Flavonols do not affect aphid load in green or senescing birch leaves but coincide with a decrease in Photosystem II functionality.

Instead of red anthocyanins, birches synthesise colourless (to human eye), UV-absorbing flavonols during autumn senescence. To test if flavonols protect against insects, and if leaves with high or low amounts of flavonols differ in their photosynthetic functions, aphid-free and aphid-infested green and senescing birch leaves were collected from outdoor-grown trees and analysed. Photosynthetic parameters were greatly affected by the leaf chlorophyll content (i.e. the phase of senescence). Photochemical quenching and the amount of functional Photosystem I decreased linearly with chlorophyll content, while FV/FM (Photosystem II functionality) decreased strongly only at the end of senescence. Non-photochemical quenching of excitation energy (NPQ) increased towards the end of senescence. However, no significant differences in the total flavonol amounts, nor in individual flavonol species, were found between aphid-free and aphid-infested leaves, suggesting that flavonols play no role in defence against aphid herbivory. Interestingly, both green and senescing leaves with a high flavonol content showed low FV/FM values. High flavonol content slowed down PSII photoinhibition and improved recovery, but only in green leaves. Previously, we proposed that anthocyanins provide an additional sink for photosynthates at the nitrogen resorption phase during autumn senescence, and the present data may suggest that flavonol synthesis plays a similar role.