Metabolite profiles of green, green/red, and red pigmented leaves of Lactuca sativa L. cultivars harvested at mature and bolting stages

Lettuce (Lactuca sativa L.) is one of the most popular vegetables in the world. Studying the metabolite content of lettuce at different stages of harvest could be useful to understand their health-benefits and stages of harvest for maximum phytochemical valorization. This study was aimed to identify, characterize, and quantify prominent phenolic compounds and estimate antioxidant activity of lettuce cultivars at mature and bolting stages. The identity and contents of the studied metabolites and antioxidant activity varied significantly. The total content of phenolic acids ranged from 18.3 to 54.6 mg/100g DW and 15.5 to 54.6 mg/100g DW; flavonoids between 9.2 & 25.9 mg/100g DW and 14.9 & 83.0 mg/100g DW in mature and bolting stage, respectively. The contents of cyanidin, lactucin, lactucopicrin, and ABTS values were in the range of 0.3 to 9.7 and 0.5 to 10.2 mg/g DW, 1.8 to 41.9 and 9.7 to 213.0 µg/g DW, 9.9 to 344.8 and 169.2 to 3888.2 µg/g DW, and 12.1 to 29.0 and 15.7 to 30.3 mg TE/g DW, respectively. The score plots of PCA revealed that the green and red pigmented cultivars were distinguished each other and located at the negative and positive sides of PC1, respectively, while the green/red pigmented cultivars were distributed throughout the four quadrants of the PCA plots with no prominent grouping. Lettuce accumulate high amount of sesquiterpene lactones, quercetin malonylglucoside, methylkaempferol glucuronide, kaempferol malonylglucoside, and 3-O-caffeoylicquinic acid at bolting compared to the mature stage. In general, red pigmented materials contained higher amount of metabolites.