Regulation of Anthocyanin Synthesis in Apple Skin. III. Involvement of Phenylalanine Ammonia-lyase

The relationship between L-phenylalanine ammonia-lyase (PAL) activity and anthocyanin synthesis in apple skin was investigated. Enzyme activity was induced by exposure to white light. After a lag phase of less than 10 h, PAL activity increased to a maximum at 30 h and then declined. Anthocyanin accumuIation, after a lag phase of 20 h, continued at a constant rate for a further 100 h. The increased rate of anthocyanin accumulation which resulted from u.v. light and cycloheximide treatment of whole fruit was preceded by comparable increases in the level of PAL activity. Wounding, which stimulated anthocyanin synthesis in skin discs, caused a far greater increase in PAL activity. In wounded tissue (skin discs) neither u.v. light or cycloheximide treatment caused any increase in anthocyanin accumulation or PAL activity. Cycloheximide inhibited anthocyanin formation in skin discs and, to a lesser extent, reduced PAL activity. When cycloheximide was applied as a single drop to a whole fruit, it inhibited anthocyanin formation but increased PAL activity in the skin to which it was applied. In a surrounding ring of skin, both anthocyanin level and PAL activity were increased. Without light, anthocyanin synthesis did not occur and there was virtually no PAL activity in whole fruit skin. However, in skin discs held in the dark, PAL activity developed to the same level as in illuminated discs, but no anthocyanin accumulated. The results indicate that, although the level of PAL activity may control the rate of anthocyanin synthesis in whole fruit, it is not the only critical enzyme regulating anthocyanin synthesis in apple skin. The mechanisms by which treatments may stimulate PAL activity and anthocyanin synthesis and the possible involvement of ethylene are discussed.