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Andreas Winkler, Max Ossenbrink, and Moritz Knoche

When mature sweet cherries (Prunus avium L.) came into contact with sweet cherry juice, cracking dramatically increased. The objectives of our study were: 1) to quantify the cracking of fruit in cherry juice, 2) to determine which constituent(s) of the juice especially promote cracking and, 3) to establish its/their mode of action in promoting cracking. Artificial juice was made up as an aqueous solution of the same five pure chemicals and at the same relative concentrations as the five major osmolytes of real sweet cherry juice. Artificial and real juice was used at half-isotonic concentrations as the real juice from that batch of fruit. Cracking of sweet cherries placed in either artificial or real juice was more rapid and occurred for lower net water uptakes than of fruit placed in half-isotonic polyethylene glycol 6000. The crack-promoting component in sweet cherry juice was malic acid. Further tests with malic acid, and other organic acids, and with different concentrations of malic acid, with and without pH control, and with the enantiomers of malic acid, showed the effects were primarily related to the pH of the incubation solution. Leakage of anthocyanin from discs of flesh was increased in the presence of malic acid and greater in hypotonic than hypertonic solutions, suggesting that malic acid increases the permeability of the plasma membrane and tonoplast and weakens the cell walls. Malic acid may be an important link (amplifier) in a reaction chain that begins with the bursting of individual epidermal cells and ends with the formation of macroscopic skin cracks.

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Thomas O. Athoo, Andreas Winkler, and Moritz Knoche

Pedicel appearance is a good indicator of freshness in sweet cherries (Prunus avium L.). Fruit with shriveled, discolored pedicels have reduced market value. Shriveled pedicels are thought to result from postharvest water loss due to transpiration. The objectives of our study were to 1) quantify the transpiration permeances of fruit and pedicel surfaces; 2) determine the role of the fruit in pedicel transpiration; and 3) identify the effects of selected factors on pedicel transpiration. Fruit with and without pedicels were incubated under controlled conditions [usually 22 °C, 75% relative humidity (RH)] and their mass losses determined gravimetrically. Pedicel transpiration was calculated by subtracting measured transpiration of fruit without pedicels from that of fruit with pedicels. Cumulative pedicel transpiration increased with time. Rates of pedicel transpiration were essentially constant over the first 0 to 1.5 hours but declined thereafter, approaching an asymptote over the subsequent period of 1.5 to 96 hours over which measurements were made. Cumulative pedicel transpiration exceeded the amount of water in the pedicel, indicating that at least some of the transpired water originated from the fruit. There was no significant effect of steam girdling on pedicel transpiration suggesting that water moved from the fruit to the pedicel through the xylem (steaming prevents phloem conduction). Abrading the cuticular membrane (CM) from a pedicel surface or extracting the cuticular wax by dipping pedicels once or five times in chloroform/methanol (1:1 v/v) increased rates of transpiration 12-, 3-, and 5-fold, respectively. The water vapor permeance of the pedicel surface determined under steady-state conditions (8.7 ± 0.4 × 10−4 m·s−1) exceeded that of the fruit (2.1 ± 0.1 × 10−4 m·s−1), possibly because of a more permeable CM and/or a higher stomatal density (38.5 ± 1.3 stomata/mm2 for pedicels vs. 1.1 ± 0.0 stomata/mm2 for fruit). Treatments known to affect stomatal opening (incubation in buffered abscisic acid at 0.1 mm or in CO2- or N2-atmospheres) had no effects on pedicel transpiration. Rates of transpiration were negatively correlated with RH but positively with temperature. There was no effect of RH and/or temperature on the permeances of pedicel or fruit surfaces. From our results it is inferred that 1) pedicel transpiration is a physical process governed by Fick’s law of diffusion, where cuticle and wax in particular represent the major rate-limiting barriers; 2) the permeances of pedicel surfaces exceed those of fruit surfaces; and 3) pedicel transpiration can be minimized by minimizing the driving force (difference in water vapor concentration) during postharvest handling and storage.

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

Rain cracking of sweet cherry (Prunus avium L.) fruit is commonly thought to result from excessive net water uptake. This excess increases flesh turgor, which then strains and eventually ruptures the skin at the weakest point. This idea—the critical turgor hypothesis—assumes the fruit comprises a semifluid flesh, held under pressure by a taut skin. The objectives of this study were to test the validity of this popular hypothesis. We investigated the effects of 1) the different pathways of water uptake and 2) the fruit’s water balance on cracking. Incubating fruit of 19 cultivars in water resulted in rapid fruit cracking. The time to 50% cracking (T50) averaged 7.5 ± 1.3 hours with considerable variability between cultivars (T50 range from 1.5 to 18.6 hours). The amount of water taken up at 50% cracking (WU50) averaged 96.5 ± 17.6 mg (WU50 range from 17.7 to 331.5 mg). There was no correlation between either the T50 or the WU50, and the rate of water uptake. Also, there was no correlation between the values of T50 (r = 0.58) and only a weak correlation between the values of WU50 (r = 0.80*) determined in different years. Comparing the value of WU50 under incubation vs. under perfusion revealed a 3.9- to 38-fold higher WU50 under perfusion (397.6 to 1840 mg) than under incubation (48.8 to 102.6 mg). This marked dissimilarity remained, regardless of pretreatments with isotonic polyethylene glycol (PEG) 6000 to induce microcracking or by manipulation of skin wetness during perfusion. Sealing the pedicel/fruit junction markedly decreased the rate of water uptake under incubation. It had no effect on the T50, and it markedly decreased the WU50. Similarly, manually induced skin defects greatly increased the rate of water uptake but, with few exceptions, had no effect on the T50, whereas, the WU50 had increased. The location on the fruit surface of the resulting cracks was not related to the region of the skin in which the manual defect was induced. Allowing the fruit to transpire increased both, the T50 and the WU50. Interestingly, the amount of water lost by transpiration exceeded the amount that was subsequently required to cause cracking up to 5-fold. Incubating fruit with their stylar ends immersed in water, whereas their remaining surfaces were in air of 0%, 28%, 75%, or 100% relative humidity (RH) resulted in net losses of water of up to 5.9 ± 0.7 mg·h−1, nevertheless their stylar ends still cracked. All our results indicate rain cracking in sweet cherries is a localized phenomenon that is not related to the net fruit water balance (the critical turgor hypothesis) but is the result of more local exposure of the fruit skin to liquid-phase water (the zipper hypothesis).

