Auxin Questions in English

(C) Apical dominance is a phenomenon in plants where the growing apical bud suppresses the growth of lateral (axillary) buds.
This occurs due to the production of auxins in the shoot apex.
When the shoot tip (apical bud) is removed,the source of auxin is eliminated,which allows the lateral (axillary) buds to grow,resulting in a bushier plant.
Therefore,$A$ is apical,$B$ is lateral,and $C$ is lateral.

(C) Phototropism is the growth or movement of a plant organ (like a coleoptile) towards a light source,which is mediated by the hormone auxin.
Auxin is produced in the apical tip and moves downwards. When light is present,auxin accumulates on the shaded side,causing faster cell elongation on that side,which results in bending towards the light.
$1$. In coleoptile $A$,the blade (blockage) is on the side facing the light. This prevents auxin from moving down the illuminated side,allowing it to accumulate on the shaded side,which promotes bending towards the light.
$2$. In coleoptile $B$,the blade blocks the entire cross-section,preventing any downward movement of auxin. Therefore,no bending occurs.
$3$. In coleoptile $C$,the blade is on the shaded side. This prevents auxin from reaching the shaded side,so the plant cannot bend towards the light.
$4$. In coleoptile $D$,there is no blockage,so auxin moves normally,and the coleoptile bends towards the light.
Thus,coleoptiles $A$ and $D$ will bend towards the light.

(A) Auxins are synthesized at the shoot apices and are transported downwards towards the base of the stem. This downward movement from the apex to the base is known as basipetal transport. In the roots,auxin is transported from the root tip towards the shoot,which is known as acropetal transport. However,in the context of the stem tip mentioned in the question,the primary movement is basipetal.

(D) The functions of auxin include:
$(i)$ Auxin helps to initiate rooting in stem cuttings,which is a widely used application for plant propagation.
$(ii)$ Auxin promotes flowering in plants like pineapples.
$(iii)$ Auxin plays a crucial role in controlling xylem differentiation and helps in cell division.
Since all three statements ($I, II,$ and $III$) are correct functions of auxin,the correct option is $D$.

(C) Auxins are a class of phytohormones that are chemically derived from indole compounds.
Common examples of naturally occurring auxins include Indole-$3$-acetic acid $(IAA)$ and Indole butyric acid $(IBA)$.
Therefore,the correct answer is Auxin.

(B) $IAA$ (Indole$-3-$acetic acid) and $IBA$ (Indole$-3-$butyric acid) are natural auxins found in plants.
$2, 4-D$ ($2, 4$-Dichlorophenoxyacetic acid) and $NAA$ (Naphthalene acetic acid) are synthetic auxins produced artificially in laboratories.

(D) Auxin is a plant hormone that plays a dual role in leaf abscission.
It inhibits the abscission of young leaves by maintaining the physiological activity of the cells in the abscission zone.
Conversely,it promotes the abscission of old leaves by stimulating the formation of the abscission layer.

(A) Auxins like $IAA$ (Indole$-3-$acetic acid) and $IBA$ (Indole$-3-$butyric acid) are used to induce parthenocarpy.
Parthenocarpic fruits are seedless fruits that develop without fertilization.
By applying these auxins in a diluted form to the flowers,the development of fruit is stimulated without the need for pollination or fertilization,as seen in tomatoes.

(C) $2, 4-D$ ($2, 4$-dichlorophenoxyacetic acid) is a synthetic auxin that is widely used as a weedicide.
It is specifically used to eliminate unwanted broad-leaved weeds in fields of monocotyledonous crops like wheat and maize without affecting the main crop.

(C) Phototropism is primarily caused by the unequal distribution of auxin in the stem. When a plant is exposed to unilateral light,auxin accumulates on the shaded side,causing that side to grow faster and resulting in the stem bending towards the light. If the stem is already saturated with auxin,the exogenous auxin overrides the natural gradient,leading to uniform growth on all sides. Consequently,the stem will grow straight upwards despite the unilateral light exposure.

(D) Auxins are plant hormones that promote cell elongation,the development of parthenocarpic fruits,and the prevention of abscission of leaves and fruits.
Reversal of genetic dwarfism is a physiological effect primarily associated with Gibberellins,not Auxins.

(A) The transport of auxins is polar in nature.
In the stem,auxins are transported from the shoot apex towards the base,which is known as basipetal transport.
Therefore,the correct direction of auxin transport is polar and basipetal.

