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T.G. McCollum

The effects of gene B on susceptibility to chilling injury (CI) in two types of summer squash (Cucurbita pepo L.) were investigated. Two pairs of near-isogenic lines with (BB) and without (B+ B+) gene B were included in the study: `Caserta' (B+ B+) and `Precocious Caserta' (BB) of the vegetable marrow type, and `Benning's Green Tint' (B+ B+) and `Benning's Yellow Tint' (BB) of the scallop type. Respiration and ethylene evolution at nonchilling temperature were consistently higher in marrows than in scallops. Gene B had no influence on respiratory rates at nonchilling temperatures; however, the presence of gene B enhanced the chilling-induced stimulation of respiration in both marrows and scallops. Temporal differences in the patterns of chilling-induced stimulation of ethylene evolution indicated a greater sensitivity to chilling in marrows than in scallops and in both types in the presence of gene B. Electrolyte leakage was decreased by storage at chilling temperature in both marrow genotypes and was not influenced by storage temperature in B+ B+ scallops, but was increased by storage at chilling temperature in BB scallops. Therefore, electrolyte leakage was not a good CI index for these summer squash.

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T.G. McCollum

Decay caused by fungal pathogens accounts for significant postharvest losses. Although the application of synthetic fungicides can reduce postharvest decay, increasing public concern over using fungicides as well as the resistance that develops to them indicate that alternative means of decay control are needed. Freedom from disease before harvest is the norm rather than the exception. Numerous defense mechanisms, both preformed and inducible, are involved in plant resistance to fungal pathogens. Understanding how natural defense mechanisms are regulated and how to maintain them in harvested products may provide the basis for new strategies to reduce postharvest losses caused by pathogens. Host–pathogen interactions have been well studied in growing plants but much less extensively in harvested organs. The interaction between host and pathogen is dynamic; changes in both organisms are required for disease development. Following harvest, the incidence of decay increases indicating that changes in the host render it more susceptible to pathogen development. Recent studies by plant physiologists and pathologists have contributed to our understanding of changes in harvested tissues that render them less resistant to decay as well as changes in the host that are induced in response to fungal infection.

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T.G. McCollum and R.E. McDonald

Storage of `Marsh' white seedless grapefruit (Citrus paradisi Macf.) for 2 weeks at 5C resulted in the development of chilling injury (CI). Electrolyte leakage from chilled fruit did not increase significantly until CI had become severe, and was therefore considered to be of limited value as an early indicator of CI. In contrast to electrolyte leakage, respiration and ethylene evolution were significantly higher in chilled than in nonchilled fruit, even before the onset of visual symptoms of CI. Respiration rates ranged from ≈8 to 11 and 5 to 7 ml CO2/kg per hour in chilled and nonchilled fruit, respectively. Ethylene evolution was not detected from nonchilled fruit, whereas chilled fruit produced from 45 to 250 nl ethylene/kg per hour. Results of this study indicate that electrolyte leakage does not increase until visible pitting of the flavedo has occurred, whereas stimulation of respiration and ethylene evolution occur early in the development of CI.

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T.G. McCollum and R.E. McDonald

Grapefruit (Citrus paradisi) flavedo is a rich source of peroxidase (POD) (EC 1.11.1.7). Changes in POD have been related to senesence and environmental stress in a variety of plant tissues. However, due to the large number of POD isoenzymes as well as the broad substrate specificity, measurement of POD activity in crude extracts is of limited value for gaining an understanding of the role of POD in vivo. We have begun to purify and characterize POD isoenzymes from grapefruit flavedo. HPLC gel permeation chromatography reveals 2 peaks of POD activity with apparent MW of 66 kD and 30 kD. Native PAGE (8% bis-acrylamide, pH 8.8) followed by activity staining indicates that the PODs differ in Pi; the 30 kD POD migrates anodally, whereas the 66 kD POD does not migrate. Isoelectric focusing has been used to separate flavedo PODs into acid (Pi ca 4.0) and basic (Pi > 8.5) forms. Treatment of grapefruit with ethylene (2 ppm 72 hours) induces a basic POD not present in freshly-harvested fruit or in nonethylene-treated controls.

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T. G. McCollum and R. E. McDonald

Storage of `Marsh' white seedless grapefruit (Citrus paradisi Macf.) for 2 weeks at 5C resulted in the development of chilling injury (CI). Electrolyte leakage from chilled fruit did not increase significantly until CI had become severe, and was therefore considered to be a poor index of CI. In contrast to electrolyte leakage, respiration and ethylene evolution were consistently higher in chilled than in nonchilled fruit, even prior to the onset of visual symptoms of CI. Respiratory rates ranged from 8.0 to 10.7 and 4.6 to 6.7 ml/kg/hr in chilled and nonchilled fruit, respectively. Ethylene evolution was not detected from nonchilled fruit, whereas chilled fruit produced from 45.6 to 249.3 ml/kg/hr ethylene. Ethylene production was maximum following 2 weeks at 5C. Results of this study indicate that increases in electrolyte leakage do not occur until considerable tissue damage has occurred, whereas stimulation of respiration and ethylene evolution occur early in the development of CI.

