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Gene Lester and Eduardo Stein

Changes in the physical and chemical properties of the plasma membrane from hypodermal mesocarp tissue of netted muskmelon (Cucumis melo L. var. reticulatus Naud.) fruit were compared in relation to the permeability changes of the same tissue during fruit maturation and storage at 4 or 24C. As muskmelon fruit progress from immaturity to maturity, and with storage of mature fruit at 4 or 24C, increased permeability of the hypodermal-mesocarp tissue occurs coincident with an increase in the saturation index of the plasma membrane phospholipids. Buoyant density of the plasma membrane from hypodermal mesocarp tissue increased from 1.13 to 1.14 g·cm-3 during fruit maturation. Vanadate-sensitive ATPase (EC activity was highest in mature fruit at harvest. After 10 days of storage, vanadate-sensitive ATPase activity was much lower in fruit kept at 24C than in those kept at 4C. The decrease in vanadate-sensitive ATPase activity in fruit stored at 24C was correlated with increased hypodermal-mesocarp membrane permeability. We suggest that biochemical changes affecting the lipid matrix of the plasma membrane influence fruit membrane permeability and possibly muskmelon storage life.

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Gene Lester

Lipoxygenase (LOX) activity was assayed on hypodermal- and middle-mesocarp tissues from netted muskmelon (Cucumis melo L.) fruit 10, 20, 30, and 40 days postanthesis and after 12 days of storage at 4 or 21C. Highest LOX activity was obtained using a phosphate buffer at pH 7 and 20C. LOX activity was detected only in hypodermal-mesocarp (hypodermic) tissue at 30 days postanthesis, and activity increased with fruit age and storage temperature. Antioxidants, which inhibit LOX, were detected only in hypodermic tissue from 10 through 30 days postanthesis fruits. Linoleic plus linolenic free fatty acids, substrates for LOX, in hypodermic tissue had declined at 30 days postanthesis, as did plasma membrane integrity, and both continued to decline in association with increased LOX activity.

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Sanalkumar Krishnan and Emily B. Merewitz

weighed to determine the dry weight (DW). RWC was calculated as (FW − DW)/(TW − DW) × 100. Electrolyte leakage (EL) was measured to approximate cell-membrane stability of leaves on ≈10 leaves per sample taken from each plant by the method of Blum and

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S.I. Shibairo, M.K. Upadhyaya and P.M.A. Toivonen

Studies were carried out to understand the effects of moisture loss on water potential and root deterioration in carrot (Daucus carota L. `Eagle') roots during short-term storage. The roots were stored at various temperatures and relative humidities (RH) to provide 0.7 (low), 3 (medium), and 9 mbars (high) of water vapor pressure deficit (WVPD). Carrots at high WVPD lost the most weight, followed by those at medium and lowest WVPD. Water potential and osmotic potential of the carrot tissue at high WVPD did not change significantly up to 6 days, but decreased thereafter. There was no change in water potential and osmotic potential for carrots at medium and low WVPD. A significant quadratic relationship (P = 0.05, r = –0.764) between water potential and carrot root weight loss was observed. Relative electrolyte leakage increased over time in carrots at the high WVPD. At medium WVPD, relative electrolyte leakage did not change up to 6 days, but increased significantly thereafter. Carrots at the low WVPD did not change in relative electrolyte leakage. Relative electrolyte leakage and weight loss correlated positively (P = 0.05, r = 0.789). The results suggest that water stress during short-term storage causes tissue deterioration that may further increase rate of moisture loss and hence reduce the shelf life of carrots.

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Fahad Al-Said and Donald J. Huber

A general feature of tomato fruit containing genetically reduced levels of polygalacmronase activity is decreased deterioration and cracking, particularly when handled at the ripe and over ripe stages. As fully ripe fruit are metabolically compromised and very prone to mechanical injuries, we investigated the influence of impact bruising on electrolyte leakage, pectin solubility, and depolymerization in ripening tomato fruit.

