Roy E. McDonald

Consequences on ethylene metabolism of inactivating the ethylene receptor sites in diseased non-climacteric fruit.

Abstract Penicillium digitatum-infected grapefruit synthesize large quantities of the stress hormone ethylene. The compound 1-methylcyclopropene (1-MCP) inhibits the binding of ethylene to the ethylene receptor site, the ethylene binding protein (EBP). Treating infected fruit with 1-MCP prevented infection-induced degreening, such that fumigated fruit retained their green immature color compared to yellow non-fumigated controls. However, 1-MCP treatment significantly increased whole fruit ethylene production. In flavedo tissue of infected non-1-MCP treated fruit, 1-aminocyclopropane-1-carboxylate (ACC) synthase transcript accumulation, ACC synthase (ACS) enzyme activity, ACC and ethylene synthesis were all significantly higher +5 mm ahead of the lesion front than in uninfected non-1-MCP treated controls, but decreased significantly with increased sampling distance away from the lesion. 1-MCP treatment increased ethylene production in infected fruit at all three sampling distances compared to the non-fumigated samples. Even in the absence of infection, 1-MCP treatment resulted in increased ethylene synthesis. The results suggest that, in the presence of a pathogenic stress, blocking the EBPs prevented regulatory control of the ethylene biosynthetic pathway that resulted in an uninhibited expression of the ACS stress-associated genes, increased ACS activity and elevated ACC accumulation and ethylene production. Blocking of the EBPs with 1-MCP did not affect progression of the pathogen through the fruit.

Temperature of water heat treatments influences tomato fruit quality following low-temperature storage

Abstract Mature-green tomato fruit (Lycopersicon esculentum Mill. cv. Sunbeam) were treated in water for 1 h 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. Treatment at 42°C reduced decay by 60%, whereas the other water temperatures were less effective. Heat treatment had no effect on time required to ripen the fruit, with 11 days required for nonchilled and 27 days required for chilled fruit (including storage time). Ripe, nonchilled tomatoes had higher respiration rates and evolved more ethylene than did chilled fruit. The 48°C treatment increased respiration and ethylene evolution compared with the other treatment temperatures. 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. With the exception of the 45°C treatment, chilled as well as nonchilled fruit previously treated at 39, 42, or 45°C were preferred in terms of taste and texture in informal taste tests over fruit treated at 27 or 48°C. Storage at 2°C led to an increase in electrolyte leakage, particularly in the 48°C treated fruit. Of the 15 flavor volatiles analyzed, the levels of five were decreased and two were increased with increasing temperature of heat treatment. Storage at the chilling temperature reduced the levels of five flavor volatiles. Heat treatments decreased sterols in the steryl ester fraction, several sterols in the free sterol, steryl glycoside, and acylated steryl glycoside fractions. Prestorage heat treatments, with the possible exception of the 48°C temperature, can reduce decay with only minimal adverse effects on tomato fruit quality.

Respiration rate, internal atmosphere, and ethanol and acetaldehyde accumulation in heat-treated mango fruit

Mature green mango (Mangifera indica L.) fruit were heated with forced-air at 46°C and 95–100% RH for 3 and 4 h or 48°C for 5 h. Respiration rate increased 312-fold and 5-fold during treatment at 46°C and 48°C, respectively. After cooling to 20°C, CO2 production remained higher in heated fruit for 4 days (46°C) or 6 days (48°C), then fell below that of control fruit. Immediately after heat treatment, the internal CO2 concentration increased to 9% (46°C) or 13% (48°C) and O2 concentration decreased to 11% (46°C) or 6% (48°C). While O2 concentration returned to control levels within 6 h after heat treatment, CO2 levels remained elevated. Acetaldehyde and ethanol concentrations increased after heat treatment at 46°C and 48°C and remained elevated 6 h after treatment in fruit treated at 48°C for 5 h. Heating mangos with hot forced-air at 95–100% relative humidity did not provide an acceptable quarantine treatment for ‘Keitt’ mangos because of internal breakdown.

Characterization of seven basic endochitinases isolated from cell cultures of Citrus sinensis (L.)

Seven endochitinases (EC 3.2.1.14) (relative molecular masses 23000–28000 and isoelectric points 10.3–10.4) were purified from nonembryogenic Citrus sinensis L. Osbeck cv. Valencia callus tissue. The basic chitinase/lysozyme from this tissue (BCLVC) exhibited lysozyme, chitinase and chitosanase activities and was determined to be a class III chitinase. While BCLVC acted as a lysozyme at pH 4.5 and low ionic strength (0.03) it acted as a chitinase/chitosanase at high ionic strengths (0.2) with a pH optimum of ca. 5. The lysozyme activity of BCLVC was inhibited by histamine, imidazole, histidine and the N-acetyl-d-glucosamine oligosaccharide (GlcNAc)3. The basic chitinase from cv. Valencia callus, BCVC-2, had an N-terminal amino acid sequence similar to tomato and tobacco AP24 proteins. The sequences of the other five chitinases were N-terminal blocked. Whereas BCLVC was capable of hydrolyzing 13.8–100% acetylated chitosans and (GlcNAc)4–6 oligosaccharides, BCVC-2 hydrolyzed only 100% acetylated chitosan, and the remaining enzymes expressed varying degrees of hydrolytic capabilities. Experiments with (GlcNAc)2–6 suggest that BCLVC hydrolysis occurs in largely tetrasaccharide units whereas hydrolysis by the other chitinases occurs in disaccharide units. Cross-reactivities of the purified proteins with antibodies for a potato leaf chitinase (AbPLC), BCLVC, BCVC-3, and tomato AP24 indicate that these are separate and distinct proteins.Mention of a trademark, warranty, propriety, or vendor does not constitute a guarantee by the U.S. Department of Agriculture and does not imply its approval to the exclusion of other products or vendors that may also be suitable.

