Thursday, October 22, 2009

So, why do leaves change colour?

If you, like me, said 'chlorophyll, something, something... ', read on.

Many people think deciduous trees lose their leaves because of cold weather and frost, a common misconception (umm.. my misconception), but actually the length of the day seems to be what determines when trees begin the process of changing colours and dropping leaves. During the shortened days of autumn there is not enough sunlight to outweigh the costs of chlorophyll maintenance and production, and the tree goes into hibernation mode. Once chlorophyll has stopped being produced, the red, golden and brown colours are "unmasked"; these colours are actually the natural colour of the leaves without the chlorophyll.

So's, why are the two pigments (carotene (responsible for yellow, orange and brown) and anthocyanins (red)) there in the first place? I mean, what is the evolutionary benefit of a leaf having these pigments? What is the benefit of having them show up there in the fall? It obviously just can't be a ho-hum, whateves decision on nature's part.

The pigments that produce yellow and orange leaves in the fall are present year-round, and help protect chlorophyll, the molecule at the heart of photosynthesis, from sunlight damage; when chlorophyll is broken down in the autumn those yellows and oranges become visible. In contrast, the red anthocyanins are produced only in the fall. It is a costly job of molecule building for the plant and an enigma to scientists, since the leaves will at that point soon be dropped entirely [BBC News]. (via discovermag)

Apparently no one knows why yet.

Possibly, the red colour could be a signal to insects that the leaves, and tree, are malnourished or have a built in chemical defense. This was postulated based on a single certain insect (aphids) that were less likely to grow to maturity living on red leaves rather than yellow or green leaves... this seems plausible, but yeah, where is the benefit to the tree?

Another possibility? David Wilkinson, an environmental scientist at Liverpool John Moores University who has published on the leaf colour debate, says that the work is not proof positive of the co-evolution theory (of insects and trees).

"I think the most likely explanation is that these [anthocyanins] are effectively sunscreens that allow the photosynthesis to continue as the machinery of photosynthesis is broken apart in the autumn.

"The idea of, as it were, 'the trees are talking to the insects', is wild and wacky and it would be rather nice if it were true.

"But I still have not seen anything that convinces me of the signaling."

(photo from Sarah used totally without permission - UPDATE: Now with permission!)

This goes under the wait until next year pile, as experiments can only be done at specific times... maybe we'll get an update soon.

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