Q.Why is abscisic acid called a stress hormone? OR Write two functions of Auxin.
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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 (ABA) is a growth-inhibiting hormone that becomes especially active when a plant faces unfavourable environmental conditions, helping it cope with and survive such stress. …
ABA is called the stress hormone because its levels rise sharply under adverse environmental conditions, triggering protective, growth-inhibiting responses such as stomatal closure and dormancy that help the plant tolerate stress.
Abscisic acid (ABA) is a general plant growth inhibitor that plays a key role in helping plants withstand various types of environmental stress:
- Under water-deficit (drought) conditions, ABA levels increase and cause the closure of stomata (by affecting guard cell turgor), reducing transpirational water loss.
- It inhibits seed germination and induces/maintains dormancy in seeds and buds, allowing plants to survive unfavourable seasons (like cold winters or dry periods) by remaining inactive until conditions become favourable.
- It promotes leaf senescence and abscission, helping the plant shed non-essential parts under stress. …
- CBSE 2026Set ANNUAL1 markQ.Write the name of any growth inhibitor.
›Reveal solutionSolution
Abscisic acid (ABA) is the standard example of a growth-inhibiting plant hormone.
Among the five classical groups of plant hormones — auxins, gibberellins, cytokinins, ethylene, and abscisic acid — most are growth promoters. Abscisic acid (ABA), by contrast, is predominantly a growth inhibitor: it antagonises the growth-promoting hormones, induces and maintains seed and bud dormancy, promotes the closure of stomata during water stress (helping the plant conserve water), and generally acts as a general plant growth inhibitor and stress hormone. Because of these roles, ABA is the standard t …
- CBSE 2024Set ANNUAL1 markMCQQ.Identify the growth hormone in plants which causes inhibitory effect.(a) Cytokinins(b) Abscissic acid(c) Gibberellin(d) Ethylene
›Reveal solutionSolution
Correct answer: (b) Abscisic acid.
ABA is the plant hormone that acts mainly as a growth inhibitor.
Among the plant growth regulators, cytokinins, gibberellins and (in most contexts) ethylene promote growth-related processes such as cell division, elongation and fruit ripening. Abscisic acid (ABA), in contrast, is the classic "stress hormone" of plants — it induces bud and seed dormancy, promotes stomatal closure …
- CBSE 2023Set ANNUAL1 markMCQQ.ABA acts as antagonistic to:(a) Ethylene(b) Cytokinin(c) Gebberellic acid(d) IAA
›Reveal solutionSolution
ABA is the principal plant growth inhibitor and 'stress hormone'; its actions on dormancy and stomatal closure generally oppose those of the growth-promoting hormone gibberellic acid (GA).
Among the five major groups of plant hormones, auxins, gibberellins and cytokinins are broadly growth-promoting, while ABA is growth-inhibiting. GA breaks seed and bud dormancy and promotes germination and elongation growth; ABA induces and maintains dormancy, inhibits seed germination and growth, and triggers stomatal closure under wate …
- CBSE 2023Set ANNUAL1 markQ.Which hormone is known as stress hormone in plants?
›Reveal solutionSolution
Abscisic acid (ABA) is called the plant stress hormone because it is produced in response to, and helps the plant cope with, adverse conditions such as drought/water stress.
ABA is generally an inhibitory growth hormone — it opposes many of the growth-promoting effects of gibberellins. Its key stress-related roles include:
- Under water-deficit (drought) conditions, ABA levels rise sharply and cause the guard cells to lose turgor, closing the stomata — this reduces transpirational water loss and helps the plant survive stress. …
- CBSE 2022Set ANNUAL1 markMCQQ._______ hormone responsible for efflux of k+ ions from guard cells and act as antitranspirant.(a) Gibberellins(b) Cytokinin(c) Ethylene(d) Abscissic acid
›Reveal solutionSolution
(d) Abscissic acid — closes stomata by causing K+ efflux from guard cells, acting as a natural antitranspirant.
Abscisic acid (ABA) is the plant's stress hormone that closes stomata by triggering K+ efflux from guard cells, reducing water loss.
Abscisic acid (ABA), often called the 'stress hormone', is synthesised in response to water deficit and acts on guard cells to promote the efflux of potassium ions (K+) out of them (along with malate and chloride); this loss of solutes reduces the osmotic pressure/turgor of the guard cells, causing the stomatal pore to close and thereby cutting down transpirational water loss — making ABA a natural antitranspirant. Gibberellins promote stem elongation and seed germination, cyto …
- CBSE 2022Set ANNUAL1 markMCQQ.Which plant hormones induces closure of stomata?(a) Abscissic acid(b) Auxin(c) Cytokinin(d) Gibberellin
›Reveal solutionSolution
Abscisic acid (ABA) is the plant hormone that induces stomatal closure, helping the plant conserve water during drought or water-stress conditions.
Abscisic acid is often referred to as the general growth inhibitor or 'stress hormone' because it accumulates in plants under adverse conditions like drought, and triggers protective responses. One of its most well-studied roles is inducing the closure of stomata: ABA promotes efflux of potassium and other ions from the guard cells, causing them to lose turgor pressure and close the stomatal pore, which reduces transpirational water loss. This is why ABA plays a central role in a plant's drought-tolerance response.
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- CBSE 2020Set ANNUAL1 markMCQQ._________ is the powerful growth inhibitor.(a) Ethanol(b) Cytokinin(c) ABA(d) Auxin
›Reveal solutionSolution
Among plant growth regulators, abscisic acid (ABA) is specifically known as the most powerful growth inhibitor, acting antagonistically to growth promoters like auxin, gibberellin, and cytokinin.
Of the four options:
- Ethanol is not a plant growth hormone at all; it is a metabolic byproduct of anaerobic respiration (alcoholic fermentation), not a regulator of growth.
- Cytokinin is a growth-PROMOTING hormone — it stimulates cell division, delays senescence, and promotes lateral bud growth (counteracting apical dominance). …
- CBSE 2018Set botany1 markQ.Abscisic acid acts as a general .............. of growth and metabolism. (Fill in the Blank)
›Reveal solutionSolution
The missing word is 'inhibitor' — abscisic acid is known as the general growth inhibitor among plant hormones.
Abscisic acid (ABA) is a plant growth regulator that generally has an inhibitory effect on plant growth and metabolic processes, in contrast to the growth-promoting hormones (auxins, gibberellins, cytokinins). Its major roles include: inducing dormancy in seeds and buds, inhibiting seed germination under unfavourable conditions, promoting the closure of stomata during water stress (thereby earning it the name 'stress hormone'), and promoting senescence and abscission o …
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