Q.31 Transpirational pull that extends down to the roots in plants can be interrupted
by
(A) Process of cavitation
(B) Process of gravitation
(C) Formation of water vapor pockets
(D) Positive pressure in xylem sap
Cavitation interrupts transpirational pull in plants by breaking the continuous water column in xylem. This process disrupts the cohesion-tension mechanism essential for water transport from roots to leaves. The correct answer is option (A).
Question Breakdown
Transpirational pull relies on continuous xylem sap under negative pressure, driven by leaf evaporation. Any break in this column halts flow to roots.
Option Analysis
-
(A) Process of cavitation: Forms vapor cavities or embolisms in xylem under tension, severing water continuity and stopping pull.
-
(B) Process of gravitation: Acts as a constant force overcome by cohesion-tension; transpiration pull routinely counters it up to tree heights over 100m.
-
(C) Formation of water vapor pockets: Describes cavitation outcome, where vapor bubbles create embolisms blocking flow.
-
(D) Positive pressure in xylem sap: Rare root-generated force pushes sap upward, aiding rather than interrupting pull during low transpiration.
Transpirational pull cavitation disrupts the vital upward water movement in plants, a key concept for CSIR NET Life Sciences aspirants studying plant physiology. This force, driven by leaf evaporation, pulls xylem sap from roots against gravity via cohesion-tension theory. Cavitation events create vapor bubbles that sever this column, halting transport.
Cavitation Mechanism
Cavitation occurs under negative xylem pressure when gas bubbles nucleate, expanding into embolisms that block conduits. Pit membranes limit spread, but embolized vessels reduce hydraulic conductance, especially under drought. Plants repair via refilling at night, though tall trees face higher risk.
Why Other Options Fail
| Option | Explanation | Role in Pull |
|---|---|---|
| Gravitation | Constant downward force balanced by tension up to 130m heights . | Overcome, not interruptive . |
| Water vapor pockets | Synonym for cavitation bubbles filling vessels . | Effect of cavitation . |
| Positive pressure | Root-driven push in low-transpiration phases like night . | Supports, doesn’t halt . |
CSIR NET Relevance
This question tests understanding of ascent of sap theories. Cohesion (water-water bonds), adhesion (water-xylem walls), and tension enable pull exceeding gravitational limits. Cavitation vulnerability links to drought tolerance, a frequent exam topic.
