Fruit Quality, Fermentation Products, and Activities of Associated Enzymes During Elevated CO2 Treatment of Strawberry Fruit at High and Low Temperatures

The effects of postharvest treatments of air and 20 kPa CO2 (in air) at 2 or 20 °C on color, fi rmness, accumulations of acetaldehyde, ethanol, and ethyl acetate, activities of pyruvate decarboxylase (PDC) and alcohol dehydrogenase (ADH) activity, and expression of an ADH gene were studied in strawberry fruit (Fragaria ×ananassa Duch. cv. Jewel). CO2 treatment enhanced strawberry fruit fi rmness at 2 °C but not 20 °C, while the rate of color changes was affected by CO2 treatment at 20 °C but not at 2 °C. Temperature also affected the accumulation of acetaldehyde, ethanol and ethyl acetate in CO2-treated fruit. All three compounds accumulated in fruits at 2 °C. At 20 °C, ethanol accumulated slightly by day 6, although ethyl acetate accumulated in fruit from both atmospheres. PDC enzyme activity was higher in CO2-treated fruit than their air-treated control at 2 °C but not at 20 °C. ADH activity and ADH mRNA accumulation of the CO2-treated berries were higher than in air at 20 °C but not 2 °C. The results, overall, indicate that patterns of change among gene expression, enzyme activities, and fermentation product accumulation were not consistent. The postharvest tolerances of most commercially important fruit and vegetables to low O2 and elevated CO2, and the optimum atmosphere combinations to achieve maximum product storability, have been established. However, wide variations in tolerances to storage atmospheres exist within and among species, cultivars, organ types, and developmental stages, these variations being a function of both metabolic and physical factors (Beaudry, 2000; Watkins, 2000). When a harvested product is subjected to atmospheres outside safe limits at any temperature/time combination, certain physiological disorders may develop. Off-fl avor, usually associated with accumulation of the fermentation products, acetaldehyde and ethanol, is often detected before visible damage and sometimes results in rejection of produce in the marketplace. Accumulation of acetaldehyde and ethanol is a major metabolic response of plant tissues to anoxia or severe hypoxia. When O2 is limited, electron transport and oxidative phosphorylation are inhibited (Kennedy et al., 1992), and fermentation then provides a mechanism for pyruvate oxidation. In the fermentation pathway, acetaldehyde is produced by pyruvate decarboxylation via pyruvate decarboxylase (PDC), and is then converted to ethanol by alcohol dehydrogenase (ADH). In plant systems, such as fruit, esters also are produced by esterifi cation of alcohols, e.g., ethyl acetate is formed from ethanol and acetyl CoA by the activity of alcohol acyltransferase (AAT) (Ke et al., 1994a; Perez et al., 1996). Most research on atmosphere effects on plant metabolism has concentrated on metabolism of growing plants, often of roots, under anoxia (Geigenberger, 2003). In fruit and vegetables the focus has been on fermentation metabolism under low O2 partial pressures (Chen and Chase, 1993; El-Mir et al., 2001; Imahori et al., 2002). Because most products respond optimally to low O2, relatively Received for publication 13 Apr. 2004. Accepted for publication 11 July 2004. This work was supported by the U.S. Dept. of Agriculture Competitive Grants Program (grant no. 99-35503-8165) and Federal Formula Funds, Regional Project NE 103. We thank Dr. Marvin Pritts for supply of fruit, and Jackie Nock