Melatonin reduces pink rib discoloration in wounded lettuce midribs

Pink rib discoloration is a physiological disorder resulting in loss of quality and marketability of minimally processed and whole head lettuce (Lactuca sativa L.). Although the biochemistry of pink rib is complex and a comprehensive understanding is lacking, multiple research efforts have focused on inhibiting or reducing pink rib through exogenous application of small molecules such as L-cysteine. In this study, the effects of naturally occurring phytochemicals [serotonin (0.5 or 1 g L−1), naringenin (0.5 g L−1), L-cysteine (0.5 g L−1), and melatonin (1 g L−1)] on pink rib expression were examined in iceberg and Romaine lettuce. The exogenous application of these natural compounds was investigated for reduced pink rib discoloration in lettuce during simulated commercial storage and examined for metabolite and enzymatic activity changes. Immersing wounded midribs for 30 s in serotonin, naringenin, or L-cysteine solutions had no or only a slight inhibitory effect to inhibit pink rib discoloration while those immersed in a melatonin solution had reduced development of pink rib in both iceberg and Romaine lettuces after 5 d at 5 °C. Phenylalanine ammonia lyase (PAL) activity and chlorogenic acid concentration increased upon pink rib discoloration, consistent with previous reports. However, melatonin-treated samples did not result in reduced PAL activity, chlorogenic acid content, or polyphenol oxidase (PPO) activity. Untargeted metabolite analysis showed that melatonin treatment resulted in a global effect on metabolite changes in the wounded lettuce midrib tissue. To compare the severity of pink rib among treatments, pink rib was quantified throughout storage visually and by converting digital images of the lettuce midribs to the L*a*b* color space. The a* component of L*a*b* represents redness versus greenness, thus its values were used to objectively measure pink rib severity. Melatonin was shown to be a promising compound to minimize development of pink rib discoloration at cut surfaces of iceberg and Romaine lettuce during commercial handling and storage.