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Diana L. Lange and Adel A. Kader

Preclimacteric avocado [Persea americana (Mill.) cv. Hass] fruit or fruit disks as well as fruit harvested in either June (midseason) or August (late season) and partially ripened were kept in air (21% O2 + 78% N2), 20% CO2 + 17% O2 (63% N2), or 40% CO2 + 13% O2 (47% N2) at either 10 or 20 °C. Ethylene production by preclimacteric fruit was completely inhibited during CO2 exposure, whereas there was only partial inhibition of ethylene production when partially ripened fruit were exposed. Compared to the fruit stored in air, O2 uptake of fruit stored in 20% CO2 was decreased by 20%, whereas the fruit stored in 40% CO2 showed 25% more O2 uptake than air-stored fruit. Fruit subjected to a storage regime of 40% CO2 at 10 °C followed by 2 d in air had the best visual quality. In general, climacteric fruit treated with 20% CO2 at 10 °C showed increased pyruvate dehydrogenase (PDH) activity and decreased cytochrome oxidase (CytOx) activity. Fruit stored in 40% CO2 had reduced CytOx activity compared to air-stored fruit, and PDH activity was variable depending on the harvest season of the fruit. Our results show that the effect of elevated CO2 on a given enzyme depends on concentration of CO2, duration of exposure, physiological state of the fruit, and type of tissue exposed.

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James R. Gorny and Adel A. Kader

The objective of this study was to compare and contrast the mode of action by which elevated carbon dioxide and/or reduced oxygen atmospheres inhibit ethylene biosynthesis. `Golden Delicious' apple fruit were placed at 0C in one of the following four atmospheres: 1) air; 2) air + 5% CO2; 3) 2% O2 + 98% N2; or 4) 2% O2 + 5% CO2 + 93% N2 and then sampled monthly for 4 months. Ethylene biosynthesis rates and in vitro ACC synthase activities were closely correlated in all treatments. In vitro ACC synthase activity and ethylene biosynthesis rates were lowest in fruit treated with 5% CO2 + 2% O2, while air-treated fruit had the highest ethylene biosynthesis rate and in vitro ACC synthase activity. Fruit treated with air + 5% CO2, or 2% O2 + 98% N2, had intermediate ethylene and in vitro ACC synthase activities. In vitro ACC oxidase was significantly different among treatments, but not as closely correlated with the ethylene biosynthesis rate as in vitro ACC synthase activity. Western blot analysis of the ACC oxidase protein was performed to determine if activity differences among treatments were correlated with the amount of enzyme present in vivo. ACC synthase and ACC oxidase mRNA transcript of abundance was determined via Northern blot analysis. Results will be discussed regarding how ethylene biosynthesis is inhibited at the molecular level by elevated CO2 and/or reduced O2.

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Diana L. Lange and Adel A. Kader

Partially ripened avocado [Persea americana (Mill.) cv. Hass] fruit harvested in either June or Aug. 1994 were kept at 10 °C in air (21% O2), 20% CO2 (17% O2, balance N2), or 40% CO2 (13% O2, balance N2) for 7 to 12 days and then were transferred to air at 10 °C for 2 to 3 days. Mitochondrial respiration was stimulated in response to elevated CO2 treatments at 10 °C. A shift to alternative pathway (Alt) respiration occurred on day 4 in experiments using avocados from both harvest dates, with a return to initial levels in only the 20% CO2-treated fruit (June-harvested fruit after return to air). Elevated CO2 at 20 °C decreased the in vitro O2 consumption of isolated mitochondria compared to mitochondria kept in air. The Alt pathway contributed less to the total O2 uptake of CO2-treated mitochondria compared to mitochondria kept in air. The respiratory control ratios of the CO2-treated fruit and mitochondria were higher and lower, respectively, than the air controls. Induction of 33 to 37 kD proteins (corresponding to the size of the alternative oxidase proteins) occurred in avocados after 4 days in 40% CO2. These results indicate that elevated CO2 has various effects depending on concentration, duration and temperature of exposure, and mitochondrial function of avocado fruit, such as increased and altered respiratory oxidation and up-regulation of alternative oxidase proteins.

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James R. Gorny and Adel A. Kader

Autocatalytic C2H4 biosynthesis in preclimacteric apple fruit (Malus domestica Borkh. `Golden Delicious') was prevented by storage in atmospheres of 20% CO2-enriched air (17% O2 + 63% N2) or 0.25% O2 (balance N2). In preclimacteric fruit, both treatments inhibited C2H2 biosynthesis by suppressing expression of ACC synthase (ACC-S) at the mRNA level. ACC oxidase (ACC-O) mRNA abundance and in vitro enzyme activity also were impaired by these treatments. However, the conversion of ACC to C2H4 never became the rate limiting step in C2H4 biosynthesis. C2H4 biosynthesis also was effectively inhibited in climacteric apple fruit kept in air + 20% CO2 or 0.25% O2. Climacteric apples also exhibited suppressed expression of ACC-S at the mRNA level, while ACC-O transcript abundance, enzyme activity, and protein abundance were reduced only slightly. ACC-S is the key regulatory enzyme of C2H4 biosynthesis and is the major site at which elevated CO2 and reduced O2 atmospheres inhibit C2H4 biosynthesis, irrespective of fruit physiological maturity. Chemical names used: 1-aminocyclopropane-1-carboxcylic acid (ACC).

