EFFECTS OF TEMPERATURE ON ETHYLENE-INDUCED PHENYLALANINE AMMONIA-LYASE ACTIVITY AND RUSSET SPOTTING IN HARVESTED ICEBERG LETTUCE

Experiments were conducted to study the effect of temperature on the ethylene inducibility of phenylalanine ammonia lyase (PAL, EC 4.3.1.5) activity and the development of russet spotting (RS) in Iceberg lettuce (Lactuca sativa L.). Tissue exposed to ethylene and held at 15 or 20C showed earlier but lower peaks in PAL activity than tissue held at 5C. Accumulation of total soluble phenolic compounds, and the development of RS paralleled increases in PAL activity after a short lag period. Lettuce tissue transferred from 5 to 15C after 1 to 3 days showed later but higher peaks in PAL activity than tissue continuously held at 15C. Transferring ethylene-exposed tissue to OC before RS symptoms developed greatly limited further rise in PAL activity and RS development. Discontinuing ethylene exposure after 4 days resulted in a rapid decrease in PAL activity. Lettuce tissue exposed to ethylene for 4 days and then transferred from 5 to OC after the development of moderate RS symptoms showed no difference in RS after 8 days compared to tissue stored continuously at 5C.