Rain cracking severely limits sweet cherry production in all regions of the world where rainfall occurs immediately before and during the harvest period (Christensen, 1996). Rain cracking is thought to be related to an excessively positive water balance resulting from 1) uptake of surface water through the fruit skin (Christensen, 1996), 2) via the vasculature of the pedicel (Measham et al., 2010; Sekse et al., 2005), and coupled with 3) the much-reduced transpiration of a wet fruit under humid conditions (Knoche and Measham, 2016).
A commonly cited model of rain cracking of soft and fleshy fruit is the critical turgor model originally proposed by Considine and Kriedemann (1972) for grape (Vitis vinifera L.) berries. This concept assumes the flesh of the berry to be held under compression by an elastically strained skin. Upon water uptake, the pressure in the fruit rises. When the fruit pressure (synonymous with fruit turgor and with flesh turgor) exceeds some critical threshold; i.e., the critical turgor pressure, the fruit skin is strained beyond its limit of extensibility and it cracks (Considine and Kriedemann, 1972). Although developed for grapes, the critical turgor model has also been used to account for rain cracking in sweet cherries (Measham et al., 2009; Sekse, 1995a, 1998a; Sekse et al., 2005). This is not unreasonable, given the similar sizes, shapes, textures, and external architectures of grapes and sweet cherries (notwithstanding the multiple small seeds of the one and the single large pit of the other).
The critical turgor model offers a logical conceptual framework for discussion of rain cracking in sweet cherry, which is consistent with the following empirical and experimental observations: 1) Sweet cherry fruit comprise two distinct tissues—the flesh and the skin. There is no published evidence (of which we are aware) that suggests the pit plays any role in cracking. The cellular components of the skin form the load-bearing structure implied in the critical turgor model (Brüggenwirth et al., 2014). 2) The sweet cherry skin is markedly strained (Grimm et al., 2012; Knoche et al., 2004). A portion of this strain is elastic (Brüggenwirth et al., 2014). According to Hook’s law, elastic strain is caused by a proportional stress and this (in a near spherical fruit) can be simply related to a biaxial strain and a proportional internal pressure. 3) Rain cracking is reported to be affected by irrigation (Sekse, 1995b) and also occurs occasionally in fruit grown under rain shelters. These observations suggest water uptake can occur along different parallel pathways—e.g., both via vascular sap flow (xylem and phloem) in the pedicel and also via surface water uptake through a wet skin (osmotic uptake)—both of these pathways potentially contributing to rain cracking (Cline et al., 1995). 4) Conversely, skin cracking is markedly decreased if surface water uptake is decreased, e.g., by incubating fruit in aqueous FeCl3 (Beyer et al., 2002; Weichert et al., 2004). Whereas the above observations are consistent with the critical turgor model (Considine and Kriedemann, 1972), the low turgor pressure recently reported in mature sweet cherry and—in particular—the lack of response of turgor to water uptake and transpiration (Knoche et al., 2014; Schumann et al., 2014) are not consistent with the critical turgor model.
The objective of the present study was to test the validity of the critical turgor model for sweet cherry rain cracking. We specifically investigated the effects of the route of water uptake and of the fruit’s water balance on cracking. The pathway of water uptake was varied by 1) perfusing sweet cherry fruit; 2) by inducing microcracking and surface defects as parallel pathways for water uptake; and 3) by sealing the pedicel/fruit junction. The fruit’s water balance was varied by allowing whole fruit or selected regions of the fruit surface to transpire before conducting cracking assays.
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