Crack initiation and propagation in sweet cherry skin: A simple chain reaction causes the crack to ‘run’

Rain cracking severely affects the commercial production of many fleshy-fruit species, including of sweet cherries. The objectives were to investigate how the gaping macroscopic cracks (macrocracks) of a rain-cracked fruit can develop from microscopic cracks in the cuticle (microcracks). Incubating fruit in deionized water is well known to cause significant macrocracking. We found that after a lag phase of 2 h, the numbers and lengths of macrocracks increased. Macrocrack number approached an asymptote at 12 h, whereas macrocrack length continued to increase. The rate of macrocrack propagation (extension at the crack tip) was initially 10.8 mm h-1 but then decreased to a near-constant 0.5 mm h-1. Light microscopy revealed three characteristic zones along a developing macrocrack. In zone I (ahead of the crack), the cuticle was intact, the epidermal cells were unbroken and their cell walls were thin. In zone II, the cuticle was fractured, the first epidermal cells died and their cell walls began to thicken (swell). In zone III, most epidermal cells had died, their cell walls were swollen and cell:cell separation began along the middle lamellae. The thickness of the anticlinal epidermal cell walls and the percentage of intact living cells along a crack were closely and negatively related. Cracks were stained by calcofluor white, but there was no binding of monoclonal antibodies (mAbs) specific for hemicelluloses (LM11, LM21, LM25). Strong binding was obtained with the anti-homogalacturonan mAb (LM19), indicating the presence of unesterified homogalacturonans on the crack surface. We conclude that macrocrack propagation is related to cell death and to cell wall swelling. Cell wall swelling weakens the cell:cell adhesion between neighbouring epidermal cells, which separate along their middle lamellae. The skin macrocrack propagates like a ‘run’ in a fine, knitted fabric

Cuticle deposition ceases during strawberry fruit development

Background: Ideally, the barrier properties of a fruit’s cuticle persist throughout its development. This presents a challenge for strawberry fruit, with their rapid development and thin cuticles. The objective was to establish the developmental time course of cuticle deposition in strawberry fruit. Results: Fruit mass and surface area increase rapidly, with peak growth rate coinciding with the onset of ripening. On a whole-fruit basis, the masses of cutin and wax increase but on a unit surface-area basis, they decrease. The decrease is associated with marked increases in elastic strain. The expressions of cuticle-associated genes involved in transcriptional regulation (FaSHN1, FaSHN2, FaSHN3), synthesis of cutin (FaLACS2, FaGPAT3) and wax (FaCER1, FaKCS10, FaKCR1), and those involved in transport of cutin monomers and wax constituents (FaABCG11, FaABCG32) decreased until maturity. The only exceptions were FaLACS6 and FaGPAT6 that are presumably involved in cutin synthesis, and FaCER1 involved in wax synthesis. This result was consistent across five strawberry cultivars. Strawberry cutin consists mainly of C16 and C18 monomers, plus minor amounts of C19, C20, C22 and C24 monomers, ω-hydroxy acids, dihydroxy acids, epoxy acids, primary alcohols, carboxylic acids and dicarboxylic acids. The most abundant monomer is 10,16-dihydroxyhexadecanoic acid. Waxes comprise mainly long-chain fatty acids C29 to C46, with smaller amounts of C16 to C28. Wax constituents are carboxylic acids, primary alcohols, alkanes, aldehydes, sterols and esters. Conclusion: The downregulation of cuticle deposition during development accounts for the marked cuticular strain, for the associated microcracking, and for their high susceptibility to the disorders of water soaking and cracking

Deposition, strain, and microcracking of the cuticle in developing 'Riesling' grape berries

The objectives of this study were to quantify deposition, strain, and microcracking of the cuticular membrane (CM) in developing 'Riesling' (Vitis vinifera L.) berries. Mass of the CM, the cutin matrix (DCM), and wax increased pre-veraison (26 to 65 days after anthesis, DAA) on a berry (+ 236, + 211, and + 332 %, respectively) and a surface area basis (+ 11, + 3, and + 43 %, respectively). Post-veraison (65 to 138 DAA), CM and DCM mass per berry remained about constant at 3.4 (± 0.16) and 2.4 (± 0.11) mg per berry, respectively, while wax mass continued to increase from 0.8 (± 0.02) to 1.1 (± 0.02) mg per berry. On an area basis, however, CM and cutin mass decreased from 5.0 (± 0.13) to 4.6 (± 0.04) g·m-2 and from 3.5 (± 0.10) to 3.2 (± 0.03) g·m-2 between 65 and 138 DAA, respectively, but wax mass remained constant at about 1.5 (± 0.04) g·m-2. The calculated rate of cutin and wax deposition peaked at about 40 DAA, and declined continuously thereafter. There was no strain and no microcracking of the CM up to veraison. Post-veraison strain of the CM and microcracking in the stylar scar region increased linearly with time. The data suggest that the cessation of cutin deposition in post-veraison berries and the ongoing berry expansion resulted in increased strain of the CM which in turn caused microcracking in the CM.

