Autumn leaves
Autumn leaves
New Zealand science writer Callum Rennick investigates the mystery of why leaves turn red in autumn
A
AAcross much of North America, few natural spectacles rival the changing of the leaves each autumn. Splendid as those colours are, scientists have puzzled for a very long time over a simple question: why should certain trees shift to yellow or orange while others take on shades of red or purple?
B
BThrough the summer, leaves stay green because they are packed with chlorophyll, the molecule that traps sunlight and turns its energy into fresh building materials for the tree. Once autumn arrives in the northern hemisphere and the supply of solar energy falls away sharply, most trees - cone-bearing evergreens being the one exception - do best to shut photosynthesis* down until spring. Keeping a set of now-useless leaves alive all winter would only waste the tree's reserves, so instead it sheds them; but not before it breaks apart their chlorophyll and draws the valuable nitrogen locked inside back into the twigs. With the chlorophyll gone, pigments that it had masked all summer are finally exposed. That uncovering accounts for the yellows and oranges of autumn, yet it cannot explain the vivid reds and purples of trees like the maple or the sumac.
C
CWhere the red comes from is no secret: it is the work of anthocyanins, water-soluble pigments that throw back the part of the visible spectrum running from red to blue. Chemically, they are sugar-based compounds of the group called flavonoids. The strange part is their timing - these anthocyanins are freshly manufactured, produced in the leaf at the very moment the tree is getting ready to cast it off. It is difficult to see the logic: why would a tree go to the trouble of building brand-new chemicals in a leaf when it is already rushing to strip out and rescue the pigments that are there?
D
DA number of explanations have been put forward for anthocyanins. They may, some have argued, work as a chemical shield against insects or fungi; or draw in the birds that eat fruit; or make a leaf better able to withstand freezing. Each idea, though, runs into trouble. One difficulty is that leaves stay red for such a brief spell that the energy poured into producing the anthocyanins would cost the tree more than any protection against fungi or plant-eaters could ever be worth.
E
EAnother suggestion is that a tree flushes its leaves a bright red to signal to plant-eating insects that it is vigorous and perfectly capable of fighting off an infestation with its chemical arsenal. An insect that took such advertising seriously might then lay its eggs elsewhere, on some duller and presumably less well-defended tree. The weakness of this account is simply that nothing yet backs it up: no one has established either that the healthiest trees carry the brightest foliage or that insects really do pick their hosts by how intense the colour is.
F
FThe most convincing proposal of all may be the one known as the 'light-screen' hypothesis. At first it seems to make no sense, for its central claim is that the red pigment forms in autumn leaves precisely in order to guard chlorophyll - the very chemical whose job is to soak up light - from receiving too much of it. Why would the finest light-absorber in nature ever need shielding? And why bother shielding it just as the tree is dismantling it to recover as much as it can?
G
GFor all the elegance with which chlorophyll has evolved to harvest the sun's energy, there are times when that energy simply overwhelms it - most of all during drought, in the cold, or when nutrients are scarce. This vulnerability to excess light grows sharper still in autumn, while the leaf is busy taking its inner machinery apart. In a healthy summer leaf, the energy that chlorophyll captures is put straight to work; in the failing autumn leaf it is not, and the leaf is then left exposed to serious harm from the oxygen that the over-excited chlorophyll sets loose.
H
HClues to all this are not hard to find. The plainest is that, on any one tree, the leaves that redden most deeply are those on the side that catches the most direct sun, and on a single leaf the colour runs richest across its upper surface. It has likewise been known for decades that the deepest reds come out when clear, dry days are followed by cold nights - exactly the conditions under which chlorophyll is left most exposed to surplus light. And in maples and their relatives, the show grows steadily more brilliant the further north one goes, where the sharper cold puts the trees under greater stress, leaving their chlorophyll more delicate and more in need of a sunblock.
I
IOne puzzle, though, is still unsolved: why do some trees go to the effort of making red pigment while others simply let their yellows and oranges show through? Might those other trees have some different way of guarding against too much light in autumn? Less of a feast for the eye though it may be, their story is bound to prove every bit as subtle and involved.
* photosynthesis: the production of new material from sunlight, water and carbon dioxide.
Question 1 - 5
Which paragraph contains the following information?
1
a reference to how long the cause of red autumn colour has remained unknown
A
B
C
D
E
F
G
H
I
J
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