Question

The amount of $\text{CO}_2$ assimilated by photosynthesis divided by amount of water transpired by transpiration process is known as}

• A. Water use efficiency
• B. Transpiration ratio
• C. Nutrient Use Efficiency
• D. Quantum efficiency

Hint:

Due to their carbon-concentrating mechanisms, C4 plants have higher water use efficiency (typically $300\text{--}400$) than C3 plants ($600\text{--}1000$), while CAM plants have the highest WUE ($50\text{--}100$).

Answer 1

The Correct Option is A

Step 1: Understanding the Concept:
Plants must open their stomata to absorb carbon dioxide ($\text{CO}_2$) for photosynthesis, which inevitably leads to water loss through transpiration. The balance between carbon gain and water loss is a key determinant of plant productivity.
Key Formula or Approach:
At the leaf level, physiological Water Use Efficiency ($\text{WUE}$) is defined as: \[ \text{WUE} = \frac{\text{Rate of Carbon Assimilation } (A)}{\text{Rate of Transpiration } (E)} = \frac{\text{Moles of }\text{CO}_2\text{ assimilated}}{\text{Moles of }\text{H}_2\text{O}\text{ lost}} \]

Step 2: Detailed Explanation:
- Water Use Efficiency (WUE) is a measure of how efficiently a plant uses water to produce biomass.
- Plants with a high WUE can assimilate more carbon per unit of water lost, which is an important adaptation for survival in arid environments.
- Transpiration Ratio (2): This is the reciprocal of WUE. It represents the amount of water transpired divided by the amount of dry matter produced (e.g., C3 plants transpire 400-900 g of water per gram of dry matter, while C4 plants transpire 250-350 g).
- Quantum Efficiency (4): Measures the moles of carbon fixed per mole of light photons absorbed.
- Nutrient Use Efficiency (3): Measures yield produced per unit of nutrient applied.

Step 3: Final Answer:
The ratio of assimilated $\text{CO}_2$ to transpired water is called Water use efficiency.