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 e thylene inducibility of phenylalanine ammonia lyase (PAL, EC 4.3.1.5) activity and the development of russet spotting (RS) in Iceberg lettuc e (Lactuca sativa L.). Tissue exposed to ethylene and held at 15 or 20 C showed earlier but lower peaks in PAL activity than tissue held at 5 C. Accumulation of total soluble phenolic compounds, and the developme nt 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 la ter but higher peaks in PAL activity than tissue continuously held at 15C. Transferring ethylene-exposed tissue to OC before RS symptoms dev eloped greatly limited further rise in PAL activity and RS development . Discontinuing ethylene exposure after 4 days resulted in a rapid dec rease in PAL activity. Lettuce tissue exposed to ethylene for 4 days a nd then transferred from 5 to OC after the development of moderate RS symptoms showed no difference in RS after 8 days compared to tissue st ored continuously at 5C.