Plant Hormone Interaction and Phenolic Metabolism in the Regulation of Russet Spotting in Iceberg Lettuce

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Abstract
Russet spotting (RS) is a physiological disorder induced in iceberg lettuce (Lactuca sativa L.) by exposure to parts per million levels of ethylene at 5 .+-. 2.degree.C. Ethylene induced phenlalanine ammonia-lyase and ionically bound peroxidase activities that correlated with development of RS symptoms. The ethylene-treated tissue had significantly higher lignin content than air control tissue with lignification localized in walls of RS-affected cells. Ethylene also caused the accumulation of the flavonoids (+)catechin and (-)epicatechin and the chlorogenic acid derivatives 3-cafeoyl-quinic acid, 3.5-dicaffeoylquinic acid, and 4,5-dicaffeoylquinic acid. These soluble phenolic compounds were readily oxidized to brown substances by polyphenol oxidase isolated from RS tissue. Ethylene substantially increased ionically bound indole-3-acetic acid (IAA) oxidase activity, while IAA application greatly reduced ethylene-induced phenylalanine ammonia-lyase, peroxidase , and IAA oxidase activities, soluble phenolic content, and RS development.