Hey all, I didn't write this, I got it from CannaTalk issue 4. I know we know some of this already, but it's still got other fascinating tidbits. Here's the article:

THE INFLUENCE OF COLORS ON PLANTS - D. Kroeze MSc, CANNA Research

The primary colors

Researchers have traditionally distinguished seven
colors. They are known as the colors of the rainbow,
red, orange, yellow, green, blue, indigo and violet,
often referred to as the primary and secondary colors.
When put together, these primary colors create
white light and only become visible when a ray of
sunshine is split by a prism.
All objects can absorb the colors within the light, reflect
them or allow them to pass through. Science
tells us that an object usually adopts the color it isnít
able to absorb itself. Because plants cannot absorb
the color green, they must themselves be green.
There are those who claim that light measurements
show that plants themselves emit light particles in
the dark and actively help create their own color.
Just like we do, plants sense the light and colors surrounding
them Ė but in a different way!

Red light

Plants are sensitive to the color red in the light spectrum,
a sensitivity that arises from the plant having
what is called a red light photoreceptor. The receptor
is a blue-green pigment called a phytochrome
present in the cells of a plant. You could compare
phytochrome with an eye that only senses red
light.
Red light impacts a plant in many ways. Plants that
are grown in plenty of red light are often large, but
in general also tall with many branches. If the photoreceptor
picks up a large quantity of natural red
light, for example in summer when thereís plenty
of natural red light, production of a plant hormone
(meta-topolin) is increased. This hormone prevents
the chlorophyll in the plant from being broken
down, so that it stays green in spring and summer;
advantageously, this is exactly the time that the
plant needs its chlorophyll to convert energy coming
from the sun into sugar. Red light also influences
a plantís flowering and seed formation.
By comparing the quantity of one frequency of red
light to the amount of another, far-red, present in
the light, makes the plant decide whether to start
flowering or not. The non-flowering period can be
extended by exposing the plant to red-containing
light during the dark period. This will, as a result,
also extend the period of time before harvesting,
which, of course, most growers would like to prevent.
This also explains why it is a bad idea to enter
the growing area when itís dark for any time, even
for a quick peek! The red color in light also influences
flavor because it increases the concentration of
special oils in plants.

Blue light

Plants see blue light as well as red light, using a photoreceptor
that is called a cryptochrome. If there is
plenty of blue light, as is the case in nature during
autumn and winter, this receptor slows down the
effect of a hormone called Auxin. This hormone is
responsible for the plantís stem and root growth.
Auxin is also responsible for what is referred to as
Ďapical dominanceí, the phenomenon which
causes the central stem to be dominant over side
stems. A branch off a main stem would be dominant
over itsí own side branches so it inhibits the
development of axillary flowers. This causes the
plant to create more side stems when exposed to
bluish light and the plant stays shorter. This helps us
to understand why plants exposed to bluish light
are often short and bushy in appearance with a
more robust structure. Experiments with blue light
resulted in plants that are wider than usual. This
can be explained by the fact that more branches
sprout because of diminished apical dominance.
Plants use the quantity of blue light to determine
how far to open their stomas. The more blue light,
the wider they open their stomas, resulting in an
acceleration of their metabolism. High levels of
blue light will increase metabolism, and as a consequence
accelerate plant growth and development.
Blue light is also responsible for leaves
growing towards the light. Blue light avoids the multiplication
of leaves around the fruits. A shortage of
blue light in the spectrum will quickly cause you to
lose 20% of your harvest. Although opinions about
this seem to differ, the optimum red to blue light ratio
should be 5:1 in general.
Green light and the other colors
Plants are hardly sensitive to green light. As far as
we know, they lack receptors for this color.
This is probably the case because in practice plants
do not absorb this color. Plants which are grown
in green light only will be exceedingly weak and
rarely grow old.
Clearly, plants only seem to sense those colors for
which they have specific receptors. This is based
on energy levels provided by each color. Plants are
not blind but, up to a certain level, they are color
blind. The way plants react to orange and yellow
light is quite similar to the reaction on red light. This
also accounts for indigo and violet which in reaction
are similar to reactions on blue light.

ĎInvisibleí light

Although plants are a bit color blind, they do sense
colors, more appropriately energy levels, that are
entirely invisible to us humans. For example, plants
can perceive far-red light. Plants often utilize the
red to far-red relationship. A seed uses this relationship
to determine whether to germinate or not.
Plants also use that relationship to determine the
number of other plants in the immediate surroundings.
Because plants absorb large amounts of red
light whilst reflecting far-red light, there will be less
red light present in a plantís immediate surrounding
if other plants are in the area too. Seeds will hold
off germinating and the plants that are already in
place will grow faster in order to emerge above the
other plants to acquire sufficient light for their photosynthesis.
The fact that far-red light has the exact
the opposite effect to that of red light makes it unsuitable
as a light for growing. The traditional light
bulb is a rich source of far-red light.
Ultra violet light (UV) also influences plants. Like is
the case with blue light, plants perceive this color
using the cryptochrome photoreceptor. It is unclear
whether other photoreceptors can perceive
UV light. When increasing the quantity of UV light,
the concentration of a purplish substance called
Anthocyanin goes up. Anthocyanins protect plants
against UV radiation, but also against micro-organisms
trying to get in. The formation of Anthocyanin
can often be seen where flaws occur such as a lack
of oxygen. UV light not only damages the plantís
DNA and membranes, but immediately disrupts the
process of photosynthesis. Therefore an excess of
UV light is unhealthy for plants as well.

Light is seeing

As we have seen, light is not just essential for plants
when it comes to supplying energy for photosynthesis.
Although this article only reveals a small part
of everything there is to know about light, the colors
in it and the effects they have on plants, we did
see how plants use colors to regulate many of their
processes. Plants are capable of perceiving those
colors that matter to them. Those colors give the
plant an indication of its general environment and
its chances of survival and reproduction. If your
plants are to develop, grow and flower well, the
composition of the light is at least as important as
its quantity. Do not forget that a plant perceives
the composition of both direct and indirect light.
Indirect light here refers to the light that is reflected
onto a plant by other objects such as walls or other
plants.

D. Kroeze, MSc.
CANNA Research