Question

The amount of carbon dioxide assimilated by photosynthesis divided by amount of water transpired is known as

• A. Water use efficiency
• B. Transpiration ratio
• C. Quantum efficiency
• D. Quantum yield

Hint:

Water Use Efficiency (WUE) is simply the "Carbon Gain Water Lost" ratio. C4 and CAM plants have much higher WUE than C3 plants because their specialized metabolic pathways allow them to minimize stomatal opening.

Answer 1

The Correct Option is A

Step 1: Understanding the Concept:
Terrestrial plants face a constant compromise: they must open their stomata to capture atmospheric \(\text{CO}_2\) for photosynthesis, which inevitably allows water vapor to escape through transpiration.
The relationship between carbon gain and water loss is a key physiological index of plant adaptation.
Key Formula or Approach:
The ratio is mathematically expressed as:
\[ \text{Water Use Efficiency (WUE)} = \frac{\text{Amount of } \text{CO}_2 \text{ Assimilated}}{\text{Amount of Water Transpired}} \]

Step 2: Detailed Explanation:
Let us analyze the definitions of the terms:
Water Use Efficiency (WUE) (A): This measures the efficiency with which a plant uses water to produce biomass.
A higher WUE means the plant can assimilate more carbon while losing less water, a trait highly developed in C4 and CAM plants.
Transpiration Ratio (B): This is the reciprocal of WUE, representing the amount of water transpired divided by the amount of \(\text{CO}_2\) fixed.
Typically, C3 plants have a high transpiration ratio ($400 - 500\text{ g } \text{H}_2\text{O}$ per $\text{g } \text{CO}_2$), while CAM plants have a very low ratio ($50 - 100\text{ g } \text{H}_2\text{O}$ per $\text{g } \text{CO}_2$).
Quantum Yield (D): This is the number of \(\text{CO}_2\) molecules fixed per photon of light absorbed, measuring light-use efficiency.
Therefore, the ratio of carbon assimilated to water transpired is water use efficiency.

Step 3: Final Answer:
The ratio is termed Water use efficiency, which corresponds to option (A).