Microcracking of strawberry fruit cuticles: mechanism and factors

Microscopic cracks in the cuticle (microcracks) are the first symptom of the strawberry fruit disorder ‘water soaking’ in which the fruit surface appears watery, translucent, and pale. Water soaking severely impacts fruit quality. The objective was to investigate the factors and mechanisms of cuticular microcracking in strawberry. Fluorescence microscopy revealed numerous microcracks in the achene depressions, on the rims between depressions and at the bases of trichomes. Microcracks in the achene depressions and on the rims were either parallel or transversely oriented relative to a radius drawn from the rim to the point of attachment of the achene. In the achene depression, the frequency of microcracks with parallel orientation decreased from the calyx end of the fruit, towards the fruit tip, while the frequency of those with transverse orientation remained constant. Most microcracks occurred above the periclinal cell walls of the epidermal cells. The long axes of the epidermal cells were primarily parallel-oriented. Microcracking increased during fruit development. Cuticle mass per fruit remained constant as fruit surface area increased but cuticle thickness decreased. When fruit developed under high relative humidity (RH) conditions, the cuticle had more microcracks than under low RH conditions. Exposing the fruit surface to increasing RHs, increased microcracking, especially above 75% RH. Liquid-phase water on the fruit surface was markedly more effective in inducing microcracking than high vapor-phase water (high RH). The results demonstrate that a combination of surface area growth strain and water exposure is causal in inducing microcracking of the strawberry cuticle

Mode of Action of Calcium in Reducing Macrocracking of Sweet Cherry Fruit

Rain cracking (hereinafter referred to as macrocracking) severely impacts the production of sweet cherry (Prunus avium). Calcium (Ca) sprays can reduce macrocracking, but the reported responses to Ca sprays are variable and inconsistent. The objective of this study was to establish the physiological mechanism through which Ca reduces macrocracking in sweet cherry fruit. Six spray applications of 50 mM CaCl2 had no effect on macrocracking (assessed using a standardized immersion assay) despite a 28% increase in the Ca-to-dry mass ratio. Similarly, during another experiment, there was no effect of up to nine Ca sprays on macrocracking, although the Ca-to-dry mass ratio increased as the number of applications increased. In contrast, CaCl2 spray applications during simulated rain (in a fog chamber) significantly reduced the proportion of macrocracked fruit. Additionally, immersion of fruit in CaCl2 decreased macrocracking in a concentration-dependent manner. Monitoring macrocrack extension using image analysis revealed that the rate of macrocrack extension decreased markedly as the CaCl2 concentration increased. This effect was significant at concentrations as low as 1 mM CaCl2. Decreased anthocyanin leakage, decreased epidermal cell wall swelling, and increased fruit skin stiffness and fracture force contributed to the decrease in macrocracking. There was no effect of CaCl2 on the cuticle deposition rate. Our results demonstrated that Ca decreased macrocracking when applied to a wet fruit surface either by spraying on wet fruit or by incubation in solutions containing CaCl2. Under these circumstances, Ca had direct access to the cell wall of an extending macrocrack. The mode of action of Ca in reducing macrocracking is primarily decreasing the rate of crack extension at the tip of a macrocrack

Peel Russeting in ‘Apple’ Mango (Mangifera indica L.): Characterization, Mechanisms and Management

