Sach developed three cardinal points for each factor affecting thephotosynthesis. These cardinals have minimum, optimum and maximum values.
- Minimum: It is the minimum value of a factor at which photosynthesis just starts. Photosynthesis does not occur below this point.
- Optimum: It is the suitable value of a factor. Maximum photosynthesis occurs at this cardinal.
- Maximum: It is the maximum value of a factor. Photosynthesis occurs at that point. But the amount of photosynthesis is much smaller. Photosynthesis stops above this point.
Plant physiologist determined minimum optimum and maximum values of different factor affecting the photosynthesis. Several factors operated simultaneously. They interact with one another. Therefore, different investigators obtained different values for these three cardinal points. F.F. Blackman (1905) solved this problem. He gave the “Principle of limiting factors”. It is stated that when many factors affect on a process the rate of the process is limited by the pace of the slowest factor.” The factor that is lesser than required for the process is called the slowest factor. The principle operates when two or more factors act simultaneously.
Experiment of Blackman
I. CO2 as limiting factor: He exposed a leaf to a light intensity. This light intensity is sufficient to utilize 5 mg of carbon dioxide per hour. If only I mg of the gas enters the leaf per hour, photosynthesis goes on at a certain rate. In this case the increase of intensity of light does not increase in the photosynthetic rate. The light is already present in excess. It is sufficient to utilize I mg of carbon dioxide per hour. He increased the supply of carbon dioxide to 2 mg per hour. The rate of photosynthesis increased at the same light intensity. This shows that the carbon dioxide concentration determine the rate of photosynthesis. Thus carbon dioxide is the limiting factor.
2. Light as limiting factor: The rate of photosynthesis goes on increasing with an increase in the concentration of carbon dioxide. Finally, the supply of carbon dioxide reaches 5mg per hour. At this point the two factors have a maximum effect on photosynthesis. Now light intensity is just enough to utilize 5 mg. Further increase in the supply of carbon dioxide will have no effect on the rate of photosynthesis. Thus light has now become the limiting factor. Rate of photosynthesis can increase only if light intensity is increased.