Auxin Questions in English

(B) $2, 4-D$ stands for $2, 4$-dichlorophenoxyacetic acid.
It is a synthetic auxin that acts as a systemic herbicide.
It is widely used in agriculture to control broad-leaved weeds without affecting monocot crops like wheat,maize,and rice.

(D) The apical bud inhibits the growth of lateral buds due to a phenomenon known as $Apical$ $Dominance$. When the apical bud is removed,the inhibitory effect is lifted,which promotes the growth of lateral branches.

(B) Apical dominance is the phenomenon where the main central stem of the plant grows more strongly than other side stems.
$Auxin$ is the primary plant hormone responsible for this process.
It is synthesized at the shoot tips and transported downwards,inhibiting the growth of lateral buds.
Therefore,the correct option is $B$.

(D) Apical dominance is a phenomenon where the apical bud inhibits the growth of lateral buds.
This process is primarily caused by the presence of $Auxin$ in the shoot apices.
When the shoot tip is removed,the inhibition is lifted,and the lateral buds begin to grow.

(C) $Auxin$ is a plant hormone responsible for phototropism and geotropism.
$1$. Bending of shoot towards light $(Phototropism)$ is caused by the unequal distribution of $Auxin$.
$2$. Bending of root towards gravity $(Geotropism)$ is also regulated by $Auxin$.
$3$. Bending of sunflower tip towards the sun $(Heliotropism)$ is also mediated by $Auxin$ distribution.
$4$. Bending of a leaf towards the direction of fragrance is not a known physiological effect of $Auxin$. Plants do not exhibit tropism towards fragrance.

(D) $Auxin$ is synthesized at the shoot tips and is then transported towards the base,which is known as $basipetal$ transport.
In roots,$auxin$ transport is generally $acropetal$ (towards the tip).
Since $auxin$ transport occurs in both directions depending on the plant organ (shoot vs root),the correct answer is $(D)$.

(D) $NAA$ (Naphthalene Acetic Acid) is a synthetic auxin. Synthetic auxins are widely used as herbicides. Examples of synthetic auxins include $1-$Naphthalene Acetic Acid $(NAA)$,$2,4-$Dichlorophenoxyacetic acid $(2,4-D)$,and $2,4,5-$Trichlorophenoxyacetic acid $(2,4,5-T)$.

(A) The bending of a plant shoot towards a light source is a growth movement known as $Phototropism$. This is a response to an external stimulus (light),which is a characteristic feature of living organisms known as irritability or sensitivity.

(A) Apical dominance is the phenomenon where the main central stem of the plant grows more strongly than other side stems.
Auxins, specifically $Indole-3-acetic$ acid $(IAA)$, are synthesized in the apical buds and transported downwards.
They inhibit the growth of lateral buds, thereby maintaining the dominance of the apical bud.
Therefore, $Auxin$ is the hormone responsible for apical dominance.

(B) The abscission layer is a specialized zone of cells that forms at the base of leaves,flowers,or fruits,leading to their detachment from the plant body.
Auxin is a plant hormone that inhibits the formation of the abscission layer and prevents premature leaf or fruit drop.
When the concentration of auxin decreases in the plant tissues,the inhibitory effect is removed,which promotes the formation of the abscission layer.
Therefore,a decrease in auxin concentration is the primary trigger for the formation of the abscission layer.

(B) $IAA$ stands for Indole-$3$-acetic acid.
It is the most common and naturally occurring auxin in plants.
It plays a crucial role in various physiological processes such as cell elongation,apical dominance,and root initiation.

(A) $2, 4-D$ stands for $2, 4-\text{dichlorophenoxyacetic acid}$.
It is a synthetic auxin that acts as a herbicide.
It is widely used to kill broad-leaved weeds without affecting mature monocot plants like cereals and grasses.

(D) The hormone $2,4-D$ ($2,4$-dichlorophenoxyacetic acid) is a synthetic auxin.
$GA_3$ and $GA_2$ are naturally occurring gibberellins.
$IAA$ (Indole$-3-$acetic acid) is the most common naturally occurring auxin in plants.
Therefore,$2,4-D$ is classified as a synthetic plant growth regulator.

(D) The shoot apex contains the apical meristem,which is the primary site of auxin synthesis. Auxin is responsible for apical dominance,a phenomenon where the main stem grows vertically and inhibits the growth of lateral buds. When the shoot apex is removed,the source of auxin is eliminated. Consequently,the inhibition on lateral buds is lifted,allowing them to grow and develop into lateral branches.

(A) The primary precursor for the biosynthesis of $IAA$ (Indole$-3-$acetic acid),which is the most common naturally occurring auxin in plants,is the amino acid $Tryptophan$.
During the metabolic process,$Tryptophan$ undergoes enzymatic conversion to form $IAA$.

