EFFECT OF SELECTED BROWNING INHIBITORS ON PHENOLIC METABOLISM IN STEMTISSUE OF HARVESTED LETTUCE

Wound-induced changes in phenolic metabolism causes stem browning (but t 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 ox idized by the enzyme polyphenol oxidase (PPO) to produce brown pigment s. This browning reaction can readily be followed by measuring a valu es. 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 compound s. The mechanism of calcium action could be to decrease PPO activity o r 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 inhib ited by acetic acid, and this may explain its role as a browning inhib itor.