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Takashi Ikeda, Kunio Okano, Yuka Sakamoto, and Shin-ichi Watanabe

This study was undertaken to investigate the water relations of tomato (Lycopersicon esculentum Mill.) fruit cracking for single-truss tomato plants. The tomato plants were cultured on a closed hydroponic system in greenhouse. Water status of culture solution and plant tissues was measured with psychrometers. Water potential of the culture solution for the stressed plant was changed from -0.06 MPa (control plants) to -0.36 MPa at 24 days after anthesis. Hardness of the fruit skin was not different significantly between the stressed plants and the control plants. Fruit cracking occurred frequently in the control plants, but not in the stressed plants. Water potential gradient between the tissue of fruit flesh and water source for the control plants was bigger than that of the stressed plants. Turgors were increased at the tissues of fruit flesh and fruit skin at the control plants between predawn and morning but not at the stressed plants. These results indicated that the water potential gradient and the increased turgor in these tissues might be a trigger for the occurrence of fruit cracking on single-truss tomato plants.

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Martin J. Bukovac, Alicia Pastor, Royal G. Fader, and Moritz Knoche

Morphological and physical characteristics of the cuticular membrane (CM) of selected cultivars of sweet cherry (Prunus avium L.) fruit were studied relative to rain-induced cracking. Two characteristics of the CM may be determinants in rain-induced fruit cracking. The surface morphology and chemistry determine surface wettability and water retention, and the morphology and physicochemical characteristics its water permeability. The fruit epidermis as well as the guard cell walls adjacent to the outer vestibule and stomatal pore are covered by a thin lipoidal CM. Stomata were present at a frequency of 0.1 to 2 per mm2 depending on cultivar and fruit surface position. However, most appeared nonfunctional with many pores partially or completely occluded with wax-like material. There was no evidence of water (containing fluorescein or AgNO3) penetration into stomatal pores following surface application or submerging fruit for short periods. There was stomatal pore penetration when submerged fruit were infiltrated by reduced pressure in the presence of 0.1% L-77. Preferential sorption of AgNO3 and fluorescein by cuticular ledges and guard cells was noted. The epicuticular wax (ECW) had no significant fine-structure. The CM was isolated enzymatically (cellulase/pectinase) and found to be 1 to 2 μm thick with an area weight of 1.2 to 2.3 g·m–2, of which 25% to 40% was chloroform/methanol (1: 1by vol.) soluble. Fractionation of the chloroform/methanol fraction indicated the presence of four groups of nonpolar constituents. The fruit surface was moderately difficult to wet, forming contact angles of 85% to 105%, and with an estimated critical surface tension in the range of 16-24 mN·m–1. Fruit water loss (transpiration) and uptake on submersion was followed and found to be complex. Transpiration increased with an increase in temperature, and both rate of transpiration and water uptake increased after removal of the epicuticular and cuticular waxes. Pathways of water uptake and the significance of our findings to rain-induced fruit cracking will be discussed.

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Stefanie Peschel and Moritz Knoche

Frequency and distribution of microcracks in the cuticular membrane (CM) were monitored in cheek, suture, pedicel cavity and stylar regions of developing sweet cherry (Prunus avium L.) fruit using fluorescence microscopy following infiltration with a fluorescence tracer (1 to 2 min in 0.1% w/v acridine orange containing 50 mm citric acid and 0.1% Silwet L-77, pH 6.5). These microcracks were limited to the cuticle, did not extend into the pericarp and were only detected by microscopy. Fruit mass and surface area increased in a sigmoidal pattern with time between 16 days after full bloom (DAFB) and maturity. The increase in frequency of fruit with microcracks paralleled the increase in fruit mass. During early development (up to 43 DAFB) the CM of `Sam' fruit remained intact. However, by 57 DAFB essentially all `Sam' fruit had microcracks in the pedicel cavity and ≈25% in the suture region with little change thereafter. At maturity percentage of `Sam' fruit with microcracks in cheek, suture, pedicel cavity and stylar end region averaged 23%, 25%, 100%, and 63%, respectively. Similar data were obtained for `Hedelfinger' (70% and 100% for cheek and pedicel cavity, respectively), `Kordia' (80% and 100%) and `Van' (100% and 100%). Generally, microcracks were most severe in pedicel cavity and stylar end region. Most of the first detectable microcracks formed above periclinal walls of epidermal cells perpendicular to their longest axis (72% and 92% in cheek and stylar regions, respectively). The other microcracks formed above the anticlinal walls were mostly oriented in the direction of the underlying cell wall. There was no difference in projected surface area, length/width ratio or orientation among epidermal cells below, adjacent to or distant from the first detectable microcracks in the CM. However, as length of microcracks increased the projected surface area of cells underlying cracks increased suggesting strain induced upon cracking of the CM. Permeability of excised exocarp segments in osmotic water uptake was positively correlated with number of stomata and number of microcracks in the CM. From our results we suggest that strain of the epidermal system during stage III of fruit growth is a factor in “microcracking” of the CM that may predispose fruit to subsequent rain-induced cracking.

