Factor Affecting Photosynthesis MCQ - Practice Questions with Answers

• Law of Limiting Factors: When several factors affect any biochemical process, Law of Limiting Factors comes into effect. It was given by Blackman in 1905.  This states the following:
• 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 its quantity is changed.

Light: 

• There is a linear relationship between incident light and CO2 fixation rates at low light intensities.
• At higher light intensities, gradually the rate does not show further increase as other factors become limiting.
• The light saturation occurs at 10 percent of the full sunlight. 
• Hence, except for plants in shade or in dense forests, light is rarely a limiting factor in nature.
• Increase in incident light beyond a point causes the breakdown of chlorophyll and a decrease in photosynthesis.

Carbon dioxide:

• Carbon dioxide is the major limiting factor for photosynthesis. 
• The concentration of CO2 is very low in the atmosphere (between 0.03 and 0.04 percent). 
• An increase in concentration up to 0.05 percent can cause an increase in CO2 fixation rates; beyond this, the levels can become damaging over longer periods.
• At high light intensities, both C3 and C4 plants show an increase in the rates of photosynthesis. 
• What is important to note is that the C4 plants show saturation at about 360 µlL-1 while C3 responds to increased CO2 concentration and saturation is seen only beyond 450 µlL-1.
• Thus, the current availability of CO2 levels is limiting to the C3 plants

Temperature:

• The dark reactions being enzymatic are temperature controlled. 
• Though the light reactions are also temperature sensitive they are affected to a much lesser extent. 
• The C4 plants respond to higher temperatures and show a higher rate of photosynthesis while C3 plants have a much lower temperature optimum.
• The temperature optimum for photosynthesis of different plants also depends on the habitat that they are adapted to. 
• Tropical plants have a higher temperature optimum than the plants adapted to temperate climates.

Water:

• Even though water is one of the reactants in the light reaction, the effect of water as a factor is more through its effect on the plant, rather than directly on photosynthesis. 
• Water stress causes the stomata to close hence reducing the CO2 availability. 
• Besides, water stress also makes leaves wilt, thus, reducing the surface area of the leaves and their metabolic activity as well.