Mitigating chilling injury of pomegranate fruit skin

Abstract

Pomegranate cv. Wonderful fruit are susceptible to chilling injury of the peel (CIp) when stored at 7 °C in modified-atmosphere bags for more than 3 months. The damage, manifested as superficial browning, appears first on the fruit skin, i.e., the outer colored layer of the peel. We previously suggested that susceptibility to CIp is associated with downregulation of PHENYLALANINE AMMONIA-LYASE (PAL) in storage, whereas POLYPHENOL OXIDASE (PPO) expression is upregulated. To further explore the role of PPO and PAL in CIp disorder, we monitored their gene expression and enzyme activity in fruit skin collected at three phases of fruit development: premature fruit 7 days before harvest when the fruit is susceptible to CIp disorder; during commercial harvest when fruit skin is aging; and during the following 4 months of storage. In addition, prestorage washing of the fruit with CaCl2 and citric acid solutions, previously shown to reduce CIp incidence in pomegranate and other fruit and to manipulate the PAL-to-PPO ratio, was applied. We determined total phenol content and antioxidative capacity of fruit skin during cold storage, and analyzed the expression of antioxidation-related genes CATALASE, SUPEROXIDE DISMUTASE, and GLUTATHIONE REDUCTASE, and key anthocyanin-biosynthesis genes CHALCONE SYNTHASE, CHALCONE ISOMERASE, and DIHYDROFLAVONOL 4-REDUCTASE. Results suggested that increased skin susceptibility to CIp is related to high total phenol content, relatively high PAL activity/gene expression, and low anthocyanin levels. Although PPO activity and gene expression increased in cold storage, this pattern was observed in all tissues tested, irrespective of their susceptibility to CIp development. Similarly, no association was evident between antioxidation-related genes’ expression levels and CIp susceptibility. On the other hand, pre-storage treatments with CaCl2 and citric acid mainly decreased CIp incidence associated with increased antioxidation-related gene expression. Hence, the mechanism governing mitigation of CIp development by prestorage washing with CaCl2 and citric acid differs from that observed in native CIp-resistant pomegranate skin.