there was no coincident increase in glucose or fructose. There was little acid invertase activity. Neutral invertase activity did not vary significantly with storage, but was slightly higher in heated, wrapped fruit than in nonheated, wrapped fruit. Sucrose synthase activity increased with storage, and was higher in heated than nonheated fruit. There was no discernible pattern of sucrose-phosphate synthase activity. No enzyme activity was correlated with the content of any sugar.