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).