“Carbon dioxide also assists with
root development on cuttings and
clones in many species and may
be applied via enrichment of
the air or through the use of
carbonated mist.”
Another often overlooked factor is CO 2
distribution around the plants. Simply
releasing or generating CO 2 for enrichment
into the growing area is often not sufficient to get
the maximum rate of photosynthesis unless this
is directed and circulated over leaf surfaces. A stale
boundary layer of moist air, depleted in CO 2 due to
photosynthesis, can form directly around the leaf surface
and this needs frequent removal and replenishment.
Whatever source of CO 2 generation is being used, it is
vital that the enriched atmosphere is thoroughly mixed so
the valuable CO 2 is delivered to plant surfaces for uptake
and assimilation. Small mixer fans can be used to gently
circulate the air away from the source of CO 2 generation
and toward the crop. To monitor this process, hand-held
CO 2 meters are useful to check levels in and around the
canopy rather than just at the point of CO 2 release. Keeping
a check on CO 2 levels inside a small growing area is vitally
important, no matter what the source of CO 2 used. It can be
difficult to judge how much CO 2 the plants are taking up and
in tightly sealed growing environments, CO 2 accumulation
can occur and cause plant damage.
CO 2 Acclimation
CO 2 enrichment is undoubtedly a great growth-promoting
tool for hydroponic growers, however, it has its limitations
and risks. Plants have the ability to adjust and adapt to
increasing CO 2 levels, so that over time, acclimation
occurs. When CO 2 enrichment is first introduced to a
crop, there is a rapid increase in photosynthesis and
growth, but as plant growth continues, the effect of the
increased CO 2 levels becomes less and less so that by
the time the crop is completed, overall yields were not
as high as the increase in early yield. Numerous studies
have reported this effect with plants grown continuously
at high CO 2 levels having a photosynthetic rate that tends
to decrease with time. If a crop grown at elevated CO 2
levels is suddenly given only ambient CO 2 , it will recover
back to normal rates of photosynthesis within five days. Some
growers have attempted to prevent this acclimation of crops
to high CO 2 levels by only supplying CO 2 intermittently, or
avoiding the use of CO 2 enrichment until a vital stage of
development, such as flowering or fruit set, has been reached
when the boost in photoassimilate is most valuable to
yields. Studies have shown the problem of CO 2 acclimation
can be reduced or eliminated if the plant has strong “sinks”
for the assimilate produced in the leaves. These sinks for
assimilate include rapidly developing tissues such as buds,
flowers, and fruits. Plants with a low sink strength often end
up with carbohydrate accumulating in the leaves under CO 2
enrichment, which in turn triggers acclimation and a reduction
in photosynthesis. Despite the issue of plant acclimation to
high CO 2 levels limiting the overall potential boost to growth,
CO 2 -enriched plants still produce photosynthetic rates higher
than those grown at ambient CO 2 levels.
CO 2 enrichment to levels of 500-1,500ppm is common for hydroponic
crops, but is species dependant.
46
Maximum Yield
Carbon dioxide enrichment is a worthwhile tool for indoor and
greenhouse growers which is well proven in a wide range of
crop species to increase growth rates and yields. However,
as with most high-tech techniques, it requires monitoring,
attention to detail, and careful consideration of the effect
on biochemical processes. If CO 2 is to be used at maximum
efficiency, correct rates of application, adjustments to light
and temperature, timing of enrichment, and consequences of
CO 2 acclimation all need consideration.