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Fall Leaf Color: Why No Blue Leaves?

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From: Gary Fleeger
City:
Marysville, PA
Can you tell me why when the leaves turn color in the fall, there are no blue leaves. We have virtually every other color of the spectrum, but no blue. There are purple and maroon leaves, and these two color have a component of blue in them, so there is blue pigment in there somewhere. But leaves are never blue. We do have blue in nature, such as blue flowers, so there are blue pigments. I am aware that the colors come from the pigments Carotenoids and Anthocyanins. The purple comes from the anthocyanins. And that they are present year round, but masked by the chlorophyll (whose green also contains a component of blue, along with yellow) in summer. But I have been unable to determine why the color blue never appears by itself, as every other color does. I have done Internet searches (which is how I found your web site), but have never found anything addressing the lack of blue leaves. In addition to the chemical reason, I am also curious if there might be some evolutionary reason causing the chemistry that results in every color but blue (or prevents blue). A friend of mine (a former park naturalist) who was the first to whom I had inquired about this hypothesized that blue pigment might filter out a wavelength that the chlorophyll needs. So it would be evolutionarily BAD to be blue, if you wanted to photosynthesize.

Perhaps yellow pigment would not do that, for some reason, but blue pigment would.

 
Extension Message
From: Jay Hayek
Extension Specialist, Forestry
Natural Resources & Environmental Sciences
jhayek@illinois.edu
Greetings Gary:

Okay, I'll give your question a shot :-) However, please note that I am definitely not a tree/plant physiologist.

Source: Pallardy, S.G. 2008. Physiology of Woody Plants, 3rd Edition.

Source: http://scifun.chem.wisc.edu/CHEMWEEK/fallcolr/fallcolr.html

Energy capture and CO2 fixation occur in the chloroplasts of higher order plants – chlorophylls (chlorophyll-a and chlorophyll-b) are the most important pigments found in chloroplasts. Additional “accessory” pigments present in plants include carotenes such as B-carotene and xanthophylls.

The absorption spectra of the chlorophylls indicate two peaks of absorption in the visible region, one in the blue and one in red wavelengths, 450-495 and 620-750 nm, respectively. The relative absence of absorption in the green region clearly illustrates why these pigments give plants and much of the world a greenish color (i.e., green wavelengths are reflected and not absorbed by the chlorophylls). Leaves of a few varieties of trees such as copper beech and Crimson King Norway and Japanese maples are red or purple because of the presence of anthocyanin pigments that occur in the cell sap of the vacuole rather than in the chloroplasts. Anthocyanin pigment, with their intense absorption of solar radiation in the photosynthetically active blue wavelengths, protects the photosynthetic apparatus from oxidative damage.

Anthocyanin pigments, responsible for the pink, red, and purple colors are related to the carbohydrates and carbohydrate accumulation favors their formation. Anthocyanins and glycosides formed by reactions between various sugars and complex cyclic compounds called anthocyanidins – they are water soluble and usually occur in the cell sap of the vacuole. Anthocyanins usually are red in acid solution and may become purplish to blue as the pH is increased. The amount of anthocyanin pigments depends primarily on the possession of certain hereditary potentialities for their production, but environmental factors also have an influence. As chlorophyll synthesis stops, the chlorophyll already present begins to decompose chemically and the newly formed anthocyanins are unmasked. In those species that do not form anthocyanin pigments, the autumn breakdown of chlorophyll unmasks the relatively more stable yellow carotene and xanthophylls pigments; or there may be an admixture of red anthocyanin pigment with yellow carotene to give a bright orange color, as in some species of maple. In other species both chlorophyll and carotenoids disintegrate simultaneously and new carotenoids are synthesized. Thus, by disintegration of green pigments and the unmasking of yellow ones, the formation of red pigments, or all three, the leaves may assume various shades of yellow, orange, crimson, purple, or red.

Carotenes (cell membranes; chloroplasts): absorb blue-green and blue wavelengths; thus the light reflected appears yellow to our eyes.

Anthocyanins (not attached to cell membranes; cell sap): absorb blue, blue-green, and green wavelengths; thus the light reflected appears reddish to our eyes.

Chlorophylls (cell membranes; chloroplasts): absorb red and blue wavelengths; thus the light reflected appears green to our eyes.

Carotene & Chlorophyll: absorb red, blue-green, and blue wavelengths; thus the light reflected appears green to our eyes.

So, a very general answer to your question may be the general theme associated with the major pigments found within native plant leaves: blue wavelength light is generally absorbed and not reflected. Hence, there are few true blue-colored summer or autumn leaves. Of course, there are exceptions.

I’m sure I will receive constructive feedback if I unintentionally misrepresented any information contained in my explanation :-)

Sources: Pallardy 2008 and scifun.chem.wisc.edu

 
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