Winter leaf reddening can be observed in evergreen herbs, shrubs, vines, and trees in Mediterranean, temperate, alpine, and arctic regions, and can persist for several months before dissipating with springtime warming. Yet, little is known about the functional significance of this color change, or why it occurs in some species but not others. Here, the physiological ecology associated with winter leaf reddening in six angiosperm, evergreen species exhibiting winter leaf reddening in the Southern Appalachian mountains, USA are compared and contrasted to those of five co-occurring, angiosperm evergreen species maintaining green leaves during winter.
Leaves of winter-red and winter-green species exhibit similar photosynthetic capacities during summer and winter. However, high light leaves of winter-red (anthocyanic) species consistently exhibit symptoms of shade acclimation during winter relative to co-occurring, high light leaves of winter-green species, consistent with a photoprotective function of red pigments. These include: higher pre-dawn Fv/Fm, decreased sustained xanthophyll de-epoxidation, lower chlorophyll a/b, higher total chlorophyll content, and lower photosynethetically-saturating PPFD. These differences abate during the summer when leaves are green. On average, winter-red species are more likely to be drought tolerators than winter-green species, though this is clearly not always the case. Winter-red and winter-green species do not differ in leaf nitrogen or low molecular weight antioxidants (LMWA) during summer or winter.
In conclusion, these results lend strong support to a photoprotective function of winter leaf reddening in angiosperm evergreen species. Red-leafed species do not appear to be photosynthetically handicapped relative to green-leafed species. Rather, synthesis of red pigments appears to be an alternative photoprotective strategy to sustained non-photochemical quenching during winter. It is not clear why a species would utilize one form of photoprotection rather than the other.