Carbon Dioxide-induced Brown Stain Development as Related to Phenolic Metabolism in Iceberg Lettuce

Abstract

Controlled atmospheres containing air + 11% CO 2 caused tissue injury and induced phenylalanine ammonia-lyase (PAL, EC 4.3.1.5) activity in iceberg lettuce ( Lactuca sativa L.) midrib tissue. Injury symptoms included brown stain (browning of epidermal tissue) and sunken epidermal areas (pitting) a few millimeters in diameter. Pitting occurred in high-CO 2 atmospheres at 5C with no increase in phenolic content, but browning did not develop until the tissue had been transferred to air at 25C. Browning developed within several hours of transfer to air and the degree of browning was correlated with the soluble phenolic content. The oxidation of soluble phenolic compounds to brown substances by polyphenol oxidase (PPO, EC 1.10.3.2) could account for tissue browning. Lignification was associated with cell wall thickening in discolored tissue and was accompanied by an increase in ionically bound and soluble peroxidase (POD, EC 1.11.1.7) activities. Exposure of tissue to elevated CO 2 increased ionically bound indoleacetic acid (IAA) oxidase activity, but reduced soluble IAA oxidase activity. Application of an aqueous solution of 1.0 m m IAA to the tissue before treatment did not significantly reduce browning. Lettuce tissue exposed to 1.5% O 2 + 11% CO 2 had reduced PAL activity and lower soluble phenolic content than lettuce exposed to air + 11% CO 2 . Depending on the sensitivity of the lettuce tissue to CO 2 injury, low-O 2 atmospheres either reduced or slightly retarded browning induced by 11% CO 2 .