Limiting Factor in Photosynthesis

Steps:

• Identify the limiting factor principle: photosynthesis rate is controlled by the resource in shortest supply relative to plant needs.
• Note normal atmospheric CO2 is 0.03%, below the level for maximum rate.
• Recognize that at this CO2 level, increasing it boosts photosynthesis, confirming limitation.
• Assume other factors like light and temperature are at non-limiting levels under standard conditions.

Why A is correct:

• Per Blackman's law of limiting factors, CO2 concentration limits the rate at normal atmospheric levels (0.03%) because it is below the saturation point for the Calvin cycle.

Why the others are wrong:

• B: Light intensity is typically saturating during normal daylight, not limiting.
• C: Temperature is usually near optimal (20-30°C) for enzyme activity, not restricting.
• D: Water availability is adequate in normal environments, without drought stress.

Final answer: A