Q.Abscisic acid controls ..................

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 textbook example when a growth inhibitor is asked for (ethylene, while it does promote senescence/abscission, is separately classified as its own gaseous hormone group rather than 'the' growth inhibitor).

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 during water stress, and generally antagonises the growth-promoting hormones. This inhibitory role is why ABA is often called the growth-inhibitor hormone.

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 water stress. Because of this directly opposing physiological role — GA drives growth/germination forward while ABA halts it — ABA is described as acting antagonistically to gibberellic acid.

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.

• It also plays a role in seed and bud dormancy, helping seeds withstand desiccation and preventing premature germination under unfavourable conditions.

• Because of this central role in the plant's stress response (and dormancy-inducing/growth-inhibiting effects), ABA earned the nickname "stress hormone" of plants.

(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, cytokinins promote cell division and delay senescence, and ethylene promotes fruit ripening and abscission — none of these directly triggers guard-cell K+ efflux for stomatal closure the way ABA does.

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.

The other hormones have opposite or unrelated effects: auxin and gibberellin generally promote growth (cell elongation) and are not associated with stomatal closure; cytokinin promotes cell division and delays senescence, and in some contexts actually promotes stomatal opening rather than closure.

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:

1. Ethanol is not a plant growth hormone at all; it is a metabolic byproduct of anaerobic respiration (alcoholic fermentation), not a regulator of growth.
2. Cytokinin is a growth-PROMOTING hormone — it stimulates cell division, delays senescence, and promotes lateral bud growth (counteracting apical dominance).
3. ABA (Abscisic acid) is the classic growth-INHIBITING hormone. It induces and maintains dormancy in seeds and buds, promotes stomatal closure during water stress (helping the plant conserve water), induces senescence and abscission of leaves/fruits, and generally antagonises the growth-promoting hormones. It is widely referred to in textbooks as the "stress hormone" and the most powerful natural growth inhibitor.
4. Auxin is a growth-PROMOTING hormone — it stimulates cell elongation, apical dominance, and root initiation (though at very high, supra-optimal concentrations it can inhibit growth, that is not its defining/typical role).

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 of leaves and fruits. Because of these broadly inhibitory effects, ABA is referred to as the general growth inhibitor of plants.