Submitted to: Postharvest Biology and Technology
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 6/16/2007
Publication Date: 2/1/2008
Citation: Zheng, Y., Fung, R., Wang, S.Y., Wang, C.Y. 2008. Transcript levels of antioxidative genes and oxygen radical scavenging enzyme activities in chilled zucchini squash in response to superatmospheric oxygen. Postharvest Biology and Technology. 47:151-158. Interpretive Summary: About one third of all fresh produce in this country is sensitive to chilling temperature. This means that these crops can not be stored at cold temperatures after harvest to preserve their freshness like other non-sensitive crops, otherwise chilling injury symptoms would occur. These symptoms include skin discoloration, pitting, flesh browning, uneven ripening, and decay. Therefore, chilling injury causes these crops to lose their market value. It is estimated that postharvest losses resulting from chilling injury amount to about $10 billion annually in the U.S. In our experiments, we treated zucchini squash, chilling-sensitive crop, with high concentration of oxygen immediately after harvest and found that this treatment helped reduce a chilling injury symptoms during cold storage and maintained the quality of the commodity for an extended period. This high oxygen treatment also enhanced the antioxidant system in the tissue. Therefore, it is possible that a high oxygen atmosphere enables the plant tissue to have a more efficient antioxidant system. This in turn increases the resistance of tissue against oxidative damages caused by chilling stress. This information is useful to other postharvest scientists and has the potential to benefit the fresh produce industry.
Technical Abstract: The transcript levels of antioxidative genes including Mn-superoxide dismutase (Mn-SOD), Cu/Zn SOD, ascorbate peroxidise (APX), and catalase (CAT) do not vary significantly during storage at 5 °C with high oxygen treatment in freshly harvested zucchini squash (Cucurbita pepo L. cv. Elite). However, the expressions of alternative oxidase (AOX) were higher in squash treated with 60% and 100% oxygen for 3 days than those in the control samples. These increases in AOX transcript levels were correlated with the increased chilling resistance in the treated squash. Their corresponding oxygen radical scavenging enzyme activities including SOD, APX, CAT and peroxidise (POD) in treated samples were also higher than those in the control for the first 3 days at 5 °C. Transcript levels of AOX increased substantially between 3 to 6 days in all treatments indicating that the importance of AOX for the defense response of squash tissue to chilling stress. All of the enzyme activities in 100% oxygen treated squash started to decline after 6 or 9 days of cold storage to a level comparable or lower than that of the control. These declines were correlated to the loss of chilling resistance in the 100% oxygen treated tissue as indicated in the chilling injury index. However, squash treated with 60% oxygen maintained elevated levels of all enzyme activities except POD and sustained the least chilling injury throughout the 15 days of storage at 5 °C. The oxygen radical absorbance capacity (ORAC) values and total phenol content remained higher in squash treated with 60% and 100% oxygen for the first 9 days, then their levels in the 100% treated samples declined sharply while those in the 60% treated samples maintained elevated, indicating that both ORAC activity and phenolic content may also contribute to the resistance of tissue against chilling injury. The 100% treated squash showed the lowest respiration rate and 60% treated samples had the lowest ethylene production. These data may also be an indication of the low chilling injury in the high oxygen treated squash.