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Moritz Knoche, Eckhard Grimm, Andreas Winkler, Merianne Alkio, and Jürgen Lorenz

Neck shrivel is a physiological disorder of european plum (Prunus ×domestica L.) fruit, characterized by a shriveled pedicel end and a turgescent stylar end. Affected fruit are perceived as of poor quality. Little is known of the mechanistic basis of neck shrivel, but microcracking of the cuticle has been implicated. The objective of our study was to quantify transpiration through the skin surfaces of european plums with and without symptoms of neck shrivel. Cumulative transpiration increased linearly with time and was greater in the susceptible european plum cultivar Hauszwetsche Wolff with neck shrivel, compared with fruit of the same cultivar but without neck shrivel and compared with fruit of the nonsusceptible unnamed clone P5-112. Cumulative transpiration of epidermal skin segments (ES) excised from symptomatic ‘Hauszwetsche Wolff’ from near the pedicel end exceeded that from ES excised from near the stylar end. The permeance of ES from near the pedicel end of ‘Hauszwetsche Wolff’ with neck shrivel (12.4 ± 2.6 × 10−4 m·s−1) exceeded that of ES from near the stylar end (2.9 ± 0.4 × 10−4 m·s−1) 4.3-fold. However, in the clone P5-112, the same difference was only 1.6-fold (1.3 ± 0.8 × 10−4 m·s−1 vs. 0.8 ± 0.3 × 10−4 m·s−1). Microscopy revealed numerous microcracks near the pedicel end of symptomatic ‘Hauszwetsche Wolff’ fruit but markedly fewer microcracks near the stylar end. The microcracks near the pedicel end were oriented parallel to the pedicel/style axis, whereas those near the stylar end were randomly oriented. Juices extracted from near the pedicel end of susceptible cultivars had consistently more negative osmotic potentials [ψS (e.g., for Doppelte Hauszwetsche −5.1 ± 0.1 MPa)] than those from near the stylar end (e.g., for Doppelte Hauszwetsche −4.0 ± 0.1 MPa) or that from fruit without symptoms of neck shrivel (e.g., for pedicel end and stylar scar regions of Doppelte Hauszwetsche −3.8 ± 0.1 vs. −3.3 ± 0.1 MPa, respectively). Our results indicate that increased transpiration through microcracks near the pedicel end may contribute to neck shrivel but that the causes of neck shrivel are likely more complex.

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Andreas Winkler, Eckhard Grimm, Moritz Knoche, Julian Lindstaedt, and Dirk Köpcke

Skin spot is a commercially important disorder of the fruit skin of ‘Elstar’ apples (Malus ×domestica Borkh.). The disorder is characterized by patches of small brownish dots (“skin spots”) that usually appear on the skin after fruit are removed from storage. Water-induced cuticular microcracks are implicated in the etiology of skin spot. The objectives of our study were 1) to establish the effect of surface wetness on the severity of skin spot; and 2) to identify possible relationships between meteorological records of rainfall over a number of seasons and the severity of skin spot in those seasons. Surface wetness treatments were imposed on fruit using overhead sprinklers installed above trees grown under a plastic rain shelter. During early fruit development [14 to 44 days after full bloom (DAFB)], surface wetness did not affect the severity of skin spot. However, during the later part of the growing season (greater than 44 DAFB), increased surface wetness increased the incidence and severity of skin spot and also the severity of cuticular microcracking. Most skin spots and microcracks were already present at harvest before storage, but skin spots and microcracking did increase slightly during subsequent controlled atmosphere (CA) storage. Over a 9-year period, the severity of skin spot in ‘Elstar’ apples grown and stored locally under standard orchard and storage conditions was positively correlated to the number of rainy days. This correlation was greater for the period between 1 Aug. to harvest than for periods 1 June to harvest or 1 July to harvest. Fruit treated with 1-methylcyclopropene (1-MCP) were more susceptible to skin spot than untreated control fruit. Calculating regression equations for the relationship between the severity of skin spot and the number of rainy days (period 1 Aug. and harvest) revealed that the (commercially important) threshold score for skin spot of 2 is predicted to occur after 44 rainy days for control fruit and after 34 rainy days for 1-MCP-treated fruit. Our data demonstrate that skin spot arises from cuticular microcracks, which in turn result from numerous exposures to surface wetness (rain, dew), especially those occurring during later stages of fruit development.