Discolouration in Lettuce
Cultivated lettuce (Lactuca sativa L.) is a diploid species (n = 9), existing in hundreds of uniform varieties and become an important staple crop for the continuously growing population. Lettuce is generally selected for leaf colour and shape, in addition to extended field and shelf life. The provisional value of home lettuce production marketed for 2005 as stated by Defra exceeded £71 million. Lettuce leaves are harvested as they are smooth and become gradually broader with additional leaf development. However, increased demand of fresh minimally processed produce had led to an increase in quantity and variety of produce available to the consumer.
Although lettuce is not an ideal model crop species for genetic studies in comparison to maize or tomato, most cultivars are highly inbred and reveal extensive genetic homozygosity. Significant progress has presently been made in the identification of genes, molecular markers and in their physical and functional relationship to each other. Numerous research groups have cooperated to develop a genetic map of L. sativa using various phenotypic and molecular markers, including commercially important genes potentially useful in breeding.
A major concern faced by commercial marketers is the limited shelf life of fresh processed lettuce due to post harvest discolouration. The shelf life of cut lettuce is regularly restricted by enzymatic browning and pinking. During storage their organoleptic characteristics are altered by the emergence of coloured pigments which generally incite an unfavourable consumer response due to the visual alteration of the product. This is in addition to the quality loss consequential of the associated decrease in content of phenolic compounds.
Baglet of lettuce, move cursor over image to see pinking and browning of tissue.
The phenylpropanoid pathway synthesises polyphenols that are sequentially oxidised by polyphenol oxidase (PPO) causing enzymatic browning. PPO catalyses the hydroxylation of monophenols to o-diphenols: monophenol monooxygenase or tyrosinase (EC126.96.36.199): and the oxidation of o-diphenols to o-diquinones; (EC188.8.131.52). This is followed by the non-enzymatic formation of brown pigments known as melanines. PPO is the main agent responsible for enzymatic browning, although a synergistic effect between PPO and peroxidases (POD) is possible. POD activity may result in oxidative actions that involve a non-specific hydrogen components in food causing discolouration.
Phenylalanine ammonia-lyase (PAL) is the rate-limiting enzyme of the phenylpropanoid pathway normally induced upon wounding of plant tissue, therefore increasing biosynthesis of polyphenols for oxidation. Cell disruption leads to decompartmentalisation involving a cascade of events, including PPO activation and promotion of PAL synthesis. PPO activation and both PAL and POD induction has been observed in six lettuce cultivars following cutting. However, there appeared to be no simple correlation between browning susceptibility and any of the parameters.
Anthocyanin pigments are secondary phenolic metabolites from the phenylpropanoid pathway which have a negative quality influence on products due to the associated red colour. Cyanidin, 3-glucoside and 3-malonylglucoside induce the red colour observed in L. sativa. Interestingly, reported high levels of PAL activity are fundamental for anthocyanin development. Although research suggests anthocyanin synthesis may occur independently of PAL activity under conditions of plentiful precursors.
Phenylpropanoid pathwaybased on Lepiniec et al., 2006. Annu. Rev. Plant Biol. 57:405–30
The prevention of discolouration in processed lettuce is presently achieved by combinations of various post harvest treatments including modified atmospheric packaging (MAP). Effectively all fresh processed products are by necessity handled in MAP to receive the necessary commercial post harvest life span. Ideally, genetic engineering would allow regulation of the pathways, avoiding treatment use. However, since widespread public opinion opposes this type of modification, exploitation of natural allelic variation is the preferred option for breeding companies.