Question

Pressure potential in the xylem of an actively transpiring plant is

• A. Zero (0)
• B. Negative (-)
• C. Positive (+)
• D. + 0.1 MPa

Hint:

Remember:
- Living plant cell $\rightarrow$ Positive pressure potential ($\Psi_p > 0$, turgor).
- Transpiring xylem $\rightarrow$ Negative pressure potential ($\Psi_p < 0$, tension).

Answer 1

The Correct Option is B

Step 1: Understanding the Concept:
Water potential ($\Psi_w$) in plant cells is the sum of solute potential ($\Psi_s$) and pressure potential ($\Psi_p$).
While pressure potential is typically positive in living cells (turgor pressure), it can become negative in dead conducting cells like xylem under tension.

Step 2: Detailed Explanation:
- Water transport in plants is explained by the Cohesion-Tension theory.
- When a plant is actively transpiring, water evaporates from the mesophyll cell walls into the leaf sub-stomatal cavities, creating a dry surface tension.
- This surface tension pulls water from adjacent cells, creating a continuous negative water potential gradient down to the roots.
- This suction force generates a negative hydrostatic pressure (tension) in the xylem conduits (vessels and tracheids).
- This negative pressure pull is similar to water being sucked through a drinking straw.
- Because the water column is under tension, the pressure potential ($\Psi_p$) in the xylem of an actively transpiring plant is negative.
- If the xylem were under positive pressure, cutting the stem would cause sap to push outward, whereas actively transpiring stems can draw air inward when cut.

Step 3: Final Answer:
The pressure potential in the xylem of an actively transpiring plant is negative.