Recent Advances in Wounded Lettuce Discoloration: A Review of Pre- and Postharvest Influencing Factors, Pathways, and...

Lettuce is highly valued for its nutritional composition and health-promoting properties. However, undesirable discoloration (i.e., pinking and browning) of wounded lettuce tissues is considered a major factor leading to economic losses in the lettuce industry. Previous reviews have primarily discussed the influencing factors of lettuce discoloration from the perspective of preharvest agronomic regulation and elaborated on the impacts of different packaging and storage methods on lettuce discoloration. However, they have not fully reviewed the research progress on discoloration throughout the entire lettuce industry chain. To fill this gap, this review systematically analyzes the pre- and postharvest factors inducing discoloration, clarifies the discoloration pathways, especially the differences and correlations between browning and pinking, characterizes other quality changes accompanying discoloration, elaborates on strategies for preventing discoloration, and proposes future prospects. In general, wounded lettuce discoloration is primarily associated with the imbalance of redox signaling involving discoloration-related enzymes and substrates. The phenylpropanoid pathway serves as the branching point of browning and pinking, with caffeic acid considered the key intermediate. Discoloration coincides with a reduction in tissue firmness, alongside the accumulation of off-flavors such as ethanol. While biological measures, natural ingredients, and hurdle strategies show promise in preventing wounded lettuce discoloration, their practical application faces challenges. In the future, the identification of genetic markers and loci, as well as the recognition and functional analysis of genomic regions and relevant genes, could collectively constitute essential theoretical underpinnings for the cultivation of lettuce varieties with low discoloration potential.