The aesthetic value and marketability of table fruits are greatly reduced by russeting, a disorder that severely affects the ‘Apple’ mango (Mangifera indica L.) cultivar in Kenya. Despite its prevalence, the underlying mechanisms and prevention strategies for russeting in mangoes are unknown. To address this gap in knowledge, this project aimed to: (1) review existing literature on russeting, (2) characterize the disorder in ‘Apple’ mango, (3) identify its mechanistic basis in comparison to a non-russet susceptible cultivar, (4) investigate the role of moisture and (5) lenticels on russeting, and (6) develop strategies to prevent the disorder. To achieve these objectives, russeting was quantified in ‘Apple’ mango within fruit and in different geographic locations in Kenya. Fruit skins and cuticles from russet susceptible ‘Apple’ and russet tolerant ‘Tommy Atkins’ mangoes were examined during fruit development, and cuticular strain was partitioned into its reversible and irreversible components. Mechanical properties of isolated cuticles from both cultivars were tested. The role of moisture in microcrack development and russeting was studied by partially wetting the fruit surface. Lenticels were characterized microscopically across cultivars and locations. Field studies were also conducted to establish the effect of pre-harvest bagging on russeting and postharvest performance. The results showed that russeting in ‘Apple’ mango increased with fruit development particularly in the stem end region. Russeting was triggered by rainfall and low temperature. The skin’s permeance to water vapor was larger in russeted than in non-russeted skin. The cuticle of ‘Apple’ mango was thinner than that of ‘Tommy Atkins’. Strains released on excision and isolation and wax extraction were higher in ‘Apple’ than in ‘Tommy Atkins’. Stiffness, fracture force, and strain at fracture were consistently lower in ‘Apple’ than in ‘Tommy Atkins’. Surface wetness induced microcracking and increased the skin's water vapor permeance, and moisture-treated fruit skins later developed russet symptoms in ‘Apple’ mango. Russeting began at lenticels and then spread across the surface, ultimately forming a network of rough, brown patches over the skin. Cross-sections of russeted areas revealed stacks of phellem cells. Pre-harvest bagging of mangoes effectively prevented russeting and lowered the rates of transpiration postharvest

Russeting in ‘Apple’ Mango: Triggers and Mechanisms

Russeting is an important surface disorder of many fruitcrop species. The mango cultivar ‘Apple’ is especially susceptible to russeting. Russeting compromises both fruit appearance and postharvest performance. The objective was to identify factors, mechanisms, and consequences of russeting in ‘Apple’ mango. Russeting was quantified on excised peels using image analysis and a categorical rating scheme. Water vapour loss was determined gravimetrically. The percentage of the skin area exhibiting russet increased during development. Russet began at lenticels then spread across the surface, ultimately forming a network of rough, brown patches over the skin. Cross-sections revealed stacks of phellem cells, typical of a periderm. Russet was more severe on the dorsal surface of the fruit than on the ventral and more for fruit in the upper part of the canopy than in the lower. Russet differed markedly across orchards sites of different climates. Russet was positively correlated with altitude, the number of rainy days, and the number of cold nights but negatively correlated with minimum, maximum, and mean daily temperatures, dew point temperature, and heat sum. Russeted fruit had higher transpiration rates than non-russeted fruits and higher skin permeance to water vapour. Russet in ‘Apple’ mango is due to periderm formation that is initiated at lenticels. Growing conditions conducive for surface wetness exacerbate russeting

Water Soaking Disorder in Strawberries: Triggers, Factors, and Mechanisms

Water soaking is an important surface disorder of strawberries that limits unprotected field production. The objective was to identify the mechanism(s) of water soaking. Symptomatic fruit show pale, deliquescent patches of skin. This damage extends into the flesh. Numerous cuticular microcracks occurred in water-soaked areas. Water soaking occurred only if the skin was exposed to liquid water. Water soaking was more rapid when the cuticle had been abraded. Water soaking, anthocyanin leakage, and water uptake all increased with incubation time. There was a lag phase for water soaking and anthocyanin leakage, but not for water uptake. Susceptibility to water soaking increased with fruit ripening and mass. Incubation in isotonic PEG 6000 increased cuticular microcracking but decreased water soaking and water uptake. Incubation in hypotonic fruit juice (natural and artificial) increased water soaking incidence and severity but reduced water uptake. Incubation in dilute citric and malic acids increased plasma membrane permeability as indexed by anthocyanin leakage and increased water soaking. Thus, water soaking involves cuticular microcracking, localized water uptake, bursting of cells, and the release of organic acids into the apoplast. The damage propagates from cell to cell

Mechanical properties of apple (Malus x domestica Borkh.) fruit skin and their potential role in fruit russeting

[no abstract

Water induces microcracks in the grape berry cuticle

Research Note