NCERT Grounding
NCERT Class 11 Biology, Chapter 13 (Plant Growth and Development), Section 13.4.3.5 devotes a compact but information-dense paragraph to ABA. The text establishes three foundational facts: ABA is a general plant growth inhibitor and an inhibitor of plant metabolism; it stimulates stomatal closure and increases stress tolerance (hence the name "stress hormone"); and it plays an important role in seed development, maturation, and dormancy, acting as an antagonist to GAs in most situations.
"ABA acts as a general plant growth inhibitor and an inhibitor of plant metabolism… it is also called the stress hormone."
NCERT Class 11 Biology, Chapter 13, Section 13.4.3.5
The NIOS Chapter 20 corroborates these points, adding that ABA "induces dormancy of buds and seeds as opposed to Gibberellin, which breaks dormancy" and specifically lists stomatal closure and leaf senescence promotion as key functions. Both sources classify ABA among the inhibitory PGRs, in direct contrast to auxins, gibberellins, and cytokinins.
Discovery and Chemistry
During the mid-1960s, three independent research groups reported the purification and chemical characterisation of three apparently different inhibitory compounds: inhibitor-B, abscission II, and dormin. All three were subsequently proved to be chemically identical. The unified compound was named abscisic acid (ABA).
Chemically, ABA is a sesquiterpene (15-carbon compound) derived from the carotenoid/isoprenoid pathway. NCERT classifies it as a "derivative of carotenoids." This biosynthetic origin is significant: carotenoids are synthesised in chloroplasts and other plastids, which is why ABA is produced primarily in leaves, stems, roots, and unripe fruits — all tissues containing plastids.
Molecular Formula Class
ABA is a sesquiterpene (15-carbon terpenoid), synthesised via the isoprenoid/carotenoid pathway in plastids. NCERT classifies PGRs by chemical origin: ABA = derivative of carotenoids; GA = terpene; IAA = indole compound; kinetin = adenine derivative; ethylene = gas.
Physiological Effects of ABA
ABA exerts inhibitory effects across multiple developmental and stress-response contexts. The six key physiological effects recognised in the NCERT syllabus are summarised below.
Classification rule: ABA belongs to the inhibitory group of PGRs. Every effect listed below either slows growth, arrests development, or promotes protective shutdown responses.
Stomatal Closure
Primary stress response. ABA triggers K⁺ efflux from guard cells → loss of turgor → pore closes.
Reduces water loss during drought; the fastest and most studied ABA response.
NEET 2025 conceptSeed Dormancy
Inhibitory substance governing dormancy. ABA inhibits synthesis of hydrolytic enzymes, keeping metabolic activity suppressed.
Dormancy protects seeds from desiccation and adverse conditions.
NEET 2020Inhibits Germination
ABA inhibits seed germination — NCERT states this explicitly.
This is the mechanism by which dormancy is maintained: germination cannot proceed while ABA levels remain high.
Promotes Senescence
ABA and ethylene both promote leaf senescence. NIOS explicitly states: "Abscissic acid and ethylene promote senescence of leaves."
Cytokinin delays senescence — directly antagonistic to ABA's effect here.
Stress Tolerance
Increases tolerance to cold, drought, and salinity. Coordinates multiple stress-response pathways.
This broad stress-management role is why ABA earned the title "stress hormone."
Seed Development
ABA plays an important role in seed development and maturation.
It promotes accumulation of storage proteins and lipids during late seed development.
Stomatal Closure — The Primary Stress Response
Mechanism: K⁺ efflux and guard-cell turgor
When a plant experiences water stress (drought), ABA is rapidly synthesised in mesophyll cells and distributed to guard cells via the apoplast and symplast. ABA binds to receptors on the guard cell plasma membrane, initiating a second-messenger cascade. The downstream effect is the opening of K⁺ (potassium) efflux channels and the closure of K⁺ influx channels in the guard cell membrane.
As K⁺ moves out of the guard cells, the osmotic potential of the guard cells rises (becomes less negative). Water follows K⁺ by osmosis, moving out of the guard cells. The guard cells lose turgor pressure and become flaccid. Flaccid guard cells collapse inward, causing the stomatal pore to close. This drastically reduces water loss through transpiration.
Figure 1. ABA-induced stomatal closure. Water stress triggers ABA synthesis; ABA causes K⁺ efflux from guard cells; guard cells lose turgor (become flaccid) and the stomatal pore closes, conserving water.
Seed Dormancy and Germination
Seed dormancy is the state of suspended metabolic activity in a seed even when external conditions (water, temperature, oxygen) are favourable for germination. NIOS states clearly that dormancy "may occur due to … presence of inhibitors like abscissic acid." ABA is the primary endogenous inhibitor that maintains this dormant state.
ABA keeps seeds dormant by suppressing the synthesis and activity of hydrolytic enzymes (notably alpha-amylase) that would otherwise break down stored starch, proteins, and lipids to provide energy for germination. It also suppresses cell expansion in the embryonic axis. As long as ABA concentrations remain high relative to GA, the seed will not germinate.
How ABA Governs Dormancy and How GA Breaks It
-
Step 1
Seed Matures
ABA accumulates during late embryo development; dormancy is established.
High ABA -
Step 2
Dormancy Maintained
ABA suppresses alpha-amylase and cell expansion; seed remains metabolically quiet.
No germination -
Step 3
Favourable Conditions
Adequate water, temperature, and light trigger ABA catabolism; ABA levels fall.
