Question

State Blackman’s Law of limiting factors.

Answer 1

Hint: Blackman in 1905 found that the rate of photosynthesis varied with the temperature at high light intensities and gave the law of limiting factors. Blackman’s law of limiting factor is the modification of Liebig's Law of the minimum.

Complete step by step answer:
The Blackman’s Law of limiting factor states, “When a process is conditioned as to its rapidity by several separate factors, the rate of the process is limited by the pace of the ‘slowest’ factor.” It means that several factors control the rate of the process. In the case of photosynthesis, there is always a minimum, optimum, and maximum for each factor. No photosynthesis will happen when there is a minimum value for a factor. Maximum photosynthesis will happen when there is an optimum value for a factor. The process of photosynthesis also comes to a halt when there is a maximum value for a factor. The factors can be light, carbon dioxide, water, temperature, oxygen, and osmotic relations.

The law can be best explained by the following illustrations:
Suppose a leaf is exposed to certain light intensity, that can utilize 5 mg. of $CO_2$ per hour in photosynthesis. If only 1 mg. of $CO_2$ enters the leaf in an hour, then the rate of photosynthesis is limited due to the $CO_2$ factor. However, as the concentration of the $CO_2$ increases from 1 to 5 mg. /hour the rate of photosynthesis is also increased along the line AB.
Any further increase in the $CO_2$ concentration will not affect the rate of photosynthesis that has become constant along the line BC. It is because the low light intensity has become a limiting factor in this case. Now the rate of photosynthesis will begin to increase further along the line BD only if the intensity of light is also increased from low to medium. At point D, the medium light intensity will again become a limiting factor and the rate of photosynthesis will again become constant along the line DE.
Similarly, at still higher light intensity and increase in $CO_2$ will bring about an increase in the rate of photosynthesis along with the line DF. And after point F a further increase in $CO_2$ concentration will have no favorable effect because the higher light intensity also becomes a limiting factor, and this leads to the rate of photosynthesis becoming constant along the line FG.
Thus, it is evident from the above illustration that the rate of photosynthesis cannot be increased by increasing only one factor. Other factors should also be increased in proper proportion for the favorable effects to be seen.

Note: A factor, which is present in the smallest amount, may not be limited because it may be required in traces. On the other hand, another factor present in larger amounts may become a limiting factor because its requirement for that physiological process might have been higher.