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11/03/2004
Tale of the Pigments
I’m starting this column on Tuesday, Election Day, having just
finished my 3-mile walk, with a stop at the polls to cast my vote.
It’s a bright sunny morning and the leaves that remain on the
trees are absolutely gorgeous, with brilliant reds, yellows and
oranges glowing in the sunlight. Our backyard is a golden
carpet, uniformly covered with the first major leaf-fall of the
season. For a golfer, it’s a mixed time of year. It’s great to be
out in the fresh air enjoying the colorful scenery, but it’s also a
time of balls lost under leaves, especially if one deviates off the
fairways as yours truly is wont to do. Hence it was a pleasant
surprise to find yesterday that the grounds crew at the Spooky
Brook course had done a remarkable job of sucking up the
leaves. I even ended up one golf ball to the good.
Regular readers will know that I shamelessly take any occasion
to mention my hole-in-one several years ago. Well, last week I
came within four feet of having another. The 164-yard drive was
dead on line with the cup and stopped just those measly four feet
short of rolling in. Then came an example of utter stupidity. I
was playing with three strangers and they offered to concede the
putt for a birdie, a term which is quite rare in connection with my
golf game. I declined the offer and, of course, missed the putt!
Oh well, I still enjoyed the brisk autumn air and the colorful
leaves. It’s an appropriate time of year to inquire as to what it is
that causes the color changes that occur. What determines the
color of an object? It’s the color of the light that’s reflected from
the object to our eyes. For transparent or translucent objects, it
could be the color that’s transmitted through the object combined
with the color of any reflected light.
In the spring and summer, the dominant pigment in most leaves
on trees in our area is chlorophyll, which absorbs red and blue
light from the white light of the Sun. The energy in this red and
blue light is used in photosynthesis, which converts carbon
dioxide from the air and water in the leaf into carbohydrates,
sugars and starches. These compounds are used by the plant
either to store energy or to grow the plant. The absorption of the
red and blue light by the chlorophyll leaves the green light to be
reflected; hence the green leaves. But chlorophyll isn’t very
stable and it gets torn apart in bright sunlight. This keeps the
plant busy making more chlorophyll to drive its growth.
Many plant leaves also contain carotene, a yellow pigment.
Carotene absorbs blue-green and blue light. If a leaf contains
both carotene and chlorophyll, it will still be a green color. The
energy that the carotene picks up from the light it absorbs is
transferred to the chlorophyll to help it do its job in
photosynthesis. Thus carotene is like an assistant energy
gatherer. Unlike chlorophyll, however, carotene is more stable
and, when chlorophyll dies out in the fall, the carotene remains
and the leaf is yellow.
Why does the chlorophyll die out in the fall? It relies on light
and warmth to do its photosynthesizing. The tree, sensing the
cooler weather and the shorter autumn days, decides that there’s
not going to be enough warmth and light to keep it growing. So,
it forms a sort of corklike growth between the branches of the
tree and the stems of the leaves. This new growth is like a plug
that cuts down on the nutrients supplied to the leaf, shutting
down its production of chlorophyll.
What about the red colors of autumn? Let’s bring in another
pigment, actually a class of compounds known as anthocyanins.
These compounds are different from the chlorophyll and
carotene, which are big molecules that aren’t soluble in the water
solutions that fill the plant cells. These compounds reside in disc
shaped structures called chloroplasts. The anthocyanins are
soluble in the sap and they absorb blue, blue-green and green
light, while reflecting red light.
The actual color of the solution of anthocyanins in sap depends
on the pH of the sap. If the sap is sufficiently acidic, the color is
bright red. If not so acidic, the color is more of a purple hue.
The anthocyanins are formed when the sugar concentration in the
sap is high. This must be a banner year in our area for sugar in
the sap of red maples, with especially bright red leaves. If the
leaf has a mix of carotene and anthocyanins, you can get the
classic autumn leaf with colors ranging from yellow through
orange and red on the same tree and even the same leaf.
Some foliage years are better than others; obviously, weather
plays a role. Chlorophyll doesn’t like low temperatures, which
encourage formation of the anthocyanins as long as they’re
above freezing. Bright sunshine hastens the demise of
chlorophyll but promotes the anthocyanins. If it’s dry, there’s
less water and the sap becomes more concentrated in sugar, again
encouraging formation of anthocyanins. Except for those red
maples and some other plants whose names I don’t know, it
seems to me this has been more of a yellow year, which would
be in line with the large amounts of rain we’ve had over the past
summer. I imagine that the sap would contain more water and
hence be less concentrated in sugar, lowering the amounts of
anthocyanins and the redness of the leaves.
One tree that produces gobs of leaves in our neighborhood is the
oak, of which we have a plethora. The leaves of the oak are very
drab, brownish in their fall attire and, in my opinion, in need of
some carotene or anthocyanins. Perhaps some biologist can
come up with a genetic modification to accomplish this.
I’m closing this column on the morning after the election and it
has just been announced that Senator Kerry is about to concede
that the predominant color on the election map is anthocyanin
red.
Allen F. Bortrum
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