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Steven J. Damon and Michael J. Havey

beneficial for growers and, combined with fewer pesticide applications, offers a potentially affordable and sustainable management strategy for this important pest. Reduced amounts of leaf epicuticular waxes (often referred to as glossy or bloomless

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Steven J. Damon, Russell L. Groves and Michael J. Havey

)] and sunflower [ Helianthus annuus L. ( Ashfaq and Aslam, 2001 )]; however, specific mechanisms were not determined. Leaf epicuticular waxes are important in plant–insect interactions and are associated with both attractive and repulsive responses

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Eduardo D. Munaiz and Michael J. Havey

, accumulates significantly less epicuticular waxes on its foliage compared with wild-type “waxy” onion, and suffers less damage by onion thrips ( Damon et al., 2014 ; Jones et al., 1934 ; Molenaar, 1984 ; Munaiz et al., 2019 ). The main epicuticular wax on

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Robert D. Belding, Sylvia M. Blankenship, Eric Young and Ross B. Leidy

Variation in amount and composition of epicuticular wax among several apple (Malus ×domestica Borkh.) cultivars was characterized by gas chromatography, thin-layer chromatography, and gas chromatography-mass spectroscopy. Across cultivars, wax mass ranged from 366 to 1038 μg·cm-2. Wax mass decreased during the 30 days before harvest. Ursolic acid accounted for 32% to 70% of the hydrocarbons that make up the epicuticular wax. Alkanes, predominantly 29-carbon nonacosane, comprised 16.6% to 49%. Primary alcohols of the hydrocarbons ranged from 0% to 14.6% of the epicuticular wax. Secondary alcohols of the hydrocarbons were the most cultivar specific, making up 20.4% of the epicuticular wax in `Delicious' and only 1.9% `Golden Delicious' strains. Aldehydes and ketones of the hydrocarbons represented a small amount of total wax, ranging from 0% and 6.0%. Percentage of primary alcohol in the epicuticular wax increased as fruit developed. Other components showed no distinct trends with fruit development. Examination of the ultrastructure of cuticular wax using scanning electron microscopy revealed structural differences among cultivars.

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Paul H. Jennings, N. Ishii and R. Rufner

Chlorotic bands across sugarcane leaves were first described as symptoms of cold chlorosis in 1926 and later described in sorghum and maize. The injury develops after exposure of seedlings to temperatures in the 0°C to 12°C range. The severity of injury in maize seedlings may be reduced by high relative humidity during the post-chilling period suggesting a temperature induced water stress. An early visible chilling response is the appearance of a glazed area in the region in which the chlorotic band will develop. This area of the young expanding maize leaf was studied with scanning electron microscopy(SEM). Maize seedlings were grown for 6 days at 24°C with a 15/9 h light/dark cycle. Plants were chilled at 10°C for 9 h during the 7th dark period and leaves sampled 39 h after the end of chilling. SEM photomicrographs revealed a gradient of epicuticular wax deposition from the tip to the base of the leaf. In the region of chill-induced chlorotic band formation, the control leaves exhibited a greater amount of wax deposition than the chilled leaves. It is suggested that the reduced epicuticular wax in a band across the chilled leaves might lead to a water stress resulting in chlorosis and eventually developing into the typical necrotic band.

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S. Wee and R.M. Beaudry

Volatile compounds produced by apple (Malus domestica Borkh) fruit partition into the cuticle and epicuticular waxes and may play an important role in superficial apple scald. Of these volatiles, α-farnesene, conjugated trienes, hydroperoxides, and 6-methyl-5-hepten-2-one have been identified as playing a crucial role in scald production. Volatiles from the epicuticular wax of four different apple cultivars have been analyzed by gas chromatography/mass spectroscopy. A correlation was found between scald incidence and 6-methyl-5-hepten-2-one content and the 6-methyl-5-hepten-2-one:α-farnesene ratio. α-Farnesene is the most-abundant volatile at the beginning of storage, whereas 6-methyl-5-hepten-2-one is present in minute quantities. These two volatile compounds appear to have an inverse relationship with respect to one another since the levels of 6-methyl-5-hepten-2-one increased and α-farnesene decreased prior to the onset of apple scald. This changing ratio may have been due to an autoxidative process resulting in the breakdown of α-farnesene to 6-methyl-5-hepten-2-one. Analysis of the volatiles emanating from the apple wax revealed a number of compounds associated with aroma that also partition readily into the fruit surface.

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Roy E. McDonald, Harold E. Nordby and T. Gregory McCollum

Differences in chilling injury (CI) susceptibility between `Marsh' grapefruit (Citrus paradisi Macf.) from interior and exterior tree canopy positions were analyzed to investigate the hypothesis that epicuticular wax morphology and composition influence CI development during low-temperature storage. The sun-exposed surface of fruit from the exterior canopy had significantly more CI and larger wax platelets than the shaded surface of the same fruit. Interior canopy fruit had significantly less CI and smaller wax platelets than exterior canopy fruit. Hydrocarbons, primarily n-alkanes, were significantly more abundant in the epicuticular wax on the surfaces of sun-exposed and exterior fruit compared with surfaces of shaded and interior fruit, respectively. Results of this study suggest that epicuticular wax plays a role in the development of external CI symptoms on grapefruit.

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Stéphane Roy, William S. Conway, Alley E. Watada, Carl E. Sams, Eric F. Erbe and William P. Wergin

`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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Stephane Roy, William S. Conway, Alley E. Watada, Carl E. Sams, Eric F. Erbe and William P. Wergin

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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Edward W. Hellman and Laurel Raines

The cuticle and epicuticular wax layer of grape berries provides a first line of defense against fungal pathogens. A relatively thick cuticle and wax layer may result in enhanced resistance to Botrytis cinerea Pers. The structure of epicuticular wax also has been postulated to play a role in Botryris resistance.

To examine the role of cuticle and wax in disease resistance, berries of diverse grape cultivars were sampled to quantify the cuticle and wax layers. Wax surface structure was examined by scanning electron microscopy. Significant differences among cultivars were found for both cuticle and wax measurements. Environmental conditions may influence development of these layers, some cultivars had significantly greater cuticle and wax layers in berries that developed in full sunlight. Visual differences in surface wax appearance were apparent among cultivars. Size, density and orientation of wax platelets varied among cultivars.