And on another lovely fall day, I thought I might take a walk back in time to science class, when I first learned why leaves change colour. Because I know it has something to do with chloroplasts and chlorophyll, but other than that I'd be hard pressed to explain it off the top of my head. Perhaps it is time for a refresher course.
So, today I aim to learn or rather re-learn why it is leaves change colour. And while I'm at it, I might as well brush up on why it is they fall off their branches too.
So, today I aim to learn or rather re-learn why it is leaves change colour. And while I'm at it, I might as well brush up on why it is they fall off their branches too.
But first, I will need to re-acquaint myself with a few key players and processes before I get down to the business of fall and leaves and colour changes. Don't worry, there will be no detailed photosynthesis schematics. I'm keeping things real simple for a Monday.
During photosynthesis, light is absorbed by chlorophyll containing proteins located within the chloroplasts of plant cells.
The energy captured is then used in the process of transforming carbon dioxide from the air and water from the ground into carbohydrates. [1]
The aforementioned pigment chlorophyll is present in the highest concentration and gives leaves their green color. [1]
The pigments carotene and xanthophyll, are also present, but in lower concentrations and help absorb sunlight, which they transfer to chlorophyll for photosynthesis. [2]
Most of the year these colors are masked by great amounts of green coloring. [1]
Although all of the cells in the green parts of a plant have chloroplasts, most of the energy is utilized by the leaves. [1]
 |
| Chloroplasts |
The behaviour of chloroplasts is strongly influenced by environmental cues. [3]
Chloroplasts contain their own DNA. [3]
But they themselves cannot be synthesized by the plant cell, and must be inherited by each daughter cell during division. [3]
WHY DO LEAVES CHANGE COLOUR?
During the summer months, leaves will make more glucose than is needed for energy and growth. The excess is turned into starch and stored. [4]
Then, what happens in the fall, is the changes in the length of daylight and temperature signal to the leaves to stop it with their food-making process. As a result, the chlorophyll starts to break down, the green color disappears, and the yellow to orange colors become visible. [2][3][6]
And what of those above mentioned stores of delicious starch? Those are what will sustain the tree through the winter months when photosynthesis can no longer occur. [2][6]
So, back to the leaves. I’ve already mentioned yellow and orange but what about the red and purple?
Whereas all trees contain chlorophyll, carotene and xanthophyll, not all of them produce the red-purple pigment anthocyanin. [2]
In fact, anthocyanins are only present in about 10% of tree species in temperate regions, although in certain areas such as New England, up to 70% of tree species may produce the pigment. [5][7]
In those that do, like the Maple tree, the anthocyanins will not be synthesized until the plant has begun breaking down the chlorophyll.[5][7]
They develop in the sap of the cells of the leaf. And their formation depends on the breakdown of sugars in the presence of bright light as the level of phosphate in the leaf is reduced. [5][8]
THEORIES ABOUT THE FUNCTION OF ANTHOCYANINS:
They may protect the leaves from excess sunlight and lower their freezing point, giving some frost protection, and enabling the trees to recover any last remaining nutrients. [2][4]
When the leaves decay, the anthocyanins may seep into the ground and prevent other plant species from growing in the spring. [4]
WHY DO LEAVES FALL OFF THEIR BRANCHES EN MASSE? [2][3][4][6]
Cued by the autumn’s decreased sunlight, deciduous trees will form a separation layer at the base of each leaf to seal it off.
From this point on, water is not being transported from the roots to the leaves and glucose is not being transported from the leaves to the tree.
And without water, as was mentioned above, chlorophyll production grinds to a halt.
As the bottom cells in the separation layer form a seal between leaf and tree, the cells in the top of the separation layer begin to disintegrate. They form a tear-line, and eventually the leaf is blown away or simply falls from the tree. This abscission layer will protect the tree during the winter months.
OTHER POINTS OF INTEREST:
Photosynthesis maintains atmospheric oxygen levels and supplies all of the organic compounds and most of the energy necessary for life on Earth. [9][1]
Evergreens keep most of their leaves during the winter. They have special leaves, resistant to cold and moisture loss.
Some, like pine and fir trees, have long thin needles.
Others, like holly, have broad leaves with tough, waxy surfaces.
On very cold, dry days, these leaves sometimes curl up to reduce their exposed surface.
Evergreens may continue to photosynthesize during the winter as long as they get enough water, but the reactions occur more slowly at colder temperatures. [4]
REFERENCES:
[1] http://en.wikipedia.org/wiki/Photosynthesis
[2] http://science.howstuffworks.com/life/botany/leaves-turn-red1.htm
Horton, Jennifer. "Why do leaves change color and turn red?" 23 September 2008. HowStuffWorks.com. <http://science.howstuffworks.com/life/botany/leaves-turn-red.htm> 15 September 2014.
[3] http://en.wikipedia.org/wiki/Chloroplast
[4] http://www.sciencemadesimple.com/leaves.html
[5] http://en.wikipedia.org/wiki/Anthocyanin
[6] http://www.esf.edu/pubprog/brochure/leaves/leaves.htm
Chloroplasts contain their own DNA. [3]
But they themselves cannot be synthesized by the plant cell, and must be inherited by each daughter cell during division. [3]
WHY DO LEAVES CHANGE COLOUR?
