1. The Blackman’s law of limiting factors states that when a process is conditioned as to its rapidity by a number of separate factors, the rate of the process is controlled by the pace of the “slowest factor”.
2. The slowest factor is that factor which is present in the lowest or minimum concentration in relation to others.
3. The law of limiting factor can be explained by taking two external factors such as carbon dioxide and light.
4. For example, a plant photosynthesizing at a fixed light intensity sufficient to utilize 10mg of C2 per hour only.
5. Photosynthetic rate goes on increasing when concentration of CO2 increases.
6. Further increase in CO2 concentration will not increase the rate of photosynthesis. In this case light becomes the limiting factor. Therefore, under such circumstances rate of photosynthesis can be increased only by increasing the light intensity.
7. This proves that the rate of photosynthesis responds to one factor alone at a time and there would be a sharp break in the curve and a plateau formed exactly at the point where another factor becomes limiting.
8. If any one of the other factors which is kept constant (e.g. Light) is increased, the photosynthetic rate increases again reaching the optimum where again another factor becomes limiting.
