Rain-induced cracking is an important limitation in sweet cherry production worldwide (Christensen, 1996). Cracking susceptibility differs among cultivars. Cracking is thought to be related to water uptake into the fruit, but the mechanistic basis for differential cracking susceptibility is not entirely clear. Theoretically, the following two groups of factors may account for variation in cracking susceptibility among cultivars: 1) the water transport characteristics of the fruit surface; and/or 2) the mechanical constitution of the load-bearing peripheral structure, presumably the exocarp. Both groups of factors are mechanistically unrelated.
The cuticular membrane plays an important role in cracking, because it represents the primary barrier to water transport (Martin and Juniper, 1970). Microscopic cracks in the CM impair this barrier function and allow for uncontrolled and rapid water transport (Knoche and Peschel, 2006). These microcracks occur at high density in sweet cherry grown without rain shelters, because the CM of sweet cherry fruit is markedly strained and thinned in the course of development (Knoche et al., 2004; Peschel and Knoche, 2005) and liquid water on or high concentrations of water vapor above the strained CM induce microcracks (Knoche and Peschel, 2006). In contrast to water transport, a role of the CM in the fruit skin integrity is less likely because the support of the CM by underlying epidermal cells is essential for the integrity of the CM (Knoche and Peschel, 2006) and abrading the CM using carborund powder had no effect on the (calculated) fruit turgor (Knoche et al., 2004). If the CM formed a structural shell that held the mesocarp under compression (Considine and Brown, 1981), the fruit turgor should decrease on abrasion of the CM. However, this was not the case. These arguments do not exclude the possibility that microcracks act as stress concentrators and extend into macroscopic cracks during cracking (Glenn and Poovaiah, 1989).
Low cracking susceptibility is an important objective in sweet cherry breeding programs around the world. Improving cracking resistance by breeding approaches requires phenotyping of cracking relevant traits, quantifying their variance components and estimating the broad-sense heritabilities. To our knowledge, such information is not available for sweet cherry.
The objectives of our study were to phenotype cracking-relevant traits in selected sweet cherry cultivars and to quantify their heritability. We focused on CM characteristics such as the CM and wax mass, the (elastic) strain of the CM, the densities of stomata and of microcracks in the exocarp, and the permeability of the exocarp to water transport in transpiration and in osmotic water uptake.
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