Question

Given below are two statements, one is labelled as Assertion A and the other is labelled as Reason R
Assertion A : The presence of thick and fleshy water-storing leaves or stems in CAM plants enhances their photosynthetic efficiency.
Reason R : The photosynthetic rates in CAM plants are limited by their vacuolar storage capacity, as malate synthesized at night through carboxylation of PEP has to be stored in the large central vacuole until the daytime.
In light of the above statements, choose the correct answer from the options given below

• 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 the correct explanation of A
• C. A is true but R is false
• D. A is false but R is true

Hint:

CAM plants are masters of "water-use efficiency," not "photosynthetic efficiency." Their growth is slow and limited because they can only store a finite amount of malic acid in their vacuoles each night.

Answer 1

The Correct Option is D

Step 1: Understanding the Concept:
Crassulacean Acid Metabolism (CAM) is a specialized photosynthetic adaptation found in succulent plants growing in arid environments.
It involves a temporal separation of carbon dioxide capture and assimilation to minimize water loss.

Step 2: Detailed Explanation:
Let us evaluate both statements to understand the physiological traits of CAM plants:
Assertion A Analysis: CAM plants possess thick, fleshy, water-storing tissues (succulence) which allow them to store substantial water reserves to survive extreme drought.
However, this adaptation does not enhance their photosynthetic efficiency.
In fact, CAM plants have extremely low photosynthetic efficiency and slow growth rates compared to C3 and C4 plants.
Succulence is an adaptation to maximize water-use efficiency and survival, not photosynthetic rate or efficiency.
Therefore, Assertion A is false.
Reason R Analysis: In CAM plants, stomata open at night to capture \(\text{CO}_2\), which is fixed by PEP carboxylase into malate.
This malate must be stored inside the large central vacuole of the mesophyll cells as malic acid to prevent cellular acidification.
During the day, stomata close, and the stored malic acid is transported out of the vacuole and decarboxylated to release \(\text{CO}_2\) for Rubisco.
Because the plant can only capture as much \(\text{CO}_2\) at night as it can store as malate in its vacuoles, the overall photosynthetic capacity is strictly limited by the vacuolar storage volume.
Therefore, Reason R is true.

Step 3: Final Answer:
Assertion A is false but Reason R is true, which corresponds to option (D).