Auxin Questions in English

(B) The excurrent or pyramidal shape of $Pinus$ is primarily due to the phenomenon of apical dominance.
Apical dominance is controlled by the hormone auxin,which is produced in the terminal buds (stem tips).
This hormone inhibits the growth of lateral buds,causing the main stem to grow vertically much faster than the side branches.
As a result,the branches are shorter at the top and longer at the bottom,giving the tree its characteristic conical or pyramidal appearance.

(A) Auxins are plant hormones that play a crucial role in root development.
In roots,low concentrations of auxins promote cell elongation and growth.
While high concentrations of auxins inhibit root growth,the physiological process of root elongation is primarily regulated by the distribution and activity of auxins within the root tissues.

(B) Zinc $(Zn^{2+})$ is essential for the synthesis of tryptophan,which is a precursor of indole$-3-$acetic acid $(IAA)$,a major plant hormone belonging to the auxin group. Therefore,zinc plays a critical role in the biosynthesis of $IAA$.

(C) Zinc is a crucial micronutrient required by plants for various physiological processes.
One of its most significant roles is in the biosynthesis of the plant growth hormone auxin,specifically Indole$-3-$Acetic Acid $(IAA)$.
Zinc acts as a cofactor for enzymes involved in the synthesis of tryptophan,which is the primary precursor amino acid for $IAA$ production.

(D) . Charles Darwin and his son Francis Darwin observed that the coleoptile of canary grass responded to unilateral illumination by growing towards the light source. Later,Peter Boysen-Jensen demonstrated that this response was due to the migration of a chemical substance produced at the tip,which was subsequently identified as auxin. Auxin accumulates on the shaded side of the coleoptile,causing cells on that side to elongate more rapidly than those on the illuminated side,resulting in the bending of the coleoptile towards the light.

(D) The process of producing seedless fruits through hormone application is known as parthenocarpy.
In many fruits like tomato,orange,and watermelon,seedless varieties are commercially desirable and preferred by consumers.
However,in pomegranate $(Anar)$,the edible part is the juicy seed coat (aril).
Therefore,obtaining seedless fruits in pomegranate is not desirable because the seeds are the primary edible component.

(B) Auxin inhibits the abscission of leaves and fruits. The abscission layer is formed when the concentration of auxin in the leaf petiole falls below a critical minimum level,which triggers the enzymatic degradation of the cell walls in the abscission zone.

(B) The first isolation of auxins was reported by $F.W. \text{Went}$ from the tips of oat seedlings $(Avena \text{ sativa})$.
$F.W. \text{Went}$ successfully extracted the growth-promoting substance, which he named auxin, using the agar diffusion method.
Therefore, the correct answer is $B$.

(A) Apical dominance is a phenomenon where the main central stem of the plant grows more dominantly than the side stems.
This process is primarily controlled by the hormone auxin,which is produced in the apical bud and inhibits the growth of lateral buds.
Cytokinins,on the other hand,promote the growth of lateral buds and help in breaking apical dominance.
Therefore,the balance between auxin and cytokinin levels determines the extent of apical dominance in higher plants.

(A) $Auxins$ promote elongation and growth of stems and roots and the enlargement of many fruits by stimulating the elongation of cells in all directions.
Therefore,the correct answer is $Auxin$.

(A) The first naturally occurring auxin,i.e.,Indole-$3$-acetic acid $(IAA)$,was isolated by Kogl and Haagen-Smit from human urine.

(A) Auxin is a plant hormone primarily responsible for growth-related movements such as phototropism (movement of shoot towards light) and geotropism (movement of roots towards soil).
Nyctinastic movements (sleep movements) are primarily caused by changes in turgor pressure within the pulvinus cells,not by auxin-mediated growth.
Therefore,nyctinastic leaf movement is not related to changes in auxin levels.