(D) Apical dominance is a phenomenon in plants where the apical bud (the main shoot tip) grows actively and suppresses the growth of lateral (axillary) buds. This is primarily mediated by the hormone $Auxin$,which is synthesized in the apical meristem and transported downwards. Because the apical bud dominates the growth,the plant grows vertically rather than branching out. If the apical bud is removed,the inhibition is lifted,and the lateral buds begin to grow,leading to a bushier plant.

(C) $2, 4-D$ stands for $2, 4$-Dichlorophenoxyacetic acid.
It is a synthetic auxin used as a herbicide to kill broad-leaved weeds.
It is widely used in agriculture to control weeds in cereal crops without affecting the crops themselves.

(B) : In low concentrations,auxins such as $2,4-D$ ($2,4$-Dichlorophenoxy acetic acid) are useful in preventing pre-harvest fruit drop and leaf drop.

(A) $Avena$ curvature test is a bioassay based on the experiments of Went $(1928)$,which can measure auxin concentration up to $300 \ mg/litre$.
Indole-$3$-Acetic Acid $(IAA)$ is a universal natural auxin,and the $Avena$ curvature test serves as an accurate bioassay for this specific plant hormone.

(C) : Auxin bioassay is a quantitative test as it measures the concentration of auxin required to produce a specific effect and the magnitude of that effect. The $Avena$ Curvature test is based on the experiments of Went $(1928)$. $A$ $10^{\circ}$ curvature is produced by an auxin concentration of $150 \ mg/L$ at $25^{\circ}C$ and $90\%$ relative humidity. This test can measure auxin concentrations up to $300 \ mg/L$. Auxin from a shoot tip or any other plant organ is allowed to diffuse into a standard-sized agar block (generally $2 \times 2 \times 1 \ mm$). Alternatively,auxin can be dissolved directly into the agar. $A$ $15-30 \ mm$ long oat coleoptile grown in the dark is held vertically over water. $A$ $1 \ mm$ tip of the coleoptile is removed without injuring the primary leaf. After $3$ hours,a second decapitation is performed at a distance of $4 \ mm$. The primary leaf is then pulled loose,and the agar block is placed against the tip of the decapitated coleoptile. After $90-110$ minutes,the coleoptile is observed to have bent. The curvature is then measured,or it can be photographed and determined via a shadow graph.

(B) : Auxins induce cell elongation. In a differentially illuminated plant,they accumulate in the shaded part,causing elongation of the cells in the shaded part. This unequal elongation on two sides causes the plant to curve or bend towards the light source,$i.e.$,phototropic curvature.

(B) The experiment conducted by $F.W. Went$ is known as the $Avena$ curvature test.
This experiment demonstrated that the substance responsible for growth (auxin) diffuses from the coleoptile tip into the agar block.
By placing the agar block on one side of a decapitated coleoptile,the differential growth caused by the auxin leads to bending.
This method allows for the quantitative measurement of the concentration of growth-promoting substances (auxins) based on the degree of curvature produced,making it the basis for the bioassay of auxins.

(A) $NAA$ ($\alpha$-Naphthalene acetic acid) and $2, 4-D$ ($2, 4$-dichlorophenoxy acetic acid) are synthetic auxins.
Normally, auxins inhibit flowering in many plants.
However, in pineapple and litchi, the application of these synthetic auxins promotes flowering.
Therefore, they are used to induce flowering in pineapple throughout the year.

(D) : Phototropic curvature is the result of uneven distribution of $auxin$.
$Charles \ Darwin$ and his son $Francis \ Darwin$ observed that the coleoptiles of canary grass responded to unilateral illumination by growing towards the light source $(phototropism)$.
After a series of experiments, it was concluded that the tip of the coleoptile contains $auxin$, which causes the bending of the entire coleoptile in relation to the direction of light.

(D) : Indole-$3$-acetic acid (also called auxin) is a phytohormone which is generally produced by the growing apices of the stems and roots,from where it migrates to the regions of its action.
It is observed that the growing apical bud inhibits the growth of the lateral (axillary) bud,a phenomenon known as apical dominance.
Since the apical meristem is the site of auxin synthesis,it is the physiological effect of the auxin that results in apical dominance.
When shoot tips are removed,it usually results in the growth of lateral buds.
This phenomenon is widely applied in tea plantations and hedge-making because,in tea plantations,the apical bud is plucked for tea processing,which stimulates the growth of more lateral buds,thus enhancing the yield for industrial purposes.

