Assertion (A): Positive xylem pressure causes exudation of sap through hydathodes, a process called guttation.

Reason (R): Guttation is noticed when transpiration is suppressed and relative humidity is high.

In the light of the above statements, choose the correct answer from the options given below:

1. Both (A) and (R) are true and (R) is the correct explanation of (A)
2. Both (A) and (R) are true but (R) is NOT the correct explanation of (A)
3. (A) is true but (R) is false
4. (A) is false but (R) is true

   Guttation in plants involves the exudation of xylem sap through hydathodes due to positive xylem pressure, typically under high humidity conditions. Both Assertion (A) and Reason (R) are true, with (R) correctly explaining (A).

   Correct Answer

   Both (A) and (R) are true and (R) is the correct explanation of (A).

   Assertion Analysis

   Assertion (A) states that positive xylem pressure causes sap exudation through hydathodes, termed guttation. This is accurate because root pressure builds positive hydrostatic pressure in xylem when roots absorb water and minerals actively, forcing sap out via hydathodes—specialized pores at leaf margins that remain open unlike stomata.

   Reason Evaluation

   Reason (R) notes guttation occurs when transpiration is suppressed and relative humidity is high. This holds true as low transpiration (e.g., at night or in humid conditions) prevents evaporative water loss, allowing root pressure to dominate and push sap outward.

   Option Breakdown

   • Both true, (R) explains (A): Correct. High humidity suppresses transpiration, enhancing root pressure’s effect, directly linking to positive xylem pressure driving guttation.​

   • Both true, but (R) does not explain (A): Incorrect. (R) precisely describes conditions amplifying the pressure in (A).

   • (A) true, (R) false: Incorrect. (R) accurately reflects guttation triggers.

   • (A) false, (R) true: Incorrect. (A) correctly defines the mechanism.

   Guttation Defined

   Guttation is the plant process where liquid sap exudes from hydathodes due to positive xylem pressure, distinct from dew or transpiration. It supplies excess water and minerals when roots over-absorb under calm conditions.​

   Role of Positive Xylem Pressure

   Roots generate positive xylem pressure via osmosis from mineral uptake, pushing sap upward. This forces droplets out of hydathodes, visible as morning beads on grasses or tomatoes.

   Conditions Favoring Guttation

   High relative humidity and suppressed transpiration (e.g., night-time) allow pressure buildup, as stomata close and evaporation halts. This explains why guttation peaks in saturated soils.

   Feature	Guttation ​	Transpiration
   Pressure	Positive (root-driven)	Negative (evaporation)
   Pathway	Hydathodes	Stomata
   Conditions	High humidity, low transpiration	Dry air, sunlight
   Occurrence	Night/morning	Daytime

   This mechanism aids nutrient cycling but risks pathogen entry if excessive. For exams like GATE Life Sciences, recognize (R) as the key trigger for (A)’s pressure effect.