Postharvest Lettuce Discoloration: Current Understandingand Future Directions

Postharvest discoloration is one of the primary factors limiting the quality, shelf life, and marketability of lettuce. Although substantial progress has been made in understanding the physiological and biochemical basis of lettuce discoloration, inconsistent terminology, subjective assessment methods, and fragmented investigation of contributing factors have limited comparisons among studies and slowed progress towards elucidating the mechanisms regulating development of this physiological disorder. This review synthesizes current knowledge of lettuce discoloration by integrating disorder classification, physiological and biochemical mechanisms, genetic regulation, preharvest and postharvest factors that influence the shelf life, objective phenotyping approaches, and mitigation strategies. While gas-induced disorders are discussed to provide a comprehensive perspective, particular emphasis is placed on wound-induced oxidative discoloration and the growing body of evidence suggesting that browning and pinking may represent related but distinct oxidative phenotypes, although their physiological relationship remains unresolved. Inconsistencies in terminology and phenotype classification across the literature are also highlighted, and suggestions are provided to improve clarity and consistency in reporting. Recent advances in digital imaging and hyperspectral sensing, coupled with machine learning and molecular biology, provide new opportunities to objectively characterize discoloration phenotypes and improve understanding of the physiological, biochemical, and genetic mechanisms underlying discoloration development. Integrating these technologies with physiological, biochemical, and genetic research will be essential for improving mechanistic understanding and developing more effective strategies to predict, mitigate, and breed for reduced postharvest discoloration in lettuce and in other fresh produce.