(B) Tea plants are pruned to remove the apical buds.
Apical buds are the primary sites of auxin synthesis.
Auxin is responsible for apical dominance,which inhibits the growth of lateral buds.
By removing the apical buds,the effect of auxin is eliminated,allowing the lateral buds to grow and resulting in a bushier plant.

(B) Auxin is a plant growth regulator that acts as a weak organic acid. It possesses an unsaturated ring structure (specifically an indole ring in the case of $IAA$). It is biosynthesized from the amino acid $Tryptophan$.

(A) When a plant is placed horizontally,the gravity-sensing organelles known as statoliths (starch-filled amyloplasts) settle at the lower side of the cells due to gravity.
This displacement of statoliths triggers a signaling pathway that causes the redistribution of the plant hormone auxin.
Consequently,auxin accumulates on the lower surface of the stem or root,leading to differential growth (gravitropism).
Therefore,both the Assertion and the Reason are correct,and the Reason provides the correct explanation for the phenomenon described in the Assertion.

(A) According to the acid growth hypothesis,auxin stimulates the proton pumps in the plasma membrane,which increases the concentration of $H^+$ ions in the cell wall,leading to acidification.
This acidic environment activates cell wall-loosening enzymes known as expansins.
These enzymes break the hydrogen bonds between cellulose microfibrils,causing the cell wall to loosen.
This loosening allows the cell wall to extend under turgor pressure,resulting in cell elongation.
Therefore,both the Assertion and the Reason are correct,and the Reason correctly explains the mechanism behind the Assertion.

(C) $2, 4-D$ ($2, 4$-dichlorophenoxyacetic acid) is a synthetic auxin used extensively as a herbicide to kill dicotyledonous weeds without affecting mature monocotyledonous crop plants.
Therefore,the Assertion is correct.
However,flowering in most plants is not initiated by high concentrations of auxin. In fact,auxin generally inhibits flowering in most plants,with the notable exception of pineapple,where it promotes flowering. The primary hormone responsible for inducing flowering is florigen.
Therefore,the Reason is incorrect.

(C) The plant hormone $2,4-D$ ($2$,$4$-dichlorophenoxyacetic acid) is a synthetic auxin.
It is widely used as a herbicide to kill dicotyledonous weeds in fields of monocotyledonous crops like wheat and maize.
It does not affect mature monocotyledonous plants,making it a selective weed killer.
Therefore,the correct option is $C$.

(A) Zinc $(Zn^{2+})$ is an essential micronutrient required by plants for various physiological processes.
Zinc is a critical cofactor for the enzyme tryptophan synthetase,which is involved in the biosynthesis of the plant growth hormone auxin (indole$-3-$acetic acid).
Therefore,a deficiency of zinc leads to a significant reduction in the biosynthesis of auxin,resulting in stunted growth and smaller leaves in plants.

(N/A) The shoot apex exhibits positive phototropism. This phenomenon occurs because of differential growth in the stem,which is caused by the unequal distribution of the hormone auxin. When light falls on one side of the shoot,auxin migrates to the shaded side. Higher concentration of auxin on the shaded side promotes faster cell elongation compared to the illuminated side,causing the plant to bend toward the light source.

(N/A) $1$. The apical bud is the primary site of $Auxin$ synthesis in plants.
$2$. $Auxin$ produced in the apical bud moves downwards and suppresses the growth of lateral or axillary buds,a phenomenon known as $Apical$ $Dominance$.
$3$. When the apical bud is removed,the source of $Auxin$ is eliminated.
$4$. Due to the absence of $Auxin$,the inhibitory effect on the axillary buds is lifted,allowing them to grow out and develop into new branches.

(N/A) In an intact plant,the endogenous level of auxins is already optimal for growth.
Adding exogenous auxins does not further enhance growth because the plant's physiological processes are already saturated with respect to the auxin concentration required for normal development.
Therefore,the plant does not show a significant growth response to additional external application.

(N/A) Auxins are a class of plant hormones that promote growth. While Indole-$3$-acetic acid $(IAA)$ is the most common and primary natural auxin,it is not the only one. Other chemical substances,such as $4$-chloro $IAA$ and phenylacetic acid $(PAA)$,have been isolated from various plant species. These compounds exhibit auxin-like physiological effects,such as cell elongation and root initiation,and are therefore classified as naturally occurring auxins.

(N/A) Auxin is a generic term for a group of plant growth hormones,primarily indole$-3-$acetic acid $(IAA)$ and other natural and synthetic compounds having growth-regulating properties.
These hormones are synthesized at the shoot and root apices (tips) and are responsible for cell elongation,apical dominance,and various other developmental processes in plants.

