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READING PASSAGE 17
Autumn leaves
Canadian writer Jay Ingram investigates the mystery of why leaves turn red in the fall
A
One of the most captivating natural events of the year in many areas throughout North America is the
turning of the leaves in the fall. The colours are magnificent, but the question of exactly why some trees turn
yellow or orange, and others red or purple, is something which has long puzzled scientists.
B
Summer leaves are green because they are full of chlorophyll, the molecule that captures sunlight converts
that energy into new building materials for the tree. As fall approaches in the northern hemisphere,
the amount
of solar energy available declines considerably. For many trees – evergreen conifers being an exception – the
best strategy is to abandon photosynthesis* until the spring. So rather than maintaining the now redundant
leaves throughout the winter, the tree saves its precious resources and discards them. But before letting its
leaves go, the tree dismantles their chlorophyll molecules and ships their valuable nitrogen back into the twigs.
As chlorophyll is depleted, other colours that have been dominated by it throughout the summer begin to
be revealed. This unmasking explains the autumn colours of yellow and orange, but not the brilliant reds and
purples of trees such as the maple or sumac.
C
The source of the red is widely known: it is
created by anthocyanins, water-soluble plant pigments reflecting
the red to blue range of the visible spectrum. They belong to a class of sugar-based chemical compounds also
known as flavonoids. What’s puzzling is that anthocyanins are actually newly minted, made in the leaves at the
same time as the tree is preparing to drop them. But it is hard to make sense of the manufacture of
anthocyanins – why should a tree bother making new chemicals in its leaves when it’s already scrambling to
withdraw and preserve the ones already there?
D
Some theories about anthocyanins have argued that they might act as a chemical defence against attacks by
insects or fungi, or that they might attract fruit-eating birds or increase a leafs tolerance to freezing. However
there are problems
with each of these theories, including the fact that leaves are red for such
a relatively short period that the expense of energy needed to manufacture the anthocyanins would outweigh
any anti-fungal or anti-herbivore activity achieved.* photosynthesis: the production of new material from
sunlight, water and carbon dioxide.
E
It has also been proposed that trees may produce vivid red colours to convince herbivorous insects that they
are healthy and robust and would be easily able to mount chemical defences against infestation. If insects paid
attention to such advertisements, they might be prompted
to lay their eggs on a duller, and presumably less
resistant host. The flaw in this theory lies in the lack of proof to support it. No one has as yet ascertained
whether more robust trees sport the brightest leaves, or whether insects make choices according to colour
intensity.
F
Perhaps the most plausible suggestion as to why leaves would go to the trouble of making anthocyanins
when they’re busy packing up for the winter is the theory known as the ‘light screen’ hypothesis. It sounds
paradoxical, because the idea behind this hypothesis is that the red pigment is made in autumn leaves to protect
chlorophyll, the
light-absorbing chemical, from too much light. Why does chlorophyll need protection when it
is the natural world’s supreme light absorber? Why protect chlorophyll at a time when the tree is breaking it
down to salvage as much of it as possible?
G
Chlorophyll, although exquisitely evolved to capture the energy of sunlight, can sometimes be overwhelmed
by it, especially in situations of drought, low temperatures, or nutrient deficiency. Moreover,
the problem of
oversensitivity to light is even more acute in the fall, when the leaf is busy preparing for winter by dismantling
its internal machinery. The energy absorbed by the chlorophyll molecules of the unstable autumn leaf is not
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immediately channelled into useful products and processes, as it would be in an intact summer leaf. The
weakened fall leaf then becomes vulnerable to the highly destructive effects of the oxygen created by the
excited chlorophyll molecules.
H
Even if you had never suspected that this is what was going on when leaves turn red, there are clues out
there. One is straightforward: on
many trees, the leaves that are the reddest are those on the side of the tree
which gets most sun. Not only that, but the red is brighter on the upper side of the leaf. It has also been
recognised for decades that the best conditions for intense red colours are dry, sunny days and coo nights,
conditions that nicely match those that make leaves susceptible to excess light. And finally, trees such as
maples usually get much redder the more north you travel in the northern hemisphere. It’s colder there, they’re
more
stressed, their chlorophyll is more sensitive and it needs more sunblock.
I
What is still not fully understood, however, is why some trees resort to producing red pigments while others
don’t bother, and simply reveal their orange or yellow hues. Do these trees have other means at their disposal
to prevent overexposure to light in autumn? Their story, though not as spectacular to the eye, will surely turn
out to be as subtle and as complex.
