Why leaves change colour in autumnApril 15th, 2009 - 1:58 pm ICT by ANI
London, Apr 15 (ANI): A new study of thousands of breeds of apple trees has tried to explain why leaves change colour in autumn.
The study bolsters a claim that red foliage evolved as a warning signal to insects in search of a winter home.
According to a theory first proposed by the late English evolutionary biologist, W.D. Hamilton, red-pigmented leaves are a clear sign of health and an indication that a tree has enough energy to fill future leaves with unappetising toxins.
He also believed that red foliage requires the production of new pigments, called anthocyanins. Other researchers suggest that these pigments might protect trees from the damaging effects of sun while their chloroplasts are breaking down. But Hamilton argued that photo-protection cannot explain why some trees flush red every autumn, while others don’t.
In a bid to put Hamilton’s claim to the test, Marco Archetti, an evolutionary biologist at the University of Oxford, compared wild apple trees that evolved in the presence of insects with domestic varieties, bred to produce sweet, large fruit and not to resist pests.
“If you take a species with autumn colours and you let it evolve without insects, then this population will no longer need autumn colours,” New Scientist quoted Archetti, as saying.
From analyses, Archetti found that just 2.8 percent of 2170 domesticated varieties of apples in England develop red leaves in the autumn. In Central Asia, where apples first evolved, 62 percent of wild apple trees turn red, while only two-fifths of domesticated breeds turn red.
The expert also found that aphids, a common pest of apple trees, tended to avoid the red-leafed trees in an apple orchard. When Archetti moved aphids to various apple trees in another orchard in the late spring, trees that would later turn red-leafed proved far more inhospitable to the insects than trees whose leaves stay green or turn yellow.
To make the case for the co-evolution between trees and insects even stronger, Archetti next looked at the prevalence of an aphid-transmitted disease called fire blight. If red leaves served as a warning to disease-carrying insects, he reasoned that varieties more susceptible to fire blight would be under stronger selective pressure to produce red leaves.
Indeed, red-leafed Central Asian breeds of apple trees tended to be far less susceptible than American apple trees, most of which are green-leafed in the autumn. Further, green-leafed American apple trees are even more susceptible to fire blight than the few red-leafed trees in the US.
The study has been published in the journal Proceedings of the Royal Society B. (ANI)
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