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

Abstract

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.

Spatial heterogeneity of flesh-cell osmotic potential in sweet cherry affects partitioning of absorbed water

“…This redistribution of water causes the 'orange peel' disorder in sweet cherry 14 . Also, a major fraction of fruit cracks in the stylar scar region which is consistent with the more negative osmotic potential measured in this study 23,24 .…”

Swelling of cell walls in mature sweet cherry fruit: factors and mechanisms

“…Extraction and complexing of Ca would also account for differential swelling among buffers of different chemistry but of the same pH. Additional indirect evidence for the involvement of pectins in swelling comes from (i) the exposure of pectins on the fracture surface of the sweet cherry fruit skin (Schumann et al 2019), (ii) the mode of fracture of the skin along and not across cell walls (Brüggenwirth and Knoche 2017) and (iii) data on other fruit crops demonstrating that swelling is associated with the pectin fraction (Redgwell et al 1997;Basanta et al 2013).…”

Effect of calcium on relieving berry cracking in grape (Vitis viniferaL.) ‘Xiangfei’

“…In the present study, the reduced depth and extent of cracks in the calcium treatment (Fig. 5) may be explained by increased cross linking of homogalacturonans by di-and trivalent ions (Schumann et al, 2019;Weichert et al, 2004). Dipping fruit with calcium solution may stimulate Ca accumulation at stem-end region…”