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E.W. Stover and T.G. McCollum

The diseases huanglongbing [HLB, associated with Candidatus Liberibacter asiaticus (CLas)] and Asian citrus canker [ACC, caused by Xanthomonas citri (Xcc)] are widespread in Florida and many other citrus-growing areas, presenting unprecedented challenges for citrus breeding. Because HLB and ACC weaken trees and compromise cropping, breeding is much less efficient using seed parents that have been exposed to these diseases. Therefore, it would be highly desirable to use unique disease-exposed selections only as pollen parents with pollen applied to disease-free trees. However, there may be a risk of introducing these diseases using such pollen sources. To assess this potential, abundance of the pathogens associated with these diseases was assessed in anthers and flowers using quantitative polymerase chain reaction. Because CLas is systemic, levels on mature leaves from the flower source trees were assessed to see if the presence of CLas in flowers was associated with leaf levels. Disease-exposed trees were tested in 10 genotypes from each of three broad genotypic categories, which reflect different levels of susceptibility to the diseases associated with the pathogens studied: Poncirus trifoliata hybrids (most resistant to HLB), Citrus maxima and hybrids (susceptible to both diseases), and C. reticulata and hybrids (considerable resistance to ACC). Of the 30 samples of each tissue type analyzed for CLas, 88% of mature leaves, 69% of flowers, and 88% of anthers had one or more CLas bacterium per sample. The trifoliate genotypic group had significantly lower levels of CLas than the pummelo and mandarin groups in mature leaf samples, but CLas levels were more similar between groups in anther and flower samples, and the pathogen was present in most of the trifoliate hybrids tested. Mean numbers of CLas detected per nanogram nucleic acid were 100 to 800 times higher in mature leaf samples, most characteristic of HLB symptoms, compared with anther samples. Xcc DNA was detected in 30% of flower samples and 23% of anther samples. No significant differences in Xcc levels were found between tissue type or genotypic group. However, regressions between Xcc levels in flowers and percent of plant pedigree derived from mandarin had a negative correlation and an r 2 of 0.159 (P = 0.029). The biology of CLas is consistent with the pathogen being present in anthers from unopened flowers, whereas the ACC pathogen detected inside flowers was likely the result of contamination despite great care in sample collection and handling. Where exceptional diligence to exclude HLB and ACC is appropriate, results suggest that there may be a risk of spreading these pathogens through use of pollen from trees on infected farms.

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R.E. McDonald, T.G. McCollum, and E.A. Baldwin

The objective of this study was to determine the effects of prestorage heat treatments on chilling tolerance of tomatoes. Mature-green `Agriset' tomato fruit (Lycopersicon esculentum Mill.), either C2H4-treated or not, were immersed in 42C water for 60 min, held in 38C air for 48 hours, or not treated, and then stored at either 2C (chilled) or 13C (nonchilled) for 14 days before ripening at 20C. Heat-treated fruit stored at 2C and transferred to 20C ripened normally while nonheated fruit decayed before reaching red ripe. Color (a*/b* ratio), lycopene content, and internal quality characteristics of fruit were similar at the red-ripe stage irrespective of method of heat treatment. In red-ripe heat-treated fruit, free sterol levels were significantly higher in chilled fruit than in nonchilled fruit. Heating fruit in 38C air resulted in significantly higher levels of some free sterols compared with heating fruit in 42C water. Of the 15 flavor volatiles analyzed, six showed significantly decreased concentrations as a result of C2H4-treatment and seven showed decreased concentrations when stored at 2C before ripening. Some volatiles were decreased by the heat treatments. Prestorage short- and long-term heat treatments could allow for storage of mature-green tomatoes at lower temperatures with little loss of their ability to ripen normally.

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R.E. McDonald, T.G. McCollum, and E.A. Baldwin

Mature green `Sunbeam' tomato fruit (Lycopersicon esculentum Mill.), were treated in varying order with C2H4, 42°C water for 60 minutes, 38°C air for 48 hours, partial ripening for 48 hours at 20°C, or not treated, and then stored at 2°C for 14 days before ripening at 20°C. Heat treated fruit stored at 2°C and transferred to 20°C ripened normally while 63% of nonheated fruit decayed before reaching red ripe. More chilling injury (CI) developed when C2H4 was applied following heat treatment rather than before. There was more CI in fruit that were 42°C water treated compared with the 38°C air treatment. Less CI developed on fruit that were partially ripened for 2 days at 20°C before a 42°C water treatment rather than following it. At red ripe, nonchilled fruit were firmer than chilled heat treated fruit. Fruit treated in 42°C water were firmer when the heat treatment was applied before the C2H4 treatment rather than following it. Chlorophyll and lycopene content and internal quality characteristics of fruit were similar at the red ripe stage irrespective of C2H4 or heat treatment. Chilling and heat treatments reduced some of the 15 flavor volatiles analyzed. Volatile levels were lower in fruit treated with C2H4 before heat treatment compared with fruit treated with C2H4 following heat treatment. Prestorage heat treatments could allow for storage of mature green tomatoes at low temperatures with little loss in their ability to ripen normally.

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R.E. McDonald, T.G. McCollum, and E.A. Baldwin

Mature green `Sunbeam' tomato fruit (Lycopersicon esculentum Mill.) were treated in water for 1 hr at 27 (ambient), 39, 42, 45, or 48°C, and then either ripened at 20°C (nonchilled) or stored at 2°C (chilled) for 14 days before ripening at 20°C. The most-effective heat treatment was 42°C, which reduced decay 67% in chilled fruit and 53% in nonchilled fruit. Heat treatment had no effect on time required to ripen the fruit. Red-ripe tomatoes had higher respiration rates and evolved more ethylene following nonchilling storage, but heat treatment had no effect on respiration or ethylene evolution. Red color development was enhanced by heat treatment, and inhibited by chilling. At red ripe, fruit were firmer as a result of storage at the chilling temperature, while heat treatment had no effect on firmness. Heat-treated fruit were preferred in terms of taste and texture over nontreated fruit in informal taste tests, with the exception of the 45°C treatment. With increasing temperature of heat treatment, there was increased electrolyte leakage following chilling storage. Of the 15 flavor volatiles analyzed, the levels of five were decreased with increasing temperature of heat treatment. Storage at the chilling temperature reduced the levels of six flavor volatiles. Prestorage heat treatments can reduce decay with only minimal adverse effects on tomato fruit quality.