Question

Which one of the following is NOT an auxin?

• A. Indole-3-acetic acid
• B. Abscisic acid
• C. Phenylacetic acid
• D. Indole-3-butyric acid

Hint:

IAA and IBA are important auxins, whereas ABA (abscisic acid) is mainly a stress hormone involved in seed dormancy and stomatal closure.

Answer 1

The Correct Option is B

Step 1: Understand auxins.
Auxins are a class of plant growth hormones mainly involved in cell elongation, apical dominance, root initiation, and tropic responses in plants.

Step 2: Analyze Indole-3-acetic acid (IAA).
Indole-3-acetic acid is the principal naturally occurring auxin found in plants. Therefore, option (A) is an auxin.

Step 3: Analyze Phenylacetic acid.
Phenylacetic acid also exhibits auxin-like activity and is considered a naturally occurring auxin in plants. Hence, option (C) is an auxin.

Step 4: Analyze Indole-3-butyric acid (IBA).
Indole-3-butyric acid is a synthetic or naturally occurring auxin commonly used for stimulating root formation in plant cuttings. Thus, option (D) is also an auxin.

Step 5: Analyze Abscisic acid (ABA).
Abscisic acid is not an auxin. It is a different class of plant hormone mainly associated with stress responses, seed dormancy, and stomatal closure.

Step 6: Compare all options.
Among the given compounds, only abscisic acid belongs to a different plant hormone category and does not function as an auxin.

Step 7: Final conclusion.
Therefore, the compound that is NOT an auxin is abscisic acid.
\[ \boxed{\text{Abscisic acid}} \]
Hence, the correct answer is option (B).