Wound-induced changes in phenolic metabolism causes stem browning (butt discoloration) in
harvested lettuce. Stem tissue near the harvesting cut exhibited increased phenylalanine ammonialyase
(PAL) activity and accumulation of caffeic acid derivatives. These o-diphenols can be oxidized
by the enzyme polyphenol oxidase (PPO) to produce brown pigments. This browning reaction can
readily be followed by measuring a* values. Browning was reduced by washing stem disks with
solutions of 0.3 M calcium chloride, 1.0 mM 2,4-dichlorophenoxyacetic acid (2,4-D), or 0.5 M acetic
acid. These browning inhibitors appear to act in different ways. Calcium chloride decreased PAL
activity to 60% of the control, but did not substantially affect the accumulation of phenolic compounds.
The mechanism of calcium action could be to decrease PPO activity or to preserve membrane
structure. PAL activity was inhibited 60% by 2,4-D, and the biosynthesis of phenolic compounds
was strongly inhibited but not suppressed. Acetic acid completely inhibited PAL activity and the
production of wound-induced phenolics. PAL was irreversibly inhibited by acetic acid, and this
may explain its role as a browning inhibitor.