This article originally appeared in the January/February 2022 issue of Cannabis Science & Technology.
It is all too common to refer to soil nutrients as “food” for plants, when the real food plants consume is carbon dioxide and photosynthetic light. The remaining parameters of growth are supporting or limiting factors.
In Earth’s atmosphere, carbon dioxide (CO2) content is around 0.04% (400ppm) depending on location and oxygen (O2) content is around 20-21%. Plants utilize both molecules for growth, but because CO2 makes up such a small percentage of air, plants have become very sensitive to fluctuations. Changes in the CO2 content of the air shift how a plant regulates gas exchange through its leaf pores and how it stores and uses carbon. When CO2 content increases, stomatal pores tend to close more and vice versa when CO2 content drops. These changes at the physiological level significantly alter the moisture load on environmental control equipment.
The first article in this series introduced a major protein in leaves: the enzyme RuBisCO. RuBisCo utilizes CO2 during photosynthesis (net gain of energy) or O2 during photorespiration (net loss of energy) to supply metabolic material for growth as well as maintain the various tissues already produced by the plant. From a grower’s perspective, photorespiration is an undesirable process as it restricts new growth by consuming captured photosynthate to keep itself functioning in undesirable conditions. This typically happens when the CO2 content of the air is low (less than earth ambient) or temperature is very high, increasing oxygen solubility in the cell.
CO2 enrichment in cannabis cultivation
Boosting cannabis growth rate with CO2 enrichment is dependent on light intensity, leaf temperature, substrate size (carbohydrate storage by roots) and nutrient concentration – primarily nitrogen. Without sufficient light intensity, nitrogen fertilizer, rootzone size and optimum leaf temperature, the growth response will be limited. If these factors are not balanced, the return on investment for supplying those resources decreases.
By enriching CO2 content to 0.06% of air, the chance of RuBisCO binding O2 is very low. In response to these conditions, binding sites for O2 on RuBisCO begin to decline. Most cannabis production facilities do not supplement CO2 beyond 0.15%. At 0.08%, the optimum return on investment has been reached and there are diminishing returns for further supplementation as the most limiting factors on carbon capture are eliminated. Supplementing further to 0.1-0.12% will fully saturate photosynthesis during highly productive growth phases (such as the initial three to five weeks of the generative phase of cannabis) but the gain in growth rate for pushing to these levels is much less than the jump from 0.04% to 0.08%.
CO2, photosynthesis and plant growth stages
There is no evidence that supplementing CO2 negatively influences cannabis growth, but there is a good chance you may be using more than needed to get the most out of your investment. During the propagation and vegetative stages, most cannabis strains are unable to utilize high light intensity. This is primarily an effect of transpiration as the root system needs to mature in order to supply enough water under a high radiation load. Each stage of growth should have its own CO2 range that best supports the growth potential under the current temperature and lighting conditions.
In the vegetative phase, 0.06-0.08% CO2 is enough to greatly increase the rate of root and shoot growth under any light intensity, but without this amount of CO2 the plant will be unable to use the total light supplied unless intensity is quite low (<200µmol m-2 s-1). The CO2 setpoint at which ROI decreases is dependent on the light intensity and growth stage. Shortly after forcing cannabis into generative growth by restricting light for 12 hours, the photosynthetic capacity greatly increases, and the total light intensity should be ramped up over time. It is critical to also increase CO2 content at this time, otherwise much of the light will be reflected or re-emitted as RuBisCO will be unable to capture carbon fast enough to keep up with the light reactions of photosynthesis without additional CO2. If CO2 is not supplemented when light intensity is increased, photosynthetic rate will decrease as the plant experiences the chronic stress of excess light in the absence of additional carbon substrate.
In an effort to ensure maximal growth, many growers keep CO2 content at 0.15%. In this case, you’ll want to pay very close attention to the plant’s response. If photosynthetic rate increases , then the amount of carbohydrate storage available in the plant will also need to increase. The light reactions of photosynthesis capture energy from air and light, but the dark reactions of photosynthesis use that energy for growth 24-hours per day.
Many scientists present “CO2 response curves,” which show how high photosynthetic rates can be at saturating light intensity when CO2 concentration is incrementally increased over a 30-45 minute period. This really shows how fast a plant can “sprint,” but it does not show how long it can sustain that rate of carbon capture. With unlimited root growth, frequent pruning/training of the canopy and high nitrogen levels, it’s likely that this rate can last a long time, but each species and varietal may need a specific pattern. Signs of damage from high CO2 are often seen as necrotic and chlorotic leaf spots as so much carbohydrate is captured that the plant starts to signal the leaves to cease photosynthesis.
Plants form a circadian rhythm based off of the photoperiod (lights on) and scotoperiod (lights off) and begin to anticipate when light cycles will end. In response to this, you may notice the leaves droop toward the end of the day as the energetic demand has been satisfied and the leaves no longer need to be upright harvesting light. If you notice this happening hours before the end of the photoperiod, it could be that you are supplying too much CO2 without enough space for additional root mass and the plant will slow activity earlier in the day. This only applies to the end of the photoperiod – if your leaves are drooping all the time, it could be one of many problems.
Just like the ability to absorb light, the potential to capture CO2 increases with a more mature root system. The roots are a major storage location for carbohydrates and respiratory activity. Mature roots tend to hold more carbohydrates stored than fine root hairs which primarily absorb moisture. Systems such as deep-water culture allow for unrestricted root growth, but introduce issues such as uncontrollable water-borne pest infection (pythium and fusarium) and risk inadequate oxygen concentration if not supplemented through injection systems.
The key takeaways are that growth enhancement from CO2 supplementation peaks at 800-1000 ppm for most growth stages, higher light intensity is required to get the most out of supplementing CO2, and sufficient root zone volume and mass is required to prevent downregulation of photosynthesis. Keep in mind, new cannabis genetics are constantly being explored and created and most have not been thoroughly studied to identify their maximum limitations and possibilities and it is possible that even higher concentrations may be merited in the future. Some genotypes are more sensitive than others to environmental conditions, so it is important to consider multiple environmental variables.
Maintaining high CO2 is easily attainable for indoor operations with well-sealed production spaces, but many greenhouse producers rely on ventilation as part of climate control and have more difficulty in maintaining a high concentration. For both indoor and greenhouse climates, focus on injecting CO2 at the plant canopy. There are diffusing tubes that can be placed in the duct to release CO2 in response to consumption. There are also Sub-Canopy Airflow Systems to help improve airflow and evenly distribute CO2 and conditioned air. Keep your sensors close to the canopy since the goal is to keep the plant canopy enriched, not necessarily every corner of the room. In greenhouses with heavy ventilation, this method of enrichment is essential to maintain a high concentration around the leaves.
With this knowledge of how CO2 affects photosynthesis, you can start to identify what concentration is right for your crop at your light intensity, temperature setpoint, nitrogen fertilization rate and rootzone volume. CO2 not only affects the rate of photosynthesis, but also has significant effects on stomatal pores, which can drastically change the transpiration rate and subsequently the size and flexibility of environmental conditioning equipment. Keep all of these factors in mind when designing your room control setpoints and the equipment you trust to maintain those variables.