Question

Assertion: Plants possessing C₄-pathway of carbon fixation have a higher net primary productivity than the C₃-pathway possessing plants.

Reason: For each unit weight of fixed carbon, C₄-pathway possessing plants require less water than the C₃-pathway possessing plants.

(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

The correct option is: (c)

Explanation:

C₃ plant exclusively fix CO₂ via the Calvin-Benson pathway. The leaf cells of C₄ plants possess two types of chloroplast, bundle sheath cells and mesophyll cells. The mesophyll cells of C₄ plants exhibit high activity of phosphoenolpyruvate (PEP) carboxylase, which catalyses the fixation of CO₂ with PEP to form oxaloacetic acid. The bundle sheath cells exhibit high RuBP carboxylase and the other enzymes of the Calvin-Benson cycle. C₄ plants are highly productive photosynthetically in view of the low affinity of RuBP carboxylase for its substrate. But CO₂ is concentrated as C₄ acids due to the activity of the PEP carboxylase, which in effect maintains high pool levels of CO₂. C₄ plants are related to very high light-harvesting saturation points. C₄ plants exhibit low rates of photorespiration only in bundle sheath cells. C₄ plants are adapted to high temperature. C₄ plants show better utilization of available water but it cannot be the cause of its higher productivity.