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 the Concept:
Guttation is the exudation of liquid water (xylem sap) from the tips or margins of leaves, through special pores called hydathodes. The question asks for the environmental condition that leads to guttation.
Step 2: Detailed Explanation:
Guttation is a result of a specific interplay between root pressure and transpiration rate.


Root Pressure: When soil moisture is high, water actively enters the root xylem due to osmotic potential. This builds up a positive hydrostatic pressure, known as root pressure, which pushes water up the xylem column from below.

Transpiration: This is the process of water vapor loss from the plant, primarily through stomata in the leaves. Transpiration creates a negative pressure or tension (transpirational pull) that pulls water up the xylem.

Guttation occurs when:

Root pressure is high: This typically happens at night when the soil is moist and mineral ions are actively transported into the root xylem, causing water to follow by osmosis.

Transpiration rate is very low: This also typically occurs at night due to high humidity, low temperatures, and closed stomata. With transpiration being negligible, the transpirational pull is absent. The positive pressure from the roots continues to push water up the xylem. Since this water cannot escape as vapor, it is forced out in liquid form through the hydathodes.

Let's analyze the options based on this understanding:

(A) Transpiration rate is very high: This would lead to rapid water loss as vapor, preventing any liquid exudation. This is incorrect.

(B) Transpiration rate is very low: This condition, combined with positive root pressure, is precisely what causes guttation. This is correct.

(C) Cell sap is pure water: Cell sap (and xylem sap) is a solution of water, minerals, and organic substances; it is never pure water. This is incorrect.

(D) Temperature is high: High temperatures usually increase the rate of transpiration (unless humidity is 100\%), which would inhibit guttation. This is incorrect.

Step 3: Final Answer:
The primary condition that allows root pressure to cause the exudation of liquid water is a very low rate of transpiration.