However, the induction of anthocyanin synthesis by high light in tissues that are unlikely to have an excess of carbon reserves, such as germinating seedlings, is inconsistent with the carbon overflow hypothesis ( Christie et al., 1994 Yanovsky et al., 1998). ![]() Anthocyanins are end products of the flavonoid pathway and the induction of anthocyanin synthesis has been suggested to result from carbohydrate “overflow” during the active recycling of photosynthetic proteins ( Matile, 2000). The prevailing view among plant physiologists is that anthocyanins are a nonfunctional by-product of leaf senescence ( Mohr and Schopfer, 1994 Archetti, 2000 Matile, 2000). Ford (1986) hypothesized that accumulation of anthocyanins may represent an excretion process to load toxins into the soon-to-be-discarded leaves. A recent study proposed that autumn anthocyanins have no direct physiological significance to plants but instead reflect co-evolutionary interactions with aphids, where anthocyanins act as “warning coloration” to deter herbivores ( Archetti, 2000). It has been unclear why anthocyanins are synthesized in autumn leaves just before they are shed ( Mohr and Schopfer, 1994 Archetti, 2000 Matile, 2000). Anthocyanins are a group of water-soluble flavonoids (glycosides of phenolic aglycons with a flavan C6-C3-C6 skeleton) produced in the cytoplasm and then transported into the vacuole ( Harborne, 1988 Marrs et al., 1995 Shirley, 1996). An exception to this is the winter accumulation of red carotenoids in the shoots of some conifers and leaves of a few evergreen flowering plants ( Ida et al., 1995). Unlike yellow and orange autumn leaves where chlorophyll breakdown unmasks the already present carotenoid pigments, most red leaves result from de novo synthesis of anthocyanins ( Matile et al., 1992 King, 1997 Kozlowski and Pallardy, 1997 Matile, 2000). Senescing leaves of many temperate deciduous plants turn brilliant red in autumn ( Wheldale, 1916 Sanger, 1971 Chang et al., 1989 King, 1997 Kozlowski and Pallardy, 1997). We suggest that optical masking of chlorophyll by anthocyanins reduces risk of photo-oxidative damage to leaf cells as they senesce, which otherwise may lower the efficiency of nutrient retrieval from senescing autumn leaves. ![]() However, red light induced a similar, prolonged decrease in PSII photon yield in both red- and yellow-senescing leaves. We found that dark-adapted PSII photon yield of red-senescing leaves recovered rapidly following illumination with blue light. A role of anthocyanins as screening pigments was explored further by measuring the responses PSII photon yield to blue light, which is preferentially absorbed by anthocyanins, versus red light, which is poorly absorbed. Because no differences were observed in light response curves of effective PSII photon yield for red- and yellow-senescing leaves, differences between red- and yellow-senescing cannot be explained by differences in the capacities for photochemical and non-photochemical light energy dissipation. Using chlorophyll a fluorescence measurements, we observed that maximum photosystem II (PSII) photon yield of red-senescing leaves recovered from a high-light stress treatment, whereas yellow-senescing leaves failed to recover after 6 h of dark adaptation, which suggests photo-oxidative damage. ![]() Measurements of leaf absorbance demonstrated that red-senescing leaves absorbed more light of blue-green to orange wavelengths (495–644 nm) compared with yellow-senescing leaves. Here, we provide evidence for red-osier dogwood ( Cornus stolonifera) that anthocyanins form a pigment layer in the palisade mesophyll layer that decreases light capture by chloroplasts. Why the leaves of many woody species accumulate anthocyanins prior to being shed has long puzzled biologists because it is unclear what effects anthocyanins may have on leaf function.
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