(B) Parthenocarpy is the development of fruit without fertilization.
Auxins are plant hormones that can induce parthenocarpy in many plants.
When auxins are applied to unfertilized ovaries,they stimulate the ovary to develop into a fruit,bypassing the need for fertilization.

(A) $F.W. Went$ discovered the $Avena$ curvature test in $1928$.
This bioassay is used to measure the concentration of auxins, specifically $Indole-3-acetic$ $\text{acid}$ $(IAA)$, based on the curvature produced in oat $(Avena$ $\text{sativa})$ coleoptile segments.
Therefore, the correct option is $A$.

(B) The correct answer is $B$. In many plants,the apical bud grows and the lower axillary buds are suppressed,a phenomenon known as apical dominance.
Synthetic auxins,when applied in high concentrations,mimic this effect or enhance it,thereby preventing the growth of lateral buds.
Removal of the apical bud results in the growth of lower buds because the source of auxin $(IAA)$ is removed,which otherwise inhibits the growth of lateral buds.

(A) $2, 4-D$ ($2, 4$-dichlorophenoxyacetic acid) is a synthetic auxin,not a naturally occurring plant hormone.
$IAA$ (Indole$-3-$acetic acid) is a natural auxin.
$GA_2$ and other gibberellins are naturally occurring plant hormones.

(B) Auxins are plant hormones that are primarily synthesized in the shoot apical meristems and young leaves.
Because they are responsible for cell elongation and apical dominance,their concentration is highest in the growing tips (apical buds) of plants.
Therefore,the correct option is $B$.

(B) Auxins are a class of plant hormones that promote growth.
Indole$-3-$acetic acid $(IAA)$ is the most common and well-known naturally occurring auxin in plants.
$2, 4-D$ $(2, 4-\text{dichlorophenoxyacetic acid})$ and $NAA$ $(Naphthalene acetic acid)$ are synthetic auxins.
Maleic hydrazide is a plant growth inhibitor.

(B) Synthetic auxins like $2,4-D$ $(2,4-\text{dichlorophenoxyacetic acid})$ are widely used in agriculture as herbicides.
These compounds are applied in high concentrations to selectively kill broad-leaved weeds without affecting mature monocotyledonous crops.
Therefore, high concentrations of synthetic auxin are primarily used for weed control.

(C) $IAA$ (Indole$-3-$acetic acid) is the most common naturally occurring auxin in plants.
Natural auxins are synthesized in physiologically active parts of plants,such as shoot apices,leaf primordia,developing seeds,buds,and embryos.
The amino acid tryptophan serves as the primary precursor for the biosynthesis of $IAA$ in plants.

(B) Auxins are plant hormones that promote apical dominance. In this phenomenon,the growth of the apical bud is stimulated,while the growth of lateral axillary buds is inhibited. This is why removing the apical bud often leads to the growth of lateral branches.

(A) Phototropism is the growth of a plant part in response to a light source.
In shoots,this phenomenon is primarily caused by the unequal distribution of the hormone $Auxin$.
When a shoot is exposed to light from one side,$Auxin$ migrates to the shaded side of the stem.
Since $Auxin$ promotes cell elongation,the cells on the shaded side grow faster than those on the illuminated side,causing the shoot to bend towards the light source.

(B) In unisexual plants such as $Cannabis$ $sativa$,the application of auxins can induce the development of male flowers on a female plant and vice-versa.
This phenomenon is known as sex-change,where plant hormones regulate the expression of floral sex characteristics.

(C) The discovery of auxins was initiated by the observations of Charles Darwin and his son Francis Darwin,who worked on the coleoptiles of canary grass. Later,$F$.$W$. Went isolated auxin from the tips of Avena coleoptiles (oat seedlings). Therefore,most of the fundamental information regarding the physiological effects and properties of auxins was derived from experiments conducted on Avena coleoptiles.

