Plant Hormones — Set 1

Correct Answer: B. Auxin

• **Auxin** = Auxin is synthesized at the shoot apex and transported downward, where it suppresses the growth of lateral buds, keeping the plant growing tall rather than bushy — a phenomenon called apical dominance. • **First discovered hormone** — Auxin (IAA) was the first plant hormone ever identified, demonstrated by Charles Darwin's experiments on coleoptiles bending toward light. • When the shoot tip is removed, auxin levels drop, lateral buds are released from inhibition, and side branches grow freely. • 💡 Option A (Ethylene) is wrong because it promotes senescence and ripening, not apical control; Option C (Cytokinin) is wrong because it actually promotes lateral bud growth and opposes apical dominance; Option D (Gibberellin) is wrong because it stimulates stem elongation, not bud suppression.

Correct Answer: B. Ethylene

• **Ethylene** = Ethylene is a gaseous plant hormone that triggers a cascade of biochemical changes during fruit ripening — softening of flesh, conversion of starches to sugars, and development of colour and aroma. • **Climacteric fruits** — Fruits like banana, mango, and tomato show a sharp rise in ethylene production (climacteric rise) just before full ripening, which is why storing unripe bananas near ripe ones speeds up ripening. • In commercial agriculture, ethephon (an ethylene-releasing compound) is sprayed on fruits to achieve uniform, controlled ripening. • 💡 Option A (Auxin) is wrong because it delays fruit drop but does not trigger ripening; Option C (Abscisic Acid) is wrong because it induces dormancy and stress responses, not ripening; Option D (Cytokinin) is wrong because it delays aging and senescence in plant tissues.

Correct Answer: D. Abscisic Acid

• **Abscisic Acid** = Abscisic Acid (ABA) is the primary stress hormone in plants; when water is scarce, ABA levels surge and trigger the closure of stomata by causing potassium ions to leave guard cells, drastically reducing water loss through transpiration. • **'Stress hormone' / 'Dormin'** — ABA was originally called dormin because it was thought to directly cause bud and seed dormancy; it inhibits cell division and elongation, opposing the effects of gibberellins and auxins. • ABA also helps plants respond to cold, salinity, and pathogen attacks by activating specific stress-tolerance genes. • 💡 Option A (Gibberellin) is wrong because it promotes growth and seed germination, the opposite of stress tolerance; Option B (Auxin) is wrong because it stimulates cell elongation and root growth; Option C (Cytokinin) is wrong because it promotes cell division and delays senescence.

Correct Answer: D. Gibberellin

• **Gibberellin** = Gibberellins stimulate both cell division and cell elongation in the internodal regions of the stem, causing a sudden and dramatic increase in stem height — a process called bolting — in rosette plants like cabbage, lettuce, and radish. • **Discovered from 'Bakanae' disease** — Gibberellins were first identified because a fungus (Gibberella fujikuroi) secreted them into rice plants, making them grow abnormally tall and spindly. • Gibberellins are widely used to increase the length of sugarcane internodes, thereby increasing juice yield per stalk. • 💡 Option A (Cytokinin) is wrong because it promotes lateral growth and cell division in leaves, not stem elongation; Option B (Auxin) is wrong because it causes cell elongation primarily in shoots but does not cause the dramatic bolting response; Option C (Ethylene) is wrong because it actually inhibits elongation and causes thickening of stems.

Correct Answer: B. Cytokinin

• **Cytokinin** = Cytokinins are plant hormones that specifically promote cytokinesis (cell division) and are essential for the formation of new cells in growing tissues; coconut milk (liquid endosperm) is one of the richest natural sources of cytokinin, which is why it is used in plant tissue culture media. • **Delay senescence** — Cytokinins actively delay leaf aging (senescence) by preventing the breakdown of proteins and chlorophyll, keeping leaves green and functional longer — a property commercially exploited to extend the shelf life of cut flowers and vegetables. • The first cytokinin discovered was kinetin (a synthetic compound), while zeatin from maize was the first natural cytokinin isolated. • 💡 Option A (Gibberellin) is wrong because it promotes stem elongation, not the cell division associated with coconut milk; Option C (Abscisic Acid) is wrong because it inhibits growth and promotes dormancy; Option D (Auxin) is wrong because it primarily drives cell elongation rather than cytokinesis.