ABA decreasing -
Step 4
GA Rises
Gibberellins accumulate in the embryo; GA antagonises residual ABA and induces hydrolytic enzyme synthesis.
GA > ABA -
Step 5
Germination
Food reserves mobilised; radicle and plumule elongate; seedling emerges.
Dormancy broken
ABA versus Gibberellin — The Key Antagonism
NCERT explicitly states: "In most situations, ABA acts as an antagonist to GAs." This antagonism is the single most NEET-tested aspect of ABA biology. The table below contrasts their opposing effects across all major physiological contexts.
Abscisic Acid (ABA)
Inhibitor
General growth inhibitor (NCERT)
- Induces and maintains seed dormancy
- Inhibits seed germination
- Promotes leaf senescence
- Causes stomatal closure
- Inhibits stem elongation and cell division
- Promotes stress tolerance (cold, drought)
Gibberellin (GA)
Promoter
Promotory PGR (NCERT)
- Breaks seed dormancy
- Promotes seed germination
- Delays leaf senescence
- Does NOT directly control stomata
- Promotes stem elongation and bolting
- Promotes fruiting and parthenocarpy
ABA and the "Stress Hormone" Label
The designation "stress hormone" reflects ABA's role as the plant's rapid-response coordinator under abiotic stress. Within minutes of soil water deficit, leaf ABA concentrations rise 20–50-fold. The stomatal closure response begins within that same timeframe. No other PGR mediates this fast, physiologically significant response to drought. Additionally, ABA promotes cold hardening and may upregulate genes encoding late embryogenesis abundant (LEA) proteins and osmoprotectants — all directed at survival under stress.
ABA and abscission: the name is misleading
Students often assume that because the hormone is called abscisic acid, it must be the primary hormone driving leaf and fruit abscission. This is historically incorrect. ABA was named when initially isolated in the context of cotton boll abscission research (as "abscission II"), but subsequent work established that ethylene is the major hormone promoting abscission of leaves and fruits. ABA promotes leaf senescence (aging), which precedes abscission, but the abscission zone development and activation is predominantly ethylene-driven.
Rule: ABA = senescence promoter; Ethylene = primary abscission promoter. Auxin PREVENTS early abscission of young leaves and fruits.
Worked Examples
Statement: "ABA is a plant growth inhibitor." Is this statement correct or incorrect?
Correct. NCERT explicitly classifies ABA as "a general plant growth inhibitor and an inhibitor of plant metabolism." It belongs to the inhibitory group of PGRs, in contrast to auxins, gibberellins, and cytokinins (all promotory). The NEET 2025 question bank includes this as a true statement in a multiple-assertion question. A companion incorrect statement in that question was "Apical dominance promotes the growth of lateral buds" — that statement is wrong because apical dominance suppresses lateral bud growth (the apical bud inhibits laterals, primarily via auxin).
A plant under severe drought closes its stomata rapidly. Which PGR mediates this response, and what is the ionic mechanism?
ABA (abscisic acid) mediates this response. Under water stress, ABA is synthesised rapidly in mesophyll cells and transported to guard cells. ABA activates K⁺ efflux channels and inhibits K⁺ influx channels in the guard cell plasma membrane. Potassium ions leave the guard cells; water follows by osmosis, reducing turgor pressure. The guard cells become flaccid and the stomatal pore closes, minimising further water loss through transpiration.
A student applies gibberellic acid (GA₃) to dormant seeds. What happens and why?
GA₃ breaks dormancy and initiates germination. Dormancy is maintained by high ABA levels that suppress hydrolytic enzyme (alpha-amylase) synthesis. GA₃ acts as an antagonist to ABA: it overcomes ABA's inhibitory effect and stimulates production of alpha-amylase and other hydrolases, which mobilise stored food reserves (starch, proteins, lipids) in the endosperm. The embryo receives energy and nutrients, radicle elongates, and germination proceeds. This GA-ABA antagonism is the molecular basis of why NCERT states: "In most situations, ABA acts as an antagonist to GAs."
Common Confusion and NEET Traps
GA is NOT an inhibitory substance governing seed dormancy
NEET 2020 Q.3 asked students to identify which of four substances is not an inhibitory substance governing seed dormancy. Options included ABA, phenolic acid, para-ascorbic acid, and gibberellic acid. Many students who are uncertain may second-guess themselves on ABA (which sounds like it could promote abscission rather than dormancy), or they may not recall that para-ascorbic acid is indeed an inhibitor. Gibberellic acid is unambiguously a growth promoter that breaks dormancy — it is not an inhibitory substance at all.
Rule: Inhibitory substances governing dormancy = ABA, phenolic acids, para-ascorbic acid, coumarin. GA breaks dormancy — it is the opposite, a promoter, and is the correct answer (4) to the 2020 question.
Senescence vs Abscission: who does what?
Students conflate two sequential processes. Senescence is the programmed aging and degradation of a leaf or organ — chlorophyll breakdown, protein remobilisation, yellowing. Abscission is the physical detachment. ABA promotes senescence. Ethylene promotes both senescence and abscission. Auxin delays abscission of young, actively growing leaves and fruits. Cytokinin delays senescence. These four relationships are each individually testable in NEET.
Rule: ABA + ethylene = senescence promoters. Ethylene = primary abscission trigger. Auxin = abscission inhibitor (young organs). Cytokinin = senescence inhibitor.