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Zohar Shaham, Amnon Lers, and Susan Lurie

`Granny Smith' apples [Malus sylvestris (L.) Mill. var. domestica (Borkh.) Mansf.] were harvested in two seasons and stored at 0 °C air storage with no pretreatment (control), after heating for 4 d at 38 °C, or after treating for 16 hours at 20 °C with 1 μL·L-1 1-methylcyclopropene (1-MCP). The effects of the two treatments on superficial scald development were consistent over both seasons. Scald began to appear after 8 weeks in control fruit, after 16 weeks in heated fruit but not on 1-MCP treated fruit. α-Farnesene accumulation and oxidation were slower in the skin of heated than in control fruit, and almost entirely absent in 1-MCP treated fruit. The activities of five antioxidant enzymes, ascorbate peroxidase, catalase, glutathione reductase, peroxidase and superoxide dismutate, were measured at two-week intervals in the apple peel, quantitatively as total activity and qualitatively by isozyme analysis. Enzyme activities either increased or remained stable during 16 weeks of storage, except for superoxide dismutase activity, which decreased. Ascorbate oxidase activity was higher in heated than control apples and there was an additional peroxidase isozyme present in activity gels. The activities of antioxidant enzymes were lower in 1-MCP treated fruit except for catalase during the first month of storage. Lipid soluble antioxidant activity was higher in 1-MCP treated fruit than the fruit of the other treatments, and water soluble antioxidant activity was higher in both treatments than in control fruit during the time that scald was developing in control apples. Both free and total phenol contents in the peel fluctuated during storage but no consistent trend was detected. The differences in enzyme activity and antioxidant content of the peel of 1-MCP and heated apples may play a role in preventing or delaying the appearance of superficial scald.

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F. Ghahramani, K.J. Scott, and R. Holmes

`Delicious' (Malus ×domestica Borkh.) apples were kept in sealed polyethylene bags (thickness 0.05 mm) and exposed to ethanol, propan-1-ol, butan-1-ol, and pentan-1-ol during storage at 0 °C. Rates of application varied from 1.85 to 120 mmol·kg-1. Complete control of superficial scald was achieved using 30 mmol·kg-1 of fruit with butan-1-ol or propan-1-ol; ethanol required 120 mmol·kg-1 to control the disorder, but at this concentration, purpling of the red skin occurred. Butan-1-ol and propan-1-ol did not affect the color. Pentan-1-ol caused severe skin injury resembling soft or deep scald of `Jonathan' apples. Apples were also kept in high purity N at 20 °C for up to 8 days before storage at 0 °C. Complete control of scald occurred with a 6- or 8-day exposure to N. Control of scald appears to be due to the accumulation of ethanol during exposure to N. Nitrogen treatments did not affect skin color.

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Cynthia L. Barden and William J. Bramlage

Antioxidants are believed to protect against the oxidation of α-farnesene to conjugated trienes in apple (Malus domestica, Borkh.) peel, thus providing resistance against superficial scald development. We conducted three experiments in which apples were a) harvested weekly, during which they were exposed to increasing hours at <10C during ripening; b) induced to ripen with no hours at <10C by applying ethephon; and c) enclosed in paper bags as they ripened. Inducing ripening with ethephon increased total water-soluble reducing compounds and percentage inhibition of lipid oxidation of peel extracts, increased concentrations of α-tocopherol, carotenoids, and ascorbic acid in peel, but only slightly reduced scald. Delayed harvests increased all of these antioxidants except ascorbic acid and greatly reduced scald development. Bagging fruit before ripening decreased α-tocopherol, carotenoid, and ascorbic acid concentrations, decreased total water-soluble reducing compounds, and increased scald development. We conclude that changes in these antioxidants probably are affected more by ripening and light intensity than by low temperature before harvest. Chemical name used: (2-chloroethyl)phosphonic acid (ethephon).

