Incidence of Physical Damage on Peach and Nectarine Skin Discoloration Development: Anatomical Studies

Additional index words. inking, abrasion, vibration damage, copigmentation, Prunus persica Abstract. Skin discoloration (SD) formation in peach (Pnmuspersica (L.) Batsch) and nectarine (Prunuspersica (L.) Batsch, var. nectarinelwas related to physical damage (abrasion) to the fruit during fruit handling (harvest and hauling operations) within the orchard and during transport to the packinghouse. Vibration and rubbing treatments increased SD formation indicating that tissue damage is involved in SD formation. Anatomical studies comparing sound and SD-injured tissues done by scanning electron and light microscopy indicated that very-low-intensity physical damage could induce brown and/or black spots because of cell disruption in the epidermal and hypodermal layers. The fact that injury was specific to the exocarp tissues (cuticle, epidermis, and hypodermis), and that mesocarp tissue located below the exocarp cells remained sound and turgid, indicated that abrasion injury is associated with SD. Similar types of visible and anatomical injury characteristics were induced by a rubbing treatment, demonstrating that physical abrasion damage affecting just exocarp cells was enough to induce SD. Skin discoloration or inking of peach and nectarine fruit has become a frequent problem in the past decade in California, as well as in other production areas. Although SD affects only the fruit's cosmetic appearance, this disorder causes considerable losses to the peach and nectarine industry each year. Of the fruit shipped to the New York market between 1972 and 1975, nearly 16% of the peaches and 13% of the nectarines showed some SD (Ceponis et al., 1987). The SD disorder appears as either dark (black staining), or dark brown, orange, or tan (brown discoloration) spots. It has been suggested that development of this disorder is associated with exogenous contamination occurring during packing operations (Baumgarder, 1985; Dennyet al., 1986; Hopfinger, 1985; Hopfmger and Frecon, 1985; Ridley et al., 1976; Van Blaricom and Senn, 1967). However, an anatomical comparison between sound and damaged tissues has never been done. Despite the importance of the SD problem, few studies have been done to try to understand the SD development mechanism(s). Denny et al. (1986) hypoth- esized that fruit injury was arequirement for SD formation because it allowed iron ions to penetrate and complex with the pigments inside the cells. Thus, most of the available data supports the hypothesis that iron or high pH contamination during the packing- house operation following physical injury is the cause of SD. Based on this information, most of the recommendations to control SD have focused on reducing metallic ion contamination and basic pH exposure during the packinghouse operation. Hudson and Christ (1981) advised that clean drying rollers need to be used during packing. Hopfinger (1985), in New Jersey, recommended reduction of iron concentration in the water used during the peach hydrocooling operation. Phillips (1988) reported that nearly 23% of the fruits of 'Elegant Lady' and 'O'Henry' had SD upon arriving at the packinghouse, suggesting that SD was related to transport injury.