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In this article we will discuss about the Blackman’s law of limiting factor of photosynthesis.
Sachs in 1860 for the first time propounded the concept of the three cardinal points. According to this concept, there is a minimum, optimum and maximum for each factor in relation to photosynthesis.
This way, with any given species there may be a minimum temperature below which no photosynthesis takes place, an optimum temperature at which the highest rate takes place and a maximum temperature beyond which no photosynthesis will take place.
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In the twentieth century Blackman (1905) proposed his principle of limiting factors. According to this principle the rate of photosynthesis controlled by several factors is only as rapid as the slowest factor permits.
He claimed that, if all other factors are kept constant the factor under consideration will affect the rate of photosynthesis, starting at a minimum below which no photosynthesis takes place and ending with an optimum at which a horizontal would be established, that is, the rate would remain constant despite further increases in that factor. At this point some other factor becomes limiting.
The explanation of Blackman’s principle can best be presented in terms of the illustration given by Blackman himself. It is to be assumed that light intensity supplied to a leaf is just sufficient to utilize 5 mg of carbon dioxide per hour in photosynthesis. As the carbon dioxide supply is increased the rate of photosynthesis is also increased until 5 mg of carbon dioxide enters the leaf per hour.
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Any further increase in the supply of carbon dioxide will have no influence upon the rate of photosynthesis. Light has now become the limiting factor and further increase in the rate of photosynthesis can be brought about only by an increase in the intensity of light.
These results are indicated graphically. Here the effects of three different light intensities have been shown on the rate of photosynthesis under increasing concentrations of carbon dioxide.
Under low light intensity, the rate of photosynthesis increasing concentrations of carbon dioxide. Concentration is raised until B is reached where further increase in carbon dioxide concentration is not accompanied by any increase in the rate of photosynthesis.
The rate of photosynthesis becomes constant along the line BC. Any further increase in the supply of carbon dioxide will have no effect single upon the rate of photosynthesis, because light intensity has now become the limiting factor. If light intensity is now further increased, the rate of photosynthesis also increases until light again becomes a limiting factor.
Here light becomes the limiting factor at the point C and there is another sharp break in the rate of photosynthesis along the line CF. Further increase in light intensity causes an increase in the rate of photosynthesis along the line CD with a Proportional increase in carbon dioxide concentration.
The rate of photosynthesis becomes constant along the line DE where light again becomes limiting factor. At the points B, C and D the increase in the rate of photosynthesis stops abruptly because one or the other factors becomes limiting.
Light and carbon dioxide are not the only factors which can be limiting in the process of photosynthesis; other factors of photosynthesis can also become limiting under certain conditions.
It becomes clear from the above discussion that when photosynthesis is under the influence of several factors simultaneously, an increase in that factor and the limiting factor will bring about an increase in the rate of photosynthesis.
