Yes! CO₂ helps, but it’s not quite as simple as you’ve been taught. Read on for some surprises. This is Chapter 35: “Effectiveness of CO₂ ” from my new book, Marijuana Cultivation Reconsidered.
Why You Need to Know: The orthodoxy that you should supplement CO₂ at 1500 ppm is wrong.
Although cannabis has not received much attention from the scientific community–owing to its preposterous legal status–it has, thankfully, received some. Before we get into that, let's review the orthodoxy that I am about to challenge: 1500 ppm is the optimal CO₂ concentration for marijuana growth. This is what we are told and most believe it to be the received Wisdom of the Weed Gods; yet I believe it to be unsupported by the scientific research. I am therefore calling bullshit on the "1500 ppm CO₂ is optimal for pot growth" heuristic.
The number itself fails to pass the smell test. Always be wary of round numbers–they are seldom to be trusted. If you don't believe me, go ahead and try to find a single scientific study demonstrating that 1500 ppm is optimal… for any plant. All plants have different CO₂ usage curves. Some simply like it more than others. Marijuana's response to CO₂ enrichment has been studied exactly one time as far as I can ascertain (and the authors who conducted that study agree that they are the first)! We are, it seems, in uncharted territory here. It's a good thing somebody has bothered to look at this. We will get to the results of that study in a bit…
One thing we haven't touched on in great detail is the fact that there is more than one kind of photosynthesis; there are actually three types, and the plants that use them are known as C3, C4 or CAM plants. (They are named for the first product of carbon fixation in the Calvin cycle that they use, where C stands for carbon, and CAM stands for Crassulacean Acid Metabolism. So, for example, a plant producing a three-carbon compound to fix CO₂ from the air is referred to as a C3 plant.) Each of the three plant types has a slightly different cellular and tissue structure. I have been presenting C3 plants in this book, since that is what concerns us (Cannabis sativa L. is a C3 plant).
C3 plants lose about 500 molecules of water for every CO₂ molecule they fix from the air, whereas C4 and CAM plants have evolved for life in dry and hot environments, respectively. A C4 plant will lose around half as many water molecules for every CO₂ molecule it fixes. For CAM plants, the number is a fraction of even that low number–usually below 100. The details of the three photosynthesis models are not important to understand (but are given in Appendix IV Plant Processes), except to note that each uses CO₂ in differing amounts, with C3 plants being the least efficient of the three.
As of mid-year 2013 ambient atmospheric CO₂ levels are at 400 ppm. Previous studies have shown that C3 plants can greatly increase, and as much as double, their photosynthetic capacity when ambient CO₂ levels are doubled (at the time of such studies, CO₂ concentrations would have been at around 350 ppm, so doubling that would put CO₂-enriched plant subjects in roughly a 700 ppm environment.) C4 and CAM plants generally do not show the same increase in efficiency when tested in CO₂-enriched environments because they are already approaching their maximum CO₂ saturation point. This is because they are better at using CO₂ to begin with.
The way that CO₂ concentrations work to enhance plant growth is intimately connected with the way the plant uses light. This makes sense, because we know that photosynthesis is the process of converting light and CO₂ into sugar. A plant can have as much CO₂ as you are able to give it, but it can't do anything with it if it doesn't have light. Conversely, when CO₂ levels are at zero, a well-lit plant can conduct no photosynthesis. Therefore, we understand that CO₂ and light are independent limiting factors. Up to a point, that is. Photo-inhibition occurs in cases of too much light. Most plants top out at around 1000 µmol m²s¹ (mol per square meter per second, because it is a flow) photon flux (see Marijuana Cultivation Reconsidered Section 3: Measuring Light). In such instances, the plant's leaves are unable to dissipate heat fast enough and its photosynthetic structures become damaged. Similarly, it is possible to exceed the amount of useful CO₂ given to the plant. At concentrations higher than the plant's ability to add CO₂ to the Calvin cycle, the additional CO₂ becomes useless.
What this shows us is that there is a balance between where light levels rise to, and then surpass, the threshold of available CO₂, and where CO₂ levels rise to, and then surpass, the available level of light. In the first case growth is light-limited; in the second case, growth is CO₂-limited. The trick is to discover the perfect balancing point.
Circling back to the issue at hand, which is optimal CO₂ plus light concentrations for the growth of marijuana, these levels have yet to be elucidated. However, as mentioned earlier, I have been able to find one study that was funded in part, amazingly, by the National Institute on Drug Abuse (NIDA), which is under the Department of Health and Human Services. (In other words, the U.S. government paid these guys to study pot, so big tip-o-the-hat to them!) This study examined the degree to which CO₂ increases cannabis growth. It’s rather technical as these things are wont to be, so let me break it down for you. The scientists measured the effect of CO₂ using several metrics, the most important of which were water use efficiency (WUE), net photosynthesis (Pn), internal(Ci)-to-external(Ca) CO₂ concentration (Ci:Ca ratio) and stomatal conductance (gs).
