Polyphenoloxidase isozymes in bitter pit affected apple tissues either natural or locally induced by ammonium oxalate injections

Abstract

Calcium-related disorders in fruits lead to the collapse of cells, resulting in tissue enzymatic browning, caused by polyphenol oxidase (PPO) and peroxidase enzymes. PPOs are responsible for the enzymatic browning reaction that occurs in damaged fruits or vegetables. It appears that oxidative browning reactions in plants generally decrease quality by changing nutritional and organoleptic properties. In this work, electrophoresis was used for detection and comparison of active PPO isoforms from apple cv. Golden Delicious fruit tissues. Three types of materials were studied: sound apple pulp, bitter pit affected pulp and corky lesions induced by ammonium oxalate subdermal injections. Three main isozyme bands revealed by catechol staining were detected in the pulp of sound apple. In the apple tissue affected by bitter pit, the intensity of the two bands of high molecular weight increased and two new isozymes appeared, one below the second of greater mass and another one in the zone of low molecular weight. However, in oxalate-induced corky tissue, the lowest molecular weight PPO isozyme found in the natural pits, did not appear while the remaining pattern was analogous. Both the natural process of bitter pit development and the chemically induced one finally resulted in cork-like spots. These dehydrated brown tissues accumulate calcium and show greater PPO activity. However, these two processes must follow different metabolic routes or the speed at which the spots appear in the oxalate-induced process, does not allow for the adaptation of the isozymatic system as it occurs in the natural process.