Plant Growth Regulators (PGRs)

"Learn all about plant growth regulators – their types, roles, and functions in plant development. Understand how PGRs impact agriculture and crop."

AMM July 28, 2025

Drought in Agriculture: Types and Impact on Farming

Plant Growth Regulators

🌱 Plant Growth Regulators (PGRs) Exam

1. Which amino acid serves as the precursor for Auxin biosynthesis?

A) Methionine

B) Adenine

C) Tryptophan

D) Violaxanthin

Explanation: Tryptophan (with Zn) is the precursor for Auxin biosynthesis in plants.

2. Which PGR is derived from Ent-Kaurene and Acetyl Co-enzyme?

A) Cytokinin

B) Gibberellin

C) Ethylene

D) Abscisic Acid

Explanation: Gibberellins are synthesized from Ent-Kaurene and Acetyl Co-enzyme.

3. Which of the following PGRs is known for inducing parthenocarpy?

A) Auxin

B) Abscisic Acid

C) Cytokinin

D) Gibberellin

Explanation: Both Auxin and Gibberellin can induce parthenocarpy (fruit development without fertilization).

4. The Richmond-Lang effect is associated with which PGR?

A) Auxin

B) Cytokinin

C) Gibberellin

D) Ethylene

Explanation: The Richmond-Lang effect refers to cytokinin's ability to delay senescence in leaves.

5. Which PGR is primarily responsible for fruit ripening?

A) Auxin

B) Cytokinin

C) Gibberellin

D) Ethylene

Explanation: Ethylene is known as the fruit ripening hormone.

6. Which of the following is NOT a function of Auxin?

A) Cell enlargement

B) Root initiation

C) Stomatal closure

D) Prevention of lodging

Explanation: Stomatal closure is regulated by Abscisic Acid (ABA), not Auxin.

7. Which PGR is derived from Violaxanthin?

A) Auxin

B) Cytokinin

C) Gibberellin

D) Abscisic Acid

Explanation: Abscisic Acid (ABA) is synthesized from Violaxanthin.

8. Which synthetic auxin is commonly used as a herbicide?

A) IAA

B) IBA

C) NAA

D) 2,4-D

Explanation: 2,4-D is used both as a fruit setting hormone and as a herbicide.

9. Which PGR is known to increase male flowers in plants?

A) Auxin

B) Cytokinin

C) Gibberellin

D) Abscisic Acid

Explanation: Gibberellins promote male flower formation in plants.

10. What is the commercial name for Chlormequat?

A) Cycocel

B) Sproutstop

C) Cepa

D) None of these

Explanation: Chlormequat is commercially known as Cycocel, a lodging preventer.

11. Which PGR is synthesized from Methionine?

A) Auxin

B) Cytokinin

C) Gibberellin

D) Ethylene

Explanation: Ethylene is synthesized from Methionine via ACC (Aminocyclopropane-1-carboxylic acid).

12. Which of the following is a natural cytokinin?

A) IAA

B) Zeatin

C) 2,4-D

D) GA3

Explanation: Zeatin is a naturally occurring cytokinin in plants.

13. Which PGR is known as the "stress hormone"?

A) Auxin

B) Cytokinin

C) Gibberellin

D) Abscisic Acid

Explanation: ABA is called the stress hormone as it helps plants cope with stress conditions.

14. Which auxin is most commonly used for root formation in cuttings?

A) IAA

B) IBA

C) NAA

D) 2,4-D

Explanation: IBA (Indole-3-butyric acid) is most effective for root formation.

15. What is the primary function of Paclobutrazol?

A) Promote shoot growth

B) Retard plant growth

C) Enhance fruit ripening

D) Increase seed germination

Explanation: Paclobutrazol is a plant growth retardant that decreases shoot growth.

16. Which PGR is used to prevent onion sprouting in storage?

A) Gibberellin

B) Malic Hydrazide

C) Cytokinin

D) Auxin

Explanation: Malic Hydrazide (sold as Sproutstop) prevents onion sprouting in storage.

17. Which of the following PGRs is gaseous in nature?

A) Auxin

B) Cytokinin

C) Gibberellin

D) Ethylene

Explanation: Ethylene is the only gaseous plant hormone.

18. Which PGR is antagonistic to Gibberellin?

A) Auxin

B) Cytokinin

C) Abscisic Acid

D) Ethylene

Explanation: ABA is known as anti-gibberellin as it opposes GA actions.

19. Which synthetic PGR is used for degreening citrus fruits?

A) NAA

B) Ethephon

C) IBA

D) Paclobutrazol

Explanation: Ethephon (trade name Cepa) is used for degreening citrus fruits.

