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. Transpiration pull
• B. Root pressure
• C. SPAC
• D. Root hydraulic conductivity

Hint:

Root pressure can force liquid water out of specialized pores at the margins of leaves (hydathodes). This process is known as guttation and is commonly observed in lawns and strawberry leaves early in the morning.

Answer 1

The Correct Option is B

Step 1: Understanding the Concept:
Water transport in plants is driven by pressure differences. While water is typically pulled upward under negative pressure (tension) generated by transpiration, a positive hydrostatic pressure can develop in the roots under certain conditions.

Step 2: Detailed Explanation:
- When a well-watered herbaceous plant is cut near the soil line, xylem sap can be observed exuding from the cut surface.
- This exudation is driven by a positive hydrostatic pressure called root pressure.
- Root pressure develops when:
1. Mineral ions are actively absorbed from the soil solution by root epidermal and cortical cells and transported into the xylem vessels.
2. This active accumulation of solutes lowers the solute potential ($\Psi_s$) inside the xylem, creating a water potential gradient ($\Delta\Psi_w$) between the root cells and the surrounding soil.
3. Water moves osmotically into the xylem, generating a positive hydrostatic pressure that forces water and dissolved nutrients upward through the stem.
- This positive pressure is most pronounced under conditions of low transpiration (such as at night or on humid days) and high soil moisture.
- Transpiration pull (A) generates a negative tension, not a positive pressure.
- SPAC (C) stands for Soil-Plant-Atmosphere Continuum, which describes the continuous water pathway.

Step 3: Final Answer:
The positive pressure in the xylem is called Root pressure.