Flavonoids in low concentrations are capable of displaying functional roles of extraordinary
significance in plant-environment interactions [6,74]. We note that during the colonization of land by plants the metabolism of dihydroxy B-ring-substituted flavonoids was already active. The whole set of genes for the biosynthesis of luteolin and quercetin derivatives has been detected in liverworts and mosses [75]. These findings lead to the hypothesis that it is unlikely that flavonoids fulfilled primary UV-B screening functions when early plants moved from water to colonize the land [1,3,6]. As already mentioned, dihydroxy B-ring-substituted flavonoids have a greater capacity than most other flavonoids to scavenge ROS, and do not display a superior capacity to absorb the shortest solar wavelengths. Actually, early land plants suffered from water and nutrient shortage in an O2-rich atmosphere under high-light conditions [76]. As a consequence flavonoid, particularly flavonol biosynthesis protected plants from the concomitant action of different environmental constraints, being high-UV-B just one of these.