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Tara Auxt Baugher, Kendall C. Elliott, and D. Michael Glenn

Three growth suppression treatments were compared during 1991 to 1993 on `Stayman' apple (Malus domestica Borkh.) trees grown in the T-trellis and the MIA trellis systems. All treatments—root pruning, K-31 fescue (Festuca arundinacea Schreb.), and K-31 fescue plus root pruning—suppressed tree growth compared to the nontreated control, but results were inconsistent between years and systems. Sod or sod plus root pruning reduced terminal shoot length in both systems in 2 out of 3 years. Root pruning decreased shoot length in the T-trellis in 1992. Sod decreased trunk cross-sectional area in the T-trellis in 1993. Treatments did not affect 3-year average yield efficiency but did appear to increase biennial bearing. Sod, with or without root pruning, decreased fruit cracking in the T-trellis 69% and 42%, respectively, in 1992, and sod plus root pruning decreased cracking in the MIA trellis 50%. Sod reduced fruit diameter in the T-trellis in 1992. Secondary effects of growth suppression treatments included increased light penetration and improved fruit color. Sod decreased leaf N and Mg and increased leaf P, K, and Cu. The Oct. 1993 stem water potential gradient from root to canopy was more negative in the sod plus root pruning treatment, and the osmotic potential of rootsucker leaves in the combination treatment was greater than in the control, indicating that sod plus root pruning alters the distribution of water within a fruit tree.

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Wade J. Sperry, Jeanine M. Davis, and Douglas C. Sanders

Two crack-resistant and two crack-susceptible fresh-market tomato (Lycopersicon esculentum Mill.) cultivars were evaluated at varied soil moisture levels for physiological fruit defects and yield. Cultural practices recommended for staked-tomato production in North Carolina with raised beds, black polyethylene mulch, and drip irrigation were used. Soil moisture levels of less than −15.0, −30 to −40, and greater than −70 kPa were maintained and monitored using daily tensiometer readings. Soil moisture level had no effect on fruit cracking, blossom-end rot, zippers, or yield. However, there-were large differences among cultivars for fruit defects and total and marketable yields.

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Annick Moing, Christel Renaud, Hélène Christmann, Lydie Fouilhaux, Yves Tauzin, Anne Zanetto, Monique Gaudillère, Frédéric Laigret, and Jacques Claverie

Rain-induced fruit cracking is a limiting factor for sweet cherry (Prunus avium L.) growers in many production areas. Although many studies have concerned this complex phenomenon, the basic mechanisms involved in fruit cracking remain unclear. We re-examined the relations between osmotic potential and cracking susceptibility in cherry fruit by comparing the osmotic contribution of the major metabolites separately in flesh and skin, in four cultivars (with different levels of susceptibility to cracking) at four stages of development. Several differences were observed between flesh and skin revealing compositional gradients in the fruit tissues. Acidity and malate concentrations were higher in flesh than in skin for all stages. The absolute value of osmotic potential was higher but the contribution of the sum of sugars to osmotic potential was lower in flesh than in skin. As determined using fruit immersion test, `Fermina' and `Regina' were less susceptible to fruit cracking than `Lapins' and `Brooks'. At commercial maturity when fruit susceptibility to cracking was highest, no clear difference appeared between `Brooks' and `Lapins' compared to `Regina' and `Fermina' for flesh or skin osmolarity and for the contribution of the major sugars or organic acids to skin and flesh osmotic potential.

