The Science Behind CO₂ Enrichment

Carbon dioxide enrichment in greenhouses is one of the most well-documented agricultural interventions, with decades of peer-reviewed research demonstrating substantial yield improvements. However, understanding how plants actually respond to elevated CO₂concentrations—and the economic trade-offs involved—requires looking beyond simple percentage increases to the underlying plant physiology.

How Plants Use CO₂ - Photosynthesis is the process by which plants convert light energy, water, and CO₂ into sugars for growth. The enzyme responsible for capturing CO₂ from the air is called RuBisCO, and it operates at the molecular level inside plant cells. At ambient atmospheric CO₂ levels of around 425ppm, RuBisCO isn't working at full capacity. It's essentially limited by the availability of its substrate—CO₂ molecules. When greenhouse operators increase CO₂ concentration to 600-1200 ppm, they're effectively removing this substrate limitation, allowing RuBisCO to function more efficiently. This accelerates theCalvin Cycle, the series of chemical reactions that fix carbon into organic compounds. The result is faster growth, larger fruit, and higher total yields.

Yield Improvements: What the Research Shows - A 2025 systematic review analyzed CO₂enrichment studies across multiple crops. The findings show that yield responses vary significantly by crop type, light availability, and target CO₂concentration. For tomatoes grown under optimal greenhouse conditions with supplemental lighting, yield increases ranged from 7% to 125%, with most commercial operations achieving 20-40% improvements. Cucumbers typically show20-30% yield gains, while lettuce and herbs respond with 15-25% increases. These aren't theoretical numbers. Commercial greenhouses in the Netherlands, which have been using CO₂ enrichment for decades, routinely achieve tomato yields exceeding 70 kg per square meter annually, nearly double the yields from non-enriched greenhouses.

The Non-Linear Response Curve - One critical aspect that greenhouse operators must understand is that photosynthesis does not respond linearly to CO₂concentration. Research shows that the relationship follows a saturation curve. At low concentrations (below 400 ppm), small increases in CO₂ produce substantial yield gains. As concentration increases above 600 ppm, the incremental benefit per additional unit of CO₂ diminishes. For cucumber grown under high light conditions(1000 μmol/m²/s PAR),photosynthesis rates can increase from approximately 4 grams CO₂ per square meter per hour at 500 ppm to about 7 grams at 1000 ppm. However, the curve flattens significantly beyond 800 ppm, meaning additional CO₂ provides progressively smaller returns. This non-linearity has important economic implications for determining optimal enrichment targets.

Target Concentrations and Economic Efficiency - Most commercial greenhouses target CO₂concentrations between 600-800 ppm during daylight hours when photosynthesis is active. Targeting 600 ppm typically represents the economic sweet spot for many crops—providing substantial yield gains while maintaining reasonable CO₂utilization efficiency. However, efficiency must be understood in the context of greenhouse leakage. Even modern, semi-closed greenhouses lose CO₂continuously through two mechanisms: infiltration through gaps in the structure(typically 5,000-10,000 cubic meters per hectare per hour), and intentional ventilation to control temperature and humidity (which can reach 60,000 cubic meters per hectare per hour on warm days). This means that maintaining a target concentration requires continuous CO₂ supply, with actual crop uptake representing only a portion of the total CO₂ supplied.

Research on Dutch greenhouse tomato production shows that maintaining 425 ppm (ambient outdoor level) requires approximately 60 tons of CO₂ per hectare per year, with utilization efficiency around 98%—meaning almost all supplied CO₂ is used by the crop.[6] Raising the target to 600 ppm increases annual consumption to approximately 282 tons per hectare, but efficiency drops to 24% due to higher ventilation losses. At 800 ppm consumption reaches 444 tons per hectare annually, with efficiency falling to just 16%.

Daily and Seasonal Variation - CO₂ demand in greenhouses is not constant—it varies dramatically with light availability. During nighttime hours, photosynthesis stops entirely, and most crops actually release CO₂ through respiration. During dark periods, CO₂ dosing is typically halted. On a clear summer day, a tomato greenhouse without supplemental lighting will show CO₂ dosing rates rising sharply after sunrise, peaking at midday when solar radiation is highest (often reaching 150-200 kg per hectare per hour), then declining as the sun sets. On cloudy days with intermittent sun, dosing rates fluctuate correspondingly, tracking the available light. This creates a practical challenge: CO₂ supply must be available precisely when plants need it most, during peak sunlight hours. Seasonal variation is equally pronounced. Dutch greenhouses consume more than 2,500 kg of CO₂ per hectare per day during bright summer months, but less than 300 kg per hectare per day during winter when light levels are low and crops grow slowly.

Practical Recommendations - For greenhouse operators evaluating CO₂enrichment, several evidence-based guidelines emerge from the research. Target concentrations of 600-700 ppm provide substantial yield benefits for most crops while maintaining reasonable supply efficiency. Enrichment should be tightly coupled with light availability, either natural solar radiation or supplemental grow lights—to maximize returns. CO₂ supply systems should be sized based on peak daytime demand during high-light periods, not annual average consumption.This prevents the system from becoming the limiting factor during periods when enrichment would provide maximum benefit. Finally, monitoring actual crop uptake and environmental losses can help optimize dosing strategies and identify opportunities to improve greenhouse sealing and reduce wasteful ventilation.

Conclusion - CO₂ enrichment remains one of the highest-return investments in greenhouse production, with well-documented yield improvements across a wide range of crops. However, realizing these benefits requires understanding the underlying plant physiology, the non-linear response curves, the importance of light as a co-factor, and the practical realities of ventilation losses and temporal demand variation. These factors collectively determine both the biological effectiveness and economic viability of different CO₂ supply approaches.