Question

Exudation of sap from the cut surface of the stem of a well-watered herbaceous plant is due to the positive pressure in the xylem, which is termed as

• A. Root pressure
• B. Transpiration pull
• C. Osmoregulation
• D. Root hydraulic conductivity

Hint:

To distinguish between the two forces: "Transpiration Pull" is a negative pressure (tension) that pulls water up during the day. "Root Pressure" is a positive pressure (push) generated osmotically in the roots, primarily at night when transpiration is low.

Answer 1

The Correct Option is A

Step 1: Understanding the Concept:
Water transport in the xylem is typically driven by negative pressure (tension) generated by transpiration.
However, under certain conditions, such as high soil moisture and low transpiration (e.g., at night or early morning), plants can generate a positive hydrostatic pressure in their roots.

Step 2: Detailed Explanation:
Let us analyze the origin and effects of this positive hydrostatic pressure:
Root Pressure (A): During the night, roots actively absorb mineral ions from the soil and transport them into the xylem vessels of the stele.
This accumulation of solute ions lowers the solute potential of the xylem sap.
In response, water moves osmotically from the soil through the root cortex into the xylem.
Because the endodermis acts as a barrier preventing these ions from leaking back out, a positive hydrostatic pressure is generated within the xylem conduits.
When the stem of a well-watered herbaceous plant is cut, this positive root pressure forces the xylem sap upward and causes it to exude from the cut surface (guttation and exudation).
Transpiration Pull (B): This is a negative tension generated by water evaporation from leaves, which pulls water upward.
It would not cause exudation from a cut stem; rather, it would draw air into the cut xylem.
Osmoregulation (C): This is the active regulation of the osmotic pressure of a cell's fluids to maintain water balance.
Root Hydraulic Conductivity (D): This is a measure of the ease with which water can move through the root tissues.

Step 3: Final Answer:
The positive pressure in the xylem driving sap exudation is termed Root pressure, which corresponds to option (A).