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.
The Color of Plants on Other Worlds
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 vapor 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
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.
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
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 vapor 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 shifted 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‹shielding 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.
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