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Moritz Knoche and Stefanie Peschel

The effect of surface water on the frequency of microcracks in the cuticular membrane (CM) of exocarp segments (ES) of developing sweet cherry fruit (Prunus avium L.) was studied. Strain of CM and ES on the fruit surface was preserved by mounting a stainless steel washer on the fruit surface in the cheek region using an ethyl-cyanacrylate adhesive. ES were excised by tangentially cutting underneath the washer. Frequency of microcracks in the CM of ES was determined following infiltration for 10 minutes with a 0.1% acridine orange solution by fluorescence microscopy before and after exposure to deionized water (generally 48 hours). Exposing the surface of ES of mature `Burlat' sweet cherry fruit to water resulted in a rapid increase in microcracks in the CM that approached an asymptote at about 30 microcracks/cm2 within 24 hours. There was no change in microcracks in the CM when the surface of the ES remained dry. Incubating ES in polyethylene glycol solution that was isotonic to fruit juice extracted from the same batch of fruit resulted in a greater increase in frequency of microcracks as compared to incubation in deionized water. The water-induced increase in microcracks was closely related to strain of the CM across different developmental stages within a cultivar [between 45 and 94 days after full bloom (DAFB); r 2 = 0.96, P ≤ 0.001, n = 9] or across different cultivars at maturity (r 2 = 0.92, P ≤ 0.0022, n = 6). Incubating ES of developing fruit in enzyme solution containing pectinase and cellulase such that the outer surface remained dry resulted in complete rupture and failure of the ES. Time to rupture and percentage of ruptured ES were closely related to the strain of the CM (r 2 = 0.92, P ≤ 0.001, n = 9 and r 2 = 0.68, P ≤ 0.0063, n = 9, respectively). Removal of epicuticular wax had no effect on frequency of water-induced microcracks. Also, temperature had no effect on frequency of water-induced microcracks, but frequency of microcracks increased exponentially when exposing the outer surface of ES to relative humidities above 75%. At 100% humidity the increase in frequency of microcracks did not differ from that induced by liquid water. Local wetting the surface of intact fruit in the pedicel cavity or stylar end region resulted in formation of macroscopically visible cracks despite of a net water loss of fruit. Uniaxiale tensile tests using dry and fully hydrated CM strips isolated from mature `Sam' sweet cherry fruit established that hydration increased fracture strain, but decreased fracture stress and moduli of elasticity. Our data demonstrate that exposure of the fruit surface to liquid water or high concentrations of water vapor resulted in formation of microcracks in the CM.

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Rémy E Milad and Kenneth A Shackel

End cracking of French prune fruits occurs when previously water stressed trees are irrigated during early July. Fruit phloem, xylem and transpiration flows (P, X and T, respectively) were measured diurnally during 72 h periods in mid June, early July and mid July (before, during and after the crack-susceptible period). Midway through each 72 h period, the previously stressed trees were irrigated. In mid June, X was larger than P, whereas P was larger than X during early July. In mid July, P and X were similar. In early July, the period preceding irrigation was characterized by an ourflow of phloem sap during the day and phloem inflow during the night. After irrigation, larger phloem inflows were observed and no phloem outflow occurred. Fruit transpiration rates were highly correlated with VPD. They exhibited a gradual decrease during the season, reaching minimum values during early July, before increasing again. The sum of P and X was virtually identical for the three periods i.e. stronger P's compensated for weaker X's and vice versa. Our results suggest that properties intrinsic to the fruit play the primary role in modulating water and photosynthate movements between the tree and the fruit. The possible role of these properties on fruit growth and cracking will be examined.

