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The answer is (b) Photorespiration

In C₃ plants the first stable product of CO₂ fixation is phosphoglyceric acid (PGA) whereas, in C₄ plants, the first stable product is oxalic acetic acid (OAA).

The red drop occurs when the light of wavelength greater (longer) than 680 nm is supplied to plant.

Stephen Hales is considered as the father of plant physiology

The light reaction takes place in the grana region of chloroplasts and dark reaction is completed in the stroma region of chloroplasts.

RuBISCO – Ribulose biphosphate carboxylase.

According to Blackman, when some process is affected by more than one factors, then the rate of the process is determined by the factor present in a minimum amount. This is called Blackman’s law of limiting factor.

In 1905, Blackman gave the Law of Limiting factors. When several factors affect any biochemical process, then this law comes into effect. 

This states that:

If a chemical process is affected by more than one factor, then its rate will be determined by the factor which is nearest to its minimal value. It is the factor which directly affects the process if this quantity is changed.

To illustrate the law, suppose light intensity supplied to a leaf is just sufficient to utilize 5 mg of CO₂​ per hour in photosynthesis. As the CO₂​ supply is increased, the rate also increases to 5 mg of CO₂​ enters the leaf per hour. After that, any further increase in the supply of CO_2​ does not have any effect on the rate. Light has now become the limiting factor and further increase in the rate of photosynthesis will occur only by increasing the intensity of light.

Blackman (1905) established the law of limiting factor in connection with photosynthesis. According to this law when a process is conditioned to its rapidity by a number of separate factors, the rate of process is limited by the pace of the lowest factor, i.e., the factor which is present in minimum amount.

The correct answer is (B) C₄ plants

Crassulacean Acid Metabolism (CAM). 

1. It is one more alternative pathway of carbon fixation found in desert plants. 

2. It was first reported in the family Crassulaceae, therefore called as CAM (Crassulacean Acid Metabolism).

3. In CAM plants, stomata are scotoactive i.e. active during night, hence initial CO₂ fixation occurs in night. 

4. Thus, C₄ pathway fix CO₂ at night and reduce CO₂ in day time via the C₃ pathway by using NADPH formed during the day. 

5. PEP caboxylase and Rubisco are present in the mesophyll cell (no Kranz anatomy). 

6. Formation of malic acid during dark is called acidification (phase I).

7. Malate is stored in vacuoles during the night. 

8. Malate releases CO₂ during the day for C₃ pathway within the same cell is called deacidification (phase II). 

9. Examples of CAM plants: Kalanchoe, Opuntia, Aloe etc. 

10. The Chemical reactions of the carbon dioxide fixation and its assimilation are similar to that of C₄ plants.

External factors which affect photosynthesis are as follows:

1. Light: 

a. It is an essential factor as it supplies the energy necessary for photosynthesis. 

b. Quality and intensity of light affects the photosynthesis. 

c. Highest rate of photosynthesis takes place in red light followed by blue light. 

d. The rate of photosynthesis considerably decreases in plants which are growing under a forest canopy. 

e. In most of the plants, photosynthesis is maximum in bright diffused sunlight. 

f. Uninterrupted and continuous photosynthesis for a very long period of time may be sustained without any visible damage to the plant. 

2. Carbon dioxide: 

a. The main source of CO in land plants is the atmosphere, which contains only 0.3% of the gas. 

b. Under normal conditions of temperature and light, carbon dioxide acts as a limiting factor in photosynthesis. 

c. Increase in concentration of CO increases the photosynthesis. 

d. Increase in CO to about 1% is advantageous to most of the plants. 

e. Higher concentration of the gas has an inhibitory effect on photosynthesis. 

3. Temperature: 

a. Like all other physiological processes, photosynthesis also needs a suitable temperature.

b. The optimum temperature at which the photosynthesis is maximum is 25-30 °C. Except in plants like Opuntia, photosynthesis takes place at as high as 55 °C. 

c. This is the maximum temperature. Minimum temperature is temperature at which photosynthesis process just starts. 

d. In the presence of sufficient light and CO , photosynthesis increases with the rise of temperature till it becomes maximum. After that there is a decrease or fall in the rate of the process. 

4. Water: 

a. Water is necessary for photosynthetic process. 

b. An increase in water content of the leaf results in the corresponding increase in the rate of photosynthesis. 

c. Thus, the limiting effect of water is not direct but indirect. 

d. It is mainly due to the fact that it helps in maintaining the turgidity of the assimilatory cells and the proper hydration of their protoplasm.