By: Due Diligence Horticulture

January 27, 2025

Key Takeaways:

  • Short-term increase in single-leaf photosynthesis at elevated CO2 is greater than the long-term increase in yield 
  • CO2 enrichment from ambient to 1200 ppm increases yield by about 40% 
  • Elevated CO2 reduces transpiration and increasers canopy temperature by up to 3 C The optimum fertilizer concentration increases at elevated CO

Introduction

Carbon dioxide (CO) is essential for photosynthesis, serving as a primary substrate that plants convert into biomass. Current atmospheric COlevels have climbed from 320 ppm in the 1960’s to 420 ppm today (Figure 1)[1]. This rise in CO2 and the associated impacts to global climate sparked widespread research into how plants will respond to rising anthropogenic CO2. Most studies focused on doubling CO2from 350 to 700 ppm, but few studies have evaluated the value of higher CO2 in optimal environments. Elevated CO₂ has been known to enhance plant growth for more than 200 years[2], but the long-term value  of CO2 enrichment in controlled environments is still not well understood. This review examines the effects of elevated CO₂ on short-term photosynthesis and long-term growth and yield of cannabis, summarizing key findings from recent studies and discussing the implications for cannabis cultivation.

line graph depicting Atmospheric CO2 at Mauna Loa Observatory
Figure 1: Atmospheric CO2 at Mauna Loa Observatory

Short-Term Effects of Elevated CO

Photosynthesis

Photosynthesis is a fundamental process involving the assimilation of COand water in the presence of  light to produce glucose and oxygen. This process occurs within the chloroplasts, where COis fixed into  organic molecules via the Calvin cycle. The rate of photosynthesis is influenced by several factors, including  COconcentration, light intensity, temperature, and nutrient availability[3]. Cannabis is known to benefit from  high light intensity, with single-leaf photosynthetic rate increasing up to a photosynthetic photon flux density  (PPFD) of 2000 mol m-2 s-1[4], but less is known regarding the response to elevated CO2

In C3 plants like cannabis, the enzyme Rubisco (ribulose-1,5-bisphosphate carboxylase/oxygenase) plays  a crucial role in the Calvin cycle by fixing CO. However, Rubisco has a dual affinity for both COand  oxygen (O). When Ois fixed instead of CO, a process called photorespiration occurs (Figure 2), leading  to the formation of phosphoglycolate—a compound that is toxic to the plant and requires energy to recycle[5].  Photorespiration reduces the efficiency of photosynthesis by consuming energy and releasing previously  fixed CO.

graphic depicting Rubisco, the enzyme responsible for CO2 fixation, can bind O2 in a process called photorespiration
Figure 2: Rubisco, the enzyme responsible for CO2 fixation, can bind O2 in a process called photorespiration, which reduces photosynthetic efficiency.

Elevated COminimizes photorespiration by increasing the COto Oratio within the chloroplasts. Higher COconcentrations favor the carboxylation reaction over oxygenation, thereby enhancing the likelihood  that Rubisco will fix COrather than O. This shift reduces photorespiratory losses and improves the overall  efficiency of the photosynthesis. 

In Cannabis, short-term single-leaf photosynthesis increased by 50% between 350 to 750 ppm CO24, which is within reported ranges for several C3 crops under similar conditions[6]. Short-term single-leaf photosynthesis measurements provide valuable insight into plant health and physiology, but they often  overestimate the long-term growth and yield response to elevated CO2[7].

Long-Term Effects of Elevated CO

Growth and Yield

Long-term studies evaluating the impact of elevated CO2 on whole plant growth and yield are scarce for most plants, and virtually nonexistent for cannabis. This is because these studies require multiple chambers with sophisticated monitoring to precise control of CO2 concentration, and there are few institutions with the infrastructure and expertise to conduct these studies. That said, I was fortunate to conduct such studies for my PhD dissertation[8] at Utah State University, and I will share the key findings here. Figure 3 shows a picture of plants at harvest after eight weeks of reproductive growth at ambient, 800, and 1200 ppm CO2

high-CBD cultivar 'Peach Goliath' at harvest
Figure 3: Photo of the high-CBD cultivar ‘Peach Goliath’ at harvest after being grown at 460 (ambient), 800, and 1200 ppm CO2 and a DLI of 43.2 mol m-2 d-1.

In my studies, there was a 39% increase in  biomass and a 43% increase in flower yield when grown at 1,400 ppm COcompared to  ambient (Figure 4). Increasing CO2 from ambient to 1200 ppm accounted for 95% of the yield increase. This response was consistent across two cultivars, and three daily light integrals, highlighting the predictable yield response of cannabis to elevated CO2 grown in optimal environments.

graphs depicting biomass and yield of plants grown at increasing CO2 from ambient (420 ppm) up to 1400 ppm
Figure 4: Biomass and yield of plants grown at increasing CO2 from ambient (420 ppm) up to 1400 ppm and a DLI of 32, 43, or 65 mol m-2 d-1. CO2 enrichment increased biomass and yield by about 40% regardless of DLI or cultivar.

While elevated COcan promote plant growth, the overall response can depend on other environmental  factors such as light intensity, temperature, water availability, and nutrient supply. According to the principle  of limiting factors, the rate of a physiological process is restricted by the least available resource. Therefore, optimal environmental conditions are essential for plants to fully capitalize on the benefits of elevated CO. For example, if nutrient availability is suboptimal, the positive effects of elevated COmay not be fully  realized.

Interactions With Other Environmental Factors

Temperature

Temperature and CO2 interact in a couple of ways. The first and most well-known interaction is that CO2enrichment increases the optimum temperature for photosynthesis (Figure 5). This shift is primarily a result of the relative change in O2 and CO2 solubility in water as temperature increases, increasing the CO2 to O2 ratio at Rubisco and effectively inhibiting photorespiration.

graph depicting how CO2 enrichment the optimum temperature for photosynthesis
Figure 5: CO2 enrichment increases the optimum temperature for photosynthesis.

