Plant Hormones — Set 2

Correct Answer: D. Gibberellins

• **Gibberellins** = The 'Bakanae' (foolish seedling) disease causes infected rice plants to grow abnormally tall and spindly before collapsing; Japanese scientists in the 1920s–30s identified that the fungus Gibberella fujikuroi (now Fusarium fujikuroi) secreted a substance responsible for this excessive growth, which was eventually isolated and named gibberellin. • **E. Kurosawa (1926)** — Eiichi Kurosawa demonstrated that sterile filtrates of the fungal culture, when applied to healthy rice seedlings, reproduced the Bakanae symptoms, establishing gibberellin as a chemical growth regulator produced by the fungus. • Gibberellins are now known to be produced naturally by all higher plants and play key roles in stem elongation, seed germination, and flowering. • 💡 Option A (Cytokinins) is wrong because they were discovered later from maize kernels and promote cell division, not excessive elongation; Option B (Auxins) is wrong because they were discovered earlier through phototropism experiments, not the Bakanae disease; Option C (Ethylene) is wrong because it was identified through observations of gas-induced fruit ripening, unrelated to the foolish seedling disease.

Correct Answer: C. Auxin

• **Auxin** = Auxin (particularly IBA — indole-3-butyric acid) is the most effective hormone for inducing the formation of adventitious roots on stem cuttings; it is applied as a rooting powder or gel at the cut base of a stem, where it stimulates cell differentiation into root initials. • **IBA in horticulture** — IBA is the preferred commercial rooting hormone because it is more stable than IAA; nurseries routinely use IBA-based rooting powders to propagate plants vegetatively, enabling faster, more reliable multiplication of ornamental and fruit plants. • Adventitious roots form from non-root tissue (stem or leaf), and auxin triggers the dedifferentiation of parenchyma cells into root primordia at the cut surface. • 💡 Option A (Abscisic Acid) is wrong because it inhibits growth and promotes dormancy, which would suppress root formation; Option B (Ethylene) is wrong because while it can promote root hair elongation, it does not induce the adventitious root initiation needed in cuttings; Option D (Gibberellin) is wrong because it promotes stem elongation and actually inhibits adventitious root formation.

Correct Answer: D. Abscisic Acid

• **Abscisic Acid** = ABA promotes the formation of the abscission zone — a specialized layer of cells that develops at the base of the petiole (leaf stalk) or fruit stalk; enzymes like cellulase and pectinase weaken the cell walls at this zone until the organ detaches and falls. • **Auxin–ABA balance** — While ABA promotes abscission, high auxin levels in a healthy leaf or young fruit actually suppress abscission by maintaining the abscission zone cells; it is the decline of auxin combined with rising ABA that triggers the separation layer to form in autumn. • The term 'abscisic acid' itself is derived from 'abscission', reflecting its original discovery as the hormone promoting leaf and fruit fall. • 💡 Option A (Gibberellin) is wrong because it promotes cell growth and fruit development, opposing abscission; Option B (Cytokinin) is wrong because it delays senescence and actively inhibits leaf yellowing and fall; Option C (Auxin) is wrong because in young, healthy organs, auxin suppresses abscission by counteracting ABA's effect on the separation zone.

Correct Answer: C. Ethylene

• **Ethylene** = The 'triple response' is a characteristic set of morphological changes seen in etiolated (dark-grown) seedlings when exposed to ethylene: (1) inhibition of stem elongation, (2) radial swelling of the stem (thickening), and (3) horizontal (diageotropic) growth — the stem bends to grow sideways rather than upright. • **Adaptive significance** — This response is believed to help seedlings push through soil or obstacles while germinating; the thickened, shortened, horizontally probing shoot is mechanically stronger and better able to navigate around physical barriers. • The triple response is the classical bioassay used to detect and measure ethylene activity, since even very low concentrations of the gas produce clear, visible symptoms in dark-grown pea seedlings. • 💡 Option A (Auxin) is wrong because it promotes elongation rather than causing the stem-shortening and swelling of the triple response; Option B (Cytokinin) is wrong because it promotes cell division and leaf expansion, with no role in this mechanical-stress morphology; Option D (Gibberellin) is wrong because it stimulates elongation in exactly the opposite direction to the inhibition seen in the triple response.

Correct Answer: D. Gibberellin

• **Gibberellin** = As a seed begins to germinate, gibberellin levels rise sharply; in cereal grains like barley, gibberellin released by the embryo travels to the aleurone layer where it induces the synthesis of alpha-amylase, the enzyme that breaks down stored starch into sugars to fuel the growing seedling. • **GA–ABA switch** — Germination is essentially a hormonal switch: ABA (dormancy hormone) levels must fall and GA (germination hormone) levels must rise; environmental cues like warm temperature and sufficient moisture trigger this shift, releasing the seed from dormancy. • Gibberellins also stimulate the production of other hydrolytic enzymes (proteases, lipases) that mobilize all classes of stored reserves (proteins, fats, starch) needed to support early seedling growth before photosynthesis begins. • 💡 Option A (Ethylene) is wrong because while it can break some types of dormancy, it is not the primary germination hormone; Option B (Abscisic Acid) is wrong because it maintains dormancy and would be declining as germination begins, not increasing; Option C (Auxin) is wrong because it is produced in the growing shoot and root tips after germination begins, not the driver of the germination process itself.

