Wound induced changes in phenolic metabolism and tissue browning are altered by heat shock

Wounding is one of many abiotic stresses that produce signals that propagate from injured into adjacent non-injured tissues and induce the de novo synthesis of specific wound-induced proteins. Some of these induced proteins are enzymes of phenolic metabolism, such as phenylalanine ammonia-lyase (PAL), whose increased activity leads to the accumulation of phenolic compounds (e.g., chlorogenic acid, dicaffeoyl tartaric acid and isochlorogenic acid) and tissue browning. Wounding of iceberg lettuce leaves increases PAL activity six- to 12-fold over 24 h at 10°C and leads to a three-fold increase in the total phenolic content within 3 days. There may be a hierarchical order to the plant's response to different abiotic stresses. Plant tissue simultaneously exposed to a heat shock and wounding responds to the heat shock in preference to wounding by producing heat shock proteins instead of PAL. A 90 s, 45°C heat shock prevents an increase in PAL activity if administered either 4 h before or 2 h after wounding. This diversion of wound-induced protein synthesis by heat shock might be used to prevent browning in other crops that normally have low phenolic content; e.g., celery and lettuce. The persistence of the ability of a heat shock to preferentially induce the synthesis of heat shock proteins (hsps) in place of wound-induced enzymes of phenylpropanoid metabolism offers a new way to control browning in lightly processed fruits and vegetables. The design of processing lines using a heat shock to extend the shelf-life of fresh-cut lettuce will need to be modified from existing designs to take full advantage of the effect of the heat treatment.