Q10. Which naturally occurring plant hormone is commonly called the stress hormone?
[RPSC Senior Teacher Grade II Science, 1 July 2017]
(A) Auxin
(B) Cytokinin
(C) Gibberellin
(D) Abscisic acid
Answer: (D) Abscisic acid
Explanation: Abscisic acid is called the stress hormone because its signalling becomes prominent during drought and other environmental challenges. It helps close stomata, conserve water and maintain seed dormancy. Its action supports survival when growth would be unfavourable.
Q9.
Q8. The Avena curvature test is used for the bioassay of which substance?
[NEET 2016, Phase 1]
(A) IAA
(B) Ethylene
(C) ABA
(D) GA3
Answer: (A) IAA
Explanation: The Avena curvature test uses the bending response of an oat coleoptile to measure auxin activity. Indole-3-acetic acid causes unequal elongation when applied asymmetrically, making curvature a biological indicator of its concentration. It is a classic experiment in plant hormone research.
Q7. Which one of the following is a natural auxin?
[WBCS Preliminary Examination 2015]
(A) NAA
(B) IAA
(C) 2,4-D
(D) IBA
Answer: (B) IAA
Explanation: Indole-3-acetic acid (IAA) is the best-known natural auxin found widely in plants. NAA and 2,4-D are synthetic auxin analogues. IBA can also occur naturally, so the wording is imperfect scientifically; IAA was the answer designated in this examination.
Q6. Which of the following is a plant hormone?
[Chhattisgarh PSC Preliminary Examination 2014]
(A) Insulin
(B) Thyroxine
(C) Estrogen
(D) Cytokinin
(E) None of the above
Answer: (D) Cytokinin
Explanation: Cytokinin is a plant growth regulator that promotes cell division and can delay leaf ageing. Insulin, thyroxine and estrogen are animal hormones with metabolic or reproductive roles. Plant hormones operate at low concentrations and coordinate development and responses to surroundings.
Q5. Which hormone stimulates plant cells to grow so that the plant bends towards light?
[UPSC NDA-I 2013]
(A) Cytokinin
(B) Auxin
(C) Gibberellin
(D) Abscisic acid
Answer: (B) Auxin
Explanation: Auxin accumulates preferentially on the shaded side of a young shoot illuminated from one direction. It stimulates greater cell elongation there, producing curvature towards the light. This directional growth response is phototropism and improves light capture for photosynthesis.
Q4. Which plant hormone exists in gaseous form?
[Chhattisgarh PSC Preliminary Examination 2013]
(A) Abscisic acid
(B) Ethylene
(C) Gibberellins
(D) Auxins
(E) None of these
Answer: (B) Ethylene
Explanation: Ethylene is a gaseous plant hormone at ordinary temperatures and can diffuse through plant tissues and the surrounding air. It regulates ripening, organ abscission and several developmental responses. The other listed hormones are not gases under normal conditions.
Q3. Seed dormancy is regulated chiefly by which of the following?
[SSC MTS 2013]
(A) Abscisic acid
(B) Gibberellic acid
(C) Indole acetic acid
(D) Ethylene
Answer: (A) Abscisic acid
Explanation: Abscisic acid helps establish and maintain seed dormancy, preventing germination when conditions are unsuitable. Gibberellins generally act antagonistically by promoting dormancy release and seed germination. The balance between these hormonal pathways determines whether many seeds remain dormant or begin growing.
Q2. Which phytohormone is mainly responsible for fruit ripening?
[UPPSC Mains 2009 (also UPPSC Prelims 2006)]
(A) Cytokinin
(B) Abscisic acid
(C) Ethylene
(D) None of the above
Answer: (C) Ethylene
Explanation: Ethylene initiates and coordinates many changes during ripening, including pigment development, softening and aroma formation in climacteric fruits. The gas can act on nearby fruits, making synchronized commercial ripening possible under controlled conditions. Cytokinins and abscisic acid have different principal roles.
Q1. Seed dormancy is due mainly to which of the following?
[AIPMT 2002]
(A) Ethylene
(B) Abscisic acid
(C) IAA
(D) Starch
Answer: (B) Abscisic acid
Explanation: Abscisic acid suppresses premature germination and supports dormancy during seed development. Gibberellins tend to counter this effect when conditions favour germination. Ethylene may help overcome dormancy in some species, while starch is a stored carbohydrate rather than a plant hormone.