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H.P. Vasantha Rupasinghe, Gopinadhan Paliyath, and Dennis P. Murr

To decipher the relation between α-farnesene metabolism and the development of superficial scald in apples, trans,trans-α-farnesene synthase, the enzyme that catalyzes the conversion of farnesyl pyrophosphate to α-farnesene, was partially purified from skin tissue of `Delicious' apples (Malus ×domestica Borkh.) and characterized. Total and specific activities of the enzyme were higher in the cytosolic fraction than in membrane fractions. α-Farnesene synthase was purified 70-fold from the cytosolic fraction by ion exchange chromatography and gel permeation, and the native molecular weight was estimated to be 108,000. The enzyme had optimal activity at a pH of 5.6 and absolutely required a divalent metal ion such as Mg2+ or Mn2+ for activity. It exhibited allosteric kinetics, S(0.5) for farnesyl pyrophosphate being 84±18 μmol·L-1, and a Hill coefficient (nH) of 2.9, indicating the number of subunits to be two or three. Enzyme activity was highest between 10 and 20 °C, while 50% of the maximal activity was retained at 0 °C. In vivo α-farnesene synthase activity was minimal at harvest, then increased rapidly during 16 weeks storage in air at 0 °C, and decreased during further storage. Activity of α-farnesene synthase, α-farnesene content, and conjugated triene alcohol (the putative scald-causing oxidation product of α-farnesene) content in skin tissue were not correlated to the inherent nature of scald susceptibility or resistance in 11 apple cultivars tested.

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Nazir Mir, Rufino Perez, and Randolph M. Beaudry

`Cortland' apples (Malus ×domestica Borkh.), either untreated or treated with diphenylamine (DPA), were stored for 120 days in air at 0 °C. Peel samples were taken from these fruit immediately after storage, placed in glass vials and incubated for 48 hours, or were isolated from fruit held 2 to 72 hours at 22 °C and incubated in the vials for 2 hours. Emission of 3,7,11-trimethyldodeca-1,3(E),6(E),10-tetraene, known as trans,trans-α-farnesene, or simply α-farnesene, and its oxidation product, MHO, were measured in the vial headspace. α-Farnesene content in the gas phase of vials with peel samples reached a maximal level 2 hours after vials were sealed and was higher in DPA-treated than untreated fruit. The content of α-farnesene in the vial headspace remained unchanged for DPA-treated fruit peel during the 2-day holding period. However, α-farnesene declined rapidly after 10 hours incubation for control samples. Incubating peel samples of control fruit under N2 atmosphere prevented the decline in α-farnesene. The MHO release by the peel of control fruit was rapid during the first 2 hours and continued to increase for 24 hours. In contrast, the MHO released from DPA-treated fruit peels was 8000-fold lower than from peel samples of control fruit. The increase in vapor phase MHO was concomitant with peel browning in controls. For whole fruit held at 22 °C for 2 to 72 hours, cumulative MHO release from fruit peels followed a pattern that was similar to the pattern of superficial scald development in these fruit.

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J. Pablo Fernández-Trujillo, Jacqueline F. Nock, and Christopher B. Watkins

`Cortland' and `Law Rome' apples [Malus sylvestris (L.) Mill var. domestica (Borkh.) Mansf.] were either nontreated or treated with the inhibitor of superficial scald development, DPA, and exposed to air or CO2 (40 or 45 kPa) in air at 2 °C for up to 12 days. Fruit exposed to air or 45 kPa CO2 were sampled during treatment, and peel and flesh samples taken for fermentation product and organic acid analyses. After treatment, fruit were air stored for up to 6 months at 0.5 °C for evaluation of disorder incidence. `Cortland' apples were most susceptible to external CO2 injury and `Law Rome' to internal CO2 injury. DPA treatment markedly reduced incidence of both external and internal injury. Fermentation products increased in peel and flesh of both cultivars with increasing exposure to CO2, but the extent of the increase was cultivar dependant. Acetaldehyde concentrations were about 10 times higher in peel and flesh of `Law Rome' than that of `Cortland' apples. Ethanol concentrations in the flesh were similar in both cultivars, but were about twice as high in `Cortland' than in `Law Rome' peels. Neither acetaldehyde nor ethanol concentrations were affected consistently by DPA treatment. Succinate concentrations, often regarded as the compound responsible for CO2 injury, increased with CO2 treatment, but were not affected by DPA application. Citramalate concentrations were reduced by CO2 treatment in `Law Rome' peel, but other acids were not consistently affected by CO2. Results indicate that acetaldehyde, ethanol or succinic acid accumulation are not directly responsible for CO2 injury in apples. Chemical name used: diphenylamine (DPA).

