The trial was conducted during the 2007 growing season on the experimental site Sheep Pens (west) at Warwick HRI, UK (Latitude: 52.183. Longitude: 1.583). The trial was designed in 3 blocks, each block containing a single plot of 28 lines randomised in an alpha design. The trial was planted on 1st May 2007, with lettuce transplants raised under glasshouse conditions. Each plot contained 12 plants (4 x 3) of the same accession with 35 cm between each plant (central plants were therefore self-guarded to avoid ‘edge’ effects).
Fencing and flappers surrounded the land to provide protection from the local fauna, and plants treated with protective sprays as information was released on the UK HDC Pest Bulletin (www2.warwick.ac.uk/fac/sci/whri/hdcpestbulletin/) according to good agricultural practice. Plants were irrigated through an oscillating line as required for establishment (although no soil moisture deficit was recorded the land was irrigated to bring it near field capacity prior to transplanting), and irrigation stopped 7 days before harvest.
The trial was harvested 30th May, 5th June, 12th June and 19th June 2007 depending on maturity of individual accessions with all replicates of an accession assessed on the same date. On harvest day the central two replicated heads per plot were cut at soil level and the untrimmed weight recorded. Excess material from each head was trimmed by removing the outermost exposed wrapper leaves and the trimmed weight recorded. Heads were processed by halving the head, removing the core, cutting lengthwise from butt to crown and again transversely then cutting into ~4 cm2 pieces . Processed material from each head was mixed thoroughly. Approximately 75g of unwashed processed material were sealed (removing any excessive air and ensuring the seal was not compromised by material) in a non-selective permeability film bag (P-PLUS 35PA240; 200 x 250mm; Amcor Flexibles P-Plus, UK) with material from one head filling two bags. The use of non-selective film ensured that the atmosphere within the pack did not reduce the natural intrinsic rate of discolouration in each product. Bags were either hung or stored vertically on a racking system at 3°. On each assessment (days 1, 3, 6, 9 and 13) bags were removed from storage and arranged under a halogen light source for assessment, ensuring that the bags had the same orientation on each assessment date. A 12-square 3 x 4 acetate grid (50 mm x 50 mm) was overlaid on the bag and (confirming each square represented material) discolouration in each square was quantified based on a set of photographic standards for pink and brown discolouration (Hilton et al., 2009). Pinking and browning were each split into 2 categories of severity, including slight and severe. When no discolouration was recorded, bags were classified as clean. When there was uncertainty about whether the discolouration was browning or pinking it was classified as 'visible'. This prevented missing data on discoloration and allowed the discolouration to be included in the general 'discoloration' classification category upon analysis. As time progresses the material becomes easier to clarify so the amount of data classified as 'visible' decreased. Each grid square was given a single score as a discolouration co-ordinate at its most intense representation (see sample grid below), resulting in 12 tallies per scoring grid per bag on each assessment day. The percentage discolouration (= 100 x (number of grid squares with discolouration / 12)) and the intensity of discolouration (mean discolouration score / number of grid squares showing the discolouration) were determined on each assessment day for pinking, browning, visible and all discolouration. Data for Day 3 gave best discrimination between accessions and is presented in the data page.