Question

The amount of water transpired by a plant divided by the amount of carbon dioxide assimilated is known as

• A. Water use efficiency
• B. Transpiration ratio
• C. Quantum Efficiency
• D. Carbon Assimilation

Hint:

Always check the numerator and denominator:
Water lost Carbon fixed = Transpiration Ratio (High values indicate less water efficiency).
Carbon fixed Water lost = Water Use Efficiency (High values indicate high water efficiency).

Answer 1

The Correct Option is B

Step 1: Understanding the Concept:
During carbon fixation, plants must open their stomata to allow \( \text{CO}_2 \) to diffuse into the leaf mesophyll.
This opening inevitably allows water vapor to escape from the saturated interior of the leaf to the drier atmosphere, a process known as transpiration.
Key Formula or Approach:
The relationship between water loss and carbon gain is quantified by two inverse indices:
1. Transpiration Ratio (TR):
\[ \text{Transpiration Ratio} = \frac{\text{Amount of Water Transpired}}{\text{Amount of } CO_2 \text{ Assimilated}} \] 2. Water Use Efficiency (WUE):
\[ \text{Water Use Efficiency} = \frac{\text{Amount of } CO_2 \text{ Assimilated}}{\text{Amount of Water Transpired}} = \frac{1}{\text{Transpiration Ratio}} \]

Step 2: Detailed Explanation:
1. Transpiration Ratio (B): This ratio indicates the amount of water a plant must transpire to produce a single gram of dry matter (or assimilate a mole of \( \text{CO}_2 \)).
For \( C_3 \) plants, the transpiration ratio is high (typically 500 to 1000), meaning they lose up to 1000 molecules of water for every molecule of \( \text{CO}_2 \) fixed.
For \( C_4 \) plants, this ratio is lower (250 to 350), and for CAM plants, it is lowest (50 to 100), reflecting their water-conserving adaptations.
2. Water Use Efficiency (A): This is the inverse of the transpiration ratio, representing the amount of carbon fixed per unit of water lost.
3. Quantum Efficiency (C): This measures the efficiency with which light energy is converted into chemical energy, calculated as the moles of \( \text{CO}_2 \) fixed per mole of photons absorbed.
Therefore, the ratio of water transpired to carbon dioxide assimilated is defined as the Transpiration Ratio.

Step 3: Final Answer:
The ratio is called the Transpiration Ratio, corresponding to option (B).