Abstract/Summary

Pinking of midribs is one of the major postharvest issues in lettuce that affects its visual quality and reduces consumer acceptance. In this study, the effects of cultivar, light intensity and time of storage on pinking discolouration and related metabolites were elucidated. Two cultivars of Romance lettuce, Keona (fast pinking) and Icarus (slow pinking) were grown under four light intensities, and we determined the effects on the pinking of leaf midribs, phenolic acids, soluble sugars and total ascorbic acid concentrations after eight days of cold storage. Differences in pinking index of the midribs of the two cultivars were only observed when the plants were grown in higher light intensities (L1 and L2, 1044 and 578 µmol m−2 s−1 respectively) compared tonlower intensities (L3 and L4, 386 and 338 µmol m−2 s−1 respectively). All assessed phenolic acids increased during storage and their contents were highest at L1. Coumaric acid concentration increased by 135.0% in Keona compared to those in Icarus (96.0%) regardless of light levels after 8 days of storage. The accumulation of caffeic acid in the midribs of Keona increased more in concentration than Icarus during storage, resulting in more concentrated caffeic acid in Keona tissues. Significant increase in chlorogenic acid that occurred in both cultivars between 0 days and 8 days of storage was only observed in plants grown under high light whilst at low light, the increase in chlorogenic acid only occurred in Keona. Higher light intensity (L1 and L2) increased soluble sugars concentrations in midribs. Keona consistently contained higher concentration of glucose, galactose and sucrose regardless of light intensity compared with Icarus after both 0 days and 8 days of storage. The concentration of sugars in plants grown under L1 remained unchanged for both cultivars during storage as opposed to those grown at lower light intensities (L3 and L4). Principal component analysis (PCA) shows that coumaric acid, caffeic acid, and chlorogenic acid were positively correlated with pinking index for both cultivars. Ascorbic acid was negatively correlated with pinking index in Keona, but the association was not found for Icarus. The results of the study revealed that pinking of the midribs of lettuce can be reduced when the plants were grown at low light intensity, but this phenomenon was coupled with low concentrations of phenolic acids, sugars and total ascorbic acid. Using low pinking cultivars offers a clear benefit in reducing pinking of lettuce after harvest, especially when the plants are grown under high light intensity.