Spray application of Calcium salts: An effective means to improve cracking susceptibility in sweet cherry?

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Rain-cracking limits the production of sweet cherries world wide. Cracking is thought to result from increased water uptake into the fruit that causes the fruit skin to rupture. Thus, cracking depends on two independent groups of factors: (1) water transport characteristics of skin and pedicel and (2) the mechanical constitution of the fruit skin. The effect of applications of Ca salts (overhead sprinkler, spray application and immersion assays) on cracking susceptibility of sweet cherry fruit has been investigated in the past. The effects obtained, however, were inconsistent. In some studies, Ca applications markedly reduced cracking, in others Ca had no effect. Unfortunately, the Ca-content of the fruit has rarely been investigated and therefore it is not known whether the lack of performance was simply related to a lack of Ca uptake through the skin. Uptake through the pedicel is often low in maturing fruit of many fruit crops and we would expect the same for sweet cherry. Because uptake of the Ca cation through the cuticle is low and highly variable, the lack of uptake is indeed a likely explanation. However, there are no studies on Ca uptake into sweet cherry fruit. The objectives of the present study are to (1) quantify Ca uptake via the pedicel, the effect of fruit transpiration thereon and the Ca distribution within the fruit in the course of development, (2) to determine the effect of Ca on the mechanical properties using biaxial tensile tests of segments of the fruit skin, anthocyanin leakage of tissue cylinders and swelling assays of isolated cell walls, (3) to study Ca penetration through the fruit skin and (4) to establish the effects of multiple applications of Ca salts on the Ca-content and distribution within the fruit and the cracking susceptibility.