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  • Author or Editor: William J. Bramlage x
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During the harvest season apples ripen and develop scald resistance. In the Northeast they usually are also exposed to cool temperatures as they mature and ripen. Experiments were conducted to study the effects of cool temperature, light and maturity on the endogenous antioxidants and subsequent scald development in Cortland and Delicious apples. Total lipid-soluble antioxidant activity in apple peel at harvest generally increased as scald incidence after storage decreased. Yet, α tocopherol, ascorbic acid and total water-soluble reducing capacity were not closely related to scald development. The absence of light (bagged fruit) decreased all measured antioxidants and increased scald development. However, ethephon applied in mid-August to induce ripening increased the levels of these antioxidants but had little effect on scald incidence in the absence of cool temperatures (hours <10°C). Cool temperatures, which decreased scald susceptibility, increased lipid-soluble antioxidant activity but had little influence on the other measured antioxidants. These data suggest that the endogenous antioxidants may be only partly responsible for natural scald resistance.

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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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Fifty four trees in a block of `Sturdeespur' Delicious on M.106 rootstock were given soil applications of various combinations of boron and gypsum. Twenty 2.9 to 3.0 inch diameter fruit and 20 unsized randomly chosen fruit were separately weighed and analyzed for Ca, Mg, K, and B concentrations. The coefficient of variation among fruit weights was greater for unsized than sized samples. Analyses of variance showed similar treatment effects for the two sampling methods. However, by using both sized and unsized fruit, effects of treatment on fruit size, and effects of relationships between fruit size and fruit mineral concentration may be separated.

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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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Seeking non-chemical alternatives to use of DPA for scald control on apples, we interrupted storage with a brief warming period. This often reduces chilling injuries of fruit. Warming `Granny Smith apples for 5 days at 20 C after 2 weeks at 0 C reduced scald as effectively as a 1000 ppm DPA treatment at that time. To better characterize this response, we tested other timings of the warming period, and also lower warming temperature. Warming at 10 C, or for shorter times at 20 C, or after longer periods at 0 C all were less effective. Maintaining a warm period before storage was not effective. During warming of `Cortland' and `Delicious' apples softening and loss of green color occurred, the extent of which increased with warming time and usually was greater if the fruit had initiated the ethylene climacteric before warming.

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Fruit from five regions worldwide were rated for scald development after four months in 0°C air plus one week at 20°C. Scald incidence was quantitatively related to preharvest temperature conditions and fruit maturity, as measured by starch index. Various low temperature cutoffs were used, and high temperatures over 30°C were also used in the equations. Where data were available light conditions and rainfall were included in the equations. Several models were developed and tested to determine if prediction equations could be of commercial value. The most successful equations explained over 50% of the variation in scald. While they could not predict exact percentages of fruit which would develop scald, equations could predict cases of very high and very low scald, thus identifying fruit requiring the greatest scald control measures and those needing minimal scald control action.

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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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Apples generally become less susceptible to scald as the season progresses and the fruit ripen. However, ripening effects may be confounded with effects of low temperature. Over 3 seasons we tested interactive effects of hrs < 10°C and ripening on scald susceptibility on Cortland apples. Ripening was induced by 500 ppm ethephon. In 1989, ethephon advanced ripening, increased endogenous antioxidant concns, and reduced scald incidence by 30% when 62 hrs <10°C had occurred before harvest. In 1990 no hrs <10°C had accumulated at harvest. Ethephon increased endogenous antioxidants and advanced ripening, but had little effect on scald development (>90% incidence). Low temps during ripening may be needed for ripening to induce scald resistance. In 1991, ethephon was applied once to apples at 6 different stages of development. Fruit were harvested a week later, after 14-156 hrs <10°C. Ethephon advanced ripening at the first 4 applications, but at the first 3 insufficient cool temperatures had occurred to expect scald reduction unless ripening had an independent effect. Results will be presented and discussed.

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