Question

Given below are two statements, one is labelled as Assertion A and the other is labelled as Reason R
Assertion A: Evapotranspiration decreases with increasing salinity level in soil.
Reason R: Low water availability is caused by high osmotic pressure of soil solution.
In light of the above statements, choose the correct answer from the options given below

• A. Both A and R are true and R is the correct explanation of A
• B. Both A and R are true but R is NOT the correct explanation of A
• C. A is true but R is false
• D. A is false but R is true

Hint:

Saline soils cause physiological drought. Even if the soil is physically wet, the high osmotic pressure prevents plant roots from absorbing water, which reduces transpiration and total ET.

Answer 1

The Correct Option is A

Step 1: Understanding the Concept:
Evapotranspiration (ET) is the sum of water lost from the soil surface via evaporation and from the plant canopy via transpiration.
Total water potential (\(\psi_t\)) in soil is the sum of matric, osmotic, gravitational, and pressure potentials:
\[ \psi_t = \psi_m + \psi_o + \psi_g + \psi_p \] In saline soils, high concentrations of soluble salts lower the soil water potential by reducing osmotic potential (or increasing osmotic pressure).
Detailed Explanation:
Let us analyze both statements:
- Assertion A: High soil salinity reduces the evapotranspiration rate of crops. When plants are grown in saline soils, they transpire less water due to stomatal closure, which helps prevent dehydration. This statement is correct.
- Reason R: The accumulation of soluble salts in the soil solution increases its osmotic pressure, lowering its osmotic potential (\(\psi_o\)). This reduces the total soil water potential, narrowing the water potential gradient between the soil and the plant roots. As a result, the roots must exert higher suction to draw water, leading to a state of physiological drought where less water is available to the plant. This statement is correct.
Since the low water availability caused by high osmotic pressure (R) directly leads to stomatal closure, reduced water uptake, and a lower transpiration rate, Reason R is the correct explanation for Assertion A.

Step 2: Final Answer:
Both A and R are true, and R is the correct explanation of A, corresponding to Option (A).