During the summer months, leaves will make more glucose than is needed for energy and growth. The excess is turned into starch and stored. [4]
Then, what happens in the fall, is the changes in the length of daylight and temperature signal to the leaves to stop it with their food-making process. As a result, the chlorophyll starts to break down, the green color disappears, and the yellow to orange colors become visible. [2][3][6]
And what of those above mentioned stores of delicious starch? Those are what will sustain the tree through the winter months when photosynthesis can no longer occur. [2][6]
So, back to the leaves. I’ve already mentioned yellow and orange but what about the red and purple?
Whereas all trees contain chlorophyll, carotene and xanthophyll, not all of them produce the red-purple pigment anthocyanin. [2]
In fact, anthocyanins are only present in about 10% of tree species in temperate regions, although in certain areas such as New England, up to 70% of tree species may produce the pigment. [5][7]
In those that do, like the Maple tree, the anthocyanins will not be synthesized until the plant has begun breaking down the chlorophyll.[5][7]
They develop in the sap of the cells of the leaf. And their formation depends on the breakdown of sugars in the presence of bright light as the level of phosphate in the leaf is reduced. [5][8]
THEORIES ABOUT THE FUNCTION OF ANTHOCYANINS:
They may protect the leaves from excess sunlight and lower their freezing point, giving some frost protection, and enabling the trees to recover any last remaining nutrients. [2][4]
When the leaves decay, the anthocyanins may seep into the ground and prevent other plant species from growing in the spring. [4]
WHY DO LEAVES FALL OFF THEIR BRANCHES EN MASSE? [2][3][4][6]
Cued by the autumn’s decreased sunlight, deciduous trees will form a separation layer at the base of each leaf to seal it off.
From this point on, water is not being transported from the roots to the leaves and glucose is not being transported from the leaves to the tree.
And without water, as was mentioned above, chlorophyll production grinds to a halt.
As the bottom cells in the separation layer form a seal between leaf and tree, the cells in the top of the separation layer begin to disintegrate. They form a tear-line, and eventually the leaf is blown away or simply falls from the tree. This abscission layer will protect the tree during the winter months.
OTHER POINTS OF INTEREST:
Photosynthesis maintains atmospheric oxygen levels and supplies all of the organic compounds and most of the energy necessary for life on Earth. [9][1]
Evergreens keep most of their leaves during the winter. They have special leaves, resistant to cold and moisture loss.
Some, like pine and fir trees, have long thin needles.
Others, like holly, have broad leaves with tough, waxy surfaces.
On very cold, dry days, these leaves sometimes curl up to reduce their exposed surface.
Evergreens may continue to photosynthesize during the winter as long as they get enough water, but the reactions occur more slowly at colder temperatures. [4]
REFERENCES:
[1] http://en.wikipedia.org/wiki/Photosynthesis
[2] http://science.howstuffworks.com/life/botany/leaves-turn-red1.htm
Horton, Jennifer. "Why do leaves change color and turn red?" 23 September 2008. HowStuffWorks.com. <http://science.howstuffworks.com/life/botany/leaves-turn-red.htm> 15 September 2014.
[3] http://en.wikipedia.org/wiki/Chloroplast
[4] http://www.sciencemadesimple.com/leaves.html
[5] http://en.wikipedia.org/wiki/Anthocyanin
[6] http://www.esf.edu/pubprog/brochure/leaves/leaves.htm
[7] Archetti, Marco; Döring, Thomas F.; Hagen, Snorre B.; Hughes, Nicole M.; Leather, Simon R.; Lee, David W.; Lev-Yadun, Simcha; Manetas, Yiannis; Ougham, Helen J. (2011). "Unravelling the evolution of autumn colours: an interdisciplinary approach". Trends in Ecology & Evolution 24 (3): 166–73. doi:10.1016/j.tree.2008.10.006. PMID 19178979.
[8] Davies, Kevin M. (2004). Plant pigments and their manipulation. Wiley-Blackwell. p. 6. ISBN 1-4051-1737-0.
[9] Bryant DA, Frigaard NU (November 2006). "Prokaryotic photosynthesis and phototrophy illuminated". Trends Microbiol. 14 (11): 488–96. doi:10.1016/j.tim.2006.09.001. PMID 16997562.[8] Davies, Kevin M. (2004). Plant pigments and their manipulation. Wiley-Blackwell. p. 6. ISBN 1-4051-1737-0.
IMAGE CREDITS
"Photosynthesis" by At09kg - Own work. Licensed under Creative Commons Attribution-Share Alike 3.0 via Wikimedia Commons - http://commons.wikimedia.org/wiki/File:Photosynthesis.gif#mediaviewer/File:Photosynthesis.gif
"Plagiomnium affine laminazellen" by Kristian Peters -- Fabelfroh - photographed by myself. Licensed under Creative Commons Attribution-Share Alike 3.0 via Wikimedia Commons - http://commons.wikimedia.org/wiki/File:Plagiomnium_affine_laminazellen.jpeg#mediaviewer/File:Plagiomnium_affine_laminazellen.jpeg
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