Question

Assertion : Due to pollution atmospheric concentration of CO₂ is increasing which will be relatively more harmful for C₄ plants that C₃ plants

Reason : C₄ plants have greater efficiency for CO₂ as CO₂ is fixed by PEP oxygenase.

(a) Both A and R are true and R is the correct explanation of A

(b) Both A and R are true but R is not correct explanation of A

(c) A is true but R is false

(d) A and R are false

Answer 1

Correct option (c) 

Explanation :

CO₂ concentration of the atmosphere is 0.036% or 360 ppm (360 μl l^-1). It is a limiting factor for C₃ 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 then 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.