Sunlight and Plants

The latest Scientific American has a article on what plant colors my 
look like under different suns. The first part of the article explains 
why our plants are the color that they are. The second part is also very 
interesting but no included here.

I thought some may be interested to add this knowledge to their 
understanding of gardening.

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The Color of Plants on Other Worlds
Scientific American
April 2008

pg. 48

The prospect of finding extraterrestrial life is no Ionger the domain of 
science fiction or UFO hunters. Rather than waiting for aliens to come 
to us, we are looking for them. We may not find technologically advanced 
civilizations, but we can look for the physical and chemical signs of 
fundamental life processes: ³bio-signatures.² Beyond the solar system, 
astronomers have discovered more than 200 worlds orbiting other stars, 
so-called extrasolar planets. Although we have not been able to tell 
whether these planets harbor life, it is only a matter of time now. Last 
July astronomers confirmed the presence of water va**** on an extrasolar 
planet by observing the passage of starlight through the planet¹s 
atmosphere. The world¹s space agencies are now developing telescopes 
that will search for signs of life on Earth-size planets by observing 
the planets¹ light spectra.

Photosynthesis, in particular, could produce very conspicuous 
biosignatures. How plausible is it for photosynthesis to arise on 
another planet? Very. On Earth, the process is so successful that it is 
the foundation for nearly all life. Although some organisms live off the 
heat and methane of oceanic hydrothermal vents, the rich ecosystems on 
the planet¹s surface all depend on sunlight.

Photosynthetic biosignatures could be of two kinds: biologically 
generated atmospheric gases such as oxygen and its product, ozone; and 
surface colors that indicate the presence of specialized pigmints such 
as  green chlorophyll. The idea of looking for such pigments has a long 
history. A century ago astronomers sought to attribute the seasonal 
darkening of Mars to the growth of vegetation. They studied the spectrum 
of light reflected off the surface for signs of green plants. One 
difficulty with this strategy was evident to writer H. G. Wells, who 
imagined a different scenario in The War of the Worlds: ³The vegetable 
kingdom in Mars, instead of having green for a dominant colour, is of a 
vivid blood-red tint.² Although we now know that Mars has no surface 
vegetation (the darkening is caused by dust storms), Wells was prescient 
in speculating that photosynthetic organisms on another planet might not 
be green.

Even Earth has a diversity of photosynthetic organisms besides green 
plants. Some land plants have red leaves, and underwater algae and 
photosynthetic bacteria come in a rainbow of colors. Purple bacteria 
soak up solar infrared radiation as well as visible light. So what will 
dominate on another planet? And how will we know when we see it? The 
answers depend on the details of how alien photosynthesis adapts to 
light from a parent of different type than our sun, filtered through an 
atmosphere that may not have the same composition as Earth¹s.

Harvesting Light

In trying to figure out how photosynthesis might operate other planets, 
the first step is to explain it on Earth. The energy spectrum of sun 
light at Earth¹s surface peaks in the blue-green, so scientists have 
long scratched their heads about why plants reflect green, thereby 
wasting what appears to be the best available light . The answer is that 
photosynthesis doesn¹t depend on the total amount of light energy but on 
the energy per photon and the number of photons that make up the light.
Whereas blue photons carry more energy than red ones, the sun emits more 
of the red kind. Plants use blue photons for their quality and red 
photons for their quantity. Tin green photons that lie in between have 
neither the energy nor the numbers, so plants have adapted to absorb 
fewer of them.     

The basic photosynthetic process, which fixes one carbon atom (obtained 
from carbon dioxide, CO2) into a simple sugar molecule, requires a 
minimum of eight photons. It takes one photon to split an 
oxygen-hydrogen bond in water H2O and thereby to obtain an electron for 
biochemical reactions. A total of four such bonds must be broken to 
create an oxygen molecule (O2). Each of those photons is matched by at 
least one additional photon for a second type of reaction to form the 
sugar. Each photon must have a minimum amount of energy to drive the 
reactions.

The way plants harvest sunlight is a marvel of nature. Photosynthetic 
pigments such as chlorophyll are not isolated molecules. They operate in 
a network like an array of antennas, each tuned to pick out photons of 
particular wavelengths. Chlorophyll preferentially absorbs red and blue 
light, and carotenoid pigments (which produce the vibrant reds and 
yellows of fall foliage) pick up a slightly different shade of blue. All 
this energy gets funneled to a special chlorophyll molecule at a 
chemical reaction center, which splits water and releases oxygen.
The tunneling process is the key to which colors the pigments select. 
The complex of molecules at the reaction center can perform chemical 
reactions only if it receives a red photon or the equivalent amount of 
energy in some other form. To take advantage of blue photons, the 
antenna pigments work in concert to convert the high energy (from blue 
photons) to a lower energy (redder), like a series of step-down 
transformers that reduces the 100,000 volts of electric power lines to 
the 120 or 240 volts of a wall outlet. The process begins when a blue 
photon hits a blue-absorbing pigment and energizes one of the electrons 
in the molecule. When that electron drops back down to its original 
state, it releases this energy‹but because of energy losses to heat and 
vibrations, it releases less energy than it absorbed.