(A) The scientific explanation for this practice is the phenomenon of apical dominance.
In many plants,the apical bud produces auxins which inhibit the growth of lateral buds.
By trimming or removing the shoot apex (apical bud),the source of these inhibitory auxins is removed.
This allows the lateral buds to develop,promoting lateral branching and making the plant appear more bushy.

(N/A) The phenomenon by which the terminal/apical bud suppresses the growth of lateral buds is known as apical dominance.
It is primarily caused by the hormone $Auxin$, which is synthesized in the apical buds and transported downwards, inhibiting the development of lateral buds.
To overcome this phenomenon, one can perform decapitation, which involves the physical removal of the apical bud.
Alternatively, the application of $Cytokinin$ can promote the growth of lateral buds, thereby counteracting the effect of $Auxin$.

(C) The synthesis of the plant growth hormone $IAA$ (Indole$-3-$acetic acid) and its synthetic analog $IBA$ (Indole$-3-$butyric acid) requires the micronutrient $Zinc$ $(Zn)$.
$Zinc$ is essential for the synthesis of $Tryptophan$,which is the precursor amino acid for the biosynthesis of $Auxins$ like $IAA$.

(B) Auxin was first isolated by $F.W. Went$ from the tips of $Avena$ (oat) seedlings. While Charles and Francis Darwin observed the phototropic response,it was $F.W. Went$ who successfully isolated the growth-promoting substance and named it auxin.

(C) Auxins are plant growth regulators.
$IAA$ (Indole$-3-$acetic acid) and $IBA$ (Indole$-3-$butyric acid) are naturally occurring auxins found in plants.
$NAA$ (Naphthalene acetic acid) and $2,4-D$ ($2$,$4$-dichlorophenoxyacetic acid) are synthetic auxins produced in laboratories.
Therefore,the correct pair for natural auxins is $IAA$ and $IBA$,and for synthetic auxins is $NAA$ and $2,4-D$.

(C) $IAA$ 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.

(B) The figure illustrates the phenomenon of apical dominance in plants.
Apical dominance is the phenomenon where the growing apical bud inhibits the growth of lateral buds.
This process is primarily regulated by the plant hormone Auxin,which is synthesized at the shoot apex.
When the apical bud is removed (as shown in the figure),the inhibition on the lateral buds is lifted,allowing them to grow into branches.
Therefore,the correct association is Auxin - Apical dominance.

(A) $2,4-D$ ($2,4$-dichlorophenoxyacetic acid) is a synthetic auxin.
It is widely used as a herbicide to eliminate broad-leaved or dicotyledonous weeds.
It does not affect mature monocotyledonous plants,making it useful for protecting cereal crops.

(A) The correct answer is $A$ (Auxin).
Auxins are known to prevent the premature abscission of young leaves and fruits by maintaining the integrity of the abscission zone.
However,at higher concentrations or in specific physiological contexts,they can also promote the abscission of older,mature leaves and fruits by stimulating the production of ethylene,which triggers the formation of the abscission layer.

(A) Auxins are plant hormones that promote apical dominance.
In gardening,pruning the apical bud removes the source of auxins,which allows lateral buds to grow,resulting in a bushy appearance.
Therefore,auxins are responsible for the growth patterns that allow gardeners to shape plants into hedges.

(B) Sol. Auxin,specifically synthetic auxins like $2,4-D$ ($2$,$4$-Dichlorophenoxyacetic acid),is used as a selective herbicide. It does not affect mature monocotyledonous plants like grasses. This is because monocots possess a different vascular structure and show limited translocation of the applied auxin,leading to its rapid degradation,thereby sparing the grass while killing broad-leaved weeds.

(D) Auxins are a class of plant hormones that promote cell elongation and root initiation.
$IBA$ (Indole$-3-$butyric acid) is a synthetic auxin that is widely used in horticulture to induce root formation in stem cuttings.
$GA$ (Gibberellic acid) is primarily involved in stem elongation and seed germination.
$ABA$ (Abscisic acid) acts as a growth inhibitor and promotes dormancy.
Kinetin is a type of cytokinin that promotes cell division.

(A) In plant tissue culture,the morphogenetic response is determined by the ratio of cytokinin to auxin.
$1$. $A$ high cytokinin to auxin ratio promotes shoot development.
$2$. $A$ low cytokinin to auxin ratio (i.e.,high auxin relative to cytokinin) promotes root development.
Therefore,a low ratio of cytokinin to auxin induces root development.