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J.R. Schupp, T.L. Robinson, W.P. Cowgill Jr., and J.M. Compton

Three experiments were conducted on `Empire' apple (Malus ×domestica Borkh.) to evaluate the effects of hard water, calcium chloride (CaCl2), water conditioners, surfactants, and captan fungicide on the growth reduction and fruit cracking caused by prohexadione-calcium (PC). Two applications of 63 mg·L-1 PC provided season-long growth control in two studies. Adding a water conditioner to PC reduced shoot growth more than an application of PC in hard or soft water in one New York study. Ammonium sulfate (AMS) and Choice were equally effective water conditioners. PC provided no growth control of water sprouts and had no effect on fruit set or yield. PC applied at 250 mg·L-1 reduced fruit size. `Empire' fruit cracking and corking was severe, despite the use of only 63 mg·L-1 PC in two of the three experiments. This damage was exacerbated by the addition of a water conditioner, however AMS applied with a surfactant but without PC had little or no effect on either the severity or extent of fruit injury. In a third experiment, the addition of surfactants, CaCl2, or captan to 250 mg·L-1 PC plus a water conditioner had no effect on the severity of fruit damage. Fruit cracking caused by PC increased preharvest drop in two of three experiments, and increased postharvest rot in the Geneva, N.Y., experiment where fruit were stored prior to grading. Application of PC plus a water conditioner reduced estimated gross return per hectare for `Empire. We conclude that the fruit injury is caused by the formulated PC product itself under certain environmental conditions, and that this product should not be used on `Empire. Chemical name used: calcium 3-oxido-4-proprionyl-5-oxo-3-cyclohexine-carboxylate [prohexadione-calcium (PC)].

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Marco Beyer and Moritz Knoche

Rain-induced cracking of sweet cherry (Prunus avium L.) fruit is thought to be related to water absorption through the fruit surface. Conductance for water uptake (gtot. uptake) through the fruit surface of `Sam' sweet cherry was studied gravimetrically by monitoring water penetration from a donor solution of deionized water through segments of the outer pericarp into a polyethyleneglycol (PEG) containing receiver solution. Segments consisting of cuticle plus five to eight cell layers of epidermal and hypodermal tissue were mounted in stainless steel diffusion cells. Conductance was calculated from flow rates of water across the segment and the difference in osmotic potential between donor and receiver solution. Flow rates were constant up to 12 hours and decreased thereafter. A log normal distribution of gtot. uptake was observed with a median of 0.97 × 10-7 m·s-1. Further, gtot. uptake was not affected by storage duration (up to 71 days) of fruit used as a source of segments, thickness of segments (range 0.1 to 4.8 mm), or segment area exposed in the diffusion cell. Osmolality of the receiver solution in the range from 1140 to 3400 mmol·kg-1 had no effect on gtot. uptake (1.45 ± 0.42 × 10-7 m·s-1), but gtot. uptake increased by 301% (4.37 ± 0.46 × 10-7 m·s-1) at 300 mmol·kg-1. gtot. uptake was highest in the stylar scar region of the fruit (1.44 ± 0.16 × 10-7 m·s-1) followed by cheek (1.02 ±0.21 × 10-7 m·s-1), suture (0.57 ±0.17 × 10-7 m·s-1) and pedicel cavity regions (0.22 ±0.09 × 10-7 m·s-1). Across regions, gtot. uptake was related positively to stomatal density. Extracting total cuticular wax by dipping fruit in chloroform/methanol increased gtot. uptake from 1.18 ± 0.23 × 10-7 m·s-1 to 2.58 ± 0.41 × 10-7 m·s-1, but removing epicuticular wax by cellulose acetate stripping had no effect (1.59 ± 0.28 × 10-7 m·s-1). Water flux increased with increasing temperature (range 20 to 45 °C). Conductance differed between cultivars with `Hedelfinger' sweet cherry having the highest gtot. uptake (2.81 ± 0.26 × 10-7 m·s-1), followed by `Namare' (2.68 ± 0.26 × 10-7 m·s-1), `Kordia' (0.96 ± 0.14 × 10-7 m·s-1), `Sam' (0.87 ± 0.15 × 10-7 m·s-1), and `Adriana' (0.33 ± 0.02 × 10-7 m·s-1). The diffusion cell system described herein may be useful in analyzing conductance in water uptake through the fruit surface of sweet cherry and its potential relevance for fruit cracking.