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  • Author or Editor: Zhanyuan Du x
  • HortScience x
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Much correlative data support the hypothesis that superficial scald on apples results from oxidation of α farnesene to conjugated trienes (CT) in the coating of apples. However, these associations are poorly defined both chemically and physiologically. α Farnesene and CT are measured as OD 232 and OD 281-290, respectively, of a hexane extract of the fruit surface. During assays, we observed anomalies in absorbance characteristics of extracts from fruit with different scald potentials, particularly in the region of 258 nm. Results suggest that absorbance near 258 nm might represent a metabolite of CT, which may be further metabolized. It appeared that under different conditions, CT metabolism could be altered, resulting in changed ratios of OD 258/OD 281. Higher ratios correlated with lower scald development, regardless of CT concentration. Thus, CT metabolism, rather than its concentration, may determine if scald occurs.

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Ethephon treatments had two opposing effects on scald induction. First, synthesis and metabolism of α- farnesene were immediately enhanced, which could increase scald development. Second, during prolonged storage the relative concentrations of two conjugated triene forms (CT281 and CT258) were altered so as to increase the CT258/CT281 ratio, which could reduce scald development. The balance between these responses determined whether ethephon increased or decreased scald. DPA treatment also had two effects, immediately suppressing ethylene and α- farnesene concentrations, and over long periods, suppressing CT281 but increasing CT258 concentrations. Both effects of DPA appeared to reduce scald development. Effects of DPA, as well as of ethephon, were at least partly ethylene - mediated, and treatment with DPA counteracted effects of an ethephon treatment.

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When Cortland apples were stored at 0 and 20C, 0C reduced ethylene production and increased accumulations of ∞-farnesene and conjugated trienes (CTs) in fruit peel, but it resulted in a lower CT258: CT281 ratio than did 20C. At 20C no fruit developed scald, but at 0C, 84% of the fruit scalded. When fruit were stored at 0C but transferred to 20C for 5 days after 0 to 8 weeks at 0C, only transfer after 2 or 4 weeks reduced scald. During warming, ethylene production and accumulations of ∞-farnesene and CTs increased, but after 20 weeks of storage, when scald developed, warming at 2 and 4 weeks reduced CT281 and increased the CT258: CT281 ratio. Warming at other intervals had no effect.

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Superficial scald of apples is believed to be caused by the oxidation of α-farnesene to conjugated trienes, which then perturb membrane lipids and cause cell death. The antioxidant diphenylamine that is used for commercial scald control is believed to inhibit α-farnesene oxidation. However, measurement of TBA-reactive substrates and total peroxides in apple peel showed that marked increases occurred as fruit senesced, but only slight changes were associated with conditions that specifically led to scald development. Similarly, changes in activity of catalase, peroxidase, and SOD occurred as fruit senescent progressed, but changes in these enzymes had no association with conditions that promoted scald development. We also found that DPA primarily affected accumulation of α-farnesene, with only a small effect on its oxidation to conjugated trienes. These results cast doubt on the hypothesis that lipid peroxidation in membranes produces superficial scald on apples.

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When `Granny Smith' apples were stored at different temperatures, they produced different patterns of ethylene synthesis. Accumulation of α-farnesene and conjugated trienes (CT) in peel of these fruit paralleled ethylene production. Both pre- and postharvest applications of ethephon to `Cortland' apples increased accumulation of α-farnesene and CT, whereas application of DPA suppressed ethylene production and accumulation of α-farnesene and CT. When both ethephon and DPA were applied, their respective effects were negated. In these experiments, inhibition of ethylene synthesis always reduced scald development after storage, but stimulation of ethylene did not always increase scald development Ethylene had two effects: an immediate stimulation of α-farnesene and CT accumulation which increased scald susceptibility, and also a slowly developing alteration of CT metabolism, which reduced scald susceptibility. The effect of high ethylene on scald development depended on the balance between these responses.

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