The second way that COand temperature interact is through changes in leaf/canopy temperature. Under higher COconcentrations, stomatal conductance decreases, leading to reduced transpiration rates[9]. Like humans, plants cool themselves by evaporating water (i.e. sweat). The reduction in transpiration at elevated CO2 can increase leaf temperature by up to 2 C and flower temperature by up to 8 C[10]. Figure 6 shows a thermal image of cannabis grown at 420 ppm (left) and 1200  ppm (right) CO2. Lighter colors indicate higher temperature. This effect of CO2 enrichment on canopy/flower  temperature is critical in determining developmental rates and cannabinoid/terpene synthesis in the flowers.

Thermal image of a canopy of cannabis after seven weeks of reproductive growth grown at 420 ppm (left) and 1400 ppm (right)
Figure 6: Thermal image of a canopy of cannabis after seven weeks of reproductive growth grown at 420 ppm (left) and 1400 ppm (right) CO2. Lighter colors indicate higher temperatures. CO2 enrichment reduces evaporative cooling and increasing canopy/flower temperature.

Nutrition

The accelerated growth and reduced transpiration rate associated with elevated COrequires a  corresponding increase in nutrient solution concentration to maintain sufficient levels in the tissue. Inadequate nutrition can limit the potential benefits of COenrichment. Studies have indicated that plants  under elevated COconditions may require twice the concentration of nutrients in solution to maintain the  same concentration in tissue[11] (see previous review on Water Use Efficiency).

Sources of CO2

CO2 tanks and CO2 generators are two common methods for CO2 supplementation[12]. CO2 tanks, often used in smaller operations, involve purchasing compressed CO2 in liquid form and distributing it through a regulated system. A 20-50 pound tank typically costs $150-$200, with refills costing $20-$50 every two weeks for a small greenhouse. While this method allows for precision in controlling CO2 levels, it requires additional equipment like regulators and timers, adding to the overall cost. Over time, the expense of frequent refills can accumulate, especially for larger operations, making this option more suitable for smaller-scale growers. CO2 generators (Figure 7), on the other hand, burn propane or natural gas to produce CO2, heat, and water, making them ideal for larger growers. These systems cost $1,000-$2,500, with additional installation expenses, and are more cost-effective in the long run as they continuously produce CO2 at a lower operational cost. However, generators may produce harmful gases if combustion is incomplete, and they generate excess heat and humidity, which can be problematic in warm, humid climates. While  the upfront costs are higher, CO2 generators are more efficient for larger  operations. On average, it costs around $0.38 per ft2 per year to run CO2 generators, making it a cost-effective option to increase yield.

Natural gas CO2 generator
Figure 7: Natural gas CO2 generator at USU Research Greenhouses.

Conclusion

Elevated COis a low cost and effective way to significantly increase yield of cannabis. Short-term  increases in photosynthetic rates are typically greater than long-term increase in growth and yield. The full  benefits of COenrichment are realized when other growth factors—including light, nutrients, water, and  temperature—are optimized. For cannabis growers aiming to maximize yield, COenrichment represents  a valuable tool within an integrated cultivation strategy. However, careful management of environmental  conditions and nutrient supply is essential to ensure that plants can fully utilize the elevated CO.

 

References 

  1. https://gml.noaa.gov/ccgg/trends/ 
  2. De Saussure, N. T. (1804). Recherches chimiques sur la vegetation. Nyon. 
  3. Taiz, L., & Zeiger, E. (2023). Plant Physiology (7th ed.). Sinauer Associates. 
  4. Chandra, S., Lata, H., Khan, I. A., and Elsohly, M. A. (2008). Photosynthetic response of Cannabis sativa L.  to variations in photosynthetic photon flux densities, temperature and CO2 conditions. Physiol Mol Biol  Plants 14, 299–306. doi: 10.1007/s12298-008-0027-x 
  5. Sharkey, T. D., Bernacchi, C. J., Farquhar, G. D. and Singsaas, E. L. (2007). Fitting photosynthetic  carbon dioxide response curves for C3 leaves. Plant, Cell & Environment 30, 1035–1040. 
  6. Lawlor DW and Mitchell RA. (1999). The effects of increasing CO2 on crop photosynthesis and  productivity: a review of field studies. Plant, Cell & Environment. Oct;14(8):807-18. 
  7. Kirschbaum, M.U., (2011). Does enhanced photosynthesis enhance growth? Lessons learned from CO2  enrichment studies. Plant physiology, 155(1), pp.117-124. 
  8. Westmoreland, F.M. (2024). Environmental Physiology of Medical Cannabis. All Theses and  Dissertations, Fall 2023 to Present. 243. https://digitalcommons.usu.edu/etd2023/243 
  9. Drake, B. G., Gonzalez-Meler, M. A., and Long, S. P. (1997). More efficient plants: a consequence of  rising atmospheric CO? Annual Review of Plant Biology, 48(1), 609-639. 
  10. Nelson, J. A. & Bugbee, B. (2015). Analysis of environmental effects on leaf temperature under sunlight,  high pressure sodium and light emitting diodes. PloS one 10, e0138930. 
  11. Langenfeld, N. J., Pinto, D. F., Faust, J. E., Heins, R., and Bugbee, B. (2022). Principles of Nutrient and  Water Management for Indoor Agriculture. Sustainability 14, 10204. doi: 10.3390/su141610204
  12. Poudel, M., and Dunn, B. (2017). Greenhouse Carbon Dioxide Supplementation. Oklahoma State  University Extension Fact Sheet. HLA-6723.