Fall – a blazon landscape comes alive with autumn’s canvas of dazzling red, orange, yellow, green, and violet, in seemingly endless combinations. Withering and falling garments marks the end of an old chapter and the beginning of a new in the book of nature. Fall is truly one of my favorite times of the year; my imagination melts into the beauty and idea that we are always surrounded by scintillating points of radiant light.
We ponder the mystery: what is responsible for color transformation in leaves? What’s the chemistry behind it all? Beneath the surface lies a multiverse of biochemical activities giving rise to these enthralling appearances. But according to chemistry, the dynamic factor behind color change is sunlight! If we sit back for a moment and think of the importance of sunlight, we would come to recognize the sun as an electromagnetic power station which drives EVERY action on the planet and space, from the tiniest movement of an insect, up to the whirling motion of the planets. Even when we talk to each other, we are using and exchanging solar energy.
To understand leaf color we have to look at what is responding to sunlight. Color is produced from a combination of pigment molecules of different classes. The color green is due to an abundance and production of chlorophyll (type a or b), which belongs to a class of pigments called porphyrins. One of the best known examples of a porphyrin is the red pigment and prosthetic, heme, found in hemoglobin. Both hemglobin and chorophyll share the same proto-porphyrin ring system. What gives different colors? While hemoglobin has porhphyrin complexed to an Fe atom, chorophyll’s porphyrin ring is complexed to a Mg atom.
Chlorophyll is found in plant organelles, tiny vessels called chloroplasts, which is vital to photosynthesis – production of sugar and O2 from CO2 + Water. By using chlorophyll, a leaf acquires solar energy which it converts to a storage form of chemical energy (such as carbohydrates), which it distributes throughout the entire plant or tree.
Chorophyll allows plants to absorb energy from the sun by absorbing wavelengths in the blue and red regions. As a result, wavelengths in the green region are reflected. But here’s the catch: chlorophyll is not stable and decomposes easily when exposed to highamounts of sunlight such as summer time. A bit puzzling? Yes! But in order to make chorophyll,
plants require sunlight and warm temperatures. So in the summer time, plants are continuously generating new amounts of chlorophyll to maintain or increase their sugar concentration. As autumn approaches, the temperature and amount of sunlight decreases, nights get longer, and the amount of chlorophyll produced begins to decrease. In addition, during this time a corky layer of cells called the abscission layer seals the leaves off from the stem, blocking water and nutrient flow to and from the leaves. As a result, chorophyll breaks down and this causes leaves appear less green, while other pigments become more pronounced. Two classes of such pigments are Carotenoids and Flavonoids. These two are really responsible for the flare of fall’s foliage.
Carotene, a member of the group carotenoid, absorbs blue and blue-green light, which reflects and produces a yellow-colored leaf. Carotene is found abundantly in carrots and is known for its antioxidant effects! When found together, chlorophyll and carotene are responsible for producing green-colored leaves. Other carotenoids include lycopene and xanthophyll. These two compounds produce a red (as found in tomatoes) and yellow (found in egg yolks) color, respectively. While chlorophyll requires sunlight for its growth and production, carotenoids do not. As a result, these pigments are always present, though only seen when chlorophyll is being broken down or ceases production. “The amount of carotenoids is pretty much constant, unlike chlorophyll,” says Robert Moreau, a plant biochemist with the U.S. Department of Agriculture’s research service branch.
Organic Chemistry 101:
More conjugated double bonds = Absorption of light at higher wavelengths
Another pigment, and class, found in leaves is anthocyanins. This compound, unlike chlorophyll and carotene, is synthesized with sugar and proteins in the cell sap. When the sugar concentration is high, sunlight induces the reaction. This explains why only the parts of an apple exposed to the sun turn red (also as found in cherries, strawberries, and cranberries). These compounds absorb blue-green and green light. As a result, anthocyanin pigments reflect and produce fiery red-colored leaves. A unique characteristic of anthocyanins is their sensitivity to changes in pH, which ultimately affects its final color. If the cell sap is more acidic, the leaves appear red. If the cell sap leans toward the basic pH, the leaves appear purple or magenta.
Pigments: A Rainbow of Antioxidants…
In recent times, natural pigments have gained attention and new applications, as food makers seek to impart a bright hue to their products. As food-processing companies and consumers are looking to move away from artificial or synthetic coloring ingredients, naturally occurring pigments such as
anthocyanins and carotenoids present a viable and healthy alternative. Healthy because most pigments act as antioxidants and may have a protective effect on cells. An example of such natural alternative is cyanidin. Cyanidin (see at right) is a glucose-bound anthocyanin, which is
responsible for the color found in purple potato, red cabbage, and dark berries. When cyanidin is added to food or beverages, depending on the product’s pH, it imparts a color that varies dramatically from red to a dull blue.
What’s in the Red?
The science of red color changes in leaves is not that straightforward. While anthocyanins are induced to be synthesized, environmental conditions also contribute to its production. According to Emily Habinck from the University of North Carolina, Charlotte:
“Autumn leaves turn fiery-red in an attempt to store up as much goodness as possible from leaves and soil before a tree settles down for the winter.
The worse the quality of soil, the more effort a tree will put in to recovering nutrients from its leaves, and the redder they get.”
Much debate exists around the question: Why would a tree or plant use energy to synthesize anthocyanin as it prepares for winter? William Hoch at Montana State University, Bozeman, noted that leaf pigments may act as a sunscreen that helps to keep leaves on for an extended time. Hoch said that “a plant on a nutrient-poor soil is going to be more concerned about keeping the nutrients it has.” To this end, leaves turn redder — producing more pigments such as anthocyanins — to protect itself from sunlight, which may cause damage. “Damaged leaves will fall more quickly and rid the tree of a nutrient supply.”
So if you are working or maybe studying hard all day long, take the time out to take a walk (or go bicycling) outside to appreciate nature and chemistry. The spectacle of foliage can help relax and inspire you! Take a trip to your local park — maybe you live in a foliage zone as seen above. This link (www.fs.fed.us/fallcolors/2012/index.shtml) from the U.S. Forest Service on 2012 fall foliage offers fun and interesting things for both adults and kids on foliage progression at national parks and much more.
“The most beautiful thing we can experience is the mysterious. It is the source of all true art and all science. He to whom this emotion is a stranger, who can no longer pause to wonder and stand rapt in awe, is as good as dead: His eyes are closed.” —Albert Einstein