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Peter D. Petracek

A peel disorder of white grapefruit that has caused substantial losses over the past several seasons was examined. This disorder, which has been identified also in Fallglo and reported for oranges and other grapefruit varieties, is characterized by scattered clusters of pits and is caused by the collapse of oil glands. Applying commonly used waxes and subsequent high-temperature storage (15°C or higher) triggered pitting. Washing and exposure to ethylene during degreening did not affect pitting. Shellac-based water emulsion waxes from three companies and a polyethylene-based wax stimulated pitting but carnauba-based wax did not. Evaluation of internal gases of the fruit showed that all but the carnauba-based wax resulted in low O2 (<5%) and high CO2 (>6%) internal atmospheres. Subjecting nonwaxed fruit to 4% O2 and 9% CO2 storage atmospheres also stimulated pitting. These results suggest that wax application, in conjunction with high-temperature storage, may stimulate pitting by affecting gas exchange and respiration.

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Peter D. Petracek

Early-season fresh citrus are routinely exposed to ethylene to trigger chlorophyll degradation (degreening) in the peel and thus improve fruit color. Recent questions about whether ethylene is trapped in the fruit by subsequent waxing have sparked interest in characterizing ethylene exchange. Internal gas samples of mature, pesticide-free `White Marsh' grapefruit were taken through septa of silicone rubber on electrical tape affixed 10 the blossom end. Gassing of the fruit in a degreening room (10 ppm ethylene) required about four hours lo reach equilibrium while degassing was completed in less than two hours and was not affected by location of the fruit in a 0.680 m3 pallet bin. Waxing with a water-soluble wax immediately following ethylene exposure increased the time for complete degassing to over 48 h. Surface gas exchange protiles were prepared by sequentially analyzing the same fruit after: (1) harvest, (2) 22 h exposure to 10 ppm ethylene, (3) exposure to ethylene and washing with an ionic cleaning surfactant, and (4) exposure to ethylene and waxing. Glass cells with interfacing silicone rubber o-rings (23 mm diam.) were strapped to the fruit following each treatment. Ethylene emanation was measured by sampling the cells which were capped 15 m after removal from ethylene. Water and CO2 were measured by flow-through cells following ethylene analysis. Ethylene emanation following the initial exposure was the same for the stem end and midsection and two fold greater than the blossom end. Washing increased the rate of emanation five fold for the stem end and about 2.5 fold for the midsection and blossom end. Waxing reduced emanation by nearly four fold for the midsection and blossom end, but only 30% for the stem end. Water loss was increased about 40% by washing, reduced about 30% by waxing, and was primarily through the stem end. Stem-end CO2 exchange doubled upon waxing.

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Peter D. Petracek and Craig Davis

Postharvest pitting of citrus fruit is a recently defined peel disorder that is caused by high-temperature storage (>10°C) of waxed fruit. We examined the anatomy of pitted white grapefruit peel to improve our understanding of this disorder and assist in its diagnosis. Scanning, light, and transmission micrographs showed that postharvest pitting is characterized by the collapse of oil glands. Cells enveloping the oil glands are the cells of primary damage. Oil gland rupture may occur anywhere around the oil gland, but often occurs in regions farthest from the epidermal cells. Adjacent parenchyma cells are damaged as the oil spreads. Epidermal and hypodermal cells are often damaged during severe oil gland collapse. In contrast, chilling injury is characterized by the collapse of epidermal and hypodermal cells. Oil glands are affected only in severe cases of chilling injury. Oleocellosis (oil spotting) is often characterized by the collapse of epidermal and hypodermal cells, but cells enveloping the oil gland are typically not damaged. Physical damage is characterized by damage of epidermal cells, a wound periderm, and presence of secondary pathogens.

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Huating Dou and Peter D. Petracek

`Fallglo' is an early season variety of tangerine that has become known among citrus packers for its susceptibility to postharvest peel disorders. Among these disorders is postharvest pitting, a disorder characterized by the collapse of the peel during the storage of waxed fruit at room temperature. In this study, the effects of wax application on selected postharvest storage characteristics were evaluated.

Fruit were either not waxed or waxed with carnauba-, polyethylene-, or shellac-based waxes obtained from four suppliers of commercial citrus coatings and were stored at 21°C. In general, waxing reduced weight loss and improved shine. Waxing with shellac-based waxes significantly decreased internal O2 levels (5% vs. 19% for non-waxed fruit) and increased internal CO2 (6% vs. 2% for non-waxed fruit) and ethanol levels. Waxing with shellac-based waxes also significantly reduced post-packing degreening and stimulated pitting. Waxing with more gas-permeable coatings (carnaubaand polyethylene-based waxes) resulted in less internal gas modification than that of the shellac-based treatments, and low incidences of pitting. Controlled atmosphere studies showed that low (4%) O2, rather than high (8%) CO2, inhibited post-packing degreening and stimulated pitting.

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Peter D. Petracek and Lymari Montalvo

`Fallglo' (Bower citrus hybrid [Citrus reticulata Blanco × (C. reticulata Blanco × C. paradisi Macf.)] × `Temple' [C. reticulata Blanco × C. sinensis L.] is an early season tangerine that is reportedly hypersensitive to ethylene exposure during degreening. The effects of ethylene exposure time, waxing, and storage temperature on `Fallglo' color were examined to assess degreening strategies. Exposure to 5 μL·L-1 ethylene for 24 or 48 hours hastened degreening, and internal ethylene levels increased following the time periods of ethylene exposure. Fruit not exposed to ethylene, or exposed for shorter periods (2 or 6 hours), degreened slowly during storage at 15.5 °C and internal ethylene levels remained low. Low-temperature storage (4.5 °C) or waxing hindered degreening after ethylene exposure but decreased water loss. Degreening after ethylene exposure was faster for fruit stored at 15.5 than 26.5 °C.