Correct Answer: C. Auxin

• **Auxin** = In phototropism, light causes auxin to migrate laterally from the illuminated side to the shaded side of the shoot; the shaded side, with more auxin, undergoes faster cell elongation, causing the shoot to curve toward the light source. • **Cholodny–Went hypothesis** — This mechanism was explained by the Cholodny–Went theory, which states that unequal auxin distribution on the two sides of a shoot or root drives tropic responses including phototropism and gravitropism. • Darwin's experiments with oat coleoptiles were the earliest evidence that a 'transmissible influence' (later identified as auxin) produced at the tip caused bending. • 💡 Option A (Ethylene) is wrong because it causes stem thickening and fruit ripening, not directional bending; Option B (Abscisic Acid) is wrong because it is a stress/dormancy hormone with no role in tropic bending; Option D (Gibberellin) is wrong because it promotes uniform elongation, not unequal bending toward light.

Correct Answer: C. Gibberellin

• **Gibberellin** = Gibberellins can stimulate fruit development without fertilization (parthenocarpy) by mimicking the hormonal signals that normally require pollination; when sprayed on unpollinated flowers, they induce ovary swelling and fruit formation without producing seeds. • **Commercial application** — This property is extensively used in grape cultivation: GA3 (gibberellic acid) sprays are applied to Thompson Seedless grapes to produce larger, elongated, more marketable berries. • Auxin can also induce parthenocarpy in some species (like tomato), but gibberellin is the more universally used hormone for this purpose in commercial horticulture. • 💡 Option A (Ethylene) is wrong because it promotes fruit ripening and senescence, not seedless fruit formation from unfertilised flowers; Option B (Abscisic Acid) is wrong because it inhibits growth and would prevent fruit development; Option D (Cytokinin) is wrong because it promotes cell division but does not independently trigger parthenocarpic fruit development.

Correct Answer: C. Cytokinin

• **Cytokinin** = Cytokinins delay senescence by actively preventing the degradation of proteins, RNA, and chlorophyll in plant cells; this keeps leaves green (delays yellowing) and maintains the structural integrity of flowers, extending their functional life after harvest. • **Richmond–Lang effect** — The ability of cytokinins to mobilize nutrients toward the site of their application and delay aging in detached leaves is called the Richmond–Lang effect, a classic demonstration in plant physiology. • Commercially, cytokinin-based sprays are used in the post-harvest industry to keep vegetables like spinach, broccoli, and cut flowers fresh for longer periods during storage and transit. • 💡 Option A (Ethylene) is wrong because it accelerates senescence, ripening, and wilting — the exact opposite of what is needed; Option B (Abscisic Acid) is wrong because it promotes leaf drop and dormancy, hastening yellowing; Option D (Auxin) is wrong because it mainly drives cell elongation and root initiation, with no significant role in delaying senescence.

Correct Answer: D. Ethylene

• **Ethylene** = When plant tissues are damaged by physical injury, insect feeding, or pathogen infection, a rapid burst of ethylene production is triggered at the wound site; this acts as a local and systemic signal that activates defence-related genes and accelerates cellular responses to limit damage. • **Wound-induced ethylene** — The wounding response involves conversion of ACC (1-aminocyclopropane-1-carboxylic acid) to ethylene, which then triggers production of defensive compounds such as protease inhibitors that deter herbivores from continuing to feed. • Ethylene also coordinates the sealing of wounds by stimulating the deposition of wound callus tissue and promoting the formation of a protective periderm. • 💡 Option A (Gibberellin) is wrong because it promotes growth and germination and has no primary role in the wound response; Option B (Auxin) is wrong because it regulates growth and tropisms, not injury-defence signalling; Option C (Abscisic Acid) is wrong because, while it handles drought and cold stress, it is not the primary responder to physical wounding or pathogen attacks.

Correct Answer: B. Abscisic Acid

• **Abscisic Acid** = ABA is the principal hormone that imposes and maintains seed dormancy; it suppresses the transcription of genes needed for germination and promotes the synthesis of storage proteins and LEA (late embryogenesis abundant) proteins that protect seeds through harsh winter conditions. • **ABA–Gibberellin antagonism** — The balance between ABA (dormancy) and gibberellin (germination) determines whether a seed germinates; as spring arrives, ABA levels decline and gibberellin levels rise, breaking dormancy and triggering germination. • ABA also promotes the accumulation of reserves in seeds during their maturation on the parent plant, ensuring they are fully stocked with nutrients before dispersal. • 💡 Option A (Gibberellin) is wrong because it breaks dormancy and promotes germination, the opposite of maintaining dormancy; Option C (Cytokinin) is wrong because it promotes cell division and delays senescence but does not maintain seed dormancy; Option D (Auxin) is wrong because it functions in cell elongation and tropic responses, not dormancy maintenance.