Autumn foliage can save trees’ lives
Under the microscope, the structure of a leaf is revealed as a multitude of chambers – “cells.” In a green leaf, most cells, rich in chlorophyll, are green. But within the newly fallen red leaf I was just observing, many of the cells were empty; only a few were green; the vast majority were pink to red.
The yellows and oranges of autumn leaves were there all summer, but masked by the more abundant chlorophyll. The reds, however, are newly made … just as the leaf is dying.
Vegetables are not only food, but medicine. Chemicals in leaves can scarf up – scavenge – free radicals, super-reactive molecules we create as byproducts when we “burn” food for energy. Reacting with DNA, proteins and other molecules, free radicals cause damage that can lead to all sorts of disease and disorders (not least of which is the “disorder” known as growing old).
Among the molecules that protect us from free radical damage, some of the most powerful are anthocyanins (an-tho-SIGH-uh-nins), the family of pigments that bestow a red color on leaves (and grapes and other fruits).
For more than a decade, David Lee of Florida International University has been trying to decipher why trees would expend energy and resources to create anthocyanins in leaves that would soon be dead.
After 4 billion years of evolution, life is far ahead of human technology in harnessing the forces of nature. For us, extracting hydrogen from water is a costly, energy-intensive process. Yet, by using chlorophyll to harness the energy of sunlight, plants can do it with ease. Chlorophyll is a very powerful molecule.
Now the days are growing short. There’s less light; temperatures are dropping. Because they lose water (which a tree cannot replace if the ground freezes) and catch the wind (putting the tree in danger of toppling), leaves in winter are a liability.
Chlorophyll and the other molecular machines that perform photosynthesis are made largely of proteins. Of the three basic molecules of life – fats, carbohydrates and proteins – the last are usually in shortest supply.
Over billions of years, life has evolved to recycle. Before dropping their leaves, trees disassemble their photosynthetic machinery and ship the proteins into trunk and roots for storage until spring.
Dismantling a bomb is serious work. Even after the bomb’s been disassembled, the parts can still be dangerous, liable to explode on their own.
Taking apart the machinery of photosynthesis can also be hazardous. Even in pieces, the chlorophyll molecule can absorb sunlight and trigger chemical reactions. But the machinery that normally absorbs the captured energy has also been disassembled; the absorbed energy is thus available for all sorts of mischief … creating free radicals, for instance.
Transported along with the recycled proteins, free radicals can do the same sort of damage to cells in roots and trunk as they do to human cells.
The recycling stage of a leaf, just before it falls, is thus a very dangerous time.
Lee and his colleagues have found that the brilliant red anthocyanins function as excellent sunscreens. Not for ultraviolet, for which the leaf already has sunblock, but for visible light, light that was essential in summer, but could now trigger the production of free radicals.
But even with sunblock, the bright autumn sunshine can still power the creation of free radicals. Anthocyanins now do double duty – they scavenge free radicals, a job at which they’re several times more efficient than the better-known scavengers, vitamins C and E.
Evidence is accumulating that the reds of autumn are, indeed, arboreal life-savers.
Alan Stahler trained as a biologist and is an amateur astronomer. He teaches enrichment classes for children and adults at Sierra Friends Center. His science programs can be heard at noon on alternate Tuesdays on KVMR-FM (89.5).
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