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WorldwidePies t1_irrvcid wrote

The pigments mainly serve their purpose during the mature part of the leaf cycle.

Having carotenoids (orange pigments) and anthocyanins (red pigments) on top of chlorophylls allow the leaf photosystems to absorb light in the 500-600 nanometers range (green light), which would be lost had it only be chlorophylls.

Once the mature part is over and the shorter, colder days of autumn set in, the pigments are metabolised for food / energy. The chlorophylls are metabolised first, revealing the other pigments that were already there.

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Team_Ed OP t1_irs95fa wrote

Is there a mechanism by which the pigments only express themselves after a certain time?

I ask because you see maples and other plants that go a vibrant flower red first go through a yellow phase. (You can see this when maples have a colour gradient from green through yellow to red in the canopy in early fall).

I don’t really understand how a leaf that always has a vibrant red pigment inside wouldn’t go straight to red from green.

(Pondering it: Something like a pigment that’s sensitive to Ph might make sense, if removing the chlorophyll changes the Ph balance in a gradual way.)

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WorldwidePies t1_irsikkm wrote

Yes, but it’s not a timed response as much as a temperature response (and admittedly, the cold always comes after the warmth, from the leaves point of view).

We know that carotenoids are present all year round and are seen in autumn because of the chlorophylls degradation. In contrast, anthocyanins are mostly produced in autumn, in response to cold stress. This could be because these pigments have a protecting effect against cold.

About carotenoid synthesis.

About anthocyanins and cold stress.

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julie78787 t1_irsc4pe wrote

I’m also wondering why the leaves which are shaded seem to go red last. One of my maples is turning red on the exposed (southeast to southwest) side first, with the leaves further to the north, shaded by the canopy, still green.

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