INDUCTION AND INACTIVATION OF PHENYLALANINE AMMONIA-LYASE IN LETTUCE IN RESPONSE TO TEMPERATURE AND PROTEIN SYNTHESIS INHIBITORS

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Activity of phenylalanine ammonia-lyase (PAL) is critical in the induction of russet spotting (RS) in leaves of Iceberg lettuce (Lactuca sativa L.). RS is a major postharvest disorder of lettuce caused by exposure to ppm levels of ethylene at = 5C. Both PAL and RS are decreased when lettuce tissue previously exposed to ethylene is stored at = 15C or is transferred from = 5C to = 15C. To study the induction and inactivation of PAL, we exposed lettuce leaves to air ± 10 ppm ethylene at 5C for four days to initially induce high PAL levels. After four days, leaves were treated with water ± 2 mg/L cycloheximide, and transferred to air at 5 or 15 C. In leaves previously exposed to ethylene, PAL activity decreased rapidly to baseline levels within two days in non-cycloheximide treated leaves transferred to 15C. PAL activity remain elevated in the same treatment held at 5C. In leaves treated with cycloheximide and transferred to 15C, PAL did not begin to decrease until after four days. Cycloheximide treated leaves held at 5C showed increased PAL activity both two and four days after treatment.