Interacting factors and the inverse square law

Interacting factors and the inverse square lawSpec 4.4.1.2

These factors interact, and any one of them may be the factor that limits photosynthesis. Which one is limiting can change with conditions, for example from the middle of a summer day to a winter morning.

To explain a graph with two or three factors, follow these steps:

1. Look at the lines (or curves) for each set of conditions.
2. Find the part where a curve is still rising. The factor on the x-axis is limiting there.
3. Find the part where a curve is flat. A factor that is not on the x-axis is limiting there.
4. If a line is higher because another factor was increased (for example a higher carbon dioxide concentration or temperature), that other factor was limiting the lower line.
5. Where two lines are on top of each other, the factor on the x-axis is limiting both, so changing the other factor has no effect there.

Light intensity and the inverse square law

Light intensity is inversely proportional to the square of the distance from the light source. This is the inverse square law. If you move a lamp twice as far away, the light intensity is one quarter as much, not one half.

If light is the limiting factor, the rate of photosynthesis is roughly proportional to light intensity. So halving the distance gives four times the light intensity, and four times the rate (provided nothing else becomes limiting).

Limiting factors and greenhouses

Limiting factors are important in the economics of enhancing the conditions in greenhouses. Growers can add heat, light or carbon dioxide to gain the maximum rate of photosynthesis, so that plants grow faster and yield more.

But each addition costs money for fuel, electricity or carbon dioxide. The grower must still maintain a profit. It is only worth adding a factor if it is the limiting factor, and if the extra crop is worth more than the cost of adding it. Increasing a factor that is not limiting is a waste of money.