`Sunny' tomato fruit harvested at the mature-green, turning, and ripe stages of development and subjected to controlled impact injury exhibited elevated ethylene production at all developmental stages. Subsequent analyses were performed on discs prepared from bruised and uninjured pericarp tissue. Discs from bruised tissues exhibited enhanced electrolyte leakage and, in bruised tissues from ripe fruit, enhanced pectin efflux. Levels of soluble pectins derived from ethanol-insoluble powders were unaffected by bruising; however, pectins from bruised ripe fruit exhibited mol wt downshifts relative to those from nonbruised tissues.

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Hua Zhang and Paul H. Jennings

The effects of heat shock duration and persistence on the induction of chilling tolerance in cucumber roots were studied using total root growth, electrolyte leakage, and membrane peroxidation as injury indices after chilling. Heat shock reduced the chilling induced electrolyte leakage, decreased membrane peroxidation as measured by MDA content, and resulted in a greater total root growth after chilling compared to the control. Heat shocks at 40°C, applied to 36 hr germinated seedlings for time periods from 1 to 15 hr, all resulted in an increase in chilling tolerance in a time-dependent manner. The heat shock induction of chilling tolerance is most effective when heat shock was imposed immediately before chilling, but the effect is persistent even 32 hr after heat shock when seedlings are held at 25°C before chilling. The possible mechanism of heat shock effect and its persistence will be discussed in relation to heat shock proteins and antioxidant enzyme systems.

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Gene E. Lester

Plasma membrane (PM) from hypodermal-mesocarp tissues of muskmelon fruits (Cucumis melo L. var. reticulatus Naud.) were compared to the electrolyte leakage changes of the same tissue during maturation and storage at 4 or 24C. During fruit maturity and storage, leakage of the hypodermal-mesocarp tissue increased, which is coincident with increased total sterol: total phospholipid ratios and increased phospholipid fatty acid saturation index of the PM. ATPase activity, a marker for the PM, indicated that the PM increased in buoyant density from 1.13 to 1.14 during maturity and ATPase activity peaked with fruit maturation. ATPase activity decreased with 10 days postharvest storage and was less at 24C vs. 4C, which was coincident with increased hypodermal-mesocarp electrolyte leakage. Biochemical changes within the sterol and phospholipid matrix of the PM are suggested to contain the processes capable of altering fruit membrane permeability and subsequent muskmelon fruit storage life.

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Karl J. Sauter, David W. Davis, Paul H. Li and I.S. Wallerstein

Yield in common bean, Phaseolus vulgaris L., can be significantly reduced by high temperature (I-IT) during bloom. Ethylene production from plant tissue increases as a consequence of various stresses, including heat stress. The inheritance of leaf ethylene evolution rate (EER) of HT-stressed (35/30C day/night) progenies from crosses among bean genotypes previously categorized as HT sensitive or tolerant, based on cell electrolyte leakage, was investigated. Evidence from generation means analysis of Fl, F2, and backcross progenies shows EER to be genetically controlled, with additive, dominance, and epistatic effects indicated for low EER. The range (0.62 to 2.52 μg-1·hr-1) of EER from field-grown lines and cultivars suggests the existence of considerable genetic variability. EER was associated (r = –0.70) with heat tolerance, as estimated by cell electrolyte; leakage.

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Reeser C. Manley and Rita L. Hummel

The index of injury (It) and tissue ionic conductance (gTi) formulas for analyzing electrolyte leakage data from freeze-stressed tissues of cabbage (Brassica oleracea L. Capitata group) were compared. The two formulas produced similar results in calculating the relative freezing responses of stem pith, lamina, and petiole tissues. Disagreement occurred only with lamina tissues when the magnitude of ion leakage was low. Vital staining of pith and petiole tissues with triphenyl tetrazolium chloride indicated that the tissue TK50 (the temperature resulting in 50% injury), derived from It data, was a reliable indicator of the freeze-killing point. These results support the use of the simpler It method for analyzing electrolyte leakage data in studies of cabbage freezing tolerance.

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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.