Plant Growth Regulators Practice Questions
20 free Plant Growth Regulators practice questions for the NCERT Biology. Tap an option to answer — you get instant feedback, the correct answer, and a detailed explanation for every question.
The term 'Auxin' was first isolated from which of the following sources?
- A Corn germ oil extract
- B Human urine samples
- C Rhizopus nigricans fungus
- D Fusarium moniliforme fungus
Correct answer: Human urine samples
Auxins were first isolated from human urine by Kogl and Haagen-Smit. In plants, the most common natural auxin is Indole-3-acetic acid (IAA), produced by the growing apices of stems and roots.
Which of the following plant growth regulators is responsible for the 'bakanae' (foolish seedling) disease in rice?
- A Indole-3-acetic acid
- B Abscisic acid
- C Gibberellic acid
- D Zeatin
Correct answer: Gibberellic acid
The bakanae disease in rice seedlings was caused by the fungal pathogen Fusarium moniliforme. E. Kurosawa identified that the symptoms were induced by a substance later named Gibberellic acid.
F.W. Went demonstrated the presence of auxins by performing experiments on the coleoptile tips of:
- A Maize
- B Oat (Avena sativa)
- C Tobacco
- D Rice
Correct answer: Oat (Avena sativa)
F.W. Went isolated auxin from the coleoptile tips of oat seedlings. He showed that the growth-promoting substance could diffuse into agar blocks and still induce curvature in decapitated coleoptiles.
Which specific combination of hormones is often used in tissue culture to induce shoot and root differentiation?
- A Maize (Zea mays)
- B Oat (Avena sativa)
- C Tobacco (Nicotiana tabacum)
- D Rice (Oryza sativa)
Correct answer: Oat (Avena sativa)
F.W. Went isolated auxin from the coleoptile tips of oat (Avena sativa) seedlings. He showed that the growth-promoting substance could diffuse into agar blocks and still induce curvature in decapitated coleoptiles.
Apical dominance, the phenomenon where the apical bud inhibits the growth of lateral buds, is primarily caused by:
- A Cytokinins
- B Gibberellins
- C Auxin (IAA)
- D Ethylene
Correct answer: Auxin (IAA)
Auxins produced at the shoot apex travel downward and inhibit the growth of axillary buds. Decapitation (removal of the shoot tip) usually results in the growth of lateral buds.
Which plant growth regulator is used to speed up the malting process in the brewing industry?
- A GA3
- B IAA
- C ABA
- D Kinetin
Correct answer: GA3
Gibberellic acid (GA3) is used in the brewing industry to increase the yield of alpha-amylase in germinating barley seeds. This enzyme breaks down starch into sugars, accelerating the malting process.
Which hormone is a gaseous plant growth regulator?
- A Abscisic acid
- B Ethylene
- C Zeatin
- D NAA
Correct answer: Ethylene
Ethylene is a simple gaseous PGR produced in large amounts by tissues undergoing senescence and ripening fruits. It is unique among the five major classes of plant hormones due to its physical state.
What is the primary effect of Abscisic acid (ABA) on stomata during water stress?
- A Induces stomatal opening
- B Increases stomatal frequency
- C Stimulates stomatal closure
- D Has no effect on stomata
Correct answer: Stimulates stomatal closure
ABA is often called the 'stress hormone' because it increases the tolerance of plants to various kinds of stresses. It stimulates the closure of stomata in the epidermis and increases the tolerance of plants to drought.
The 'triple response' in seedlings (shortening of the axis, thickening of the axis, and horizontal growth) is a characteristic effect of:
- A Auxin
- B Cytokinin
- C Ethylene
- D Gibberellin
Correct answer: Ethylene
Ethylene induces a specific growth pattern in dicot seedlings known as the triple response. This includes inhibition of stem elongation, stimulation of radial swelling, and horizontal growth/apical hook formation.
Which synthetic auxin is widely used as a weedicide to kill broad-leaved dicotyledonous weeds?
