Rain cracking of sweet cherry fruit is an important limitation in crop production worldwide and is thought to be related to water uptake through the fruit surface. This uptake occurs by diffusion through the cuticle and by viscous flow along an aqueous continuum across the sweet cherry fruit exocarp that is referred to as the polar pathway (Weichert and Knoche, 2006a). Polar pathways are formed by hydration and orientation of polar functional groups in the cuticular membrane [CM (Beyer et al., 2005; Schönherr and Bukovac, 1970; Schönherr and Schmidt, 1979; Schönherr, 2006)] and allow for rapid water uptake into the fruit (Weichert and Knoche, 2006a).
Incubating sweet cherry fruit in aqueous solutions of AgNO3, CuCl2, HgCl2, AlCl3, EuCl3, and FeCl3 was recently shown to markedly decrease water uptake and subsequent cracking (Beyer et al., 2002; Weichert et al., 2004). Unfortunately, none of these salts can be used in horticultural practice to control cracking. Nevertheless, understanding their mechanism in decreasing water uptake and fruit cracking may be helpful in identifying compounds that are equally effective as the above salts, but also acceptable from an ecotoxicological perspective.
Of the salts cited above, the most information is available on FeCl3. The data demonstrated that decreased water uptake is related to decreased permeability of the polar pathways (Weichert and Knoche, 2006b), which most likely results from a pH-dependent precipitation in the exocarp. Aqueous solutions of ferric salts [e.g. FeCl3, Fe(NO3)3, and Fe2(SO4)3] are strongly acidic. During penetration into the exocarp, ferric ions encounter a microenvironment of increasing pH until the pH of the fruit's apoplast is reached (Weichert and Knoche, 2006b). The increase in pH causes formation of colloidal gels and precipitates, comprising complex hydrated amorphous ferric-oxides and -hydroxides, most likely FeO(OH) (Greenwood and Earnshaw, 1990) that are expected to form a significant barrier to water transport across the sweet cherry exocarp (Weichert and Knoche, 2006b).
Indirect evidence for this pH-dependent precipitation reaction comes from experiments by Weichert and Knoche (2006b) where diffusion of 3H2O was studied using an infinite dose diffusion system (Bukovac and Petracek, 1993). In this system, 3H2O diffusion was followed from a donor solution containing FeCl3 (pH 2.6) through an interfacing ES into a water receiver of pH values adjusted to pH 2.0 to 6.0. In presence of FeCl3 in the donor, 3H2O diffusion through the ES into the receiver markedly decreased at receiver pH ≥ 3.0, but there was no effect of FeCl3 on 3H2O diffusion at the receiver pH 2.0 (Weichert and Knoche, 2006b). According to the above hypothesis, we would expect formation of precipitates at receiver pH ≥ 3.0 that must have decreased water uptake and fruit cracking.
To obtain direct evidence for this mechanism, we now studied the effect of receiver pH on penetration of 55Fe from ferric salts through excised ES or isolated CM into a receiver solution using the infinite dose diffusion system, and the occurrence of ferric or ferrous precipitates in the sweet cherry exocarp.
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