Question

Law of limiting factor is
(a) Law of maximum
(b) Law of minimum
(c) Law of optimum
(d) All of the above

Answer 1

Hint: The Law of limiting factor was given by scientist Blackman in 1905 when he studied the effect of various factors like $CO_2$ concentration, light intensity, and temperature on the rate of photosynthesis.

Complete 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 even though a process is controlled by several factors, it is the limiting factor that directly affects the rate of that process. In other words, the rate of a physiological process is limited at a given time by one and only one factor which is deficient. Blackman suggested that 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. Thus, the Law of limiting factor is also known as the law of the minimum.

Additional Information: -Blackman exposed a leaf to a temperature of 20℃ and 0.01% $CO_2$. He observed that in the absence of light, no photosynthesis occurred. On exposing the leaf with a light of low intensity, photosynthesis occurred. The rate of photosynthesis kept on increasing with the increase in light intensity until it leveled off finally.
-This was because, previously light was the limiting factor, affecting the rate of photosynthesis. But now, as it is available in sufficient quantities and $CO_2$ has become the limiting factor. Thus, optimum light intensity and an increase in $CO_2$ will bring about an increase in the rate of photosynthesis.
-When both were provided in sufficient quantities, now the rate of photosynthesis, after increasing initially, again reached a peak and could not be increased further. This time, the temperature has become the limiting factor.
-Thus, it became evident from his experiments 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.
So, the correct answer is ‘Law of minimum.’

Note: -External factors that influence photosynthesis are $CO_2$ concentration, light intensity, quality of light, duration of light, temperature, oxygen concentration, water, and minerals.
-The internal factors or the leaf factors influencing photosynthesis includes the age of the leaf, production of hormones, presence or chlorophyll, and leaf anatomy.