RELATION OF CHILLING STRESS TO POLYAMINES IN ZUCCHINI SQUASH

There is increasing evidence that elevated polyamine levels are beneficial in maintaining postharvest quality of fruits and vegetables. Controlled atmosphere storage has been shown to increase polyamine levels in apples (Kramer et al., 1989), Chinese cabbage (Wang, 1988), and zucchini squash (Wang and Ji, 1988). Temperature conditioning also increased polyamine levels in zucchini squash (Kramer and Wang, 1989). Postharvest treatment with polyamines retarded softening in Golden Delicious and McIntosh apples (Kramer et al., 1991). The reduced rate of softening in long-keeping tomato cultivars has also been correlated with elevated polyamines (Dibble et al., 1988). The purpose of the studies reported here was to determine if the elevated polyamine levels induced by temperature conditioning and low oxygen storage are related to the reduction of chilling injury, and if direct treatments of polyamines alleviate chilling injury in zucchini squash.

Zucchini squash (Cucurbita pepo L., cv. Ambassador) were freshly harvested from a local farm near Beltsville, MD. For low oxygen storage, samples were stored in sealed stainless steel chambers and flushed with 1% O₂ at the rate of 100 mL min^-1 at 2.5C. Another group of squash was similarly stored but flushed with air to serve as control. For temperature conditioning, samples were placed at 10C for the first 2 days of storage and then moved to 2.5C for the remainder of the study. The control fruit were placed at 2.5C immediately after harvest until the end of the experiment. The nonchilled control group was stored at 12.5C. Polyamine treatment of the fruit involved the use of pressure infiltration. The squash were immersed in 10 mM spermine and subjected to 82.7 kPa of air pressure for 3min. Samples were taken at various intervals throughout the storage period for chilling injury evaluation and polyamine analysis as described previously (Kramer and Wang, 1989).

The development of chilling injury symptoms progressed rapidly in squash after 3 days in air storage at 2.5C. Severe pitting and slight decay were observed after 12 days of exposure to chilling temperature. Both low O₂ storage and temperature conditioning treatments were effective in delaying the development of chilling injury symptoms. The difference in the severity of surface pitting between the control and treated squash was evident after 6 days of exposure to 2.5C. The squash stored under 1% O₂ maintained higher levels of spermidine and spermine than those stored in air throughout the experiment. Temperature conditioning also significantly increased spermidine and spermine levels. These results indicate that the increased resistance to chilling injury in

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