plant factories 2
in plant factories lit solely by artificial
light, it will be necessary to establish specific
lighting characteristics for specific crops.”
LED lighting on NFT lettuce.
Temperature becomes a limiting factor because both the
biochemical part of the photosynthesis process and the
transport of the sugars away from the chloroplasts are temperature dependant. Increasing the temperature will further
increase the rate of photosynthesis.
The above has been the basic philosophy for the supplementary use of carbon dioxide in greenhouse production for the
past 50 years, and it has worked exceedingly well. However,
there are new theories suggesting this model may not hold
up when growing plants in an enclosed environment. In a
recent presentation at the 2014 International Conference on
Healthcare in Brisbane, Australia, Duggan-Jones and myself
showed that the Gaastra model might not apply. When measuring plant biomass as dry matter, there was always a good
correlation between photosynthesis and dry matter production, although obviously more photosynthesis will result in
a greater quantity of dry matter, but plant morphology may
play a major role in determining what the actual end point
will be. We found the optimal temperature for dry matter
accumulation was 77°F, when compared to 68 and 86°F,
irrespective of light or CO2 levels. We also found there was
little increase in dry matter accumulation above CO2 levels
of 1,000 ppm. This was in spite of a clear-cut increase in dry
matter accumulation with increased light levels.
Our results, which differ fundamentally from those of
Gaastra, may be due to the fact that in our experiment,
we were able to compare three light intensities with three
temperature and three CO2 levels. Gaastra only used two
temperature and two CO2 levels. One possible explanation for our results is that when growing plants at a
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continuously high level of CO2, the stomata may not open
so wide, or, alternatively, a smaller number of (or smallersized) stomata might develop under a high-CO2 environment. The lack of response to higher temperatures—even
at high CO2 levels—is even more unsettling, but it should
be noted that increasing temperature tends to make leaves
expand faster. If carbohydrates are limited, the leaves will
be thinner. This certainly appeared to be the case with lettuce and cabbage seedlings.
Conclusion
It is starting to appear that in plant factories lit solely by artificial light, it will be necessary to establish specific lighting
characteristics for specific crops. This is also likely to involve
consideration of different temperature regimens and carbon
dioxide levels. To date, virtually all of the commercialization
in plant factories has involved leafy vegetables and yet the
major greenhouse crops grown worldwide are fruit
vegetables, tomatoes, sweet peppers and cucumbers. Clearly, we have a long way
to go to fully replace current
greenhouse technology.
This article originally appeared in
Practical Hydroponics & Greenhouses.