Question

The condition required for transpiration (water to move through the plant from the soil to the air) is
{Note: soil water potential (\( \psi_{\text{soil}} \)), root water potential (\( \psi_{\text{root}} \)), leaf water potential (\( \psi_{\text{leaf}} \)), water potential in the atmosphere (\( \psi_{\text{atmosphere}} \))\

• A. \( \psi_{\text{soil}} > \psi_{\text{root}} > \psi_{\text{leaf}} > \psi_{\text{atmosphere}} \)
• B. \( \psi_{\text{root}} > \psi_{\text{soil}} > \psi_{\text{leaf}} > \psi_{\text{atmosphere}} \)
• C. \( \psi_{\text{root}} > \psi_{\text{soil}} > \psi_{\text{atmosphere}} > \psi_{\text{leaf}} \)
• D. \( \psi_{\text{root}} > \psi_{\text{atmosphere}} > \psi_{\text{soil}} > \psi_{\text{leaf}} \)

Hint:

Water always moves from high potential to low potential.
For water to flow from the soil, through the plant, and out into the air, the water potential must become increasingly negative along the path: Soil \( \rightarrow \) Root \( \rightarrow \) Leaf \( \rightarrow \) Atmosphere.

Answer 1

The Correct Option is A

Step 1: Understanding the Concept:
Water moves spontaneously down a thermodynamic gradient, from a region of higher water potential (less negative, closer to zero) to a region of lower water potential (more negative).
The continuous movement of water from the soil, through the plant, and into the atmosphere is described by the Soil-Plant-Atmosphere Continuum (SPAC).

Step 2: Detailed Explanation:
For transpiration to occur continuously without requiring active cellular energy, a continuous decrease in water potential must exist along the entire pathway:
1. Soil Water Potential (\( \psi_{\text{soil}} \)): This must be the highest (least negative) water potential in the continuum to allow water to enter the root hairs. Under well-watered conditions, \( \psi_{\text{soil}} \) is typically close to zero (e.g., \( -0.1\text{ MPa} \)).
2. Root Water Potential (\( \psi_{\text{root}} \)): To draw water from the soil into the root xylem, the water potential of the root cells must be lower (more negative) than that of the soil (e.g., \( -0.3\text{ MPa} \)).
3. Leaf Water Potential (\( \psi_{\text{leaf}} \)): To pull water up through the xylem of the stem and into the leaves against gravity and frictional resistance, the water potential of the leaf mesophyll must be lower than that of the root (e.g., \( -0.8\text{ MPa} \)).
4. Atmospheric Water Potential (\( \psi_{\text{atmosphere}} \)): For water to evaporate from the leaf intercellular spaces into the surrounding air, the atmosphere must have the lowest (most negative) water potential.
Even at a high relative humidity of \( 90\% \), the water potential of the atmosphere is extremely negative (e.g., \( -14\text{ MPa} \)), and it drops further at lower humidities.
Thus, the gradient must follow the sequence:
\[ \psi_{\text{soil}} > \psi_{\text{root}} > \psi_{\text{leaf}} > \psi_{\text{atmosphere}} \]

Step 3: Final Answer:
The required water potential gradient is \( \psi_{\text{soil}} > \psi_{\text{root}} > \psi_{\text{leaf}} > \psi_{\text{atmosphere}} \), corresponding to option (A).