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  • Author or Editor: William S. Conway x
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Changes in tissue water relations, cell wall calcium (Ca) levels and physical properties of Ca-treated and untreated `Golden Delicious' apples (Malus×domestica Borkh.) were monitored for up to 8 months after harvest. Pressure infiltration of fruit with CaCl2 solutions at concentrations up to 0.34 mol·L-1 reduced both fruit softening and air space volume of fruit in a concentration-dependent manner. Turgor potential-related stress within the fruit persisted during storage and was higher in Ca-treated than in untreated fruit. Fruit that were pressure infiltrated with CaCl2 solutions between 0.14 and 0.20 mol·L-1 and then waxed to reduce water loss during storage showed no peel injury. Calcium efflux patterns from apple tissue disks indicated two distinct Ca compartments having efflux kinetics consistent with those for cell wall Donnan-phase bound and water free space soluble Ca. At Ca concentrations up to 0.20 mol·L-1, cell wall bound Ca approached saturation whereas soluble Ca showed a linear dependence. At higher external Ca concentrations, only soluble Ca in the tissue increased. During 8 months of cold storage, cell wall Ca-binding capacity increased up to 48%. The osmotic potential of apples harvested over three seasons ranged between-1.32 and -2.33 MPa. In tissue disks, turgor potential changes caused by adjusting the osmolality of the incubation solution with CaCl2 or sorbitol were accompanied by changes in the osmotic and water potentials of the tissue. In CaCl2 solutions up to 0.34 mol·L-1, turgor potential was ≥0.6 MPa in tissue incubated in 0.14 or 0.17 mol·L-1 solutions of CaCl2 and was more than 3 times higher than in tissues incubated in low (≤0.03 mol·L-1) or high (≥0.27 mol·L-1) concentrations of CaCl2. At osmotically equivalent concentrations, turgor potential was up to 40% higher in Ca-than in sorbitol-treated tissue. The results suggest that postharvest treatment with 0.14 to 0.20 mol·L-1 solutions of CaCl2 are best for maintaining fruit water relations and storage life of `Golden Delicious' apples while minimizing the risk of salt-related injuries to the fruit. While higher concentrations of CaCl2 may better maintain firmness, these treatments adversely affect fruit water relations and increase the risk of fruit injury.

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Three polyamine biosynthesis inhibitors, α-difluoromethylornithine (DFMO), α-difluoromethylarginine (DFMA), and α-methylornithine (MeOrn), alone and in combination with CaCl2, were tested for their ability to reduce in vitro growth and soft rot development in apple (Malus domestica Borkh.) fruit caused by Botrytis cinerea Pers.:Fr. and Penicillium expansum Link. All three inhibitors reduced the in vitro growth of the pathogens. Calcium had no effect on fungal growth in vitro. Pressure infiltration of millimolar concentrations of DFMO or DFMA or 25 g·L-1 CaCl2 solutions into apples reduced subsequent soft rot development by B. cinerea and P. expansum >40%. A combination treatment of Ca and DFMO or DFMA reduced decay >67%. Treatment of apples with MeOrn was less effective at inhibiting decay development. None of the inhibitors affected polyamine levels in apple cortical tissues. Some injury to the fruit surface was observed with Ca treatments. Fruit treated with Ca and any of the inhibitors were less firm than those treated with Ca alone. Specific polyamine biosynthesis inhibitors in combination with Ca may prove useful in reducing postharvest decay in apples.

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Abstract

The rate of postharvest softening of ‘McIntosh’ apples (Malus domestica Borkh.) was reduced by storage in a 1% O2 atmosphere at 1 or 3.5C. Apples stored in controlled atmosphere (CA) also maintained higher levels of the polyamines putrescine (PUT), spermidine (SPD), and spermine (SPN) in both skin and flesh tissues than those stored in air. The levels of putrescine and spermidine increased by 2- to 6-fold in CA-stored apples, while spermine decreased, but remained 2- to 5-fold higher than in air-stored fruit at both temperatures. Polyamines were also found to inhibit the in vitro activity of the cell wall-degrading enzyme polygalacturonase (PG). SPD and SPN were more effective than PUT, with SPN possessing the greatest inhibitory activity. These results are consistent with a hypothesis that increased polyamine levels are involved in the beneficial effects of CA storage and that polyamine activity could include the inhibition of cell wall degradation.

Open Access

Abstract

‘Golden Delicious’ apples (Malus domestica Borkh.) were pressure-infiltrated (68.9 kPa) at two harvest dates with 0%, 1%, 2%, or 4% (w/v) solutions of CaCl2 and stored at 0C for 2, 4, or 6 months followed by 1 week at 20C. Calcium concentrations, axial compression profiles, and Magness-Taylor firmness were measured. Calcium chloride infiltration increased all measures of tissue strength immediately and relative increases persisted during storage. A 1-week difference in harvest date markedly affected Ca uptake and textural responses; however, for both dates, 2% CaCl2 was effective in firming the apples. Apples from the second harvest, which were treated with 2% CaCl2 and stored for 6 months, had textural measurement values equal to or greater than those of comparable apples infiltrated only with water and measured before storage. Calcium chloride at 4% had a greater firming effect, but caused severe surface damage. Differential reponses to CaCl2 levels and storage durations by various textural measurements indicate that supplemental Ca not only increased firmness retention during storage, but also induced patterns of textural change different from those that occurred under the influence of the endogenous Ca alone.

