Question

Identify the crop with the highest Water Use Efficiency (WUE) :

• A. Sorghum
• B. Alfalfa
• C. Sugarbeet
• D. Pineapple

Hint:

Hierarchy of WUE: CAM $>$ C4 $>$ C3.
If you see a succulent or desert-adapted plant like Pineapple or Cactus in a WUE question, it's almost certainly the correct answer!

Answer 1

The Correct Option is D

Step 1: Understanding the Question:
Water Use Efficiency (WUE) is defined as the amount of dry matter produced per unit of water transpired. The question asks which crop among the list is most efficient in using water.
Detailed Explanation:

• Comparison of Photosynthetic Pathways:
WUE is largely determined by a plant's photosynthetic pathway (C3, C4, or CAM).
C3 plants (Alfalfa, Sugarbeet): These are the least efficient. They keep stomata open during the day and lose significant water through transpiration. Typical WUE: 1-3 $mg/g$ $H_2O$.
C4 plants (Sorghum): More efficient than C3. They concentrate $CO_2$ around RuBisCO, allowing them to keep stomata partially closed. Typical WUE: 2-5 $mg/g$ $H_2O$.
CAM plants (Pineapple): The most efficient. They open stomata only at night when temperatures are low and humidity is high, minimizing water loss. They fix $CO_2$ as malic acid and use it during the day. Typical WUE: 10-40 $mg/g$ $H_2O$.

• Analyzing the Crop:
Pineapple is a classic Crassulacean Acid Metabolism (CAM) plant. Because of its night-time $CO_2$ fixation strategy, it has a much higher WUE than C3 crops (Sugarbeet, Alfalfa) or even C4 crops (Sorghum).

Step 2: Final Answer:
Pineapple, being a CAM plant, has the highest water use efficiency among the options provided.