- A IAA (Indole-3-acetic acid)
- B IBA (Indole butyric acid)
- C 2,4-D (Dichlorophenoxyacetic acid)
- D GA3 (Gibberellic acid)
Correct answer: 2,4-D (Dichlorophenoxyacetic acid)
2,4-Dichlorophenoxyacetic acid (2,4-D) is a synthetic auxin used to control dicotyledonous weeds in cereal crops. It does not affect mature monocotyledonous plants.
Bolting, the internode elongation just prior to flowering in rosette plants like beet and cabbage, is induced by:
- A Auxins
- B Gibberellins
- C Cytokinins (kinetin)
- D Ethylene
Correct answer: Gibberellins
Gibberellins promote internode elongation in plants with a rosette habit. Treatment with GA leads to a sudden elongation of the stem followed by flowering.
Kinetin, a modified form of adenine, was first discovered from:
- A Immature coconut endosperm
- B Autoclaved herring sperm DNA
- C Corn kernel extracts
- D Liquid fungal culture media
Correct answer: Autoclaved herring sperm DNA
Skoog and Miller crystallized kinetin from autoclaved herring sperm DNA. It was the first cytokinin identified, though it does not occur naturally in plants; Zeatin was the first natural cytokinin found.
Which hormone is used to induce parthenocarpy (seedless fruit formation) in tomatoes?
- A Auxins
- B Cytokinins
- C Ethylene
- D ABA
Correct answer: Auxins
Auxins are known to induce parthenocarpy in several species. Applying auxins to unpollinated flowers results in the development of fruits without fertilization, such as in tomatoes.
The respiratory climacteric, an increase in respiration rate during fruit ripening, is triggered by:
- A Abscisic acid
- B Ethylene
- C Auxin
- D Gibberellin
Correct answer: Ethylene
Ethylene is highly effective in fruit ripening. It enhances the respiration rate during ripening of the fruits; this rise in rate of respiration is called respiratory climacteric.
Which hormone acts as a general plant growth inhibitor and antagonist to Gibberellins?
- A Auxin (IAA)
- B Ethylene gas
- C Cytokinin hormone
- D Abscisic acid
Correct answer: Abscisic acid
ABA inhibits plant metabolism and growth, including seed germination. In many situations, it acts as an antagonist to GAs, which promote germination and growth.
Cytokinins are primarily synthesized in regions where:
- A Senescence is occurring
- B Rapid cell division occurs
- C Water stress is high
- D Fruit ripening is happening
Correct answer: Rapid cell division occurs
Natural cytokinins are synthesized in regions where rapid cell division occurs, such as root apices, developing shoot buds, and young fruits. They help in producing new leaves and chloroplasts.
Ethephon, the most widely used source of ethylene, is primarily used to:
- A Delay leaf senescence in crops
- B Hasten fruit ripening in tomatoes
- C Inhibit lateral bud growth in trees
- D Promote seed dormancy in cereals
Correct answer: Hasten fruit ripening in tomatoes
Ethephon is an aqueous solution absorbed and transported within the plant to release ethylene slowly. It hastens fruit ripening in tomatoes and apples and accelerates abscission of flowers and fruits.
Which of the following hormones helps to overcome apical dominance?
- A Auxins
- B Cytokinins
- C Gibberellins
- D Ethylene
Correct answer: Cytokinins
While auxins promote apical dominance, cytokinins act antagonistically by promoting the growth of lateral (axillary) buds even in the presence of the apical bud.
Spraying sugarcane crops with which hormone can increase the yield by as much as 20 tonnes per acre?
- A Auxin hormone
- B Cytokinin hormone
- C Gibberellin hormone
- D Ethylene hormone
Correct answer: Gibberellin hormone
Sugarcane stores carbohydrates as sugar in their stems. Spraying the crop with gibberellins increases the length of the stem, thus increasing the yield.
The physiological effect of 'Thinning' in cotton, cherry, and walnut is achieved using:
- A Abscisic acid (ABA)
- B Ethylene
- C Cytokinin
- D Gibberellin (GA)
Correct answer: Ethylene
Ethylene (via Ethephon) promotes female flowers in cucumbers and accelerates the abscission (thinning) of fruits in cotton, cherry, and walnut to improve crop quality.