Interpreting Limiting Factor Graphs

Exam questions often give a graph of rate of photosynthesis against one factor, with several curves for different levels of a second factor. Work through it in steps.1. Read the axes. The x-axis factor (for example light intensity) is being varied. Each curve is a different level of another factor (for example carbon dioxide concentration or temperature).2. Rising part of a curve. Where the rate increases as the x-axis factor increases, the x-axis factor is limiting. Where the curves overlap at low light, the other factor is not limiting.3. Plateau. Where a curve levels off, the x-axis factor is no longer limiting; some other factor is.4. Compare curves. If the curve with more carbon dioxide has a higher plateau, then carbon dioxide concentration was limiting on the lower plateau. If two curves have the same plateau, the factor that differs between them is not limiting there.Worked example: at 20 °C and 0.04% CO₂, the rate plateaus at 20 units. At 20 °C and 0.4% CO₂, it plateaus at 50 units. At 30 °C and 0.4% CO₂, it plateaus at 80 units. On the first plateau, carbon dioxide concentration is limiting (raising it increased the rate). On the second, temperature is limiting (raising it increased the rate further).Always quote values from the graph, name the limiting factor at a particular point, and explain it by the effect on the stage of photosynthesis concerned.