Q.ABA acts antagonistic to
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Abscisic Acid (ABA) — The Plant's Stress Manager
Imagine you are a plant. Life is good — plenty of water, warm sun, and you are growing tall. Then the rain stops. The soil dries. If you keep your leaves wide open, you will lose water faster than your roots can pull it up. You need to shut down, conserve, and wait for better times.
That is exactly what abscisic acid does. It is the plant's stress hormone — the one that says "stop growing, close up, and survive."
The Intuition: A Chemical Brake Pedal
Gibberellins are the accelerator — they tell seeds to germinate, stems to elongate, and leaves to expand. Abscisic acid is the brake. It counteracts gibberellin at nearly every turn. When a seed is mature and the environment is harsh (too cold, too dry), ABA keeps it dormant. When a leaf is losing water faster than it can replace it, ABA tells the guard cells to close the stomata. When a plant faces salt, cold, or drought, ABA ramps up protective proteins.
In short: ABA is the plant's "emergency stop" button.
The Precise Statement
Abscisic acid is a sesquiterpenoid plant hormone (15-carbon molecule) that acts as a growth inhibitor. Its primary functions are:
- Stomatal closure — triggers guard cells to lose turgor, closing the pore
- Seed dormancy — prevents premature germination under unfavourable conditions
- Stress tolerance — induces expression of genes for protective proteins (e.g., dehydrins)
- Antagonism to gibberellin — opposes GA in seed germination, stem elongation, and enzyme induction
How It Works: The Mechanism
Stomatal closure is the most dramatic and fastest action. When a leaf senses water deficit, ABA is synthesised in the roots and transported to the leaves. It binds to receptors on guard cells, causing an efflux of potassium ions (K⁺) and chloride ions (Cl⁻). Water follows by osmosis, the guard cells become flaccid, and the stomatal pore closes. This happens within minutes.
Seed dormancy is a longer-term effect. During seed maturation, ABA levels rise and keep the embryo in a dormant state. Only when ABA is broken down (or its levels fall relative to gibberellin) can germination proceed. This is why seeds of desert plants often require a cold, wet period (stratification) — it degrades ABA and allows GA to take over.
Stress tolerance involves slower, gene-level changes. ABA triggers the expression of genes that produce dehydrins (proteins that protect cellular structures from dehydration), antioxidant enzymes (to handle oxidative stress), and compatible solutes (like proline) that help cells retain water.
A common mistake is to think ABA causes abscission (leaf drop). Despite its name, ABA does not primarily control leaf fall — that is mainly ethylene. The name "abscisic" is historical and misleading.
The Antagonism with Gibberellin
This is a key exam point. In many processes, ABA and GA have opposite effects:
| Process | Gibberellin (GA) | Abscisic Acid (ABA) | …
Abscisic acid generally works as an antagonist to gibberellic acid: wherever gibberellin tends to promote a process, abscisic acid tends to inhibit it. This is consistent with their opposite roles — gibberellins are growth promoters, while ABA is a general growth inhibitor associated with …
ABA and gibberellic acid pull in opposite directions on the same growth processes, which is exactly what makes their relationship antagonistic.
Abscisic acid (ABA) functions overall as a general plant growth inhibitor and an inhibitor of plant metabolism. It inhibits seed germination, stimulates the closure of stomata, increases tolerance of plants to various stresses (which is why it is also called the stress hormone), and plays a central role in seed development, maturation and dormancy. …
Method: Identifying Hormone Antagonist Pairs
Plant hormones are frequently tested in opposing pairs — the technique is to recall each hormone's overall direction (promoter vs. inhibitor) and match it to its most consistently opposite-acting partner.
Steps
Step 1: Classify each hormone as a general promoter or general inhibitor.
Gibberellins are growth promoters (elongation, delayed senescence, germination). ABA is a general growth inhibitor (dormancy, stomatal closure, stress tolerance).
Step 2: Check for a documented pattern of opposing each other's effect on the same processes. …
- TG EAPCET 2026Set ap-2026-05-04-FN1 markMCQQ.In most situations, hormone acts as antagonist to Gibberellins (A) Naphthalene acetic acid (B) Indole butyric acid (C) Kinetin (D) Abscisic acid
›Reveal solutionSolution
The hormone that acts as an antagonist to gibberellins is abscisic acid (ABA) — it opposes gibberellin’s growth-promoting effects, especially in seed dormancy and stem elongation.
The key idea here is hormonal antagonism in plants. Gibberellins are growth-promoting hormones — they stimulate seed germination, stem elongation, and fruit development. Their effects are often counterbalanced by other hormones that inhibit these processes. The question asks which of the given options is known to act against gibberellins.
Let’s look at each option:
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Naphthalene acetic acid (NAA) — This is a synthetic auxin. Auxins generally promote cell elongation and root initiation, but they don’t directly oppose gibberellins. In fact, auxins and gibberellins often work together (e.g., in fruit development). So NAA is not an antagonist.
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Indole butyric acid (IBA) — Another synthetic auxin, used mainly to stimulate root formation in cuttings. Again, no antagonistic relationship with gibberellins — they can even synergize in some processes.
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Kinetin — A cytokinin. Cytokinins promote cell division and delay senescence. While cytokinins and gibberellins have overlapping roles (e.g., both promote growth), they are not direct antagonists. In some contexts they cooperate, not oppose. …
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- TG EAPCET 2024Set ap-2024-05-07-AN1 markMCQQ.Assertion (A): Abscisic acid is called stress hormone Reason (R): Abscisic acid increases the tolerance of plants to various kinds of stresses The correct answer is (A) (A) and (R) are correct. (R) is the correct explanation of (A) (B) (A) and (R) are correct, but (R) is not the correct explanation of (A) (C) (A) is correct but (R) is not correct (D) (A) is not correct but (R) is correct
›Reveal solutionSolution
ABA is named the "stress hormone" because it increases the plant's tolerance to stresses — closing stomata, inducing dormancy and inhibiting growth. Both statements are true and R is the reason behind A: option (A).
The concept first — what ABA actually does
Abscisic acid was discovered independently as "abscisin II" and "dormin", and it acts largely as a growth inhibitor, antagonising gibberellins. Its key roles:
- Stomatal closure — when the leaf loses turgor, ABA accumulates in the guard cells, causing solute efflux, loss of guard-cell turgor and closure of the stomatal pore. Transpiration falls and the plant conserves water. This is the single most-cited ABA action.
- Seed dormancy and maturation — ABA induces dormancy, allowing seeds to survive desiccation and germinate only when conditions are favourable.
- Bud dormancy — helps perennials survive winter.
- General inhibition of growth and metabolism during adverse conditions.
Because every one of these actions is about surviving adverse conditions, ABA earned the label stress hormone.
Step-by-step
- Assertion: "Abscisic acid is called stress hormone." This is the standard name given in the textbook. → Correct. …
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