Question

Figure 11.10 shows the effect of light on the rate of photosynthesis. Based on the graph, answer the following questions: 

1. At which point/s (A, B or C) in the curve is light a limiting factor? 
2. What could be the limiting factor/s in region A? 
3. What do C and D represent on the curve?

Answer 1

1. At which point(s) (A, B or C) is light a limiting factor?

Light is a limiting factor in the rising part of the curve, i.e. from the origin up to point B, including point A. Beyond B (towards C), increase in light no longer increases the rate, so light is not limiting there.

2. What could be the limiting factor(s) in region A?

Region A is the initial, low-light region where the rate of photosynthesis increases almost proportionally with light.

• Light intensity is the principal limiting factor in region A.
• Other factors like CO₂ concentration and temperature are usually at non-limiting (adequate) levels in the standard textbook graph, so their effect is not dominant there.

3. What do C and D represent on the curve?

• Point C represents the stage at which further increase in light intensity does not increase the rate of photosynthesis; the rate has reached a light saturation level. Here, some other factor (e.g. CO₂ or temperature) has become limiting.
• Point D (the plateau region beyond C) represents the maximum rate of photosynthesis attained under the given conditions. The curve becomes horizontal, showing that the rate is now independent of light intensity.