(C) Auxins are a class of plant hormones. The most common natural auxin is Indole$-3-$acetic acid $(IAA)$. However,in specific biochemical literature regarding the chemical structure of certain auxin derivatives or related compounds,the formula $C_{18}H_{32}O_5$ is identified as the molecular formula for specific auxin-related substances. Based on the provided options,$C_{18}H_{32}O_5$ is the correct chemical representation.

(A) Phenyl acetic acid $(PAA)$ is a naturally occurring auxin found in plants. It acts as a plant growth regulator,specifically belonging to the class of auxins,which are responsible for cell elongation and various developmental processes in plants.

(C) $NAA$ stands for $Naphthalene$ $acetic$ $acid$.
It is a synthetic auxin,which is a type of plant hormone.
It is widely used in agriculture and horticulture for rooting cuttings,preventing premature fruit drop,and promoting flowering.

(A) Auxins are a class of plant hormones that promote growth. Synthetic auxins are widely used in agriculture and horticulture. $\alpha$-naphthalene acetic acid $(NAA)$ is a well-known synthetic auxin. While $\beta$-naphthalene acetic acid is an isomer,it does not possess the same physiological activity as the $\alpha$-form. Therefore,only $\alpha$-naphthalene acetic acid exhibits auxin-like properties.

(D) $2, 4-D$ stands for $2, 4$-dichlorophenoxy acetic acid.
It is a synthetic auxin used as a herbicide to kill broad-leaved weeds.
It is widely used in agriculture and horticulture to control unwanted plant growth.

(C) Indole propionic acid $(IPA)$ is a naturally occurring indole derivative. It is structurally related to the plant hormone Indole$-3-$acetic acid $(IAA)$,which is a type of auxin. Therefore,it is considered a substance related to or derived from plant hormones.

(D) Auxins are plant growth regulators that are primarily synthesized in the shoot apices (tips of stems) and root apices. From these sites,they are transported to other parts of the plant where they promote cell elongation and division. Auxins are also found in high concentrations in developing tissues such as growing fruits and developing embryos,where they play a crucial role in growth and development. Therefore,all the mentioned locations are sites where auxins are active.

(A) According to $F. \text{ Went}$,auxins exhibit polar transport in plants.
This means that auxins move primarily in a specific direction,typically from the apex (shoot tip) towards the base (downward).
This polar movement is essential for processes like apical dominance and phototropism.

(C) Auxins,such as $IAA$ (Indole$-3-$acetic acid) and $IBA$ (Indole$-3-$butyric acid),are widely used in plant propagation.
One of the most common practical applications of auxins is to initiate rooting in stem cuttings.
When stem cuttings are treated with auxin solutions,it promotes the development of adventitious roots,which facilitates the growth of new plants.

(A) In many plants,the shedding of leaves and fruits occurs due to the formation of an abscission layer.
Auxin is a plant hormone that prevents the formation of the abscission layer.
Therefore,the application of auxin can delay the fall of fruits and leaves until they are fully matured.
In the absence of auxin,this premature abscission occurs.

(B) In tobacco plants,the application of $Auxins$ (a type of plant growth regulator) is known to inhibit or retard certain enzymatic activities,particularly those related to senescence or specific metabolic pathways. This is a common physiological response used in agricultural practices to manage plant growth and development.

(A) $IBA$ stands for Indole$-3-$butyric acid.
It is a synthetic form of auxin, a plant hormone that promotes root initiation and growth.
Therefore, $IBA$ is classified as an auxin.

(D) Many new plants are usually propagated by stem cutting. If we dip the lower cut end of a cutting in a dilute solution of auxins (especially $IBA$),a large number of roots are developed on the cut ends very soon,due to which these cuttings develop into successful plants. Among the auxins,$IBA$ (Indole$-3-$butyric acid) is widely used in horticulture for inducing root formation in stem cuttings.

(C) Auxins were first isolated from human urine. Later,various forms of auxins were identified in plants. Specifically,from $Phaseolus \ vulgaris$ (French bean),$4$ types of auxins have been isolated and identified.

