Question

Assertion : Due to pollution atmospheric concentration of CO₂ is increasing which will be harmful for C₄ plants whereas productivity is going to increase for C₃ plants.
Reason : C₄ plants have greater efficiency for CO₂ as CO₂ is fixed by PEP oxygenase.

• A. If both assertion and reason are true and reason is the correct explanation of assertion
• B. If both assertion and reason are true but reason is not the correct explanation of assertion
• C. If assertion is true but reason is false
• D. If both assertion and reason are false

Answer 1

Answer: (C)

CO₂ concentration of the atmosphere is 0.036% or 360 ppm (360 μl . l^-1 ). It is a limiting factor for C₃ plants as the available CO₂ concentration is lower than the optimum for photosynthesis. Increase in its concentration upto 0.05% increases the rate of photosynthesis in most C₃ plants. When CO₂ concentration is reduced, there comes a point at which illuminated plant parts stop absorbing carbon dioxide from their environment. It is known as CO₂ compensation point or threshold value. At this value CO₂ fixed in photosynthesis is equal to CO₂ evolved in respiration and photorespiration. The value is 25-100 ppm in C₃ plants and 0-10 ppm in C₄ plants. The reason for low compensation value for C₄ plants is the greater efficiency of CO₂ fixation through PEP‐carboxylase.