Phenotypic characterization and inheritance of browning on cut surfaces of stems and leaf ribs of romaine lettuce

Authors: PENG, HUI - University Of California; Luo, Yaguang - Sunny; TENG, ZI - University Of Maryland; Zhou, Bin; BORNHORST, ELLEN - Oak Ridge Institute For Science And Education (ORISE); Fonseca, Jorge; Simko, Ivan

Submitted to: Postharvest Biology and Technology
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 7/10/2021
Publication Date: 7/19/2021
Citation: Peng, H., Luo, Y., Teng, Z., Zhou, B., Bornhorst, E.R., Fonseca, J.M., Simko, I. 2021. Phenotypic characterization and inheritance of enzymatic browning on cut surfaces of stems and leaf ribs of romaine lettuce. Postharvest Biology and Technology. 181. Article 111653. https://doi.org/10.1016/j.postharvbio.2021.111653.
DOI: https://doi.org/10.1016/j.postharvbio.2021.111653

Interpretive Summary: Lettuce is one of the most important leafy vegetables commercially cultivated in North America, with a farm value of $3.5 billion in 2019 in the U.S.A. This vegetable is sold as whole heads, partially prepared “Romaine Hearts”, or in ‘ready-to-eat’ packaged fresh-cut salad products. Tissue browning (or “pinking”), is a major postharvest defect of lettuce quality. While browning is mostly restricted to the cut surface of the stem on the whole head, it can be pervasive on the cut edges of lettuce leaf ribs in salad products. Twelve lettuce cultivars and breeding lines were grown at the Agricultural Research Station in Salinas, California and harvested at the commercial maturity stage. The most intensive browning was observed mainly around the vascular bundles dispersed along the rim area of the stems. Browning on the cut surfaces of the stems was significantly correlated with cut leaf ribs. These results showed that browning in leaf ribs (which is difficult to measure) can be predicted by the browning degree of the stems (which is easier to measure). The reported findings revealed a method to predict browning, which can be used to simplify lettuce cultivar screening and to decipher the genetics of lettuce browning in order to accelerate the breeding of browning resistant cultivars.

Technical Abstract: Enzymatic browning is a major postharvest quality defect of romaine lettuce (Lactuca sativa L.) and is pervasive on the cut surfaces of leaf ribs and stems. In this study, we provide for the first time an analysis of the relationship between the browning of leaf ribs and stems across major cultivars. Twelve lettuce cultivars and breeding lines were grown at the Agricultural Research Station in Salinas, California and harvested at the commercial maturity stage. Digital images of the cut surfaces of lettuce stems and leaf ribs were analyzed for browning index (BI) and CIELAB color values via machine vision. While all samples showed a progressive increase in BI and decline in lightness (L*) and hue (h°) during 5 °C storage, significant differences (p < 0.01) in the rate of browning development were observed among cultivars. The most intensive browning was observed mainly around the vascular bundles dispersed along the rim area of the stems. Browning on the cut surfaces of the stems was significantly correlated with cut leaf ribs, as indicated by the Pearson correlation coefficients of 0.886 for BI, 0.891 for L* and 0.866 for h° (p < 0.001). These results showed that browning in leaf ribs (which is difficult to measure) can be predicted by the browning degree of the stems (which is easier to measure). High genetic similarity was found among four cultivars with limited browning (Darkland, Parris Island Cos, Green Towers, and Hearts Delight) and also between cultivars with severe browning (King Henry and Tall Guzmaine). The highest broad-sense heritability (H2) in both trials (July and November 2018) was 0.88 and 0.92 for stem and rib browning, respectively. The reported findings revealed a method to predict browning, which can be used to simplify lettuce cultivar screening and to decipher the genetics of lettuce browning in order to accelerate the breeding of browning resistant cultivars.