APOPLASTIC LEVELS OF HYDROXYL RADICALS IN FOUR DIFFERENT APPLE CULTIVARS ARE ASSOCIATED WITH SEVERITY OF CUT-EDGE BROWNING

Abstract

Past research has shown that severity of cut-edge browning in apples used to make fresh slices is highly dependent on the cultivar. 'Ambrosia' and 'SPA343' are resistant to browning, while 'Delicious' and 'Spartan' are very susceptible. Prior experiments have shown a relationship of browning severity with superoxide dismutase/peroxidase activity ratios and peroxide contents in the apoplast. However, peroxides themselves are not sufficiently active to cause lipid peroxidation and membrane degradation. Lipid peroxidation and membrane degradation are the processes that lead to the intermingling of phenolics and polyphenol oxidase, thus leading to the initiation of browning reactions. Hydroxyl radicals can directly initiate lipid peroxidation and they are formed in vivo through the interaction of the superoxide anion with hydrogen peroxide. This research used a newly-developed method to measure hydroxyl radicals in vivo to evaluate whether hydroxyl radical accumulation in the apoplast could be associated with severity of cut-edge browning. The results clearly show that apoplastic levels of hydroxyl radical are closely related to the severity of cut-edge browning, whereas peroxide levels, water soluble antioxidants and phenols are not. A model is presented to explain the results.