Question

Identify the crop plants based on the amount of water consumed per gram of $\text{CO}_2$ fixed:
(A) Cowpea < Wheat < Redgram
(B) Tomato < Pineapple < Sorghum
(C) Pineapple < Maize < Rice
(D) Soybean < Sugarcane < Brinjal
Choose the correct answer from the options given below:

• (A) and (B) only
• (B) and (C) only
• (C) only
• (A) only

Hint:

To arrange crops by water consumption per gram of $\text{CO}_2$ fixed:
- Smallest consumption: CAM (Pineapple, Agave).
- Moderate consumption: $\text{C}_4$ (Maize, Sorghum, Sugarcane).
- Largest consumption: $\text{C}_3$ (Rice, Wheat, Legumes).

Answer 1

The Correct Option is C

Step 1: Understanding the Concept:
The amount of water consumed per gram of $\text{CO}_2$ fixed is known as the transpiration ratio (the reciprocal of water-use efficiency, WUE).
Plants with different photosynthetic pathways ($\text{C}_3$, $\text{C}_4$, and CAM) vary significantly in their transpiration ratios due to differences in stomatal opening patterns and carbon-concentrating mechanisms.

Step 2: Detailed Explanation:
Let us compare the transpiration ratios of the three major photosynthetic pathways:
CAM plants (such as Pineapple) are highly efficient in water use.
They open their stomata at night when temperatures are low, resulting in a very low transpiration ratio, typically ranging from $50 - 100\text{ g }H_2O\text{/g }CO_2$ fixed.
$\text{C}_4$ plants (such as Maize, Sugarcane, and Sorghum) concentrate $\text{CO}_2$ inside their bundle sheath cells, allowing them to keep their stomata partially closed during the day.
They have a moderate transpiration ratio, typically ranging from $250 - 300\text{ g }H_2O\text{/g }CO_2$ fixed.
$\text{C}_3$ plants (such as Rice, Wheat, Soybean, Cowpea, Tomato, and Brinjal) must keep their stomata fully open to capture atmospheric $\text{CO}_2$, exposing them to significant water loss.
They have a high transpiration ratio, typically ranging from $500 - 1000\text{ g }H_2O\text{/g }CO_2$ fixed.
Therefore, the amount of water consumed per gram of $\text{CO}_2$ fixed follows the order:
\[ \text{CAM} < \text{C}_4 < \text{C}_3 \]
Let us evaluate Statement (C):
"Pineapple (CAM) < Maize ($\text{C}_4$) < Rice ($\text{C}_3$)"
This sequence aligns with the correct transpiration ratio order ($50 < 250 < 600\text{ g}$).
Other statements contain incorrect combinations of C3, C4, and CAM plants.

Step 3: Final Answer:
Only Statement (C) represents the correct sequence.
This corresponds to Option (C).