A layman's explanation, from a layman...
Light is transmitted by particles called "photons", that we can think
of as being little packets of energy. Curiously, these little packets
travel in waves. Different photons have different energy, and the more
energy they have the shorter their wavelength. It is this difference
in energy or "wavelength" that we perceive as colors. Infrared light
has relatively low energy, followed by red, yellow, green, blue,
violet and near-ultraviolet, then hard-ultraviolet, which represents
extremely energetic photons. X-rays and gamma rays have even more
energy.
When plants convert carbon dioxide to sugar (i.e. photosynthesize) the
reactions require energy. In fact, the reaction happens molecule by
molecule, and each part of the reaction that makes a single molecule
of "sugar" requires just the right amount of energy - in other words
just the right color of light. The process of going from carbon
dioxide and water to "sugar" does not happen all in one step, and
various pieces of the reaction desire slightly different wavelengths.
Because of this, plants absorb light at several points in the color
spectrum. It also takes certain colors of light in order for the plant
to be able to manufacture the substances used in photosynthesis, such
as chlorophyll. Colors that correspond to photons with the "wrong"
energy level are rejected, or in other words reflected. The green
color we see reflected from the plant corresponds to these "unneeded"
frequencies.
What happens when we shine the "wrong" color on a plant? One possible
result is that the plant will starve to death and fail to grow,
especially if it is missing the nice "ultraviolet" colors. It is these
high energy ultraviolet photons that help the most in the production
of chlorophyll. It's not good enough to give the plant three
low-energy photons instead of one high-energy one. That would be sort
of like giving you three five-foot ladders to scale a 15-foot wall. In
theory you could tie the pieces together to make a 15-foot ladder, but
the result would be very wobbly and unstable, and the whole process
would be awkward and would take a long time and a lot of effort. The
plant feels the same way if the "high energy" ultraviolet colors are
missing.
Ultraviolet photons from "normal" lightbulbs are very few. No matter
how much "normal" light you shine on the plant it still finds itself
starved for energy. A plant grown in a room without a window and with
nothing but incandescent light tends to be very light green or
whitish, skinny and tall... it cannot produce enough chlorophyll, so
uses what little energy it has growing up as high as it can, trying to
reach "daylight". When it fails to find daylight, it exhausts itself
and dies. This is why "Grow-lights" work. They generate the
high-energy ultraviolet photons that the plant needs. Notice how
Grow-bulbs are "bluer" than normal incandescent bulbs?
So plants need a certain amount of the "right" colors, or wavelengths.
Other wavelengths, though, can be damaging to the plant. For example,
if you were to shoot lots of x-rays or gamma rays through the plant,
the photons would have so much energy that if they hit a cell they
would cause reactions that the plant would not want - for example,
breaking down chlorophyll, tearing apart cells, or causing reactions
in proteins that the plant needs to keep intact.
All light will be converted at least partially to heat when it hits
the plant. Cosequently, too much light can cause the plant to
overheat. This is especially true of red light (specifically infra-red
light, below the visible spectrum). Infrared and red light photons
tend to be absorbed by water, and are released as heat. Notice how
heat lamps are red? This is because red light (and specifically
infrared light)is very effective for heating. Ever notice how deep-sea
pictures are blue? This is largely because the red light is absorbed
by water, leaving only the blue light at depth. If you were to shine
intense infrared or red light on a plant the water molecules in the
plant would absorb the red light, convert it to heat, and would
actually cook the plant. Microwaves, by the way, are found below the
infrared end of the spectrum, so maybe you can see what I'm talking
about.
The conclusion? Plants need a certain amount of light at certain
specific colors between red and ultraviolet. If the plant is missing
critical colors (especially those in what we call the near-ultraviolet
range of the spectrum) they will not be able to produce chlorophyll,
and will starve. If they are exposed to intense light that is too
energetic (too far in the blue end of the spectrum, in regions that we
would call hard-ultraviolet or x-rays) the plants will suffer damage
to their cells. If they are exposed to intense light in the red end of
the spectrum, or massively intense light of any color, they will heat
up and cook.
Here's an excellent reference for you:
http://www.venturelighting.com/WhatsNew/lighting_for_plant_growth.htm |