Open Access

Pressure infiltration of `Golden Delicious' and `McIntosh' apples (Malus domestica Borkh.) with polyamides resulted in an immediate increase in firmness. `Golden Delicious' apples were 2.7 N (0.25 mM spermidine) to 6.7 N (1.0 mM spermine) firmer, while `McIntosh' apples were 2.2 N (0.25 mM spermidine) to 5.3 N (1.0 mM spermine) firmer than the water-treated control. During 28 weeks of storage at 0C, the differences between the polyamine-treated and water-treated apples were even larger. Similar results were observed with a 3% Ca treatment, but the Ca treatment reduced the rate of softening to a greater extent than did the polyamine treatments in `Golden Delicious'. Polyamides increased the endogenous levels of the polyamides infiltrated; however, the levels declined rapidly with time in storage. Both polyamine and Ca inhibited the development of chilling injury symptoms (brown core) in `McIntosh'. The influence of polyamines on ethylene production was negligible in both cultivars. The Ca treatment, however, inhibited ethylene evolution in `Golden Delicious'. Polyamides, thus, may affect apple softening through rigidification of cell walls rather than through interactions with ethylene metabolism.

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Apples (Malus domestica Borkh cv. `Delicious') were stored after harvest for 2 weeks at 0 C. Fifteen fruit (5 fruit per replication, 3 replications) were then removed from storage and heated (38 C, 3 days) and 15 fruit remained in storage (control). After treatment the fruit were returned to storage for 2 months at 0 C. The fruit were then removed from storage and assayed for 1-aminocyclopropane-l-carboxylic acid (ACC) activity and ethylene production. The ability of the fruit tissue to convert ACC to ethylene was also determined by utilizing ACC spiked samples. Heat significantly lowered the ethylene production rate. However, the conversion of ACC to ethylene was not different between treatments when samples were spiked with 1 μM of ACC. This indicates that ethylene forming enzyme (EFE) activity was not affected by the heat treatment. The effect of heat treatment on ACC concentration and the conjugation of ACC into 1-malonylaminocyclopropane-l-carboxylic acid (MACC) will be discussed.

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Fruit from five apple (Malus domestica Borkh.) cultivars were pressure-infiltrated at 103 kPa for 6 min with a 0%, 0.73%, 1.46%, 2.91%, or 5.82% (w/v) Ca-equivalent solution of CaCl2, Ca EDTA chelate, or buffered CaCl2 solution (Stopit). The fruit were stored at 0 ± 1C for 18 weeks and then evaluated for Ca content, firmness, and injury. Fruit treated with Ca chelate had no increase in fruit Ca content and were injured at all treatment levels. No significant differences occurred in fruit Ca levels between CaCl2 and Stopit treatments across all cultivars tested. Apples treated with Stopit were firmer than apples treated with CaCl2, when averaged across cultivars. Fruit Ca levels, firmness, and incidence of injury were positively correlated with concentrations of CaCl2 and Stopit for all cultivars.

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Frozen hydrated buds and epicarp of `Golden Delicious' apple (Malus domestica Borkh.) were observed with a low-temperature, field emission scanning electron microscope (SEM). In addition to observing surface features of these specimens, holders were modified to observe fractured specimens. A modified hinged holder retained both halves of a fractured specimen for examination of the complementary faces of frozen hydrated tissues. Low-temperature SEM avoided artifacts, such as extraction, solubilization, and shrinkage, which are normally encountered with chemical fixation, dehydration, and drying, respectively. The technique allowed observations of well-preserved frozen hydrated structures, such as the platelets of epicuticular wax; loosely associated organisms on plant surfaces, such as spider-mite eggs; delicate structures, such as fungal hyphae; and partially hydrated tissues, such as fruit epicarp and winter bud scales.

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Conventional scanning electron microscope (SEM) have greatly expanded our knowledge about the ultrastructure of plant tissues; however, the fixation, dehydration and drying procedures required for SEM preparation of specimens, are slow, extract soluble materials and cause shrinkage. Recent improvements in a technique referred to as low temperature (LT) SEM now allow samples to be observed in a frozen hydrated state, thereby avoiding the artifacts associated with conventional specimen preparation. To evaluate this technique, healthy and Botrytis cinerea infected `Golden Delicious' apple fruit were collected, frozen in liquid nitrogen, examined and photographed in a Hitachi S-4100 SEM equipped with and Oxford CT 1500 Cryotrans System. Results indicated that LT SEM (1) retained soluble materials such as platelets of the epicuticular wax; (2) preserved tissues in varying degrees of hydration and (3) maintained loosely associated structures such as spores and hyphae. These results suggest that LT SEM has potential applications to many horticultural problems.

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The viability of Penicillium expansum Link conidia in sporulating culture declined rapidly when exposed to 38 °C, and when conidia were exposed to 38 °C prior to inoculation of apple fruits (Malus ×domestica Borkh.), the resulting lesions were smaller than those on fruit inoculated with nonheated conidia. `Gala' apples were heated after harvest (38 °C for 4 days), pressure infiltrated with a 2% solution of CaCl2, or treated with the antagonist Pseudomonas syringae van Hall, alone or in combinations to reduce postharvest decay caused by Penicillium expansum. After up to 6 months in storage at 1 °C, no decay lesions developed on fruit that were heated after inoculation with P. expansum, or any combination of P. expansum, antagonist, or Ca. Parallel lots of heat-treated and nonheated fruit that were either infiltrated or not infiltrated with Ca were stored up to 6 months. They were then inoculated with P. expansum alone, or with the antagonist followed by P. expansum. Prior heat treatment did not influence lesion size. Calcium alone, the antagonist alone, and heat plus Ca all reduced the incidence of decay by ≈25%, whereas heat plus the antagonist reduced it by 70%. Calcium plus the antagonist or Ca plus the antagonist and heat reduced decay incidence by 89% and 91%, respectively. The integrated strategy of heat-treating fruit, followed by Ca infiltration and then treatment with an antagonist, may be a useful alternative to controlling postharvest decay with fungicides.

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