Hydrolife Canada February/March 2017

grow
“WITHOUT FANS
ACTIVELY mixing and
replenishing the air
in contact with your
plants, they will run low
on CO 2 , no matter how
much is available in the
surrounding room.”
There are several variations on how
plants like cannabis fix carbon.
The most common of these is termed
C3 photosynthesis. Cannabis, and all
other crops that benefit from CO 2 fertilization,
uses this pathway. Structurally,
think of the inside of a cannabis
plant's leaves as composed of tiny
reaction sites stacked on top of one another,
with empty space and plumbing
in between. These sites harness light
and turn raw materials into energyrich
building blocks to fuel plant
growth. The building blocks generated
by these reaction sites are simple sugars,
and CO 2 is a key ingredient.
CO 2 molecules present within the leaf
need to be channelled to provide a constant
supply of fuel for photosynthesis.
The answer to this need is the enzyme
RuBisCO, which binds to CO 2 molecules
and transfers them to the photosynthetic
machinery. Under ambient CO 2 concentrations
(about 400 ppm) and otherwise
favourable conditions, the activity of
RuBisCO is the limiting factor on photosynthetic
productivity. This means that
when temperature or light intensity rises
above the cannabis plant's tolerance
level, RuBisCO is unable to keep up
with the CO 2 demands of the reaction
sites and the excess energy becomes
stressful. By adding additional CO 2 to
the equation, we boost the activity of
RuBisCO. It encounters CO 2 molecules
more often and can transfer them more
efficiently, allowing the plant to extend
productivity beyond normal limits.
AIR CIRCULATION AND
CO 2 UPTAKE
As RuBisCO uses up CO 2 inside the leaf,
more is drawn in through diffusion—
the natural movement of molecules
from higher to lower concentrations.
To enter the leaf, additional CO 2 must
pass through tiny pores called stomata.
Since this is a passive process, only CO 2
contained in the air that immediately
surrounds the leaf, (known as the
boundary layer) is available. Poor air
circulation leads to stagnant boundary
layers that are rapidly depleted of CO 2.
This concept is critical to maximizing
CO 2 enrichment. Without fans actively
mixing and replenishing the air in
contact with your plants, they will run
low on CO 2, no matter how much is
available in the surrounding room.
In addition to facilitating the passage
of CO 2, stomata also regulate water
loss through transpiration. Leaves close
stomata to reduce water loss, but doing
so reduces CO 2 uptake. It's a dry world
out there, and C3 plants constantly
regulate stomatal openings to balance
CO 2 uptake against water loss. Due to
the large moisture gradient between
leaves and the surrounding air, taking in
CO 2 is costly in terms of water.
Dr. Suman Chandra, lead author
in several studies on cannabis
physiology, found that when CO 2
concentrations are raised well
above ambient, cannabis responds
by partially closing its stomata.
Without the need for CO 2 driving
them to open, the stomata naturally
close to conserve water. This is
important for several reasons. It
means that cannabis water use,
per unit area, may decrease with
CO 2 fertilization. It also makes air
mixing even more important, since
partially closed stomata will slow
CO 2 uptake. Finally, this can lead
to higher leaf temperatures by
restricting transpiration.
TEMPERATURE AND
LIGHT INTENSITY
CO 2 fertilization allows cannabis
to thrive at higher temperatures
and utilize higher light intensities,
but these two factors need to be
considered together. Light comes
with more heat, especially in HID
illuminated environments. Both
parameters shift the photosynthetic
machinery into higher gear and CO 2
enrichment allows it to run faster
and cleaner. However, even with
CO 2, pushing too hard with light
and/or temperature can send your
plants into stressful conditions.
The general recommendation
for maximizing CO 2 fertilization
in greenhouse crops is to raise
the growth temperature by three
to six degrees Celcius above the
ideal temperature in the absence
of CO 2 enrichment. For cannabis,
this means that the ideal bloom
temperature is shifted into the high
20s to low 30s. It is important to note
that ambient grow temperature
does not usually represent the
temperature that the plant canopy is
experiencing. A room temperature in
the mid 20s will translate to canopy
temperatures closer to the ideal for
growth with CO 2 enhancement. Some
strains may enjoy an even higher
temperature, but I don't recommend
running your space above 28°C
unless you know your strains will
respond favourably and you have
tight control of other environmental
parameters. Be cautious when
pushing the temperature envelope,
the difference between ideal and
harmful can be a few degrees.
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