Photosynthesis is the process by which plants, algae, and certain bacteria convert light energy into chemical energy in the form of sugars. This conversion requires three primary inputs: water absorbed from the soil, light captured by pigments, and carbon dioxide ($\text{CO}_2$) taken from the atmosphere. The rate of this conversion, known as the photosynthetic rate, is directly influenced by the availability of these raw materials, particularly the concentration of atmospheric $\text{CO}_2$.
Carbon Dioxide as the Essential Reactant
Carbon dioxide is the sole source of carbon atoms that build structural and energy-storing carbohydrate molecules, such as glucose. The gas enters the leaf interior through microscopic pores called stomata, diffusing into the chloroplasts within the plant cells. Carbon fixation occurs here, specifically in the stroma, during the light-independent reactions (the Calvin cycle).
The process of incorporating inorganic $\text{CO}_2$ into an organic molecule is catalyzed by the enzyme ribulose-1,5-bisphosphate carboxylase/oxygenase, known as Rubisco. Rubisco binds $\text{CO}_2$ to a five-carbon sugar, ribulose-1,5-bisphosphate (RuBP), initiating the cycle that synthesizes sugars. Because Rubisco is the primary gatekeeper for carbon entry, the availability of its substrate, $\text{CO}_2$, largely determines the initial velocity of the photosynthetic process.
The Limiting Factor Curve
The relationship between the concentration of $\text{CO}_2$ and the rate of photosynthesis follows a characteristic curve defined by limiting factors. At very low atmospheric $\text{CO}_2$ concentrations, the rate of photosynthesis increases directly with any increase in the gas. Carbon dioxide is the factor in shortest supply, bottlenecking the reaction rate due to the lack of available substrate for the Rubisco enzyme.
As the $\text{CO}_2$ concentration continues to rise, the rate of photosynthesis increases rapidly until it reaches a saturation point. Beyond this level, adding more $\text{CO}_2$ fails to increase the photosynthetic rate, causing the curve to plateau. At this point, the enzyme active sites are fully occupied, or the supply of another input, such as light or water, becomes the new limiting factor. For most plants, this saturation occurs at concentrations significantly higher than the current ambient atmospheric level (approximately 420 parts per million).
How Other Conditions Modify the $\text{CO}_2$ Effect
The concentration at which the $\text{CO}_2$ curve plateaus is not fixed; it is highly dependent on the availability of other photosynthetic inputs. For instance, a plant under low light intensity will reach its saturation point at a much lower $\text{CO}_2$ concentration compared to the same plant under high light. This interaction demonstrates that high $\text{CO}_2$ can only boost the photosynthetic rate if sufficient light energy is available to fuel the light-dependent reactions.
Temperature also significantly modulates the plant’s ability to utilize higher $\text{CO}_2$ levels because the Calvin cycle relies on enzymes, including Rubisco, whose activity is temperature-sensitive. If the temperature is too low, enzyme reaction speeds are slow, and the plant cannot process the excess $\text{CO}_2$ effectively. Conversely, in an environment with enriched $\text{CO}_2$, the optimal temperature for maximizing photosynthesis tends to shift higher, enabling the plant to maintain a high rate.
Real-World Applications and Global Significance
Understanding the $\text{CO}_2$ saturation curve has applications, particularly in controlled-environment agriculture like commercial greenhouses. Growers often employ $\text{CO}_2$ enrichment by injecting pure carbon dioxide into the air, raising the concentration from ambient levels to 1,000 to 1,300 parts per million. This practice boosts the yield of C3 crops, such as tomatoes and cucumbers, by moving the photosynthetic rate past the current atmospheric limiting factor. The result is a substantial increase in growth rate, earlier flowering, and higher fruit production, provided that light, water, and temperature are maintained at optimal levels.
On a global scale, the rise in atmospheric $\text{CO}_2$ concentration, a driver of climate change, produces a phenomenon known as carbon fertilization. This effect describes the increase in global plant growth and biomass accumulation resulting from the increased availability of $\text{CO}_2$ as a substrate. Satellite data has observed a “greening” of vegetated areas due to this increased photosynthetic activity. However, this fertilization effect is often constrained in real ecosystems by a lack of other nutrients, especially nitrogen, and by insufficient water availability, which limits the plant’s ability to capitalize on the excess carbon.