16 April 2009

New clue that autumn leaf colours are defensive weapon

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A 2,000-year-old experiment has provided new evidence for the idea that the brilliant autumn colours of leaves are a defence against insect pests.
Autumn leaves

Red or dead: The red colour of leaves might indicate that they have poisonous chemical defences, little nutrients or some other quality detrimental to insects. Credit: iStockphoto

SYDNEY: A 2,000-year-old experiment has provided new evidence for the idea that the brilliant autumn colours of leaves in cold climates are a defence against insect pests.

Marco Archetti, an evolutionary biologist with Harvard University in Boston, USA, compared thousands of varieties of domesticated apple trees with wild apple trees to test the idea that the red colour in these leaves is a warning.

“It has been suggested that autumn colours may be a warning signal towards insects that use the trees [for food],” he said, “Like the bright colours of poisonous tropical frogs or butterflies. The meaning of the colour is ‘stay away from me, I’m dangerous’.”

Poisonous defences

Archetti said the red colour might mean that these leaves have poisonous chemical defences, little nutrients or some other quality detrimental to insects.

An ideal test of this hypothesis is to let populations of trees evolve with and without insect pests for many generations. If autumn colours were a signal to insects, we might expect red colouration to be lost where there were no insects, he said, because it is no longer needed.

“This experiment would take too long to perform, of course, but a similar test was already started two thousands years ago with the domestication of fruit trees,” said Archetti. “What I did was to check the results.”

As predicted by the hypothesis, the study found that vibrant autumn colours are common in wild apple trees but have been lost in cultivated varieties, which are relatively protected from attack by insects.

Central Asian apples

Archetti looked at wild apple trees in Central Asia, which were the ancestral stock for the domestic varieties we eat today.

He found that while 60% of these wild trees turn red, just 40% of domestic trees in that region do. With 2,170 domesticated varieties tested in Britain, the effect was even more profound, with less than 3% of trees turning red.

A second test found twice the number of aphids on yellow and green leaves than red, and confirmed that aphids on yellow and green leaves were more likely to reach reproductive age, hinting that the red colour was backed up by some other character of the leaves.

The study is published this week in the British Journal, the Proceedings of the Royal Society B.

Kevin Gould, a physiological ecologist from Victoria University of Wellington, in New Zealand, said that the study is the first to do a good experimental test of the anti-insect hypothesis.

However, he noted that this is a question surrounded by much controversy, as many competing theories exist as to the explanation for autumn colours.

Another explanation with considerable support is that red pigment is produced as a chemical sunscreen to shield leaves from the harmful effects of sunlight at cold temperatures. Some earlier studies have shown that red pigments can protect leaves from “light stress”, said Gould, allowing trees to re-absorb nutrients before the leaves drop.

Linda Chalker-Scott, a horticulturalist from Washington State University, in Puyallup, disagreed with the conclusions of the new study.

Sun exposure

She said that while it’s true that dying leaves may be less desirable, many plants have red foliage throughout the year, yet are still attacked by insects.

“Red colouration is associated with several environmental factors including Sun exposure and relative water content,” she said, adding that the red pigment might aid plants in retaining water in their leaves.

Other theories include that the red pigment allows leaves to stay on the tree longer, that it makes green insect pests more visible to predators, and that it acts as a protective antioxidant against metabolic damage.

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