(D) Auxins are a class of plant hormones that play a crucial role in regulating plant growth and development.
One of the primary physiological effects of auxins,such as $NAA$ (Naphthalene Acetic Acid) and $IBA$ (Indole$-3-$Butyric Acid),is the prevention of premature abscission of leaves,flowers,and fruits.
In commercial horticulture,synthetic auxins are sprayed on fruit trees like apple,pear,and orange to prevent the premature drop of fruits,thereby increasing the yield.
Therefore,the correct option is $D$.

(D) Auxins,such as Indole$-3-$acetic acid $(IAA)$ and Indole$-3-$butyric acid $(IBA)$,are widely used in plant tissue culture and horticulture to induce and accelerate root formation in stem cuttings. This property makes them essential for vegetative propagation.

(B) The formation of intrafascicular cambium is primarily induced by the plant hormone $Auxin$.
$Auxin$ promotes cell division and differentiation in the vascular cambium,which is essential for secondary growth in plants.
While other hormones like $Cytokinin$ and $Gibberellins$ also play roles in plant growth and development,$Auxin$ is specifically known to stimulate the activity of the cambium.

(D) Auxins are plant hormones that play a crucial role in regulating plant growth.
Specifically,auxins prevent the premature abscission (falling) of fruits by inhibiting the formation of the abscission layer at the pedicel.
Additionally,auxins are known to break seed dormancy and promote rapid seed germination.

(C) $IAA$ (Indole$-3-$acetic acid) is a naturally occurring auxin that induces parthenocarpy.
It promotes cell division,cell differentiation,and cell elongation in the ovary wall,which leads to the development of the ovary into a fruit without fertilization.

(D) The term $auxin$ was coined by $F.W. Went$ in $1928$.
$F.W. Went$ isolated $auxin$ from the tips of $Avena$ coleoptiles.
$Charles$ $Darwin$ and his son $Francis$ $Darwin$ observed the coleoptiles of $Phalaris$ $canariensis$ and noted that they responded to unilateral illumination by growing towards the light source.
$Boysen$ $Jensen$ demonstrated that the stimulus was a chemical substance that could diffuse through gelatin.
$Paal$ showed that the growth curvature was due to the unequal distribution of a growth-promoting substance.

(A) The primary precursor of $IAA$ (Indole$-3-$acetic acid) in plants is $Tryptophan$.
$IAA$ is the most common natural auxin produced in plants.

(B) In plant physiology,synthetic auxins are widely used to regulate growth and development.
$NAA$ (Naphthalene Acetic Acid) is commonly used to induce flowering in $Litchi$.
$2, 4-D$ ($2$,$4$-Dichlorophenoxyacetic acid) is effectively used to induce flowering in $Pineapple$.
Therefore,the combination of $NAA$ and $2, 4-D$ is the correct answer for these specific applications.

(B) $2, 4-D$ ($2, 4$-dichlorophenoxyacetic acid) is a synthetic auxin.
It is widely used as a selective herbicide (weedicide) to eradicate broad-leaved dicotyledonous weeds from monocotyledonous crop fields like wheat or maize.
It does not affect the monocotyledonous crops, making it an effective tool for agricultural weed management.

(B) In plants,the apical bud produces auxin,which is transported downwards and inhibits the growth of lateral (axillary) buds,a phenomenon known as apical dominance.
As the distance from the apical bud increases,the concentration of auxin decreases.
Therefore,the lower axillary buds receive a very low concentration of auxin compared to the upper ones.
Due to this reduced concentration of auxin,the inhibitory effect is minimized,allowing the lower branches to grow larger and more profusely.

(A) Apical dominance is a phenomenon where the main central stem of the plant grows more dominantly than the side (lateral) buds.
This process is primarily controlled by $Auxins$, specifically $Indole-3-acetic \text{ } acid$ $(IAA)$.
$Auxins$ produced in the apical meristem inhibit the growth of lateral buds, thereby maintaining the dominance of the main shoot.
Therefore, the correct option is $A$.

(C) $2, 4-D$ is a synthetic auxin and does not occur naturally in plants.
$GA$ (Gibberellins), $ABA$ (Abscisic acid), and $IAA$ (Indole-3-acetic acid) are all naturally occurring plant hormones.
Therefore, the correct option is $C$.

(D) The correct answer is $D$.
Apical dominance is a phenomenon in plants where the main central stem of the plant grows more dominantly than the side stems (lateral buds).
This process is primarily controlled by the hormone $Auxin$,which is synthesized at the shoot tip (apical meristem).
These $Auxins$ are transported downwards and inhibit the growth of lateral buds,thereby maintaining the dominance of the apical shoot.