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Difficulty: MediumTransport Mechanisms in Plants

During humid nights when atmospheric humidity is high and transpiration is minimal, liquid water droplets are observed exuding from leaf margins of herbaceous plants. Which transport mechanism and plant structure account for this liquid exudation?

  1. Root pressure forcing water out through hydathodesAnswer
  2. B
    Transpiration pull drawing sap out through stomata
  3. C
    Active translocation of organic sucrose through sieve tubes
  4. D
    Osmotic suction releasing water through lenticels

Answer

Root pressure forcing water out through hydathodes
The phenomenon described is guttation, which occurs when low transpiration rates combined with high soil moisture cause positive root pressure to push water up the stem and out through specialized pores called hydathodes at leaf margins.

Step-by-Step Solution

1
Analyze the environmental conditions described in the stem.
High atmospheric humidity and nighttime conditions reduce transpiration to near zero, preventing tension-driven transpiration pull.
Stomata close or atmospheric humidity limits the water potential gradient required for evaporation.
2
Identify the primary force driving xylem sap upward under low transpiration.
Active accumulation of mineral ions in root xylem causes osmotic water influx, generating positive hydrostatic root pressure.
Root pressure pushes water upward through the xylem when upward pulling forces are absent.
3
Determine the exit point for liquid water exudation (guttation).
Excess liquid sap is exuded through specialized pores called hydathodes located at leaf tips and margins.
Hydathodes are permanently open vascular terminal structures designed for liquid water discharge.

Key Concept

Guttation and Root Pressure
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Topics

Transport Mechanisms in Plants

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