Unsurprisingly, net photosynthesis increased when CO₂ concentrations were increased. We all expected that. What you might not have expected, however, was that as CO₂ levels were increased, stomatal conductance decreased, and this, in turn, increased water-use efficiency. To repeat: as CO₂ increased, stomatal conductance decreased–the plant did not “breathe” more heavily; it breathed more efficiently. This seems counterintuitive because, as those of us who have experienced a CO₂ enriched environment know, when CO₂ concentrations are increased, the room feels warmer and more humid. We may assume that the plant is transpiring more water in the form of vapor. In absolute terms, it is. In relative terms, however, that is not the case. The room feels warmer because CO₂, a greenhouse gas, does not reflect long wavelength radiation well and, because water has a high heat capacity, that radiation sticks around in the moistened air longer. In a CO₂ enriched environment, plants are able to function more efficiently in much the same way as you would if you were to supplement oxygen while you, say, ran on a treadmill. We can see from this study that internal leaf CO₂ concentrations increase, so that's where the CO₂ is going. It's accumulating in the leaf mesophyll.
Unfortunately, this study stopped testing at 700 ppm of CO₂ supplementation. We therefore do not know where the improvements plateau. But, what does the study show us? Check out the results.
In the left column is the metric measured for each of four different strains: HPM (High Potency Mexican Variety), K2, MX and W1 (from Switzerland). Across the top you can see the color coding for increase in CO₂ (where umol/mol can be read as ppm because they are fractional equivalents). It’s pretty clear that CO₂ works to increase photosynthesis at least as far as the studied 700 ppm level. Unfortunately, the middle level tested (545 ppm) did not show a statistically significant increase over baseline, so we can't even extrapolate past 700 ppm.
To my knowledge, the point of diminishing returns has never been rigorously studied. We know that there is such a point, because at high enough levels of CO₂, the Calvin cycle simply cannot fix the CO₂ fast enough. What level? Depends on the plant. In many species of plant, what we would consider to be a fairly normal level of CO₂ supplementation is known to decrease photosynthesis. Rice, for example, tops out at around 1200 ppm and then begins to decline, resulting a net reduction in yield of 25 percent by 2500 ppm.
It has also been found that the ability of CO₂ to enhance growth is dependent upon the source form of N that is being provided to the plant. When the source form of N is NO3- instead of NH4+, CO₂ slows down growth enhancement rather dramatically (i.e., the improvement curve is cut–half as steep, in this study). The other thing that happens in elevated CO₂ environments is that heavy-metal uptake is increased. Thankfully, cannabis, though a fairly aggressive accumulator of heavy metals, tends to concentrate them in the root tissue and not in the buds or shoots. Finally, it has been observed that CO₂ growth stimulation is often a short-term phenomenon. The effects of CO₂ enrichment over the long term have yet to be studied.
In conclusion, I hastily point out the obvious: plants in a room that is not supplemented with either plenty of fresh air or CO₂ are going to underperform because they are being slowly suffocated. They need CO₂ and it seems obvious from both anecdotal evidence and scientific literature that CO₂ supplementation works. But the stunning fact is that how much we should supply to our marijuana plants and for how long has not yet been determined. I'm predicting that the answer isn't going to be "1500 ppm of CO₂ all the time."
In Summary: CO₂ supplementation is not as simple and straightforward as the cannabis community has been led to believe for decades. The fact is, we just don't know how much to add or at what intervals. It is obvious to me that supplementation does help, but I have long suspected that the peak level is closer to 900 ppm than the canonical 1500 ppm. That's just my guess, but the limited research that is available hints that I could be right.
138. Photosynthetic response of Cannabis sativa L., an important medicinal plant, to elevated levels of CO₂. Suman Chandra & Hemant Lata & Ikhlas A. Khan & Mahmoud A. El Sohly Physiol Mol Biol Plants (July-September 2011) 17(3):291-295 ↩
139. http://www.esrl.noaa.gov/gmd/ccgg/trends/weekly.html ↩
140. Blackman, Frederick Frost. “Optima and Limiting Factors.” Annals of Botany 2 (1905): 281-96. ↩
141. Blackman, Frederick Frost. “Optima and Limiting Factors.” Annals of Botany 2 (1905): 281-96. ↩
142. Taiz, Lincoln and Eduardo Zeiger. Plant Physiology, Fifth Edition. Sinauer Associates, Inc., 2010. ↩
143. Photosynthetic response of Cannabis sativa L., an important medicinal plant, to elevated levels of CO₂. Suman Chandra & Hemant Lata & Ikhlas A. Khan & Mahmoud A. El Sohly Physiol Mol Biol Plants (July-September 2011) 17(3):291-295 ↩
144. Adv Space Res. 1994 Nov;14(11):257-67. CO₂ crop growth enhancement and toxicity in wheat and rice. Bugbee B(1), Spanarkel B, Johnson S, Monje O, Koerner G. ↩
145. http://5e.plantphys.net/article.php?ch=&id=158 ↩
146. Blackman, Frederick Frost. “Optima and Limiting Factors.” Annals of Botany 2 (1905): 281-96. ↩
147. Ibid. ↩
If you are finding this information useful, why not pick up a copy of Marijuana Cultivation Reconsidered? It contains this information, plus so much more. Marijuana Cultivation Reconsidered is over 300 pages, with over 90 images and illustrations. Danny Danko describes it as “meticulously researched and enormously useful. . . a must-read for any grower striving to learn more than the basics and think outside the parameters of ‘conventional wisdom’ and horticulture folklore.”