(D) The correct answer is $(d)$.
Many auxins have been synthesized that exhibit properties similar to natural auxins.
Examples of synthetic auxins include naphthalene acetic acid $(NAA)$,indole-$3$-butyric acid $(IBA)$,$2,4$-dichlorophenoxyacetic acid $(2,4-D)$,and $2,4,5$-trichlorophenoxyacetic acid $(2,4,5-T)$.
While $IBA$ is often found naturally in plants,it is also produced synthetically and is widely used in agriculture for rooting cuttings.

(C) : Pruning is the process of cutting shoot tips to promote the lateral growth of branches.
Removal of shoot tips involves the removal of apical buds.
In the shoot tips,auxins are produced,which are growth-promoting phytohormones.
They cause apical dominance by promoting the growth of apical buds and suppressing the growth of axillary buds.
When the auxins produced in the shoot tips are removed by decapitation,it results in lateral growth,and plants thus show a bushy appearance.
This occurs because of the removal of the source of auxin that was inhibiting the lateral buds.

(B) Zinc is an essential micronutrient for plants. It is required for the synthesis of the amino acid tryptophan,which is a precursor for the biosynthesis of the plant growth hormone auxin (specifically Indole$-3-$acetic acid or $IAA$). Therefore,a deficiency of zinc leads to a reduced production of auxin,resulting in stunted plant growth.

(A) The isolation of auxin was first performed by $F.W. Went$. He extracted the growth-promoting substance from the tips of $Avena$ (oat) coleoptiles and demonstrated its ability to cause curvature in coleoptiles. While $Charles$ $Darwin$ and his son $Francis$ $Darwin$ observed the phototropic response in canary grass coleoptiles,it was $Went$ who successfully isolated the chemical substance,which he named auxin.

(B) Parthenocarpy is the development of fruit without fertilization. In many plants,including tomatoes,parthenocarpy can be artificially induced by the application of plant growth regulators. Auxins,such as $IAA$ (Indole$-3-$acetic acid) or synthetic auxins like $2,4-D$,are commonly used to induce parthenocarpy in tomatoes. While gibberellins can also induce parthenocarpy in some species,auxins are the primary hormones used for this purpose in tomatoes.

(D) Charles Darwin and his son Francis Darwin conducted experiments on the coleoptiles of canary grass $(Phalaris \, canariensis)$.
They observed that the coleoptiles responded to unilateral illumination by growing towards the light source.
This phenomenon of growth towards light is known as phototropism.
Their experiments demonstrated that the tip of the coleoptile was the site of light perception, which then transmitted a signal to the lower part to cause bending.

(A) The term 'Auxin' is a collective term used for substances that promote cell elongation and growth in plants.
$IBA$ (Indole$-3-$butyric acid) is a well-known synthetic auxin.
$C_2H_4$ (Ethylene) is a gaseous plant hormone involved in fruit ripening.
$ABA$ (Abscisic acid) is a plant growth inhibitor.
$GA_3$ (Gibberellic acid) is a type of gibberellin,which is a different class of plant growth regulators.
Therefore,$IBA$ is the correct answer as it belongs to the auxin class.

(D) The experiments conducted by Charles Darwin and his son Francis Darwin on canary grass coleoptiles demonstrated that the coleoptile tip is the site of light perception.
When the tip was removed or covered,the coleoptile did not bend towards the light.
This indicated that the tip produces a transmittable influence (later identified as the hormone auxin) that moves downwards and causes the bending of the entire coleoptile towards the light source.
Therefore,the correct answer is $A -$ tip.

(A) Natural auxins are those that are naturally synthesized by plants. $IAA$ (Indole$-3-$acetic acid) and $IBA$ (Indole$-3-$butyric acid) are examples of natural auxins.
Synthetic auxins are artificially produced compounds that mimic the activity of natural auxins. $NAA$ (Naphthalene acetic acid) and $2,4-D$ ($2$,$4$-Dichlorophenoxyacetic acid) are examples of synthetic auxins.
Therefore,$A$ matches with $1$ and $3$,and $B$ matches with $2$ and $4$.

(B) $F$. $W$. Went isolated auxin from the tips of coleoptiles of oat seedlings $(Avena \, sativa)$.
He demonstrated that the substance could diffuse into agar blocks and induce curvature in decapitated coleoptiles, which led to the discovery of the first plant growth hormone, auxin.

(D) Auxins are plant hormones that promote growth. They are categorized into natural and synthetic auxins.
$IAA$ (Indole$-3-$acetic acid) and $IBA$ (Indole$-3-$butyric acid) are natural auxins.
$NAA$ (Naphthalene acetic acid) and $2,4-D$ ($2$,$4$-dichlorophenoxyacetic acid) are synthetic auxins.
$GA$ (Gibberellic acid) is a different class of plant growth regulator,not an auxin.
Therefore,$NAA$ is the correct synthetic auxin among the given options.

