Epicuticular Wax on Leaves of the Rosette and Inflorescence of Lettuce

Lettuce is popular leafy vegetable that can harbor foodborne human-pathogens under conducive production conditions. Recent studies reveal that the epicuticular waxes on lettuce can effect the attachment of foodborne pathogens such as Escherichia coli and Salmonella enterica. However, little is known about what developmental stages in lettuce are most likely to harbor these pathogens, and whether differences in surface waxes during development can effect pathogen attachment. Epicuticular wax chemical composition and contact angles on lettuce leaves were analyzed on immature and mature rosette leaves, and leaves at three different attachment points in the inflorescence stem of lettuce (upper, middle, and lower leaves). Wax composition was similar for both immature and mature rosette leaves as revealed by gas chromatography–mass spectrometry. The total amount of wax on the upper and middle inflorescence leaves was similar, and about two fold higher than for lower leaves, which in turn were five fold higher than rosette leaves. These upper and middle leaves produced significantly higher amounts of long chain alcohols than rosette stage leaves. Specifically, the amount of hexacosanol  (C₂₆), the primary wax component, was much higher on upper and middle inflorescence leaves than lower inflorescence leaves, which in turn were much higher than on rosette leaves. Leaf contact angles and epicuticular wax crystal density (revealed using scanning electron microscopy) were positively correlated with total wax amount. No wax crystals were observed on either immature or mature rosette leaves, whereas lower inflorescence leaves had sparsely distributed wax crystals, and middle and upper inflorescence leaves had a very high density of surface wax crystals. Hexacosanol recrystallized in vitro produced plate-like crystals similar to those found in vivo on lettuce leaves, suggesting a possible role for wax composition in crystal morphology. Whether the heavier wax depositions we observed on upper and middle inflorescences inhibit attachment of dangerous human-pathogens, and whether lettuce waxes might be modified (either genetically or via cultural practices) to reduce the threat of such infections on lettuce is the subject of ongoing studies.