CO2 as Limiting Factor in Photosynthesis Graphs

Steps:

• Photosynthesis rate vs. light intensity rises steeply at low light (light-limited), then plateaus at high light (CO2-limited).
• Rate vs. CO2 concentration rises at low CO2 (CO2-limited), plateaus at high CO2 (light-limited).
• Rate vs. temperature rises to optimum, then falls; at optimum, CO2 limits if light is saturating.
• Points 2 and 5 mark plateaus or low-CO2 regions where CO2 restricts further rate increase.

Why C is correct:

• C identifies points 2 (light saturation plateau) and 5 (temperature optimum or low CO2), where CO2 is the scarcest factor per Blackman's law of limiting factors.

Why the others are wrong:

• A: Point 1 is light-limited (steep rise), not CO2.
• B: Includes point 1 (light-limited) and 6 (likely temperature decline, enzyme-denatured).
• D: Points 4 and 6 are CO2 saturation (light-limited) and high-temperature drop, respectively.

Final answer: C