(C) Auxins,such as $IAA$ (Indole$-3-$acetic acid) and synthetic auxins like $IBA$ (Indole$-3-$butyric acid) and $NAA$ (Naphthalene acetic acid),are widely used in agriculture and horticulture.
Specifically,the induction of root formation in stem cuttings is a primary commercial application of auxins,which allows for the vegetative propagation of plants.
While auxins are involved in callus formation and stem curvature,the induction of rooting is the most significant commercial use among the given options.

(B) $2, 4-D$ ($2, 4$-dichlorophenoxyacetic acid) is a synthetic auxin used as a selective weedicide.
It is highly toxic to broad-leaved plants.
It increases carbohydrate metabolism to such an extent that it causes uncontrolled growth and eventual death in dicot weeds,while leaving monocot crops largely unaffected.

(A) Plant hormones,also known as phytohormones,are chemical substances that regulate plant growth and development.
$1$. Indole acetic acid $(IAA)$ is a type of auxin that is synthesized in the shoot tips and is translocated to other parts of the plant to promote cell elongation and growth.
$2$. While gibberellins and cytokinins also promote growth,$IAA$ is the primary hormone classically associated with polar transport and translocation for growth regulation.
Therefore,$IAA$ is the correct answer.

(A) The $Avena$ curvature test is a classic bioassay used to detect and measure the concentration of auxin.
In this test,the coleoptile tips of $Avena$ $sativa$ (oat) seedlings are removed,and agar blocks containing auxin are placed asymmetrically on the cut surface.
The auxin diffuses into the coleoptile,causing differential growth and resulting in the curvature of the coleoptile towards the side opposite to the agar block.
The degree of curvature is proportional to the concentration of auxin present.

(B) The substances that originate at the tip of the stem and control growth in other parts of the plant are known as plant hormones or phytohormones.
Specifically,$Auxins$ are produced in the apical meristems (tips) of stems and are transported to other parts of the plant to regulate growth,cell elongation,and development.
Therefore,the correct option is $(b)$.

(C) Auxins are a class of plant hormones (phytohormones) that play a crucial role in the growth and development of plants.
They are primarily involved in cell elongation,root initiation,and apical dominance.
Since they promote various growth processes in plants,they are classified as growth hormones.

(D) Auxins are plant growth regulators that are primarily synthesized in the growing apices of stems and roots. However,they are produced in the highest concentrations in the apical meristems of the shoot,which are responsible for primary growth and apical dominance. Therefore,the meristematic region of the shoot is the primary site of abundant auxin production.

(B) $Auxin$ is a plant hormone that is primarily responsible for cell elongation in stems.
It promotes the growth of the stem by stimulating cell division and elongation in the apical meristem region.
Therefore,$Auxin$ is the substance that accelerates growth in the stem.

(C) Auxins are a class of plant hormones (phytohormones) that play a crucial role in plant growth and development.
They are primarily synthesized in the actively growing regions of the plant,specifically the apical meristems located at the root and shoot tips.
From these sites of synthesis,auxins are transported to other parts of the plant to regulate processes such as cell elongation,apical dominance,and root initiation.

(D) Auxins are plant hormones that promote apical dominance,meaning they inhibit the growth of lateral buds. Therefore,inducing lateral bud formation is not a characteristic of auxin; rather,it is a characteristic of cytokinins. Auxins exhibit polar transport,delay leaf and fruit abscission,and promote cell elongation.

(A) The chemical structure shown is Indole$-3-$acetic acid,commonly abbreviated as $IAA$.
$IAA$ is the most common and naturally occurring auxin found in plants.

(A) Apical dominance is a phenomenon in plants where the main central stem of the plant grows more dominantly than the side (lateral) stems.
This process is primarily regulated by the plant hormone $Auxin$.
$Auxin$ is synthesized in the apical meristem and transported downwards,where it inhibits the growth of lateral buds,thereby maintaining the dominance of the apical bud.