Light limitation occurs in the initial rising phase of the photosynthesis rate vs. light intensity graph

Steps:

• Recall the standard light response curve: rate increases linearly with light intensity until saturation, where other factors limit.
• Identify points where the curve has a positive slope, indicating light as the limiting factor.
• Point 1 is in the initial steep rise; point 5 is in a similar rising segment on another graph or condition.
• Confirm points 2, 3, 4, and 6 are at plateaus or declines, not light-limited.

Why B is correct:

• Light limits photosynthesis when rate is directly proportional to intensity (per the limiting factor principle), as at points 1 and 5 before saturation.

Why the others are wrong:

• A includes 3, a plateau where CO2 limits, not light.
• C includes 2, past initial rise where saturation begins.
• D includes 4 and 6, both in non-light-limited regions like temperature or CO2 plateaus.

Final answer: B