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Dana F. Faubion and Adel A. Kader

California grown `Hass' avocado fruit were stored at 5C, in air or a controlled atmosphere (CA) of 2% oxygen and 5% carbon dioxide. Fruit were evaluated at 0, 3, 6, and 10 weeks, both immediately upon removal from storage and after 5 days at 20C. Severe chilling injury developed in the air-stored fruit after six weeks, while only moderate symptoms were observed in CA stored avocado fruit after 10 weeks. Lipid peroxidation breakdown products increased during storage and ripening in both air and CA treatments. Sterols, sterol esters, glycolipids, and phospholipids were analyzed. There was a shift in composition during storage towards increasingly saturated fatty acids. The fatty acid shift was greater in air, than in CA stored fruit. Results will be discussed concerning their relevance to chilling injury development.

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Diana L. Lange and Adel A. Kader

Changes in cytosolic and vacuolar pH, ATP, ADP, and the ATP : ADP ratio were measured in whole fruit or mesocarp disks of avocado [Persea americana (Mill.) cv. Hass] during brief exposures to elevated CO2. Intact climacteric fruit exposed to air (21% O2), 20% CO2 (17% O2, balance N2), or 40% CO2 (13% O2, balance N2) had cytosolic pH values of 7.0, 6.6, and 6.4, respectively, while mesocarp disks had cytosolic pH values of 6.9, 6.7, and 6.4, respectively. The ß-ATP levels of intact climacteric fruit exposed to 20% CO2 or 40% CO2 for 2 h were reduced by 25% or 43%, respectively, relative to air-exposed fruit. HPLC analysis of nucleotide phosphates from preclimacteric avocados revealed that ATP levels and the ATP : ADP ratio increased in 40% compared to the air-stored fruit. However, 1 day after transfer to air, the effects of elevated CO2 had dissipated. These modifications in cellular state could alter the activity of respiratory enzymes in fruit exposed to elevated CO2 atmospheres.

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James R. Gorny and Adel A. Kader

Ethylene biosynthesis of Golden Delicious apple fruit at 20°C is rapidly inhibited by a controlled atmosphere of air + 20% CO2. However, in vitro ACC oxidase activity and ACC content were not significantly different between air and air + 20% CO2 treated fruit, To determine the in vivo effects of CO2 treatment, both in vivo and in vitro enzyme activity essays were performed in en atmosphere of air or air + 20% CO2. Western blots were also performed to quantify the amount of ACC oxidase protein present in the air and air + 20% CO2 treated fruit.

We believe that in vivo cytosolic pH changes, induced by CO2, may reduce the in vivo catalytic capacity of ACC oxidase, end hence significantly reduce ethylene biosynthesis in climacteric tissue,

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James R. Gorny and Adel A. Kader

The optimal `Bartlett' pear ripeness stage for fresh-cut processing based on flesh firmness ranges between 44.5 and 58 N (10 and 13 lbf). Use of softer pears reduces postcutting life due to flesh browning. Firmer pears may have longer postcutting life but lack good flavor. Dipping pear slices in a mixture of 2% (w/v) ascorbic acid + 1% (w/v) calcium lactate + 0.5 (w/v) cysteine (pH 7) for 5 min at 20 °C extended their shelf-life by inhibiting flesh softening and surface browning during storage at 0 °C for 10 days. After 3 days at 0 °C, ascorbic acid and cysteine residues dropped below detectable levels, while calcium content was double that of untreated slices. Preliminary sensory evaluation indicate no negative impact on flavor from this dip treatment. Exposure of intact pears to heat (35 or 40 °C) or controlled atmospheres (0.25 kPa O2 and/or 40 kPa CO2) for 24 or 48 h did not influence postcutting cut surface browning of pear slices. Storage of `Bartlett' pears at -1 °C in 2 kPa O2 (balance N2) resulted in longer postcutting life of the slices as compared to those made from air-stored pears at -1 °C. The longer the storage duration of whole pears, the shorter the shelf-life of their slices was. Fruit size did not affect the postcutting life of the pear slices, provided that they were treated with the ascorbic acid + calcium lactate + cysteine mixture. Untreated slices made from small pears exhibited surface browning faster than those made from large pears.

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Dangyang Ke, Lili Zhou, and Adel A. Kader

`Chandler' strawberries (Fragaria ananassa Duck.) were kept in air, 0.25% O2, 21% O2 + 50% CO2, or 0.25 O2 + 50% CO2 (balance N2) at 5C for 1 to 7 days to study the effects of controlled atmospheres (CAs) on volatiles and fermentation enzymes. Concentrations of acetaldehyde, ethanol, ethyl acetate, and ethyl butyrate were greatly increased, while concentrations of isopropyl acetate, propyl acetate, and butyl acetate were reduced by the three CA treatments compared to those of air-control fruit. The CA treatments enhanced activities of pyruvate decarboxylase (PDC) and alcohol dehydrogenase (ADH) but slightly decreased activity of alcohol acetyltransferase (AAT). The results indicate that the enhanced PDC and ADH activities by CA treatments cause ethanol accumulation, which in turn drives the biosynthesis of ethyl esters. The increased ethanol concentration also competes with other alcohols for carboxyl groups for esterification reactions. The reduced AAT activity and limited availability of carboxyl groups due to ethanol competition decrease production of other acetate esters.

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Dana F. Faubion and Adel A. Kader

Tightly stacked pallets of wrapped, place-packed `Anjou' pears cooled slower than those that were tray packed. Half-cooling times ranged from 2.0 to 15.7 days for wrapped, place-packed pears vs. 1.7 to 7.4 days for tray-packed pears. More time was required to remove heat from the middle than from the top or bottom of the pallets in both packaging systems; however, the difference in temperature between the middle and the top or bottom of the pallet was greater for a longer period of time when pears were wrapped and place packed. The large range in temperature within a pallet illustrated that a pallet is not a uniform unit. A change in how fruit are packed can dramatically change how fruit cool within a pallet. Accumulation of carbon dioxide and ethylene in slower-cooling boxes suggests that tray packing can enhance `Anjou' quality by facilitating faster product cooling.