Limiting factor in photosynthesis

Steps:

• Recall that photosynthesis rate is limited by the scarcest resource among light, CO2, temperature, and water.
• At normal atmospheric CO2 (0.04%), this concentration is below the saturation point for the Calvin cycle.
• Evaluate other factors: light intensity is typically sufficient in daylight; temperature is moderate; water is adequate in non-drought conditions.
• Conclude that CO2 limits the rate as it's the bottleneck under standard conditions.

Why A is correct:

• Carbon dioxide is the limiting factor per Blackman's law of limiting factors, as normal atmospheric levels restrict CO2 fixation in the Calvin cycle before other resources saturate.

Why the others are wrong:

• B: Light intensity is not limiting at normal levels, as plants receive ample sunlight for maximum rate.
• C: Temperature is not limiting under typical ambient conditions (15-25°C), which support optimal enzyme activity.
• D: Water availability is not limiting in non-stressed environments, as stomata remain open for CO2 uptake.

Final answer: A