Singlet oxygen scavenging by leaf flavonoids contributes to sunlight acclimation in Tilia platyphyllos

• Sun leaves contain more singlet oxygen antioxidants than shade leaves.
• Quercetin:kaempferol ratio is higher in sun leaves which also contain myricetin.
• Quercetin and myricetin glycosides are better 1O2 antioxidants than corresponding kaempferols.
• Quercetin and myricetin glycosides do not provide better UV-B screening than corresponding kaempferols.
• UV inducible flavonoids may protect leaves from by high PAR as 1O2 antioxidants.

Both high photosynthetically active radiation (PAR) and ultraviolet radiation (UV) are capable of causing photooxidative stress, but leaves are equipped with an array of protective mechanisms making life under full sunlight possible. Comparing acclimation strategies of Tilia platyphyllos leaves we found that sun leaves were better protected against stress than shade leaves by having (i) more efficient regulated non-photochemical quenching, (ii) a higher capacity to neutralize singlet oxygen, a reactive oxygen species known to be capable of promoting oxidative damage by excess PAR and (iii) containing more UV absorbing pigments. HPLC–MSn analysis showed both quantitative and qualitative differences in higher flavonoid contents: sun leaves contained 4.2-times more flavonoids than shade leaves and the quercetin:kaempferol ratio was also higher in the former. In addition, sun leaves also contained significant amounts of myricetin, which was detectable only in traces in shade leaves. Flavonols were mainly present as rhamnosides and in vitro tests of these compounds showed that quercetin and myricetin glycosides were much better singlet oxygen antioxidants than kaempferol glycosides. Thus a shift from monohydroxylated flavonols (kaempferol derivatives) towards dihydroxylated quercetin or trihydroxylated myricetin improves the singlet oxygen targeted antioxidant potential of leaves and potentially improves protection against photoinhibition by high PAR. On the other hand, experiments with pure test compounds also showed that multihydroxylated flavonol rhamnosides do not have better UV-B absorption than monohydroxylated ones. Thus the observed difference in flavonoid composition is not expected to contribute to the observed better UV-B absorbing capacity of sun leaves. Our data suggest that responses to high intensity PAR and to solar UV-B are closely connected and that UV-inducible flavonols play a key role in the successful acclimation of sun leaves to high PAR as efficient singlet oxygen antioxidants.