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Van den Berg, Abby Katrien

Plant Biology

2007

PhD

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. Three individual studies were conducted with sugar maple (Acer saccharurn Marsh.) leaves to investigate the principal hypotheses of the physiological functions of anthocyanin pigments - that they provide photoprotection from photooxidative stress through visible light screening or antioxidant activity. The first study tested the hypothesis that anthocyanins in sugar maple leaves provide photoprotection through visible light screening by monitoring the response and recovery of chlorophyll a fluorescence parameters in senescing red and non-senescing green leaves exposed to high irradiances and low temperatures. Light was applied to either the adaxial (D) or abaxial (B) leaf surface to assess physiological responses both with and without potential light attenuation by anthocyanins located in the palisade mesophyll (PM). Following treatment, B- and D-illuminated red leaves recovered to the same level of pretreatment photochemical efficiency. In addition, for all chlorophyll fluorescence parameters measured, the difference between B- and D-illuminated leaves was not greater in red than in green leaves. These results indicate visible light absorption by anthocyanins in the PM did not provide significant photoprotection above the basic screening of light passing through these cells and suggest visible light absorption by anthocyanins may not be a significant source of photoprotection in senescing sugar maple leaves.
The visible light screen hypothesis was examined more directly in the second study, in which chlorophyll fluorescence was measured in both juvenile and senescing sugar maple leaves to assess the influence of anthocyanins in the PM on light absorption profiles within leaves. An effective anthocyanic light screen would be expected to reduce light absorption, particularly of green wavelengths, by chloroplasts in the spongy mesophyll (SM). However, the SM of both anthocyanic juvenile and senescing leaves absorbed approximately 10% more green light than the SM of corresponding nonanthocyanic leaves. These results indicate that chloroplasts in the SM of both leaf types were not shaded by an anthocyanic light screen in the PM and provide further evidence that anthocyanins in sugar maple leaves do not provide a significant source of photoprotective visible light screening. The third study examined the hypothesis that anthocyanins scavenge free radicals by measuring the contribution of anthocyanins to the antioxidant capacity of juvenile and senescing sugar maple leaves. The antioxidant activity of juvenile and senescing anthocyanic leaves was not greater than that of corresponding nonanthocyanic leaves. These results indicate anthocyanins did not enhance total antioxidant activity relative to the activity observed in nonanthocyanic leaves and suggest anthocyanins do not provide a significant, additional source of photoprotection in anthocyanic leaves through free radical scavenging. The results of these studies provide evidence that anthocyanins in both juvenile and senescing sugar maple leaves do not provide a significant, additional source of photoprotection through either visible light screening or antioxidant action.