HOLBROOK, N. MICHELE1, TAYLOR S. FEILD1, and DAVID W. LEE2.* 1Department of Organismic and Evolutionary Biology, Harvard University, Cambridge, MA 02138; 2Department of Biological Sciences, Florida International University, Miami, FL 33199. - Why leaves turn red in autumn: The role of anthocyanins in senescing leaves of red-osier dogwood (Cornus stolonifera).
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. Here we provide
evidence for Cornus stolonifera that anthocyanins form a pigment layer
in the palisade mesophyll layer that decreases light capture by
chloroplasts. Measurements of leaf absorbance demonstrated that
red-senescing leaves absorbed more light of blue-green to orange
wavelengths (495 - 644 nm) compared to yellow-senescing leaves. Using
chlorophyll a fluorescence measurements, we observed that maximum PSII
photon yield of red-senescing leaves recovered from a high light
stress treatment whereas in yellow-senescing leaves failed to recover
after 6 hours of dark-adaptation which suggests photo-oxidative
damage. 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 nonphotochemical
light energy dissipation. 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. We found that red-senescing leaves
recovered rapidly to dark-adapted PSII photon yield following
illumination with blue light. However, red light induced a similar,
prolonged decrease in PSII photon yield in both red- and
yellow-senescing leaves. 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.
Key words: anthocyanin, Cornus stolonifera, fluorescence, leaf senescence, nitrogen retranslocation, photoprotection