Post harvest discolouration in lettuce is an increasingly important problem due to the shift in the market for prepacked processed salads. Variation in post harvest discolouration was recorded in a lettuce diversity set of 28 accessions representative of the lettuce genepool. The parents of the WHRI lettuce mapping population, Saladin and Iceberg were included in the diversity set. They showed significantly different responses for discolouration and the difference between them was representative of a major part of the variation seen in the diversity set. F7 RILs derived from a cross between Saladin and Iceberg were suitable for genetic analysis of post harvest discolouration. As a precursor to the genetic analysis, a good quality linkage map based on the F7 Sal x Ice population was generated. Significant genetic variation in the post harvest response was demonstrated for these RILs. Twenty-one significant QTL were identified for post harvest discolouration traits, and the markers linked to the QTL can be used for marker assisted selection. Significant but weak correlations were recorded between discolouration and important agronomic traits, however as these were not highly correlated this means that post harvest discolouration and agronomic traits can generally be independently selected for by breeders without having to compromise on other traits. Research was also initiated to understand the metabolic changes underlying the phenotype change. Significant variation in levels of metabolites related to post harvest discolouration including phenylalanine ammonia lyase (PAL), polyphenol oxidase (PPO) and total phenolic content (TPC) was observed in RILs with extreme phenotypes. The differences in metabolite levels were significantly correlated with the discolouration phenotypes. Work was also initiated to identify candidate genes associated with the biosynthetic pathway responsible for discolouration (phenylpropanoid pathway) in an attempt to identify genes underlying QTL. Twenty-three genes have been placed on the Sal x Ice map using comparative genomic approaches. Some of these co-locate within the region of a discolouration QTL and are therefore candidate genes for the QTL effect. Mutants with altered post harvest discolouration phenotypes were also confirmed in this study; however the point of mutation could not be identified although it is thought to be downstream of PPO. This indicates that a desired phenotype with reduced levels of post harvest discolouration can be achieved by two approaches to breeding; using natural and induced variation. This study has provided the tools and knowledge to do this.
| Identifer | oai:union.ndltd.org:bl.uk/oai:ethos.bl.uk:535316 |
| Date | January 2010 |
| Creators | Atkinson, Laura Denise |
| Publisher | University of Warwick |
| Source Sets | Ethos UK |
| Detected Language | English |
| Type | Electronic Thesis or Dissertation |
| Source | http://wrap.warwick.ac.uk/35146/ |
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