Supplementary Light Intensity and Harvest Date Affect Midrib Oxidative Pinking and Related Metabolites in Two Romaine Lettuce Cultivars with Contrasting Discolouration Sensitivities

This study elucidates the variations in phenolic acids, soluble sugars, and pinking development of midribs of two cultivars of Romaine lettuce (Keona—high pinking and Icarus—low pinking) under two light intensities (high L1—558 and low L2—244 µmol m−2 s−1) harvested at two harvest dates (M1—42 and M2—49 days after transplanting, DAT). The pinking index of Keona was higher than that of Icarus on 8 days of storage (5 °C). The concentrations of cinnamic acid were reduced in most treatments for both cultivars during storage, except for Keona grown in L2 with M2 harvest. Upon storage, the concentrations of coumaric acid in Keona were similar regardless of light intensities and harvest dates. Coumaric acid and caffeic acid concentrations in Icarus in L1 harvested at M2 were the highest. Low light intensity with M1 harvest enhanced the concentration of chlorogenic acid in Keona, but a similar situation reduced its content in Icarus during storage. Icarus contained higher initial concentrations of glucose under both light intensities, regardless of harvest dates, compared to Keona. In conclusion, high pinking was associated with high phenolic acids except for cinnamic acid. High light intensities and more advanced harvests increased the pinking of Keona but not of the Icarus.