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Martin Brüggenwirth and Moritz Knoche

the physiological inside to prevent uncontrolled water uptake from that side—this could have confounded the test results ( Simon, 1977 ). In contrast to the more common, uniaxial tensile tests of engineering, biaxial tensile tests better mimic the

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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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Martin Brüggenwirth and Moritz Knoche

) temperature on the mechanical properties of sweet cherry fruit skin using a biaxial tensile test. Materials and Methods Plant material. Fruit of sweet cherry cultivars Adriana, Kordia, Sam, and Sweetheart were harvested at commercial maturity from field

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Martin Brüggenwirth and Moritz Knoche

biaxial tensile tests such as were first described by Bargel et al. (2004) . In this test, an excised segment of the fruit surface is pressurized from its inner side. As a result, the segment bulges and its surface area increases thereby mimicking the

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Bishnu P. Khanal, Rejina Shrestha, Leonie Hückstädt, and Moritz Knoche

microscope (MZ10F; Leica Microsysteme GmbH, Wetzlar, Germany; camera DP71; Olympus) and image analysis (Software Cell^P; Olympus). The release of biaxial strain ( ; %) was calculated as: Tensile test. Strips (5 mm wide) were excised from CM or DCM discs (24

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Bishnu P. Khanal and Moritz Knoche

of epidermal segments (ES) of the fruit skin for tensile testing. At 141 DAFB when fruit were commercially mature, all fruit were harvested and put into cold storage [1.7 °C, 92% relative humidity (RH)] for up to 118 d (equivalent to 259 DAFB). At

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F.R. Harker and I.C. Hallett

The relationships between cellular characteristics of cortical tissue from `Braeburn' apple fruit (Malus domestica Borkh.) that had been harvested at two maturities and changes in texture that occurred during storage at 0C were studied. Tensile tests were used to measure adhesion between neighboring cells, and turgor pressure was manipulated to determine the pressures required to burst cells. Apples of advanced maturity became mealy during cool storage, while those of less advanced maturity did not. Mealiness was associated with low adhesion between neighboring cells, and a relatively high resistance to cell rupture.

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F.R. Harker and I.C. Hallett

Kiwifruit [Actinidia deliciosa (A. Chev) C.F. Liang et A.R. Ferguson] flesh firmness can decline by as much as 94% during fruit ripening. This phenomenon was investigated at the cellular level, with the aim of characterizing changes in the physiological condition and mechanical properties of cells. The tensile strength of kiwifruit outer pericarp tissue was measured, and low-temperature scanning electron microscopy was used to examine the mode of cell failure at fracture surfaces. The propensity with which cells ruptured was determined by incubating tissue discs in hypertonic and hypotonic solutions, and water potentials, osmotic potentials, turgor pressures, and tissue density were measured. An initial rapid reduction in flesh firmness—from 80 to 27 N during 6 weeks of storage at 0C—was related to a reduction in the adhesion between neighboring cells. Following tensile tests, an examination of fracture surfaces indicated that cells from freshly harvested fruit had ruptured, exposing the cell interior. After 6 weeks of storage, neighboring cells separated from each other without breaking open. With 23 additional weeks of storage at 0C, flesh firmness decreased from 27 to 5 N. The final softening stage was associated with an increase in the proportion of cells that separated at the middle lamella and an increase in the plasticity of the cell wall.

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Moritz Knoche, Bishnu P. Khanal, and Matej Stopar

isolation. The number of replicates was 10. Tensile tests. Uniaxial tensile tests were performed using CM strips (20 × 6 mm 2 ) excised and isolated at 75 and 138 DAFB. CM strips were mounted in a frame made from paper and masking tape (Tesa Krepp R ; tesa

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Andreas Winkler, Stefanie Peschel, Kathleen Kohrs, and Moritz Knoche

of the skin in biaxial tensile tests, whereas—based on the critical turgor model—fracture strains would be expected to decrease ( Brüggenwirth and Knoche, 2016a ). Fifth, there is a more than 20-fold difference between the critical turgor values