Fig. 3 The action spectrum of photosynthesis of a single leaf as determined by Drs. McCree and Inada.
EFFICIENCY: WHAT IS GOOD
FOR THE FIXTURE MAY NOT
BE GOOD FOR THE PLANT
To understand this distinction of
efficiency, one needs to further dissect
how the photoreceptors in a plant
interact with green light, particularly
at the most basic level: the photon. A
basic precept of plant photobiology
is how green photons are mostly
reflected when hitting chlorophylls
at the surface of the leaf, which gives
plants their greenish color. However, not
all green photons are reflected. Some
will pass through air interfaces in the
chloroplasts and will even transmit
through chlorophylls.
Because green photons are not
absorbed completely by photoreceptors
at the surface of the leaf (Fig. 2), green
light is able to penetrate deeper
through a leaf to drive photosynthesis
in chloroplasts located toward the
bottom surface of the leaf and beyond.
This reaction with green light occurs
more effectively at high photosynthetic
photon flux density (PPFD) than red
light emitted at a comparable PPFD.
In fact, as PPFD increases, light
energy that is absorbed in the upper
chloroplasts will reach a saturation
point and will be dissipated as heat,
while penetrating green light increases
photosynthesis by exciting chloroplasts
located deep in the mesophyll. And
since green light penetrates much more
effectively to the lower canopy, green
light will help drive photosynthesis
across the whole plant as it is absorbed
by leaves in the lower canopy not
exposed to red or blue light.
58
Maximum Yield
“For general growing applications, broad
spectrum light, which includes red,
blue, and green light should be used
to ensure a plant can achieve its
full Potential.
One last thing to keep in mind is red
light is more energy efficient for light
fixtures to emit at high PPFD, but as is
evidenced by the heat dissipation of red
light, it is not used as efficiently by the
plant. Green light, on the other hand,
is not as efficient to emit, but is more
effectively used by the whole plant.
SEEING GREEN, AGAIN
All this is not to say red or blue light
do not have their own uses. Certain
photomorphogenic effects can be
achieved when narrow-band lighting
is employed as a supplemental
light. However, for general growing
applications, broad spectrum light,
which includes red, blue, and green light
should be used to ensure a plant can
achieve its full potential. While red and
blue light align with peak absorption for
chlorophylls A and B and can result in
adequate yields, all the other antenna
photoreceptors that impact secondary
metabolite production are neglected
and result in poor quality for the entire
plant. Indeed, high-intensity discharge
lights (Fig. 4 a and b) have all included
green light in the past, and these bulbs
typically all possess strong green
wavelength emissions.