Question

Pressure potential in the xylem of an actively transpiring plant is

• A. Zero
• B. Negative
• C. Positive
• D. ~ 0.1 MPa

Hint:

Think of the xylem during active transpiration as a drinking straw. When you sip water through a straw, you create a vacuum (negative pressure) to pull the water up. This is exactly how transpiration pulls water up through the xylem.

Answer 1

The Correct Option is B

Step 1: Understanding the Concept:
Water movement through the xylem of tall plants is described by the cohesion-tension theory.
According to this model, water is pulled upward from the roots to the leaves under tension rather than being pushed from below.

Step 2: Detailed Explanation:
During active daytime transpiration, water evaporates from the cell walls of mesophyll cells into the substomatal cavities.
This evaporation creates a surface tension (negative pressure) in the microcapillary spaces of the cell wall.
Because water molecules are bound together by strong cohesive forces (hydrogen bonding) and are attracted to the hydrophilic cell walls of the xylem (adhesive forces), this tension is transmitted downward through the continuous water column in the xylem conduits.
This pull creates a strong negative pressure potential (tension) within the xylem vessels.
This negative pressure potential can reach values of \(-0.5\text{ to } -2.0\text{ MPa}\) (or even lower in tall trees), which is significantly lower than atmospheric pressure.
In contrast, a positive pressure potential (turgor pressure) is typical of living cells, while root pressure is a minor positive pressure generated only under low transpiration.

Step 3: Final Answer:
The pressure potential in the xylem of an actively transpiring plant is negative, corresponding to option (B).