Oxidative discolouration in whole-head and cut lettuce: biochemical and environmental influences on a complex phenotype and potential breeding strategies to improve shelf-life

Lettuce discolouration is a key post-harvest trait. The major enzyme controlling oxidative discolouration has long been considered to be polyphenol oxidase (PPO) however, levels of PPO and subsequent development of discolouration symptoms have not always correlated. The predominance of a latent state of the enzyme in plant tissues combined with substrate activation and contemporaneous suicide inactivation mechanisms are considered as potential explanations for this phenomenon. Leaf tissue physical properties have been associated with subsequent discolouration and these may be influenced by variation in nutrient availability, especially excess nitrogen and head maturity at harvest. Mild calcium and irrigation stress has also been associated with a reduction in subsequent discolouration, although excess irrigation has been linked to increased discolouration potentially through leaf physical properties. These environmental factors, including high temperature and UV light intensities, often have impacts on levels of phenolic compounds linking the environmental responses to the biochemistry of the PPO pathway. Breeding strategies targeting the PALand PPOpathway biochemistry and environmental response genes are discussed as a more cost-effective method of mitigating oxidative discolouration then either modified atmosphere packaging or post-harvest treatments, although current understanding of the biochemistry means that such programs are likely to be limited in nature and it is likely that they will need to be deployed alongside other methods for the foreseeable future

The Impact of Polyphenols on Human Health

Polyphenols are found ubiquitously in plants and are therefore abundant in human diet. Increased polyphenol consumption has been associated with a reduced risk of development of a range of chronic diseases, such as cancer, cardiovascular and neurodegenerative disorders.Initially the antioxidant property of polyphenols was believed to underlie their beneficial effects in vivo. However, recent evidence suggest that polyphenols may express their beneficial properties through their interaction with cellular signalling pathways and related machinery that mediate cell function under both normal and pathological conditions. In thisChapter we aim to provide an overview of the different classes of polyphenols and their sources and to highlight the role that polyphenols play in the prevention of, cancer,cardiovascular disease and neurodegeneration. We present epidemiological data, human intervention study finding, as well as animal and in vitro studies in support of these actions and in each case we consider how their actions at the cellular level may underpin their physiological effects