What they did not expect was that their model would also explain the colors of other photosynthetic forms of life too. Their findings point to an evolutionary principle governing light-harvesting organisms that might apply throughout the universe. They also offer a lesson that — at least sometimes — evolution cares less about making biological systems efficient than about keeping them stable.
The mystery of the color of plants is one that Nathaniel Gabor , a physicist at the University of California, Riverside, stumbled into years ago while completing his doctorate. Extrapolating from his work on light absorption by carbon nanotubes, he started thinking of what the ideal solar collector would look like, one that absorbed the peak energy from the solar spectrum.
In , Gabor and his colleagues modeled the best conditions for a photoelectric cell that regulates energy flow. But to learn why plants reflect green light, Gabor and a team that included Richard Cogdell , a botanist at the University of Glasgow, looked more closely at what happens during photosynthesis as a problem in network theory.
The first step of photosynthesis happens in a light-harvesting complex, a mesh of proteins in which pigments are embedded, forming an antenna. The efficiency of this quantum mechanical first stage of photosynthesis is nearly perfect — almost all the absorbed light is converted into electrons the system can use.
But this antenna complex inside cells is constantly moving. Quick fluctuations in the intensity of light falling on plants — from changes in the amount of shade, for example — also make the input noisy.
The delayed greening has been studied as an adaptive strategy for the plant. One line of thought is that the young leaves are not yet performing photosynthesis, so they are not capturing energy from the sun and making food, therefore they are without much nutritive value to the plant.
There also is little nutritive value to an herbivore. The plant is investing energy to grow the new leaf, so avoidance of herbivory allows the investment a better chance to mature. If being red decreases the risk that the new growth will be eaten by herbivorous animals, then the plant has used a successful strategy. The chlorophyll breaks down, the green color disappears, and the yellow to orange colors become visible and give the leaves part of their fall splendor.
At the same time other chemical changes may occur, which form additional colors through the development of red anthocyanin pigments. Some mixtures give rise to the reddish and purplish fall colors of trees such as dogwoods and sumacs, while others give the sugar maple its brilliant orange.
The autumn foliage of some trees show only yellow colors. Others, like many oaks, display mostly browns. All these colors are due to the mixing of varying amounts of the chlorophyll residue and other pigments in the leaf during the fall season.
As the fall colors appear, other changes are taking place. At the point where the stem of the leaf is attached to the tree, a special layer of cells develops and gradually severs the tissues that support the leaf. At the same time, the tree seals the cut, so that when the leaf is finally blown off by the wind or falls from its own weight, it leaves behind a leaf scar. These creatures pollinate the plants. Pollination is a process which allows plants to reproduce by carrying a substance called pollen between them.
Some leaves change colour to yellow or brown and fall off trees. When this happens, it means the leaf has died. The leaf changes colour because the chlorophyll in the leaf has broken down. This means the plant can no longer photosynthesise to produce the chemicals needed for its survival. The yellow or brown colour remaining comes from other pigment molecules that are within the leaf.
These pigments do not absorb sunlight as effectively as chlorophyll. If we can identify this, we can provide the plant with what it needs to recover.
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