Flavonoid concentrations in the inner leaves of head lettuce genotypes

Typical HPLC profiles of phenolics from head lettuce (butter lettuce) genotypes showed quercetin (Qu) compounds Qu 3-O-glucuronide, Qu 3-O-(6-malonylglucoside), an acidic luteolin (Lu) glycoside, chlorogenic acid and chicoric acid as the main compounds in outer and - except the Lu conjugate - inner leaves. The outer leaves of some genotypes additionally contained cyanidin 3-O-(6-malonylglucoside). The generally marginal flavonoid concentrations of the cores of lettuce heads increased between 10-fold and 20-fold during the illumination of opened inner leaves. This indicated the competence of inner leaves of lettuce heads to perform the light-dependent flavonoid biosynthesis. After hydrolysis of flavonoid extracts from 15 head lettuce varieties, Qu concentrations were detected between 19.4 and 152.1 mug/g dry plant material (DW) in inner leaves and from 417.3 to 2482.7 mug/g DW in outer leaves. While the Qu concentrations of the inner leaves of most genotypes, including the commercial varieties, amounted to about 3% of those found in outer leaves, these proportions were 15% and 19% in two genotypes. Two other varieties showed relatively high Qu values of cores in connection with maximum flavonoid levels of outer leaves. It is indicated that such genotypes may be useful in increasing the flavonoid concentrations in the inner leaves of commercial head lettuce varieties by conventional breeding methods.