Abstracts Biological Sciences

Superficial scald in apples

by John B. Golding

Comparative physiological and biochemical studies were conducted with apple varieties which were susceptible (Granny Smith and Lady Williams) and resistant (Crofton) to superficial scald, to understand the mechanisms of scald aetiology. This provided a basis for the development of alternatives to diphenylamine to control the disorder in cool stored apples. The research confirmed earlier research, which showed that the typical scald symptoms were the result of the deposition of brown pigments on the tonoplast in hypodermal and epidermal cells. It has been generally accepted that these pigments were formed by the action of polyphenol oxidase (PPO) following its release from chloroplasts, as a result of membrane damage caused by the oxidation products of alpha-farnesene. However, intact and functional chloroplasts were observed in severely scalded peel tissue. It was proposed that peroxidase (POD) might play a more important role in scald development. The concept that the concentration of alpha-farnesene or its products was related to the severity of scald was examined, and results indicated that differences in scald susceptability were related to the ability of apple tissue to cope with oxidative stress at low temperatures rather than the concentration of alpha-farnesene. If alpha-farnesene has a scald promoting role, it may be as a participant in a free radical chain reaction rather than an initiator. Studies were conducted on the phenolic compounds present in apple peel, which led to the proposal that phenolics might confer scald resistance by conserving other antioxidants and preventing peroxidation of membrane lipids. An holistic model of scald development was proposed, which indicated that scald results from a chilling stress since scald only occurs at temperatures below 4 degrees C and only in apples in which the capacity to protect membranes from oxidative stress is low. The model proposed that POD may have a more important role than PPO in scald development than previously thought. Doctor of Philosophy (PhD)