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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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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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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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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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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.

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R.E. McDonald, W.R. Miller and T.G. McCollum

Irradiation is being evaluated as a quarantine treatment of grapefruit (Citrus paradisi Macf. `Marsh'), but it can cause damage to the fruit. Research was conducted to determine if preirradiation heat treatments would improve fruit tolerance to irradiation as they improve tolerance to low temperature injury and to determine if canopy position influenced fruit tolerance to irradiation. Initially, grapefruit were irradiated at 0 or 2.0 kGy at a dose rate of 0.14 kGy·min-1 and selected biochemical changes were monitored over time. There was a marked increase in phenylalanine ammonia-lyase (PAL) activity following irradiation. Maximum activity (≈18-fold increase) was attained 24 hours after irradiation. Subsequently, grapefruit were harvested from interior and exterior canopy positions and irradiated at 0 or 1.0 kGy at a dose rate of 0.15 kGy·min-1 before storage for 4 weeks at 10 °C. Following storage, pitting of flavedo was the most evident condition defect noted as a result of irradiation. Pitting was observed on 27% and 15% of irradiated exterior and interior canopy fruit, respectively, whereas there was no pitting on nonirradiated fruit. Heat treatment before irradiation decreased susceptibility of fruit to damage. Pitting was 26%, 19%, and 17% when fruit were held 2 hours at 20 (ambient), 38 or 42 °C, respectively. Irradiation-induced PAL activity was reduced by temperature conditioning at 38 or 42 °C. Exterior canopy fruit flavedo contained higher levels of total phenols, including flavanols and coumarins compared with interior canopy fruit. Deposition of lignin was not related to canopy position. Neither irradiation nor heat treatment had an effect on total phenols or lignin deposition. Generally, cholesterol, campesterol, stigmasterol, β-sitosterol, and isofucosterol were found to be higher in four steryl lipid fractions in exterior canopy fruit compared with interior canopy fruit. Irradiation increased campesterol in the free sterol and steryl glycoside fractions and decreased isofucosterol in the free sterol fraction. Heat treatments had no effect on individual sterol levels. It seems that irradiation causes a stress condition in the fruit, which leads to pitting of peel tissue. Heat treatment before irradiation reduced damaging effects of irradiation.

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R.E. McDonald, W.R. Miller and T.G. McCollum

Irradiation is being evaluated as a quarantine treatment of grapefruit (Citrus paradisi, Macf.), but it can cause damage to the fruit. We wanted to determine if pre-irradiation heat treatments would improve fruit tolerance to irradiation as they improve tolerance to low-temperature injury. `Marsh' grapefruit were harvested from interior and exterior canopy positions and irradiated at 0 or 1.0 kGy at a dose rate of 0.148 kGy·min-1 before storage for 4 weeks at 10 °C. Following storage, pitting of flavedo tissue was the most evident condition defect noted as a result of irradiation. Pitting was noted on 15% and 27% of irradiated interior and exterior canopy fruit, respectively, whereas there was no pitting on non-irradiated fruit. Temperature conditioning before irradiation decreased the susceptibility of fruit to damage. Pitting was 26%, 19%, and 17% when fruit were held 2 h at 20 (ambient), 38 or 42 °C, respectively. There was a marked increase in phenylalanine ammonia-lyase (PAL) activity following irradiation. Maximum activity (≈18-fold increase) was attained 24 h after irradiation. Irradiation-induced PAL activity was reduced significantly by temperature conditioning at 38 or 42 °C. Exterior canopy fruit flavedo contained higher levels of total phenols, including flavanols and coumarins, compared with interior canopy fruit. The deposition of lignin was not related to canopy position. Neither irradiation nor heat treatment had an effect on total phenols or lignin deposition. It seems that irradiation causes a stress condition in the fruit, which leads to pitting of peel tissue. Heat treatment before irradiation reduced the damaging effects of irradiation.