(D) Phototropism is the growth of a plant part in response to a light stimulus.
According to the $Cholodny-Went$ theory, when a plant shoot is exposed to unilateral light, $Auxins$ (specifically $Indole-3-acetic$ $\text{ } acid$ or $IAA$) migrate to the shaded side of the stem.
This higher concentration of $Auxins$ on the shaded side stimulates greater cell elongation compared to the illuminated side.
As a result, the stem bends towards the light source, a phenomenon known as phototropic curvature.

(C) Auxins are plant hormones that play a crucial role in regulating various growth and developmental processes.
In the case of pineapple ($Ananas$ $comosus$),flowering is often irregular or difficult under natural conditions.
Synthetic auxins like $NAA$ (Naphthalene Acetic Acid) and $2,4-D$ ($2$,$4$-Dichlorophenoxyacetic acid) are widely used in agriculture to induce flowering in pineapple plants.
By applying these auxins,growers can synchronize and promote flowering,thereby enabling the production of fruits throughout the year.

(D) Auxins are plant hormones that have several physiological effects.
$1$. Auxins like $IAA$ and $IBA$ are used to initiate rooting in stem cuttings,which is a common practice in plant propagation.
$2$. Auxins promote flowering in certain plants like pineapple.
$3$. Auxins are widely used in tea plantations for 'tea plucking' and 'hedge making' because they promote apical dominance.
$4$. Bolting (internode elongation just prior to flowering) is primarily promoted by gibberellins,not auxins. Therefore,promoting bolting is not an effect of auxin.

(D) $2,4-D$ ($2$,$4$-dichlorophenoxyacetic acid) is a synthetic auxin.
It is widely used as a selective herbicide.
It acts by causing uncontrolled growth in dicotyledonous plants,leading to their death.
However,it does not significantly affect mature monocotyledonous plants,making it useful for protecting crops like wheat or maize from broad-leaved weeds.

(D) Auxin was first isolated by $F. W. Went$ from the tips of $oat$ ($Avena$ $sativa$) coleoptiles.
$F. W. Went$ performed the $Avena$ curvature test to demonstrate the presence of a growth-promoting substance,which he named auxin.
While $Charles$ $Darwin$ and his son $Francis$ $Darwin$ observed the phototropic response in canary grass coleoptiles,the actual isolation of the substance was achieved by $F. W. Went$ using oat seedlings.

(A) $Auxin$ is a plant hormone that primarily promotes cell elongation and division. It is synthesized in the shoot and root apices (growing tips) of plants. Because it is produced in these regions to facilitate growth,the highest concentration of $Auxin$ is consistently found in the growing apices of the plant.

(C) Phototropism (growth towards light) and geotropism (growth in response to gravity) are primarily regulated by the plant hormone $Auxin$.
$Auxin$ is synthesized at the shoot tips and root tips.
It promotes cell elongation in the shoot and inhibits it in the root,leading to differential growth that results in the bending of plants towards light or in response to gravity.

(C) $1$. $Auxin$ is a plant hormone responsible for phototropism and geotropism.
$2$. The bending of shoots towards light (phototropism) and roots towards gravity (geotropism) are classic responses mediated by $Auxin$ distribution.
$3$. The movement of the sunflower apex towards the sun (heliotropism) is also regulated by $Auxin$ gradients.
$4$. There is no biological evidence or physiological mechanism where $Auxin$ causes a leaf to bend towards a 'fragrance'. Therefore,option $C$ is not an effect of $Auxin$.

(A) Apical dominance is the phenomenon where the main central stem of the plant grows more dominantly than the side (lateral) stems.
This process is primarily controlled by the hormone $Auxin$,specifically $Indole-3-Acetic Acid$ $(IAA)$.
$IAA$ is synthesized in the apical meristem and transported downwards,where it inhibits the growth of lateral buds.
Therefore,the correct option is $A$.

(D) The apical bud produces the hormone auxin,which inhibits the growth of lateral buds,a phenomenon known as apical dominance. When the apical bud is removed,the source of auxin is eliminated. This releases the lateral buds from inhibition,allowing them to grow and develop into lateral branches.

(B) Auxin transport in plants is primarily polar. In the stem,auxin is transported from the shoot apex towards the base,which is known as basipetal transport. In the root,it is transported from the root tip towards the base,which is also basipetal. However,in certain contexts like the vascular tissue,it can show complex patterns. Given the standard physiological context of auxin movement in the stem and root tips,it is characterized as basipetal transport.

(B) Auxins,such as $NAA$ (Naphthalene Acetic Acid),are known to inhibit leaf abscission by preventing the formation of the abscission layer at the base of the petiole. While ethylene promotes abscission,auxins act as antagonists to this process in young leaves and fruits,thereby delaying the shedding process.