Question

Guttation occurs when:

• A. Transpiration rate is very high
• B. Transpiration rate is very low
• C. Cell sap is pure water
• D. Temperature is high

Hint:

Remember the key difference: Transpiration is the loss of water as {vapor} from stomata, driven by a pull from above. Guttation is the loss of water as {liquid} from hydathodes, driven by a push from below (root pressure). Guttation happens when the "pull" (transpiration) is weak or absent.

Answer 1

The Correct Option is B

Step 1: Understanding Guttation:
Guttation is the release of liquid water (xylem sap) from leaf tips or edges via hydathodes. The question concerns the environmental condition promoting this phenomenon.
Step 2: Guttation Explained:
Guttation arises from the interaction of root pressure and transpiration rate.


Root Pressure: When soil is moist, water enters root xylem due to osmosis, generating positive hydrostatic pressure (root pressure). This pressure pushes water upwards.

Transpiration: This is water vapor loss from the plant, mainly through stomata. Transpiration creates tension (transpirational pull) that draws water up the xylem.

Guttation occurs when:

Root pressure is high: This happens when the soil is moist, and mineral ions are actively transported into the root xylem, drawing water in by osmosis.

Transpiration rate is low: Typically at night, high humidity, low temperatures, and closed stomata reduce transpiration. The absence of transpirational pull, combined with positive root pressure, forces water out through hydathodes as liquid.

Analyzing the options:

(A) Transpiration rate is very high: This causes rapid water loss as vapor, preventing guttation. Incorrect.

(B) Transpiration rate is very low: This, with positive root pressure, causes guttation. Correct.

(C) Cell sap is pure water: Cell sap (and xylem sap) contains water, minerals, and organic substances; it is never pure water. Incorrect.

(D) Temperature is high: High temperatures usually increase transpiration (unless humidity is 100%), inhibiting guttation. Incorrect.

Step 3: Conclusion:
Guttation, the exudation of liquid water due to root pressure, occurs primarily when the transpiration rate is very low.