Ripe-red apples, blueberry pie, a carafe of Merlot and an autumnal dogwood tree. What do these have in common? Lunch al fresco, perhaps. Anthocyanins, definitely.
In climates such as ours in upstate NY, the shorter days and lower temperatures of September prompt all creatures to prepare for winter. Squirrels store nuts; bears gorge themselves and people burrow in cedar chests and closets to unearth wool sweaters. To ready themselves for the rigors of winter, most trees shed their leaves. Little photosynthesis factories, humming all summer long to manufacture nourishing sugars and starches, grind to a halt. Green chlorophyll infusing the leaves fades away to reveal butter yellows and ginger golds produced by carotenoids, pigments that were hiding in the leaf the whole time.
Rather than heedlessly casting their greenery to ground, the thrifty trees endeavor to retrieve as many nutrients as possible from each leaf before discarding it. Those with ample reserves will grow better and produce more fruit the following year. Oddly, though, in the midst of trying to salvage every spare molecule of nitrogen and phosphorus, some trees spend extra energy to make anthocyanins - pigments that create those luscious shades of red, pink and blue - in the very leaves that they're prepared to shed. This is exemplary thrift?
As a leaf's ability to photosynthesize dwindles with the onset of fall's crisp temperatures, the light that nourishes becomes the light that destroys. No longer able to photosynthesize, cells die in the process of trying to shuttle nutrients to others tissues. Rather than lose the whole of its potential pantry, a tree spends a portion of it to create a sunscreen of anthocyanin. Beneath a parasol of pigment, the lines of transportation from leaf to twig remain intact. This hot flash of crimson and scarlet appears to be reserved for colder climes, which seems to make sense. Oaks from the northern US flaunt reddened leaves whereas oaks from Europe, generally, do not. In one study, dogwoods growing in sun were ruddier than those growing in shade. Conversely, there is a great deal of color variation even between trees of the same species within the same area and no one is yet sure why. Such variations raise questions for plant breeders. For example, would a maple selectively bred for brighter fall color be inherently hardier?
The question remains, "If anthocyanins are so important, why don't all tree species growing in cold climates make them?" Birch, sassafras, and poplar, tossing their blonde autumn tresses, seem to need no extra protection from the sun. Some have speculated that trees that require more light in summer are less likely to be damaged by it in fall. Apparently, no one has informed the sun-worshiping sumacs, which blush scarlet at the thought. The multitasking anthocyanins also appear in response to a number of stress triggers, such as drought, or wounds. So, the answers are elusive.
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This needn't distress us. As autumn sets the hills aflame and we exclaim over vivid tints to vie Clairol's shades, we have new clues to follow as we embark on our hunt for red October.
Questions, comments, or other feedback to Judy Sullivan.