Question

Blackman’s law of limiting factor is applied to _____
(a) Respiration
(b) Transpiration
(c) Photorespiration
(d) Photosynthesis

Answer 1

Blackman’s law of limiting factor is applied to a process whose end product is the basis of survival for various organisms. This process is affected by factors like $CO_2$ concentration, light intensity, and temperature.

Complete step by step answer:
The law of limiting factors was given in 1905 by a plant physiologist named Blackman. He studied the effect of various factors like $CO_2$ concentration, light intensity, and temperature on the rate of photosynthesis and came to a conclusion that 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.”In other words, the rate of a physiological process like photosynthesis is limited at a given time by one and only one factor which is deficient. He suggested that maximum photosynthesis will happen only when there is an optimum value for a factor.

Additional Information:
- Blackman exposed a leaf to a temperature of $20^\circ \ C$ 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.
- Respiration is a catabolic process where an energy substrate like carbohydrates is broken down in the presence of oxygen to yield chemical energy in the form of ATP.
- Photorespiration is a non-essential process as it does not produce energy as in a normal respiration pathway. It occurs in an environment where oxygen is high.
- Transpiration is a physical process where a plant loses water molecules from the openings of stomata in the form of water vapors.
So, the correct answer is ‘Photosynthesis.’

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.