Blackman’s Law of Limiting Factors and his Criticism | Photosynthesis

In this article, we will discuss about 1. Illustration of Blackman’s Law of Limiting Factors and 2. Criticism of the Blackman’s Law of Limiting Factors.

Illustration of Blackman’s Law of Limiting Factors:

It had been used to study the effect of individual factors on the rate of photosynthesis in terms of minimum, optimum and maximum (the three cardinal points). Ear­lier investigators could not realise the importance of simultaneous effect of different factors upon the rate of photosynthesis and obtained different values for these cardinal points under different conditions. For example, they found the optimum CO₂ concentration to be greater at high light intensities than at low light intensities.

This problem was first tackled satisfactorily by F.F. Blackman who in 1905 enunciated the law of limiting factors. He states that “When a process is conditioned as to its rapidity by a number of separate factors, the rate of the process is limited by the pace of the ‘slowest’ fac­tor”. To explain this principle, Blackman gave the following illustration which is also shown diagrammatically in Fig. 11.24.

Suppose a leaf is exposed to a certain light intensity which can utilize 5 mg. of CO₂ per hour in photosynthesis. If only 1 mg. of CO₂ enters the leaf in an hour, the rate of photosynthesis is limited due to CO₂ factor. But as the concentration of the CO₂ increases from 1 to 5 mg./hour the rate of photosynthesis is also increased along the line AB.

Any further increase in the CO₂ concentration will have no effect on the rate of photosynthesis which has become constant along the line BC. It is because the low light intensity has become a limiting factor. Now the rate of photosynthesis will increase further along the line BD only if the intensity of light is also increased from low to a medium. At point D, the medium light intensity again becomes limiting factor and the rate of photosynthesis will again become constant along the line DE.

In the same way, at still higher light intensity an increase in CO₂ will bring about an increase in the rate of photosynthesis along the line DF. And after the point F when the higher light intensity also becomes a limiting factor, further increase in CO₂ concentration will have no favourable effect on the rate of pho­tosynthesis which becomes constant along the line FG.

Thus, it is quite evident from the above illustration that the rate of photosynthesis cannot be increased by increasing only one factor. The Other factors should also be increased in proper proportion for favourable effect. Besides CO₂ and light, other factors which af­fect rate of photosynthesis such as temperature, water etc. may also become limiting fac­tors under certain conditions.

Blackman’s law of limiting factors is applicable to any physiological process which is affected by more than one factors. While explaining the principle of limiting factors it should be clearly borne in mind that the absolute magnitude of the factors is not so important as their relative magni­tude.

A factor which is present in smallest amount may not be limiting because its re­quirement may be in traces. On the other hand, another factor present in larger amount may become a limiting factor because its requirement for the physiological process might have been higher. And once the rate of a process has become constant due to a limiting factor, the rate of that process can only be increased by increasing only that factor which is limiting and none else.

Criticism of the Blackman’s Law of Limiting Factors:

In his illustration of the principle of limiting factors Blackman showed abrupt breaks in the rate of photosynthesis along the lines BC, DE and FG (Fig. 11.24) after the par­ticular light intensity had become a limiting factor. This has been criticised by many workers such as James, Harder and others. According to them the rate of photosynthesis declines gradually and not abruptly whenever a factor has become limiting (see dotted curves in fig.).

It is because all the chloroplasts are not under similar environmental conditions at any given time. Some of them might be receiving more light than the others which are deep seated in the leaves. Similarly, some of them might be receiving more CO₂ than the others.

Therefore, if light or CO₂ factors have become limiting for photosynthesis in only some chloroplasts, that factor will not be limiting for photosynthesis in remaining chloro­plasts and hence, the total rate of photosynthesis will decline gradually and not abruptly. Under such conditions more than one factors may become limiting simultaneously.