The pigment molecule releases its energy not in the form of another 
photon but in the form of an electrical interaction with another pigment 
molecule that is able to absorb energy at that lower level. This 
pigment, in turn, releases an even lower amount of energy, and so the 
process continues until the original blue photon energy has been 
downgraded to red. The array of pigments can also convert cyan, green or 
yellow to red. The reaction center, as the receiving end of the cascade, 
adapts to absorb the lowest-energy available photons. On our planet¹s 
surface, red photons are both the most abundant and the lowest energy 
within the visible spectrum.

For underwater photosynthesizers, red photons are not necessarily the 
most abundant. Light niches change with depth because of filtering of 
light by water, by dissolved substances and by overlying organisms 
themselves. The result is a clear stratification of life-forms according 
to their mix of pigments. Organisms in lower water layers have pigments 
adapted to absorb the light colors left over by the layers above. For 
instance, algae and cyanobacteria have pigments known as phycobilins 
that harvest green and yellow photons. Nonoxygen producing (anoxygenic) 
bacteria have bacteriochlorophylls that absorb far-red and near-infrared 
light, which is all that penetrates to the murky depths.

Organisms adapted to low-light conditions tend to be slower-growing, 
because they have to put more effort into harvesting whatever light is 
available to them. At the planet¹s surface, where light is abundant, it 
would be disadvantageous for plants to manufacture extra pigments, so 
they are selective in their use of color. The same evolutionary 
principles would operate on other worlds.

Just as aquatic creatures have adapted to light filtered by water, land 
dwellers have adapted to light filtered by atmospheric gases. At the top 
of Earth¹s atmosphere, yellow photons (at wavelengths of 560 to 590 
nanometers) are the most abundant kind. The number of photons drops off 
gradually with longer wavelength and steeply with shorter wavelength. As 
sunlight passes through the upper atmosphere, water va**** absorbs the 
infrared light in several wavelength ands beyond 700 nm. Oxygen produces 
absorption lines‹narrow ranges of wavelengths that the gas blocks‹at 687 
and 761 nm. We all know that ozone (O3) in the stratosphere strongly 
absorbs the ultraviolet (UV). Less well known is that it also absorbs 
weakly across the visible range.

Putting it all together, our atmosphere demarcates windows through which 
radiation can make it to the planet¹s surface. The visible radiation 
window is defined at its blue edge by the drop-off in the intensity of 
short-wavelength photons emitted by the sun and by ozone absorption of 
UV. The red edge is defined by oxygen absorption lines. The peak in 
photon abundance is ****fted from yellow to red (about 685 nm) by ozone¹s 
broad absorbance across the visible.

Plants are adapted to this spectrum, which is determined largely by 
oxygen‹yet plants are what put the oxygen into the atmosphere to begin 
with. When early photosynthetic organisms first appeared on Earth, the 
atmosphere lacked oxygen, so they must have used different pigments from 
chlorophyll. Only over time as photosynthesis altered the atmospheric 
composition, did chlorophyll emerge as optimal.

The firm fossil evidence for photosynthesis dates to about 3.4 billion 
years ago (Ga), but earlier fossils exhibit signs of what could have 
been photosynthesis. Early photosynthesizers had to start out 
underwater, in part because water is a good solvent for biochemical 
reactions and in part because it provides protection against solar UV 
radiation‹****elding that was essential in the absence of an atmospheric 
ozone layer. These earliest photosynthesizers were underwater bacteria 
that absorbed infrared photons. Their chemical reactions involved 
hydrogen, hydrogen sulfide or iron rather than water, so they did not 
produce oxygen gas. Oxygen-generating (oxygenic) photosynthesis by 
cyanobacteria in the oceans started 2.7 Ga. Oxygen levels and the ozone 
layer slowly built up, allowing red and brown algae to emerge. As 
shallower water became safe from UV, green algae evolved. They lacked 
phycobilins and were better adapted to the bright light in surface 
waters. Finally, plants descended from green algae emerged onto land‹ 
two billion years after oxygen had begun accumulating in the atmosphere.

And then the complexity of plant life exploded, from mosses and 
liverworts on the ground to vascular plants with tall canopies that 
capture more light and have special adaptations to particular climates. 
Conifer trees have conical crowns that capture light efficiently at high 
latitudes with low sun angles; shade-adapted plants have anthocyanin as 
a sunscreen against too much light. Green chlorophyll not only is well 
suited to the present composition of the atmosphere but also helps to 
sustain that composition‹a virtuous cycle that keeps our planet green. 
It may be that another step of evolution will favor an organism that 
takes advantage of the shade underneath tree canopies, using the 
phycobilins that absorb green and yellow light. But the organisms on top 
are still likely to stay green.
-- 

Billy
http://www.youtube.com/watch?v=9KVTfcAyYGg&ref=patrick.net
http://au.youtube.com/watch?v=7WBB0svwMdY&feature=related