About Plant Photosynthesis Types (C3, C4, and CAM)

Plants have evolved different strategies to capture CO2 for photosynthesis, which affects their ability to survive in various climates and atmospheric conditions. The "starvation threshold" is the point at which CO2 levels are too low for the plant to gain more carbon than it loses through respiration.

• C3 Plants: This is the oldest and most common photosynthetic pathway, used by about 85% of plants, including trees, wheat, rice, and soybeans. They are most efficient in cool, wet climates with ample CO2. However, their process is wasteful in hot, dry conditions, and they have a relatively high CO2 starvation threshold of around 150 ppm. During Earth's ice ages, when CO2 dropped to 180 ppm, C3 plants were under severe stress.
• C4 Plants: Evolved more recently, C4 plants like corn, sugarcane, and many tropical grasses are more efficient in low-CO2 environments. They use a special enzyme to concentrate CO2 within their leaves, allowing them to thrive in hot, dry conditions where C3 plants struggle. Their starvation threshold is much lower, at around 50 ppm. The decline of CO2 in the last 30 million years likely drove the evolution and expansion of C4 plants.
• CAM Plants: This pathway is an adaptation for extremely dry conditions, used by plants like cacti, succulents, and pineapples. To conserve water, they open their pores (stomata) to collect CO2 only at night. This makes them highly water-efficient, and like C4 plants, they can survive at very low CO2 levels (around 50 ppm).

The CO2 Fertilization Effect in C3 Plants

For C3 plants, today's atmospheric CO2 concentration of over 420 ppm offers a significant advantage compared to the stable, pre-industrial level of 280 ppm. This is known as the CO2 fertilization effect, which boosts growth through several mechanisms:

• Increased Photosynthetic Efficiency: The most critical benefit is the reduction of a wasteful process called photorespiration. The enzyme used by C3 plants, RuBisCO, can mistakenly capture oxygen (O2) instead of CO2, costing the plant energy. At higher CO2 concentrations, RuBisCO is more likely to capture CO2, making photosynthesis much more efficient.
• Faster Growth: With more efficient photosynthesis, plants can produce more carbohydrates (sugars), which fuels faster growth and leads to greater overall biomass.
• Improved Water-Use Efficiency: Plants absorb CO2 through small pores (stomata) which also release water vapor. In a CO2-rich environment, plants can get the carbon they need without opening their stomata as wide or as long, thus conserving water.

Diminishing Returns and Saturation

However, the benefits of increasing CO2 are not infinite. The efficiency gains follow a curve of diminishing returns and eventually plateau as other factors become the main bottleneck on growth.

• CO2-Limited Zone (approx. 150-400 ppm): In this range, CO2 is the primary limiting factor. As seen on the third chart, this is where increasing CO2 provides the most significant boost to plant growth.
• Co-Limitation "Knee" (approx. 400-1000 ppm): As CO2 levels rise further, the benefits begin to taper off. The plant's internal machinery—such as the speed of its enzymes and its ability to use the extra sugars—starts to become the new bottleneck, however increases in CO2 still increase plant efficiency.
• Saturation Zone (>1000 ppm): At very high concentrations, the plant becomes fully CO2-saturated. Adding more CO2 gives no further benefit, as growth is now entirely limited by factors like light intensity or nutrient availability in the soil (e.g., nitrogen and phosphorus).