20. Which PGR helps in breaking dormancy of seeds?

A) Auxin

B) Cytokinin

C) Gibberellin

D) Ethylene

Explanation: Both Gibberellin and Ethylene can break seed dormancy.

21. Which PGR is primarily responsible for apical dominance?

A) Auxin

B) Cytokinin

C) Gibberellin

D) Ethylene

Explanation: Auxin maintains apical dominance by suppressing lateral bud growth.

22. Which PGR is synthesized in roots and transported upwards?

A) Auxin

B) Cytokinin

C) Gibberellin

D) Abscisic Acid

Explanation: Cytokinins are primarily synthesized in roots and transported upwards.

23. Which PGR is used to avoid alternate bearing in mango?

A) Gibberellin

B) Auxin

C) Paclobutrazol

D) Ethylene

Explanation: Paclobutrazol helps in avoiding alternate bearing in mango trees.

24. Which PGR increases the number of female flowers in plants?

A) Auxin

B) Cytokinin

C) Gibberellin

D) Ethylene

Explanation: Auxin, Cytokinin, and Ethylene all promote female flower formation.

25. Which PGR is known to cause the triple response in plants?

A) Auxin

B) Cytokinin

C) Gibberellin

D) Ethylene

Explanation: Ethylene causes the triple response: reduced stem elongation, increased lateral growth, and abnormal horizontal growth.

26. Which PGR is synthesized in mature leaves and transported to other parts?

A) Auxin

B) Cytokinin

C) Gibberellin

D) Abscisic Acid

Explanation: ABA is synthesized in mature leaves and transported to other plant parts.

27. Which PGR is used to prevent premature fruit drop in apples?

A) NAA

B) GA3

C) ABA

D) Zeatin

Explanation: NAA (Naphthalene Acetic Acid) is used to prevent premature fruit drop.

28. Which PGR is primarily responsible for phototropism?

A) Auxin

B) Cytokinin

C) Gibberellin

D) Ethylene

Explanation: Auxin distribution mediates phototropic responses in plants.

29. Which PGR is synthesized in response to water stress?

A) Auxin

B) Cytokinin

C) Gibberellin

D) Abscisic Acid

Explanation: ABA levels increase dramatically during water stress conditions.

30. Which PGR is known to promote cell division in tissue culture?

A) Auxin

B) Cytokinin

C) Gibberellin

D) Ethylene

Explanation: Both Auxin and Cytokinin are essential for cell division in tissue culture, with cytokinin being more directly involved.

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Precursors of Plant Growth Regulators (PGRs)

🌱 Auxin - Tryptophan (Zn)
🌱 Gibberellin - Ent-Kaurene (Acetyl Co-enzyme)
🌱 Cytokinin - Adenine (Adenosine Monophosphate - AMP)
🌱 Ethylene - Methionine (Aminocyclopropane-1-carboxylic acid - ACC)
🌱 Abscisic Acid (ABA) - Violaxanthin

Auxin

📍 Inhibit root elongation and promote cell division in tissue culture
📍 Cell enlargement
📍 Prevention of lodging
📍 Induce Parthenocarpy
📍 Delay abscission
📍 Promote root initiation
📍 Increase female flower

IAA: Prevents premature fruit drop

IBA: Root formation

NAA: Fruit thinner, prevents fruit drop

2,4-D: Fruit setting hormone, also used as a herbicide

Cytokinin

📍 Richmond-Lang effect
📍 Dormancy breaker, stimulate cell division, induce flowering
📍 Delay senescence, increase female flower

Gibberellins (kaurene & Acetyl Co-enzyme)

📍 Cell elongation
📍 Increase fruit size
📍 Increase germination
📍 Internodal elongation
📍 Induce parthenocarpy
📍 Delay ripening
📍 Increase male flower, dormancy breaking

Abscisic Acid (ABA)

📌 Dormancy induced stomata closure, anti-gibberellin

Ethylene

📍 Ripening of fruits
📍 Isodiametric growth of stem and roots
📍 Available in two forms: Ethrel, Ethephon
📍 Dormancy breaking, increase female flower

Other Important Chemicals

Chlormequat

📌 Lodging preventer, also known as Cycocel
📌 Dwarfing effect

Malic Hydrazide

📍 Growth retardant
📍 Prevent onion sprouting in storage
📍 Sold under trade name Sproutstop

Ethephon

📌 Trade name: Cepa (liquid form)
📌 Used as a fruit ripener and for degreening in citrus

Paclobutrazol

📌 Plant growth retardant, decreases shoot growth, increases root growth
📌 Antagonist effect to GA3, inhibits GA3 biosynthesis
📌 Avoids alternate bearing in mango
📌 Also reduces frost sensitivity in plants