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I am curious to learn what are the biological mechanisms that cause leaves of deciduous plants to change colour? What happens to the chlorophyll?

What environmental phenomena (temperature/air pressure/length of sunlight) are they responding to?

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While we're waiting for an answer, I'll point out this page from the University of Illinois, which has a superb and comprehensive explanation. Essentially, it's a combination of pigments being revealed as the chlorophyll is broken down and pigments being produced for a variety of reasons. –  Oreotrephes Aug 1 '13 at 11:14
    
@Oreotrephes thank you for that, it is indeed these mechanisms and the triggers that I am interested in. –  user3795 Aug 1 '13 at 11:31
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up vote 2 down vote accepted

Here is a quite nice Chemistry of Autumn Leaf Color. And here is a less chemical, but more detailed explanation regarding the triggering of colour change, which is quite similar to the one from the University of Illinois that @Oreotrephes already gave.

As I understand it, during sunny days a lot of chlorophyll is broken down, but also built up. When after the summer the days get cold and dark, chlorophyll is built up slower (or not any more at some point) and you start to see the colour from the carotenoids and anthocyanins. A harsh change from sunny to cold and dark is supposedly bringing up the brightest colours. At some point, also the carotenoids break down and leaves get brown. The faster the change to cold and dark the larger is the frame between having all chlorophyll broken down and having the other colouring molecules broken down (-> bright colours).

But that's the amateurs explanation.

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