Light limits photosynthesis where rate rises with intensity

Steps:

• Examine graph sections: rising slopes indicate light as limiting factor, as rate directly depends on light availability.
• Identify P and R: these show plateaus or declines, where other factors (e.g., temperature, CO2) limit rate.
• Identify Q and S: these exhibit steep increases, confirming light limitation before saturation.
• Compare temperatures: both curves share light-limited phases in Q and S, despite differing maxima.

Why B is correct:

• Per limiting factor principle, light limits when photosynthetic rate is directly proportional to light intensity (initial linear phase of curve).

Why the others are wrong:

• A: P and R are plateau regions, limited by temperature or CO2, not light.
• C: R is non-limiting (plateau); Q is limiting, but pair mismatches.
• D: S is limiting, but P is not (plateau phase).

Final answer: B