
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 CO2
Introduction
Carbon dioxide (CO₂) is essential for photosynthesis, serving as a primary substrate that plants convert into biomass. Current atmospheric CO₂ levels 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.

Short-Term Effects of Elevated CO₂
Photosynthesis
Photosynthesis is a fundamental process involving the assimilation of CO₂ and water in the presence of light to produce glucose and oxygen. This process occurs within the chloroplasts, where CO₂ is fixed into organic molecules via the Calvin cycle. The rate of photosynthesis is influenced by several factors, including CO₂ concentration, 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 CO₂. However, Rubisco has a dual affinity for both CO₂ and oxygen (O₂). When O₂ is fixed instead of CO₂, 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 CO₂.

Elevated CO₂ minimizes photorespiration by increasing the CO₂ to O₂ ratio within the chloroplasts. Higher CO₂ concentrations favor the carboxylation reaction over oxygenation, thereby enhancing the likelihood that Rubisco will fix CO₂ rather than O₂. 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 CO₂
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.

In my studies, there was a 39% increase in biomass and a 43% increase in flower yield when grown at 1,400 ppm CO₂ compared 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.

While elevated CO₂ can 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 CO₂. For example, if nutrient availability is suboptimal, the positive effects of elevated CO₂ may 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.

The second way that CO₂ and temperature interact is through changes in leaf/canopy temperature. Under higher CO₂ concentrations, 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.

Nutrition
The accelerated growth and reduced transpiration rate associated with elevated CO₂ requires a corresponding increase in nutrient solution concentration to maintain sufficient levels in the tissue. Inadequate nutrition can limit the potential benefits of CO₂ enrichment. Studies have indicated that plants under elevated CO₂ conditions 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.

Conclusion
Elevated CO₂ is 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 CO₂ enrichment are realized when other growth factors—including light, nutrients, water, and temperature—are optimized. For cannabis growers aiming to maximize yield, CO₂ enrichment 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 CO₂.
References
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