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Hanling Yu and Claude Willemot

We examined the relationship between reduced galactolipid content in tomato fruit at 4C and chilling injury. Galactolipid biosynthesis from 14C-acetate was compared in pericarp discs of cold-tolerant `New York 280' (`NY') and -sensitive `Early Cherry' (`EC') at 4C and 20C. Labeled lipids were separated by 2D-TLC. Labeled monogalactosyldiglyceride (MGDG) molecular species were hydrolyzed using a position-specific lipase; the fatty acids released were hydrogenated and separated according to chain length by reverse-phase TLC. At 4C, the relative amount of radioactivity was reduced in MGDG and enhanced in phosphatidylcholine (PC) in both cultivars, in comparison with labeling at 20C. In discs from fruit chilled for 6 h, labeling was similar in `NY' and `EC'. In fruit held at 4C for 8 days, labeling of MGDG was reduced and that in PC was enhanced to a greater extent in chilling-sensitive `EC' than in `NY'. The proportion of the MGDG label in eukaryotic species (i.e., the ratio in C18/C16 fatty acids in position sn-2), was less in `EC' at 4C than at 20C, even for fruit held at 4C for only 6 h. The ratio was little affected in `NY'. The data indicate that biosynthesis of eukaryotic MGDG was inhibited in tomato fruit at chilling injury-inducing temperatures.

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France Côté and Claude Willemot

Five tomato cultivars were tested for tolerance to chilling. After exposure of varying times to chilling at 3 °C, the fruits were returned to ambient temperature for development of chilling injury (CI) symptoms (uneven ripening and pitting). Ripening was assessed by measuring carotenoids. Electrical conductivity (EC) of leachate from pericarp discs, an indirect measure of membrane damage, was used to determine CI. During chilling EC greatly increased in the three sensitive cultivars, but hardly in the tolerant ones, in good correlation with the development of CI symptoms after rewarming. However, this correlation broke down after returning the fruit to 20 °C. While slightly injured fruit showed a large increase in EC, surprisingly EC was drastically reduced in the extensively injured fruit. Calcium pectate production due to cell wall degradation may explain the lack of correlation between EC and CI after rewarming. We conclude that EC is not always a reliable measure of membrane damage.

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Claire Hébert and Claude Willemot

Plant antioxidants have gained considerable interest because of their importance for the preservation of produce and also because of their therapeutic properties. There is increasing evidence that these compounds protect plant tissues from stress and that they delay senescence. Seven strawberry cultivars were analyzed to investigate the possible relationship between their antioxidant potential and fruit shelf-life. The antioxidant defense systems studied were free radical scavenging enzymes (SOD, catalase, glutathione reductase, GSH, ascorbate peroxidase, ascorbate free radical reductase), ascorbic acid, and ellagic acid. Enzyme assays were performed using spectrophotometric kinetic measurements. Ascorbic acid and ellagic acid were determined by HPLC. The antioxidant potential of the tissues had an incidence on fruit quality and shelf-life. The impact of these antioxidative parameters will be discussed with respect to breeding criteria for reduced perishability of strawberries.

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Richard Voisine, Claude Willemot, and Louis Vézina

Cauliflowers (Brassica oleracea) were irradiated at 0, 2, and 4 kGy and stored 8 days at 13°C. Development of yellow color and browning of the in florescence, increase in membrane electrolyte leakage and reduction of protein recovery in microsomal membranes were observed over the storage period. Changes in membrane free fatty acids, lipid phosphorus content, peroxydation level, and fatty acid composition of polar lipids also occurred. These results indicate an important modification of cellular membranes. The direct effect of gamma rays on membrane lipids via free radical production and subsequent destabilization of the lipid bilayer during storage could be responsable for earlier onset of senescence.

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Jeanne Bernardin, Claude Willemot, and Clement K. Sankat

The effect of Ca on breadfruit (Artocarpus altilis) postharvest storage was investigated. Mature-green breadfruits were hand-harvested in Blanchisseuse, Trinidad, dipped in 0%, 2%, 5%, and 10% CaCl2-2H2O solutions for 0.5, 1.0, 3.0, 6.0, and 12.0 hours, and stored at 16C for 9 days. Calcium content was shown to increase in both peel and pulp with increasing concentration and length of treatment. The 5% and 10% Ca treatment had a detrimental effect on color and texture as determined by sensory evaluation. The 2% treatment delayed fruit softening, particularly for 3-, 6-, and 12-hour dips. At the end of storage, total soluble solids content was affected little by the treatments, while pectin solubilization was delayed. Breadfruit shelf life was extended from 4 to 9 days with 2% treatments. Peel browning remains the limiting factor for storage.

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Hélène Lambert, Claude Willemot, John E. Thompson, and Joseph Makhlouf

This research is aimed at the identification of volatile compounds from the isolated membranes fractions, microsomes, and deteriosomes. Fractions were isolated from tomato pericarp by ultracentrifugation at 252,000x g during 1 hour, followed by 362,000x g during 12 hours. The supernatant was infiltrated through a membrane of 300,000 D cut off to concentrate the deteriosomes. The volatiles from the fractions were analyzed by dynamic headspace and GC-MS. Our results suggest that the isolated fractions contained most tomato volatiles. Analysis by GC-MS identified two groups: compounds originating from fatty acids [e.g., hexanal and (E)-2-hexenal] and compounds coming from amino acids (e.g., 2 and 3-methyl butanal). Both microsomes and deteriosomes were highly enriched in volatiles on a protein basis. The increase in volatile compounds in these fractions was influenced by fruit maturity and correlate closely with volatile development in the intact fruit. Volatiles may be generated in the microsomes and released from the membranes via deteriosomes.

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Han-Ling Yu, Claude Willemot, Serge Yelle, Yves Castonguay, and Paul Nadeau

Translatable mRNAs from two tomato (Lycopersicon esculentum Mill.) cultivars differing in chilling tolerance were compared after 16 days of chilling at 4C and after return to 20C for 1 and 5 days. Before chilling, the translation products, resolved by 2D NEPHGE, showed significant differences between more tolerant `New York 280' (NY) and less tolerant `Early Cherry' (EC). In NY, chilling reduced the level of five to 10 mRNAs and enhanced or induced that of several other mRNAs. After transfer to 20C, the trend was progressively reversed. Changes in the levels of two low-molecular-weight basic peptides were most noticeable. One, absent in NY before chilling, was strongly expressed after chilling and 24 h after transfer to 20C, but disappeared 5 days after transfer. The level of this peptide increased slightly in EC at low temperature and was maintained after transfer to 20C. The level of the other, high in NY before chilling, was sharply reduced after chilling. In contrast, the level of this polypeptide was low in EC under all treatments.