Why are Autumn Leaves so colorful?

Most people know, at least in broad brush strokes, HOW? autumn leaves turn out to be so colorful. I was wondering WHY? and Why so many colors?


Just to clear up the HOW? part...

At the most general level, when the weather turns colder, deciduous (leafy) trees stop making green pigment and the remaining underlying colors are revealed.


More specifically, as the in the late summer, the veins in leafy trees start to constrict and the production of the compound responsible for photosynthesis, chlorophyll, is down regulated through the reduction of a single protein FtsH6. As the chlorophyll synthesis is reduced, the remaining chlorophyll is converted to a colorless set of compounds called NCCs (nonfluorescent chlorophyll catabolites) As the chlorophyll ebbs, three specific pigments are revealed:

• Carotonids are the yellow and orange colors in leaves. They are present all year long and concentrated in structures called plastids. Note that this is a class of compounds and lots of the yellow colors in nature are due to Carotonids-like carrots, bananas and egg-yolks.


• Anthocyanins are responsible for the red and purple colors in leaves. Anthocyanins are produced in the late summer and early fall in the process of the break-down of sugars and energy production. As one might expect, many fruits are full of anthocyanins-like apples and cranberries.
  
• Cell Walls are kind of brown..the brown color left behind is due to the "super-structure" of the leaves.

As an uber-geeky aside: From a chemical perspective these colors are generated from chromophores in which an two general ways. Many biological chromophores produce color by complexation with a metal atom. Chlorophyll contains a chemical structure called a porphyrin which can "grab on" to a magnesium atom to absorb specific wavelengths. Similarly Hemoglobin in blood complexes with iron to produce red pigment. Other biological pigments are caused by chemical structures containing a series of conjugated double bonds. This is the case with both carotenoids and anthrocyanins. Interestingly, the anthrocyanins produce such a wide range of colors for such a small set of compounds, they use other mechanisms as well. Specifically, there is a pH dependent metal complexation with Aluminum or Iron that can produce some blue colors in flowers.

So that's the HOW?

But here's the interesting part, WHY? do leaves turn color in Autumn.

Of course, "Colors are what is left over" may be an acceptable explanation. Or you might want to go with the "For our viewing pleasure...". However, it seems pretty complicated and energy intensive to by coincidental. It turns out that there are three main biological theories to account for Autumn Leaf Beauty.

From an evolutionary explanation perspective, the real question is around the pigments in anthocyanins. Yellows and carotenoids are easy to explain-in fact, it is easier to leave them around than to remove the pigment in the fall, so the yellows are revealed. But why produced a new pigment in the Autumn?

1. Photoprotection-The photoprotection theory maintains that anthocyanins protect the leaves from photooxidation and photdegradation at low temperature and low light levels. This is especially important in the Autumn because the energy production (in the absence of chlorphyll) is low, and the organism has to protect those mechanisms that foster nutrient readsorption. Most evolutionary botanists seem to favor this rather boring idea...

2. Co-Evolution-The basic idea in coevolutionary theory is that leaves have adapted with colors that protect them from certain predators like aphids by producing colors that repel those insects. It isn't clear why aphids don't like red... There is strong evidence for this theory in the historical record. In America, the mountain ranges run predominantly north to south. As the seasons change, animals (and insects) migrate north to south and consequently, trees have evolved, and maintained this defense mechanism. There are lots of red and purple leaves in North America. By contrast, in Europe, mountain ranges are mostly east to west. As the seasons changes and ice ages come and go, species of trees and predatory insects didn't evolve this migratory adaptation. There are more yellow leaves in Europe. By the way, this theory (even in a more rational statement than mine) is still pretty controversial. I think the dominant view of this theory is that its "Wild and Wacky and wouldn't it be nice if it were true".

3. Alleopathy-Another possible influence for the production of anthocyanins is a competitive advantage that some plants might get by producing red and purple colors that may stunt the growth of nearby saplings and thus allow the main plant to maintain access to sunlight and soil nutrients.

On the other hand, you may prefer the explanation that in the Autumn-

Jack Frost
Who comes creeping in the night
When the moon is clear and bright?
Who paints the tree leaves red and gold
When the autumn days turn cold?
Up the hill and down he goes.
In and ou the brown corn rows.
Making music crackling sweet.
With his little frosty feet.
Jack Frost!