As we elucidate in Section 3.2, the plants compared were in very different growth stages and previously published results suggest that lettuce plants have higher concentrations of caffeoyl

derivatives in early than in later growth stages (Romani et al., 2002). Hence, we do not suppose that the elevated concentrations can be interpreted Ruxolitinib manufacturer as the plants’ response to low temperatures but rather interpret this as a developmental bias. Of the three phenolic acids that were evaluated, only the concentration of caffeoylmalic acid differed between plants cultivated in different temperature regimes, and only regarding small heads. This heterogeneity is in agreement with previously published results, indicating differences amongst phenolic acids regarding their response to environmental impacts (Oh et al., 2009) and amongst results obtained DAPT ic50 by different

studies (Grace et al., 1998, Løvdal et al., 2010 and Zidorn, 2010). Caffeoylmalic acid does not comprise the highest number of antioxidant structures per molecule (only one ortho 3′,4′-dihydroxy moiety whereas chicoric acid comprises one in each of the two caffeic acid moieties). Thus, we suppose the accumulation of caffeoylmalic acid in small heads has a function different from the commonly described antioxidant. Furthermore, there is no special similarity structure-wise between caffeoylmalic acid and cyanidin-3-O-(6″-O-malonyl)-glucoside which could explain why these two phenolic compounds were present in higher concentration in cool- than in warm-cultivated small heads. Unlike anthocyanins, phenolic acids do not absorb radiation in the wavelengths relevant for photosynthesis. Phenolic acids generally have their absorption maximum in the UV waveband and are therefore often considered UV protectants. 3-mercaptopyruvate sulfurtransferase However it is not very likely that UV played a role in our experiment as the applied radiation contained hardly UV radiation (HPS lamps; about 0.7% UV A and 0% UV B). Løvdal et al. (2010) detected the

strongest accumulation of caffeoyl derivatives in tomato leaves in response to a combination of high light, low nitrogen supply and low temperatures, indicating that temperature alone is not the trigger. Hence, the low-key impact we detected in our experiment might be due to our constant PPFD, the close monitoring of nutrient solution, and application of the lowest temperature outside the photoperiod. We were able to confirm the hypothesis that low temperatures increase the concentration of flavonoids and phenolic acids in lettuce only for cyanidin-3-O-(6″-O-malonyl)-glucoside and caffeoylmalic acid: Their concentration was higher in cool-cultivated than in warm-cultivated small heads.