Parka® Has No Effects on Cuticle Deposition, Strain Relaxation, Water Movement, or Cracking of Sweet Cherry Fruit

Rain cracking is a severe problem for producers of sweet cherry fruit. With the aim of reducing fruit cracking, a number of different proprietary products are applied as whole canopy sprays but with variable success. Parka ® is a product believed to function as a cuticle supplement. The objective of this study was to determine the effects of Parka ® on cuticle deposition, strain relaxation, water movement, and fruit cracking. Cracking was quantified following simulated rain in a green house and using classical cracking assays. Fruit mass increases sigmoidally with time between 36 and 105 d after full bloom (DAFB). Cuticle synthesis and deposition—quantified as the amount of 13 C-oleic acid incorporated into the cutin fraction—occurs only during stage I and II development and not thereafter. There were no effects of Parka ® applied at stage I (petal fall) and at early stage III (straw yellow) on the mass of the cuticular membrane (CM) or of the dewaxed CM (DCM) in the cheek region of a range of sweet cherry cultivars including Annabella, Hedelfinger, Regina, Sam, Schneiders, and Sweetheart. In the stylar end region, the effects were variable. The wax mass per unit area was unaffected. Multiple applications of Parka ® (12 times in weekly intervals beginning 14 DAFB) increased CM and DCM mass per unit area in the cheek and the stylar scar region; the effect on wax mass was significant only in the cheek region. There was no effect of Parka ® on strain relaxation of the cuticle following excision of the epidermal segment and isolation of the CM. Multiple sprays of Parka ® (12 times) left heavy spray deposits in all regions of the fruit surface. When applied only twice, light spray deposits were observed. With few exceptions, Parka ® had no effects on water uptake flow rates or water flux densities or cracking. When whole trees were sprayed with Parka ® and later exposed to simulated rain in a fog chamber, 12 Parka ® applications increased cracking in one year of two, while two Parka ® applications had no effect at all. Parka ® sprays had no effect on the permeance of polycarbonate films to water vapor. Based on our results Parka ® does not function as a cuticle supplement in sweet cherry.