Limiting Factors Change With Conditions
Definition
Limiting factor
A factor in shortest supply that restricts the rate of a process.
- Temperature, light intensity and carbon dioxide concentration interact because the factor in shortest supply sets the rate.
- If the limiting factor is increased, the rate rises until another factor becomes limiting.
- Increasing a factor that is not limiting produces little or no change in rate.
- Different combinations of conditions therefore produce different maximum rates.
Key Idea
The limiting factor can change as conditions change, so you must identify it from the evidence at the point being considered.
Compare Curves at the Same Point
- Graphs often show rate against light intensity using separate curves for different carbon dioxide concentrations or temperatures.
- Compare the curves at the same light intensity so only the second condition differs.
- If the higher-carbon-dioxide curve has a higher rate, carbon dioxide limited the lower curve at that light intensity.
- If two curves overlap at low light intensity, light is limiting both, so extra carbon dioxide has no effect.
- If a curve plateaus, increasing light no longer helps because carbon dioxide concentration or temperature now limits the rate.
Example
- At 200200200 arbitrary light units, a plant photosynthesises at 888 units with 0.04%0.04\%0.04% carbon dioxide and 121212 units with 0.10%0.10\%0.10% carbon dioxide.
- The higher carbon dioxide concentration raises the rate by 12−8=412 - 8 = 412−8=4 units, so carbon dioxide was limiting at 0.04%0.04\%0.04%.
Control Conditions for Crop Yield
- Growers can add artificial light, enrich the air with carbon dioxide and control temperature to increase photosynthesis.
- Increasing photosynthesis can produce more glucose for growth, so crop biomass and yield may increase.
- Heating, lighting and carbon dioxide enrichment cost money.
- The most profitable conditions are those where the value of the extra yield exceeds the cost of controlling the environment.
Common Mistake
Do not assume the fastest possible rate gives the greatest profit, because the extra crop may be worth less than the energy and equipment used.
Recall the Interactions
- At low light intensity, extra carbon dioxide or warmth may have no effect because light is limiting.
- At high light intensity, carbon dioxide concentration or temperature is more likely to become limiting.
Self review
- Why can the limiting factor change?
- Why might two curves overlap at low light intensity?
- How should two curves be compared fairly?
- Why do growers balance yield against cost?