Correct Answer: A. Cytokinin

• **Cytokinin** = Cytokinins drive the expansion of leaf area by stimulating cell division throughout the leaf blade (lamina); unlike auxin, which primarily promotes cell elongation in a single dimension, cytokinins trigger cell division in multiple planes, resulting in a broad, flat leaf surface. • **Interaction with auxin** — In plant tissue culture, the ratio of cytokinin to auxin determines organ development: a high cytokinin-to-auxin ratio promotes shoot and leaf formation, while the reverse promotes root formation — demonstrating cytokinin's dominant role in above-ground vegetative development. • Cytokinins are synthesised in the root tips and transported upward, ensuring that actively growing shoots receive a steady signal to produce new leaves. • 💡 Option B (Ethylene) is wrong because it inhibits leaf expansion and promotes senescence; Option C (Auxin) is wrong because it causes cell elongation primarily in the stem and root rather than the two-dimensional expansion of leaf blades; Option D (Gibberellin) is wrong because it elongates internodes and stems but does not specifically drive the lateral expansion of leaf area.

Correct Answer: B. Cytokinin

• **Cytokinin** = Cytokinins are synthesised predominantly in the root meristems and are transported upward through the plant via the xylem and phloem; this root-to-shoot translocation carries signals about root growth status and soil nutrient availability to the aerial parts of the plant. • **Nutrient status signal** — When roots detect high nitrogen availability, cytokinin synthesis and export to shoots increases, promoting leaf growth and chloroplast development; under nutrient-poor conditions, cytokinin levels drop, slowing leaf expansion and signalling the plant to conserve resources. • Cytokinin transport is not polar (unlike auxin), meaning it can travel in multiple directions depending on the vascular pathway and concentration gradients. • 💡 Option A (Abscisic Acid) is wrong because ABA moves primarily in the xylem from roots to leaves in response to drought, not in the phloem as a regular root-to-shoot developmental signal; Option C (Auxin) is wrong because it is produced at shoot tips and moves polarly downward (basipetally), the opposite direction; Option D (Gibberellin) is wrong because it is synthesised in young leaves and seeds and moves non-polarly, not specifically from root to shoot through the phloem.

Correct Answer: A. Auxin

• **Auxin** = In geotropism (also called gravitropism), statoliths (starch-filled plastids) in root cap cells settle under gravity, triggering a redistribution of auxin to the lower side of the root; crucially, roots are far more sensitive to auxin than shoots — at the higher concentration on the lower side, auxin inhibits root cell elongation rather than promoting it, causing the lower side to grow more slowly and the root to curve downward. • **Shoot vs. root sensitivity** — The same gravitational redistribution of auxin promotes shoot elongation on the lower side (shoots are less sensitive), causing shoots to grow upward while roots grow downward — a perfect, opposing dual tropism from a single hormone. • This concentration-dependent response of roots (inhibition at high auxin) vs. shoots (promotion at high auxin) is a classic example of differential tissue sensitivity to the same signal. • 💡 Option B (Gibberellin) is wrong because it promotes elongation uniformly and is not redistributed in response to gravity for tropic bending; Option C (Ethylene) is wrong because it inhibits elongation generally and does not mediate the directional gravity response; Option D (Cytokinin) is wrong because it promotes cell division and has no mechanism for the gravity-sensing directional growth of roots.

Correct Answer: C. 2,4-D

• **2,4-D** = 2,4-Dichlorophenoxyacetic acid (2,4-D) is a synthetic auxin that, when applied at high concentrations, causes uncontrolled, disorganised growth in broad-leaved (dicot) weeds, ultimately killing them; cereals and grasses (monocots) are relatively unaffected because their growing point is protected by surrounding leaf sheaths. • **Selective action** — The selectivity of 2,4-D between dicots and monocots makes it the world's most widely used herbicide; it was also a component of Agent Orange, the defoliant used in the Vietnam War. • At low concentrations, 2,4-D mimics natural auxin and promotes root formation; it is the high dose that makes it a herbicide, illustrating how the same compound can have opposite effects depending on concentration. • 💡 Option A (IAA) is wrong because it is the natural auxin produced by plants themselves, not used as a herbicide; Option B (ABA) is wrong because it is abscisic acid, a stress hormone completely unrelated to synthetic auxins or weed control; Option D (GA3) is wrong because gibberellic acid promotes growth and is used to enhance crop yield, not to kill weeds.

Correct Answer: B. Cytokinin

• **Cytokinin** = Cytokinins delay leaf senescence by preventing the breakdown of proteins, chlorophyll, and cellular membranes; they also act as a 'nutrient sink', attracting and mobilizing mineral nutrients (especially nitrogen) toward the cytokinin-rich area, keeping the leaf metabolically active and green. • **Directed transport of nutrients** — When a single leaf or leaf spot is treated with cytokinin, amino acids, sugars, and ions migrate toward that area from other parts of the leaf, demonstrating cytokinin's powerful ability to redirect nutrient flow — a phenomenon with potential applications in agriculture. • The anti-senescence effect of cytokinins is directly opposed by ethylene and ABA, making the cytokinin-to-ethylene balance a key regulator of leaf longevity. • 💡 Option A (Abscisic Acid) is wrong because it promotes senescence, stomatal closure, and leaf abscission — accelerating aging rather than preventing it; Option C (Auxin) is wrong because it prevents leaf drop by suppressing the abscission zone but does not actively delay chlorophyll breakdown or mobilize nutrients into leaves; Option D (Ethylene) is wrong because it is the premier pro-senescence hormone, directly causing chlorophyll breakdown, yellowing, and abscission.