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  • Author or Editor: Eric F. Erbe x
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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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Structural changes in the cuticle could be partially responsible for the differences in uptake of infiltrated Ca in apple fruit. We examined the relationship between the surface structure of epicuticular wax of `Golden Delicious' apple and Ca uptake by the fruit. Apples were nontreated or pressure infiltrated with distilled water, or with 0.14 or 0.27 mol·L-1 CaCl2 solutions 2 weeks before optimum harvest time, at optimum harvest, or after 2, 4, or 6 months of storage at 0 °C. Examination of the fruit surface with low-temperature scanning electron microscopy revealed that cracks in the epicuticular wax became wider and deeper as storage duration increased. After 6 months of storage, the cracks extended through the cuticle. Uptake of Ca by the infiltrated fruit was greater after 6 months of storage than after shorter storage intervals. These data indicate that as storage duration increased, epicuticular wax cracks became deeper and Ca uptake by the fruit increased.

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`Golden Delicious' apples (Malus domestica Borkh) were pressure-infiltrated at harvest with a 4% CaCl2 solution either without prior heat treatment or following 4 days at 38C. Examination of the apple surfaces from both treatments by low-temperature scanning electron microscopy revealed that heat treatment changed the pattern of epicuticular wax. The epicuticular wax of nonheated fruit exhibited numerous deep surface cracks that formed an interconnected network on the fruit surface. The epicuticular wax of heat-treated fruit did not exhibit a similar network of deep cracks. This apparent obstruction or elimination of deep cracks may limit the CaCl2 solutions from entering the fruit. The heated fruit contained significantly less Ca than the fruit that were pressure-infiltrated with CaCl2 solutions but not heated. These results indicate that cracks on the fruit surface may be an important pathway for the penetration of CaCl2 solutions.

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Prestorage heat treatment of apples has been shown to have a positive effect on fruit quality in storage. Postharvest treatment of apples with CaCl2 also beneficially affected fruit during storage. However, calcium uptake seems limited in heat-treated apples which indicates that the surface of the fruit may have been affected by the heat treatment. This study examined the effect of heat treatment on the subsequent uptake of CaCl2 solutions and related this process to the ultrastructure of the epicuticular wax surface of the fruit. Apples were pressure infiltrated with a 4% CaCl2 solution either without heat treatment or following 4 days at 38°C. Examination of the apple surfaces with low temperature scanning electron microscopy revealed that heat treatment changed the pattern of epicuticular wax. The epicuticular wax of non heated fruit exhibited numerous deep surface cracks. The epicotictdar wax of heated fruit did not exhibit similar cracks. This apparent obstruction or elimination of deep cracks may limit the CaCl2, solutions from entering the fruit. The heated fruit contained significantly less calcium compared to the fruit that were pressure infiltrated with the CaCl2 solution but not heated. These results indicate that cracks on the fruit surface may be a” important pathway for the penetration of CaCl2 solutions.

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