Anthocyanin influence on light absorption within juvenile and senescing sugar maple leaves

Red anthocyanin pigments are hypothesized to provide a variety of beneficial physiological functions to leaves, however the function of the anthocyanins synthesized transiently in developing and senescing leaves has not been extensively studied. One hypothesis is that anthocyanins function as photoprotective visible light screens, preventing over-excitation of the photosynthetic system and decreasing the likelihood of photooxidative stress by absorbing green-yellow 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 hypothesized 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, the spongy mesophyll of both anthocyanic juvenile and senescing leaves absorbed approximately 10% more green light than the spongy mesophyll of corresponding nonanthocyanic leaves. These results indicate that chloroplasts in the spongy mesophyll were not shaded by an anthocyanic light screen in the palisade mesophyll and suggest that anthocyanins in juvenile and senescing sugar maple leaves do not provide a significant source of photoprotective visible light screening.