Question

Given below are two statements, one is labelled as Assertion A and the other is labelled as Reason R.
Assertion A: CAM plants are present in dry and arid environments.
Reason R: The CAM pathway is adapted to minimize water loss and photorespiration.

In light of the above statements, choose the most appropriate answer from the options given below:

• A. Both A and R are correct and R is the correct explanation of A
• B. Both A and R are correct but R is NOT the correct explanation of A
• C. A is correct but R is not correct
• D. A is not correct but R is correct

Hint:

Key differences in Photosynthetic Pathways:
- C3: No spatial or temporal separation (high photorespiration).
- C4: Spatial separation (Kranz anatomy) between mesophyll and bundle sheath cells.
- CAM: Temporal separation (night vs. day) of carbon fixation.

Answer 1

The Correct Option is A

Step 1: Understanding the Concept:
Crassulacean Acid Metabolism (CAM) is a specialized photosynthetic carbon fixation pathway that has evolved in certain plants as an adaptation to arid conditions.

Step 2: Detailed Explanation:
Let us analyze both statements:
Assertion A:
CAM plants (such as cacti, pineapples, agaves, and sedums) are highly abundant in dry, arid, and desert environments.
This distribution is due to their specialized physiology, which allows them to maintain photosynthetic activity under extreme water limitations.
Thus, Assertion A is correct.
Reason R:
The CAM pathway is structurally and temporally adapted to conserve water:
1. Temporal Separation: CAM plants open their stomata at night (nocturnal) when temperatures are lower and relative humidity is higher, minimizing water loss via transpiration.
2. They capture $CO_2$ at night, converting it into malic acid, which is stored in vacuoles.
3. During the day, stomata remain closed to prevent transpiration. Malic acid is then decarboxylated to release $CO_2$ directly to RuBisCO, creating a high internal $CO_2$ concentration.
This high local $CO_2$ concentration suppresses the oxygenase activity of RuBisCO, minimizing photorespiration.
Thus, Reason R is correct and directly explains why these plants are uniquely adapted to and present in dry, arid environments.

Step 3: Final Answer:
Both A and R are correct, and R is the correct explanation of A.
This corresponds to Option (A).