Effect of high temperature on cell wall modifications associated with tomato fruit ripening

Abstract Tomato fruit (cv. ‘Sunny’) were harvested at the mature green stage and kept for 4 days at either 21°C or 40°C, then subsequently stored at 21°C. Red color development was inhibited in heated fruit, as was softening. Fourteen days after treatment, heat-stressed fruit were twice as firm as the controls. The cell wall uronide content increased in heat-exposed fruit while remaining unchanged in samples stored constantly at 21°C. Heat treatment inhibited ripening-associated cell wall modification so that 14 days after treatment, stressed fruit contained only one-third the amount of soluble polyuronides that was present in the control fruit. Furthermore, the rate of loss of cell wall galactose and arabinose was reduced in heat-stressed tomatoes. Postharvest exposure of unripe tomato fruit to high temperature appears to reduce the rate of cell wall degradation, while the synthesis of polyuronide components continues normally.

Some citrus chitinases also possess chitosanase activities

Several acidic chitinase and chitosanase isoforms were found in 4-week-old nonembryogenic sweet orange (‘Valencia’ [Citrus sinensis (L.) Osbeck]) callus tissue. Two isoforms (designated A1-CF1 and A1-CF2) were purified to homogeneity using HPLC size exclusion, anion exchange, and chromatofocusing techniques. Both hydrolase isoforms exhibited activity with either colloidal chitin or solubilized shrimp shell chitosan. Specific activities for the purified isoforms could not be calculated because of the lack of protein and contamination of ampholytes. However, the specific activities for chitinase and chitosanase after anion exchange were respectively 404 nmol GlcNAc per min per mg protein and 2,475 nmol GlcN per min per mg protein. The Mr for both enzymes was 30,500. The homogeneous proteins cross-reacted in western blots with antiserum against a basic class I potato leaf chitinase.

Quantitative fluorometric analysis of plant and microbial chitosanases

A quantitative fluorometric assay for chitosanase activity in bacterial and plant tissues was developed. The assay can be conducted with either finely milled preparations of chitosan in suspension or dissolved chitosan; activity is based on measurements of glucosamine (GlcN) or oligomers of GlcN. GlcN is detected fluorometrically after reaction with fluorescamine with detection in the nanomole range. Fluorescence measurements of chitosanase activity and radioassay of chitinase in commercial preparations of chitinase from Streptomyces griseus revealed that both activities were present. Specific activities for the S. griseus chitosanase using suspended and soluble chitosans were respectively 1.24 and 6.4 mumol GlcN.min-1.mg protein-1. Specific activity of the S. griseus chitinase was 0.98 mumol GlcN.min-1.mg protein-1. Sweet orange callus tissue was tested for chitosanase and chitinase activity. It was necessary to remove small amine-containing molecules from the callus preparations before chitosanase activity could be assayed. The specific activity for chitinase and chitosanase in desalted extracts of nonembryogenic Valencia sweet orange callus tissue was determined to be 18.6 and 89.4 nmol GlcN.min-1.mg protein-1, respectively.

Squalene in grapefruit wax as a possible natural protectant against chilling injury.

The influence of temperature conditioning on stored grapefruit against chilling injury (CI) as related to the fruits neutral lipids in the peel was investigated. Squalene, a highly unsaturated C30 isoprene hydrocarbon, was found to be present in the epicuticular wax of grapefruit. The optimal temperature for biosynthesis of squalene in grapefruit was 15°C; this is also the temperature reported previously as the optimum temperature for conditioning grapefruit against chilling injury. Control and temperature-conditioned grapefruit were stored monthly over three seasons from 1986 to 1989. Fruits were rated for chilling injury and the levels of squalene were determined. An inverse relationship found between CI and squalene level suggested that squalene may protect grapefruit from CI.

Preharvest applications of gibberellic acid delay senescence of Florida grapefruit

SummaryPeel senescence of ‘Marsh’ grapefruit (Citrus paradisi Macf.) was reduced by gibberellic acid (GA) treatment at three production sites in Florida as indicated by resistance to puncture and the retention of green colour. Applications of GA at 49 g ha−1 plus 0.05% Silwet L-77 were equivalent to 25 g ha−1 plus 0.1% Silwet L-77. GA at 49 g ha−1 plus 0.05% Silwet L-77 was more effective than the maximum label amount of GA (145 g ha−1) without surfactant. GA applications just prior to colourbreak in August and September were found to be most effective. GA treatments increased peel oil compared with controls, but did not affect fruit quality. Chlorophyll and carotene concentrations in October and November were higher in GA-treated fruit than fruit harvested in the remainder of the season. In contrast, there was only a small effect of GA on time to degreen fruit. Leaf drop was accelerated by GA treatment, but was not different from controls at the end of season in two of the three sites tested. The data su...

Changes in grapefruit flavedo cell wall noncellulosic neutral sugar composition

Abstract Analysis of grapefruit flavedo cell walls detected seven neutral sugars. Arabinosyl and galactosyl residues were most abundant, followed by xylosyl, mannosyl, glucosyl and fucosyl residues. The percentage of the cell wall composed of neutral sugars (38%) decreased during development and ripening to 27%. There was a change in cell wall neutral sugar composition from June to April with a reduced percentage of flavedo cell wall galactosyl and arabinosyl residues as the season progressed. Degreening of fruit accelerated the decrease in flavedo cell wall galactosyl and arabinosyl residues, and a single preharvest spray with gibberellic acid, three months prior to harvest, reduced the decrease in flavedo cell wall galactosyl and arabinosyl residues, as well as reducing the rate of peel softening and loss of green colour.