Induction of Phenylalanine Ammonia-Lyase and Increase in Phenolics in Lettuce Leaves in Relation to the Development of Russet Spotting Caused by Ethylene

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Abstract
Russet spotting (RS), consisting of numerous small brown spots on the midrib of head lettuce (Lactuca sativa), is a physiological disorder induced by exposure to ethylene. In leaves suffering RS, the increase in spotting was accompanied by a parallel increase in the amount of phenolic compounds. Of these, chlorogenic acid and isochlorogenic acid were identified. Ethylene induced high phenylalanine ammonia-lyase (PAL) activity and RS formation in the susceptible cultivar Salinas, but not in the resistant cultivar Calmar. In the absence of ethylene, neither significant PAL induction nor RS occurred. No correlation was found between the increase in polyphenol oxidase or peroxidase and the development of RS. The increase in PAL activity was closely correlated with the development of RS. The increase in PAL activity preceded the development of RS, and the extent of RS was directly related to the level of PAL. Three temperature (0.5, 5.5 and 12.5.degree. C) were compared on the basis of their influence on RS and PAL induction. At the lowest temperature (0.5.degree. C) neither PAL induction nor RS occurred to a significant extent. At the highest temperature (12.5.degree. C) an initial rapid increase in PAL activity and an earlier development of spotting was observed, but subsequently there was a decrease in PAL activity and the rate of RS development. At the medium temperature (5.5.degree. C) PAL activity and RS increased progressively with time. The decline of PAL activity at a higher temperature might be attributed to inactivation of the enzyme. A temperature favorable for induction of PAL activity by ethylene was also favorable for RS. These observations indicate a close interrelationship between the induction of PAL activity and the development of RS in response to ethylene, and suggest a causal relationship between the 2 events. PAL serves as a useful biochemical marker for the RS reaction.