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Albert C. Purvis

Diphenylamine has been used to reduce low-temperature-induced storage scald of apples for decades. Its effectiveness in reducing scald has been attributed to its antioxidant properties. Oxidative reactions have also been implicated in chilling injury of other commodities, including green bell peppers (Capsicum annuum L.). Diphenylamine was applied as a dip at rates of 500 to 2000 ppm to green bell peppers prior to storing them for 7 days at 1 °C. The development of sheet pitting, the most common visible symptom of chilling injury in bell peppers, was inhibited almost completely by diphenylamine. Diphenylamine, however, only slightly reduced the chilling-induced decrease in chlorophyll fluorescence ratios. Darkening of the vascular tissues of the calyxes and seed darkening, which are also symptoms of chilling injury, were not prevented by diphenylamine. Thus, diphenylamine either did not get into all of the sites of oxidative reactions or some of the manifestations of chilling injury are initiated by processes other than oxidative ones.

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Albert C. Purvis

Mature, unripe peaches were stored at 1°C in air or in 1% O2 plus 2.5%, 5.01, or 10.0% CO2 for 25 days and then ripened in air at 15°C for 3 days. Fruit stored in air or 2.5% CO2 developed browning of the flesh during ripening but fruit stored in 5.0% or 10.0% CO2 turned yellow. Only the fruit stored in 10.0% CO2 softened during ripening, but water soluble pectin content increased and protopectin content decreased in peaches stored in 5.0% or 10.0% CO2. Low temperature storage apparently interferes with the levels and/or activities of the pectin degrading enzymes during subsequent ripening of peaches. High levels of CO2 during low temperature storage appears to reduce chilling injury by protecting the capacity of the fruit to produce adequate levels of the pectolytic enzymes at ripening temperatures.

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Albert C. Purvis and George Yelenosky

Potted Marsh grapefruit [Citrus paradisi (Mad.)] trees with mature fruit were subjected to successive 7-day exposures to 20, 15, 10, and 5C, followed by 7 days at 25C in controlled-environment walk-in rooms. Circular depressions (pits), characteristic visible symptoms of chilling injury, developed in the peel of the fruit. To the best of our knowledge, this is the first report of inducible chilling injury of grapefruit on trees under artificial environments. The ability to induce chilling injury in the peel of grapefruit on trees enhances opportunities to study the mechanism of resistance to chilling injury that can be developed by various preharvest strategies.

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Robert L. Shewfelt and Albert C. Purvis

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Robert L. Shewfelt and Albert C. Purvis

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Albert C. Purvis and James W. Gegogeine

The mechanism by which diphenylamine (DPA) controls superficial scald in apples and reduces chilling injury in green bell peppers [Capsicum annuum L. var. annuum (Grossum Group)] has been assumed to be related to its antioxidant activity. In the present study, DPA inhibited the respiratory activity of green bell pepper fruit as well as oxygen uptake by the mitochondria isolated from them. When the alternative oxidase was inhibited with n-propyl gallate or disulfiram during state 4 respiration, DPA did not further inhibit O2 uptake. Treating green bell peppers with DPA before storage did not alter the induction and abundance of the alternative oxidase protein in mitochondria which was maximally induced in peppers stored at 4 °C. Whether added before or after the uncoupler, 2,4-dinitrophenol, DPA negated the enhanced O2 uptake associated with uncoupling of electron transfer in isolated mitochondria. These observations indicate that DPA inhibits the flow of electrons through the cytochrome path, probably somewhere in the cytochrome bc 1 complex. Although the secondary amine function of DPA makes it a powerful antioxidant, the effectiveness of DPA in reducing chilling injury in green bell peppers and superficial scald in apples [Malus sylvestris (L.) Mill. var. domestica (Borkh.) Mansf.] also may be due, in part, to its inhibition of respiration.

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Juan C. Díaz-Pérez, Albert C. Purvis and J. Thad Paulk

Bolting causes significant economic losses in sweet onion (Allium cepa L.) production. Although temperature and photoperiod are considered to be the main factors that initiate bolting in onions, preliminary results suggested that low N fertilization rates increased bolting. The objective of our study was to determine the relationships of bolting, yield and bulb decay with N fertilization rates. The N fertilization rates applied ranged from the infraoptimal to the supraoptimal (from 102 to 302 kg·ha-1 N). Shoot and bulb N content increased with increasing N rates, but there were no differences in the respective shoot and bulb N contents among cultivars. Bolting incidence declined steadily with increasing N fertilization rates up to 197 kg·ha-1 N. Bolting incidence was among the highest in the cultivar Pegasus. The percent of decayed bulbs also increased at a steady rate with the rate of N applied. Total (14.7 t·ha-1) and marketable (0.8 t·ha-1) yields at the lowest N rate (102 kg·ha-1 N) were lower (P ≤ 0.01) than those at higher N rates. Rates of N ≥145 kg·ha-1 had no significant effect on either total (mean = 33.6 t·ha-1) or marketable (mean = 21.6 t·ha-1) yields. Losses in marketable yield were primarily a combination of bolting and bulb decay and were minimized at 162 kg·ha-1 N. Yield losses at low N rates were mostly due to bolting while yield losses at high N rates were mostly due to decay. Thus, excess applications of N fertilizer should be avoided since they have little effect on yields or bolting but they increase bulb decay.

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Brenda A. del Rosario, Robert L. Shewfelt and Albert C. Purvis

Evidence is accumulating that mitochondria possess defense mechanisms which effectively protect component membranes from the attack by active oxygen species which are produced continuously within the organelle. This study compared the stability of microsomal (from bell pepper fruit pericarp and cauliflower florets) and mitochondrial (from bell pepper fruit pericarp) membranes against peroxidative challenge systems (cumene hydroperoxide and iron-ascorbate). Protein concentration, (i-tocopherol levels, and total lipids were observed to decrease for both membranes when challenged. The onset of peroxidation was observed to be earlier and at higher levels in microsomes than in mitochondria. These results demonstrate the increased stability of mitochondrial membrane fractions to peroxidative challenge and suggest that the level of antioxidants and not fatty acid composition is the critical factor in resistance to oxidative stress in plant mitochondria.