Which factors can limit the rate of photosynthesis and how is the limiting factor determined?

Which factors can limit the rate of photosynthesis and how is the limiting factor determined?

• A. Light intensity, CO2 concentration, and temperature; the rate is limited by the factor in shortest supply at any time.
• B. Water availability only; drought is always the limiting factor.
• C. Soil nutrients; the most limiting is the one with lowest concentration.
• D. Leaf size; larger leaves always have higher rates.

Answer: (A)

The rate of photosynthesis is controlled by the factor that is least available at a given moment. In most leaves the key factors are light intensity, carbon dioxide concentration, and temperature, because each one affects different parts of the process. If light is the limiting factor, the light-dependent reactions can’t produce enough ATP and NADPH to drive the Calvin cycle, so boosting CO2 or warming won’t significantly increase the rate until more light is provided. If CO2 is limiting, the carboxylation step catalyzed by enzymes like Rubisco slows down, so even with ample light the cycle can’t proceed quickly until more CO2 is available. If temperature is outside the optimal range, enzyme activities and membrane processes slow down or become unstable, lowering the rate regardless of light and CO2 levels. You can determine which factor is limiting by changing one variable at a time while keeping the others constant and observing the effect on the rate. If increasing that variable raises the rate, it was the limiting factor. Once that factor is no longer limiting, another one may become limiting, so the limiting factor can shift with changing conditions. So the overall idea is that the rate is governed by the factor in shortest supply at that moment.

The rate of photosynthesis is controlled by the factor that is least available at a given moment. In most leaves the key factors are light intensity, carbon dioxide concentration, and temperature, because each one affects different parts of the process.

If light is the limiting factor, the light-dependent reactions can’t produce enough ATP and NADPH to drive the Calvin cycle, so boosting CO2 or warming won’t significantly increase the rate until more light is provided. If CO2 is limiting, the carboxylation step catalyzed by enzymes like Rubisco slows down, so even with ample light the cycle can’t proceed quickly until more CO2 is available. If temperature is outside the optimal range, enzyme activities and membrane processes slow down or become unstable, lowering the rate regardless of light and CO2 levels.

You can determine which factor is limiting by changing one variable at a time while keeping the others constant and observing the effect on the rate. If increasing that variable raises the rate, it was the limiting factor. Once that factor is no longer limiting, another one may become limiting, so the limiting factor can shift with changing conditions.

So the overall idea is that the rate is governed by the factor in shortest supply at that moment.