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Bhaskar Bondada, Peter D. Petracek and Jim Syvertsen

Recent interest in reducing nitrate levels in ground water has stimulated the re-examination of foliar application of urea on citrus trees. Because the cuticle is the primary barrier to foliar uptake, we examined the diffusion of 14C-urea through isolated citrus leaf cuticles. Cuticles were enzymatically isolated from leaves of the four youngest nodes (1 month to 1 year old) of pesticide-free grapefruit trees. The diffusion system consisted of a cuticle mounted on a receiver cell containing stirred buffer solution. Urea (1 μL) was pipetted onto the cuticular surface, and buffer solution was sampled periodically through the side portal of the receiver cell. The time course of urea diffusion was characterized by lag (time to initial penetration), quasi-linear (maximum penetration rate), and plateau (total penetration) phases. Apparent drying time was less than 30 min. Average lag time was about 10 min. The maximum penetration rate occurred about 40 min after droplet application and was about 2% of the amount applied per hour. Rewetting stimulated further penetration. The total penetration averaged about 35% and tended to decrease with leaf age. Dewaxing the second node cuticles by solvent extraction significantly increased maximum penetration rates (30% of the amount applied per hour) and total penetration (64%).

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Peter D. Petracek and D. Frank Kelsey

High-pressure washing (>689 to 3446 kPa or 100 to 500 psi at the spray nozzle) has been used recently in citrus packinghouses to improve the action of surfactant solution and brushing on the removal of dirt and superficial molds. Although high-pressure washing has no obvious detrimental effect on citrus fruit (e.g., no cellular breakage), its effects on physiology have not been fully examined. In this study gas samples were taken from the fruit core of `Orlando' tangelos, `Hamlin' oranges, and `Ruby Red' and white `Marsh' grapefruit prior to and following washing. An apparent wound ethylene response was measured for all varieties and was a function of prolonged exposure (>20 s) and excessive pressure (>2067 kPa). For the responding fruit, internal ethylene was initially detected about 3 h after washing, reached a maximum around 24 h (range: 0.1 to 0.6 ppm), and diminished to near background levels (0.0 ppm) after 48 h. No wound ethylene was observed when fruit were washed for the recommended exposure time (10 s) and pressure (1379 kPa). Concurrent decreases in internal O2 and increases in CO2 were observed for white and red grapefruit. High-pressure washing (1379 or 2757 kPa) did not affect water loss and water, O2, and CO2 exchange. The effects of subsequent waxing of the fruit (increased internal ethylene and CO2 levels and reduced of internal O2 levels) were amplified by washing at the higher pressure (2757 kPa).

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Steven Pao, Peter D. Petracek and G. Eldon Brown

Peeling and storage characteristics of citrus fruit infused with water or enzyme solution were compared. Fruit were vacuum- or pressure-infused with water or water-containing pectinase. The enzyme treatment did not affect peeling times of white or red grapefruit, oranges, or tangelos. Pressure and vacuum infusion methods produced similar results. Grapefruit and oranges infused with water had significantly less juice leakage and were firmer than fruit infused with enzyme. Microbial levels and respiration rates and ethylene emanation during storage were the same for enzyme- and water-treated fruit.

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Peter D. Petracek, Moritz Knoche and Martin J. Bukovac

Despite the widespread use of surfactants to enhance the performance of foliar applied chemicals, the mechanisms for this enhancement are poorly understood. The penetration of surfactant per se through the cuticular membrane (CM) may play a pivotal role. Thus, we examined CM penetration by octylphenoxy surfactants (Triton X series) using a finite dose (Franz) diffusion cell. The effect of hydrophile length was studied using 14C surfactant (15.9 mm in 20 mm citrate buffer: pH 3.2) with 3, 9.5, 12, 16, and 40 ethylene oxide units per molecule (EO). One 5-μl droplet of surfactant solution was applied to the outer morphological surface of CM enzymatically isolated from mature tomato fruit. The inner CM surface remained in contact with stirred buffer at 25°C. The buffer was sampled periodically through a side portal over 648 h. Penetration curves (time vs. % penetrated) for all surfactants were characterized by three phases: lag, linear, and asymptotic. Lag: There was no effect of EO on the length of the lag phase (average 5 h) Linear: Steady state penetration (0.6 to 1.1% / h) was inversely related to log EO content. Asymptotic: About 70% of applied short EO (3 to 16) surfactants penetrated while 25% of the 40 EO penetrated in 648 h.

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Huating Dou, Peter Petracek, Mohamed Ismail, Ashok Alva and David Calvert

Effects of N, K, and water relation on the incidence and severity of postharvest pitting in white grapefruit were evaluated in two field experiments. In the first experiment, a factorial combination of 3 N (56, 168, and 336 kg·ha–1) and 3 K (52, 156, and 312 kg·ha–1) rates were used with three broadcast applications per year. In the second experiment, there were two irrigation regimes (at 30% and 60% depletion of available soil moisture, ASM, content) with three subtreatments of variable N and K rates (Kg·ha–1) at 56: 52; 112: 104, and 336: 312 kg·ha–1. The fruit were harvested three times each season, waxed with shellac wax, and stored at 70°F for evaluation of pitting. The pitting incidence was lower at the optimal N and K rates than that at the low or high rates. The irrigation at 30% ASM significantly reduced pitting incidence. The higher incidence of pitting was found in an area in the grove with higher water table. This study suggested that effects of water may play an important role on peel physiology and pitting.