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It is autumn out there. Green, brown, red, yellow, and mixed-colored leaves drop from the trees to the ground. What determines the colors of the leaves?

Wikipedia tells me

Leaf senescence is the cause of autumn leaf color in deciduous trees.

Ok. What does the color code? If a leaf is red, why is it red and not yellow or brown or green or mixed-colored?

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There is no color code for the leafs - the color results from biochemical reactions. Basically there are three colors: Green, yellow and red.

  • Green color is caused by the chlorophyll inside the chloroplasts, when the leafs are active in photosynthesis.
  • Yellow color is caused by Carotenoids, which are present in the leafs all the time, but are masked by the chlorophyll. The carotenoids become visible when the amount of chlorophyll in the leafs is reduced, which usually happens in the fall.
  • The red color is caused by Anthocyanins, which are usually not present in the leafs. They are made during the late summer and can also mix up with the carotenoids (color mix) to get intermediate colors.
  • Brown color is caused by the cell wall of dry leafs which have no other color.

For further reading these articles are interesting:

  1. Unravelling the evolution of autumn colours: an interdisciplinary approach.
  2. The Science of Color in Autumn Leaves

There is also the hypothesis around, that the bright colors are a warning sign to insects, this is called the co-evolution theory of autumn colors. The color should warn the insects that the tree has a high level of defense and thus reduce its parasitory load. You can read more about this hypothesis here:

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What

There are two theories according to Jennifer Wells, Leaf Color Change:

  1. Competition theory - states that nutrients are redistributed to important parts of the plant. Lack of nutrients in the leaves will decrease the chlorophyll synthesis thus the leaves change their color. The redistribution does not happen equally that's why some leaves are multicolored (see Ecology and the Environment at Cornell Blogs).

  2. Mutual enhancement - states that specific hormones (senescence factors) work in conjunction and cause the color change.

All these happen under the influence of light - the absorbed dose of light is modulated by internal factors. The result is a reversed balance between carotenoids and chlorophyll (increase in carotenoids).

These carotenoids (fat-soluble) turn the leaf into orange and yellow depending on their concentration. There are also anthocyanins (water-soluble) which are responsible for red coloration.

Why

Chlorophyll and its derivatives, if not carefully processed by organized biochemical pathways in green cells, can be toxic when illuminated.

According to: Howard Sidney Thomas, Leaf senescence and autumn leaf coloration. Check also Figure 3 from this link (on the last page).

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