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Susan Lurie, Amnon Lers, Zohar Shacham, Lilian Sonego, Shaul Burd, and Bruce Whitaker

Untreated control, 1-methylcyclopropene (1-MCP)-treated, and heated fruit of the superficial scald-susceptible `Granny Smith' cultivar of apple [Malus sylvestris (L.) Mill. var. domestica (Borkh.) Mansf.] were compared with respect to scald incidence, internal ethylene concentration (IEC), α-farnesene metabolism, expression of the genes AFS1, which encodes α-farnesene synthase, the final, rate-limiting enzyme in the α-farnesene biosynthetic pathway, and HMG2 and HMG3, which encode isozymes of 3-hydroxy-3-methylglutaryl-coenzyme A reductase, the proposed rate-limiting enzyme in the mevalonate pathway of isoprenoid synthesis. The incidence of scald in untreated `Granny Smith' apples after 16 weeks at 0 °C plus 1 week at 20 °C was 100%; 1-MCP treatment prevented scald development, whereas heat treatment delayed and reduced scald development. 1-MCP also inhibited both α-farnesene and IEC, suggesting that ethylene induces transcription of key genes involved in α-farnesene biosynthesis. Heat treatment reduced levels of α-farnesene and and its oxidation products, conjugated trienols (CTols), but not to the extent of 1-MCP. Internal ethylene concentrations in heated apples did not differ from those in the controls. In both control and heated fruit, a sharp increase in AFS1 mRNA during the first 4 weeks of storage preceded an increase in α-farnesene and a subsequent increase in CTols. AFS1 transcript was absent from 1-MCP-treated apples for the first 10 weeks of storage, and even at 16 weeks was lower than in heated and untreated control fruit. Levels of the HMG2 and HMG3 transcripts varied during storage and among treatments, and were not correlated with the incidence of scald. HMG2 mRNA transcript accumulation was low at harvest and increased in abundance during storage in all treatments, with the greatest increase occurring in 1-MCP-treated fruit. In contrast, HMG3 transcript was constitutively present at all storage times, although it too was slightly more abundant in 1-MCP-treated fruit.

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Sarah A. Weis and William J. Bramlage

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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Zhenyong Wang and David R. Dilley

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Thair F. Alwan and Christopher B. Watkins

Fruit of `Cortland', `Delicious' and `Law Rome' were warmed for 24 hours at 20°C either weekly, once every 2 weeks, or once every 3 weeks during storage. The effect of these treatments on fruit ripening and concentrations of alpha-farnesene and conjugated trienes in hexane extracts of the skin were measured during storage. Without warming, scald incidence of the cultivars was 70%, 14%, and 85%, respectively. Intermittent warming treatments resulted in a marked reduction of scald though effectiveness was affected by cultivar. In `Cortland', scald was reduced only by the weekly warming treatment (10%) as was less effective than DPA (1%). In `Delicious', all warming treatments were equally effective. In `Law Rome', weekly warming resulted in better control of scald (3%) than DPA (14%) and less frequent warming was proportionately less effective in controlling the disorder. Concentrations of conjugated trienes at 281 nm did not relate consistently to scald incidence after storage. However, ratios of conjugated trienes of 258 nm or 269 nm with 281 nm strongly support a hypothesis that non-toxic and toxic oxidation products of alpha-farnesene interact and influence the effectiveness of postharvest treatments on scald control (Du and Bramlage, 1993; JASHS 118:807-813). A hypothesis relating the interactions between ripening and scald development will be presented. Supported in part by USDA Specific Cooperative Agreement 58-1931-5-017.