Question

The amount of water transpired by a plant divided by the amount of carbon dioxide assimilated is
(A)
(B)
(C)
(D)

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

Hint:

Transpiration Ratio and Water Use Efficiency are reciprocals of each other!
$TR = 1 / WUE$
Always read the question carefully to see which variable is in the numerator.

Answer 1

The Correct Option is B

Step 1: Understanding the Question:
The question asks for the technical term for the ratio of moles of water lost during transpiration to the moles of $CO_2$ fixed during photosynthesis.
Key Formula or Approach:
There are two ways to express the relationship between water and carbon:
1. Water Use Efficiency (WUE) = $\frac{\text{Carbon Fixed (CO}_2\text{)}}{\text{Water Lost (H}_2\text{O)}}$
2. Transpiration Ratio = $\frac{\text{Water Lost (H}_2\text{O)}}{\text{Carbon Fixed (CO}_2\text{)}}$

Step 2: Detailed Explanation:

• Understanding the Values:
Plants are generally quite inefficient. For every molecule of $CO_2$ fixed, a $C_3$ plant might lose 500-1000 molecules of water.
Therefore, the Transpiration Ratio is a high number (e.g., 500).

• C3 vs C4 Plants:
$C_4$ plants have evolved to be more efficient. They fix $CO_2$ more effectively even when stomata are partially closed.
A $C_4$ plant might have a Transpiration Ratio of 200-300, whereas a CAM plant is even more efficient.

• Clarifying WUE:
If the question had asked for $CO_2$ divided by $H_2O$, the answer would be Water Use Efficiency (WUE). Since it asks for $H_2O$ divided by $CO_2$, it is the Transpiration Ratio.

Step 3: Final Answer:
The ratio of water transpired to carbon dioxide assimilated is called the transpiration ratio.