Question

Photorespiration is found to be zero (or) negligible in these crop plants
(A) Sunflower
(B) Pineapple
(C) Amaranthus
(D) Fingermillet
Choose the correct answer from the options given below:

• A. (A) and (C) only
• B. (B) and (D) only
• C. (A) and (D) only
• D. (B), (C) and (D) only

Hint:

To identify plants with negligible photorespiration:
- Look for $\text{C}_4$ plants (Maize, Sugarcane, Sorghum, Amaranthus, Millets).
- Look for CAM plants (Pineapple, Agave, Orchids, Cacti).
$\text{C}_3$ plants (Wheat, Rice, Barley, Soybean, Sunflower) always have high photorespiration.

Answer 1

The Correct Option is D

Step 1: Understanding the Concept:
Photorespiration ($\text{C}_2$ cycle) occurs when the enzyme RuBisCO binds to oxygen ($\text{O}_2$) instead of carbon dioxide ($\text{CO}_2$), leading to a loss of fixed carbon and energy.
Plants have evolved different photosynthetic adaptations ($\text{C}_4$ and CAM pathways) to concentrate $\text{CO}_2$ around RuBisCO, which suppresses photorespiration and increases water-use efficiency.

Step 2: Detailed Explanation:
Let us analyze the photosynthetic pathways of the listed crop plants:
Sunflower (*Helianthus annuus*) is a $\text{C}_3$ plant.
It lacks any physical or temporal carbon-concentrating mechanisms, meaning RuBisCO is exposed directly to atmospheric oxygen.
As a result, sunflower exhibits high rates of photorespiration, which can consume up to $25\%$ of its fixed carbon under warm, dry conditions.
Pineapple (*Ananas comosus*) is a CAM (Crassulacean Acid Metabolism) plant.
It opens its stomata at night to fix $\text{CO}_2$ as malate, which is stored in vacuoles.
During the day, stomata close, and malate is decarboxylated inside the cells, creating a very high internal concentration of $\text{CO}_2$ that suppresses photorespiration.
Amaranthus (*Amaranthus* species) and Fingermillet (*Eleusine coracana*) are $\text{C}_4$ plants.
They use a spatial separation mechanism where $\text{CO}_2$ is initially fixed in mesophyll cells as a 4-carbon acid and then transported to bundle sheath cells.
In the bundle sheath cells, $\text{CO}_2$ is released directly around RuBisCO, maintaining a high concentration of $\text{CO}_2$ that effectively eliminates photorespiration.
Therefore, photorespiration is zero or negligible in Pineapple, Amaranthus, and Fingermillet.

Step 3: Final Answer:
Photorespiration is negligible in (B), (C), and (D) only.
This corresponds to Option (D).