Anthocyanin influence on light absorption within juvenile and senescing sugar maple leaves – do anthocyanins function as photoprotective visible light screens?

Foliar anthocyanins are hypothesised to function as photoprotective visible light screens, preventing over-excitation of the photosynthetic system, and decreasing the likelihood of photo-oxidative stress by absorbing green light and reducing the amount of light available to be absorbed by chloroplasts in deeper tissue layers. Chlorophyll fluorescence imaging was used to test the hypothesis that anthocyanins in the palisade mesophyll of juvenile and senescing sugar maple ( Acer saccharum Marsh.) leaves function as visible light screens by assessing their influence on light absorption profiles within leaves. We hypothesised that an effective anthocyanic light screen should reduce light absorption, particularly of green wavelengths, by chloroplasts in the spongy mesophyll. Both anthocyanic juvenile and senescing leaves absorbed greater amounts of green light than corresponding nonanthocyanic leaves. However, profiles of green light absorption by chlorophyll within anthocyanic leaves were not shifted to reflect reduced absorption of green light by spongy mesophyll chloroplasts. Further, the spongy mesophyll of both anthocyanic juvenile and senescing leaves absorbed proportions of green light equal to or greater than the spongy mesophyll of corresponding nonanthocyanic leaves. These results indicate that though they may provide a general source of photoprotection by reducing the total quantity of light available to be absorbed by chlorophyll, the anthocyanins in juvenile and senescing sugar maple leaves do not attenuate light in a manner consistent with that expected for an anthocyanic screen in the palisade mesophyll.