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  • Author or Editor: Zhenyong Wang x
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AVG, as ReTain™, an inhibitor of ethylene biosynthesis, was used alone or with a subsequent application of ethephon (Ethrel™), an ethylene-releasing chemical, to determine if red color development could be enhanced without over-ripening `Gala' and `Jonagold' apples. Treatments included: 1) AVG alone; 2) AVG followed by ethephon; 3) ethephon alone; and 4) control. Silwet L-77 surfactant was included in all treatments. Application of AVG delayed the onset of the ethylene climacteric and red color development of both cultivars. Application of AVG followed by ethephon similarly delayed the onset of the ethylene climacteric, but red color development at the commercial harvest date was only marginally reduced or not affected. The results were similar in both 1998 and 1999, although environmental stress during the growing seasons differed (1998—heat; 1999—moderate temperatures). The delay of fruit maturation and ripening observed at harvest following AVG +/- ethephon treatments improved storability of fruit in controlled atmosphere (CA) storage, as demonstrated by low internal ethylene levels after storage, and high retention of flesh firmness and shelf-life, while control fruit and those treated only with ethephon entered the ethylene climacteric during storage, and flesh firmness subsequently declined during shelf-life evaluation. Chemical name used: aminoethoxyvinylglycine (AVG).

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AVG applied alone to `Gala' and `Jonagold' apples delayed maturity and the onset of the ethylene climacteric and delayed red color development. AVG followed by ethephon delayed maturity and the onset of the ethylene climacteric, but promoted red color development of both cultivars. Ethephon applied alone advanced maturity, ethylene production, ripening, and red color development compared to AVG alone. In other studies, the ripening-related effects of these treatments were reflected in the storability of fruit in CA storage. AVG - and AVG + ethephon-treated fruit were still at preclimacteric ethylene levels after 6 months in CA storage, with excellent retention of flesh firmness and shelf-life, while ethephon and control fruits had significantly higher ethylene levels and softened more during storage and shelf-life evaluation. Collectively, our results indicate that an ethephon application following AVG treatment may be useful to overcome the delay of red color development of apples treated with AVG only and that this can be achieved without overly stimulating fruit ripening. Thus, a once-over harvest of `Gala' and `Jonagold' apples may be achieved with a significant reduction in harvest costs. We attribute the promotion of red color development of apples receiving AVG treatment with a follow-up application of ethephon to the action of ethylene temporally-released from ethephon stimulating the development of the anthocyanin biosynthetic pathway, while AVG inhibits the development of the endogenous ethylene climacteric. Inhibiting endogenous ethylene production delays fruit from producing their own ethylene. We attribute maturation uniformity to the action of AVG allowing the less mature fruits to gain maturity while slowing maturity development of the more mature fruits. Improved storability of AVG + ethephon-treated fruit is attributed to the same ethylene-related phenomena.

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Multiple harvests are often necessary to achieve maximum yield of well-colored high-quality apples. This is true for most cultivars, and particularly for `Gala'. Multiple harvests add significantly to the cost of producing apples. We tested our hypotheses that anthocyanin production of ReTain™-treated apples may be enhanced by ethephon without overly stimulating other ripening processes and ReTain™ may promote uniform maturation of apples within and between trees by delaying maturation and ripening processes. Experiments were conducted with `Gala', `Empire', and `Jonagold' apples at the MSU CHES in 1997 and 1998 employing the rootstock/training systems research plot. Treatments were 1) ReTain™ (50g/ac.) applied 3 to 4 weeks before harvest, 2) ReTain™ followed by ethephon (3/4 pt/ac.) applied 1 to 2 weeks before harvest, 3) ethephon, and 4) control (Silwet® L-77 surfactant only). ReTain™ applied alone delayed the onset of the ethylene climacteric and red color development of `Gala' apples. ReTain™ followed by ethephon delayed the onset of the ethylene climacteric and red color development at the commercial harvest date was not significantly affected. Similar results were obtained with the `Empire' and `Jonagold'. Results with ReTain™ and ReTain™ + ethephon in 1998 on `Gala', `Empire', and `Jonagold' apples were more profound than in 1997; we attribute this to less environmental stress on the trees, which were well-irrigated in 1998. The ripening-related effects of treatments were reflected in the storability of fruit 1997 in air and particularly during CA storage where the action of ethylene in ripening can be attenuated. ReTain™ - and ReTain™ + ethephon-treated fruit were still at preclimacteric ethylene levels after 6 months in CA with excellent retention of flesh firmness and shelf-life, while ethephon and control fruit had higher ethylene levels and softened more during storage and shelf-life evaluation.

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We are investigating alternative strategies to control scald on apples. Ethanol vapors were applied to `Law Rome' and `Red Delicious' apples in the storage chambers by ventilating air through aqueous solutions of ethanol at different concentrations, and in modified atmosphere packages by adding various initial concentrations of ethanol vapor. Fruits in storage chambers treated with ethanol vapor at 1600 ppm for about 2 months showed no scald when stored for an additional period in air storage whereas the scald index in control was up to 2.33 (the highest is 3). The similar results in the modified atmosphere experiments confirmed that ethanol vapor could prevent apple scald. Ethanol vapor treatment was also correlated with a reduction of α-farnesene production by the fruits. α-farnesene is an isoprenoid metabolite in the pathway to carotenoid synthesis that has been implicated indirectly as a factor in scald development. Evidence for this based on diphenylamine (DPA) reducing the level of a conjugated terpene product of α-farnesene oxidation. Our results suggested that the control of scald by ethanol vapor treatment may be related to the reduction of α-farnesene production and its subsequent oxidation. Ethanol vapor treatment resulted in accumulation of ethanol in the fruits in direct proportion to the ethanol concentration administered and reduced the rate of ethylene production, and the internal ethanol levels dropped rapidly when fruits were returned to air without ethanol vapor.

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The “scald-like” disorder of `Empire' apples is a CO2-linked disorder. In this study we investigated the factors of acclimatization of preclimacteric fruits at 3°C in air or low O2 levels prior to elevating the CO2 concentration. Fruits were harvested at the immature stage, which had higher potential to suffer the disorder. Establishing CA conditions of 5% CO2 + 1.5% O2 at 3°C within 1 day of harvest resulted in the highest incidence of the `Empire' disorder, followed by 3% CO2 + 1.5% O2 > 5% CO2 + 3% O2 > 3% CO2 + 3% O2. The fruits had to be acclimated at 3°C for 3 to 4 weeks at 1.5% or 3% O2 to become insensitive to 5% or 3% CO2. Holding the fruits in air for 1 week prior to administering 3% or 5% CO2 was insufficient to control the disorder, whereas the fruits tolerated 3% or 5% CO2 after holding the fruits in air for 2 weeks. But, the 2-week delay to CA resulted in accelerating flesh softening. Supplemental carbon dioxide was not necessary for at least 1 month to maintain flesh firmness at near harvest values where fruits were kept at 1.5% O2. Whereas at 3% O2, the presence of 3% or 5% CO2 from the beginning of the storage period was required to retard flesh firmness decrease. Fruits stored for the entire storage duration without CO2 softened markedly. In summary, we conclude that establishing CA conditions of 3% to 5% CO2 with 1.5% or 3% O2 at 3°C within 1 week of harvest can lead to serious incidence of the `Empire' disorder with preclimacteric fruits. Moreover, fruits acclimate better in air than at low O2 levels with respect to tolerating CO2. This suggests a requirement of oxidative metabolism in acclimatization for CO2 tolerance. Beyond 7 days, acclimatization in air at 3°C before CA is established results in excessive flesh softening, even though it does prevent the disorder incidence.

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