Effect of gamma radiation on phenylalanine ammonialyase activity and accumulation of phenolic compounds in citrus fruit peel

External layers of citrus fruit peel evinced a marked increase in phenylalanine ammonia-lyase (PAL) activity after irradiation with 200 krad doses of gamma radiation. Maximum enzyme activity was attained one day after irradiation. Thereafter PAL activity rapidly decreased to control values in fruits which did not show visible damage symptoms later upon storage, but remained relatively high for several days in those fruits which later showed peel damage. Accumulation of phenolics in cells was demonstrated and may be explained by the enhanced PAL activity. The connections between irradiation, enzyme activity, phenolics and damage are briefly discussed.