Auxin Questions in English

(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.

(B) $Zinc$ is an essential micronutrient required for the synthesis of $Auxin$ (specifically $Indole-3-acetic$ $acid$ or $IAA$) in plants. $Zinc$ acts as a cofactor for the enzymes involved in the tryptophan-dependent pathway of $Auxin$ biosynthesis. Therefore,a deficiency of $Zinc$ leads to a reduction in the production of $Auxin$,which subsequently affects plant growth and development.

(A) Apical dominance is a phenomenon where the apical bud (shoot tip) suppresses the growth of lateral (axillary) buds.
This is primarily mediated by the hormone auxin,which is produced in the shoot apex and transported downwards.
When the apical bud is removed (pruning),the inhibitory effect on the lateral buds is lifted,allowing them to grow and develop into branches.
Therefore,the presence of the apical bud keeps the lateral buds in a dormant or inhibited state.

(D) Phototropic curvature in plants is primarily caused by the uneven distribution of the hormone $Auxin$ (specifically $Indole-3-acetic$ $acid$ or $IAA$).
When a plant is exposed to unilateral light,$Auxin$ migrates to the shaded side of the stem.
This higher concentration of $Auxin$ 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 phototropism.

(B) Dr. $F$. Went's experiment demonstrated that a chemical substance diffuses from the coleoptile tip into the agar block. When this agar block is placed on a decapitated coleoptile,it induces curvature. This experiment proves that a growth-promoting substance is produced in the plant in very small quantities,which can be isolated and transferred through agar.

(B) The bioassay for $Auxin$ is the $Avena$ coleoptile curvature test.
This test was developed by $F.W. Went$ to measure the activity of $Auxin$ (specifically $IAA$).
In this experiment,the tip of the $Avena$ coleoptile is removed,and an agar block containing $Auxin$ is 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.

(D) In plant tissue culture,the process of organogenesis (development of shoots and roots) is controlled by the balance of plant growth regulators in the culture medium.
$1$. Cytokinins are primarily responsible for promoting shoot development (caulogenesis).
$2$. Auxins are primarily responsible for promoting root development (rhizogenesis).
$3$. By manipulating the ratio of Auxin to Cytokinin,researchers can induce the formation of roots,shoots,or undifferentiated callus tissue. Therefore,a combination of Auxin and Cytokinin is essential for organ differentiation.

(B) The Avena curvature test is a classic bioassay used to detect and measure the activity of auxins,specifically Indole$-3-$acetic acid $(IAA)$.
This test was developed by Frits Went in $1928$.
In this experiment,agar blocks containing the substance to be tested are placed asymmetrically on the cut coleoptile tips of Avena sativa (oat) seedlings.
The auxin diffuses into the coleoptile,causing differential growth (curvature) on the side where the block is placed.
The degree of curvature is directly proportional to the concentration of the auxin present in the agar block.

(C) Auxins are plant hormones that play a crucial role in regulating plant growth and development.
One of the primary functions of auxins,such as $IAA$ (Indole$-3-$acetic acid) and $NAA$ (Naphthalene acetic acid),is to prevent the premature abscission (falling off) of fruits and leaves.
They inhibit the formation of the abscission layer at the base of the petiole or fruit stalk,thereby keeping them attached to the plant for a longer duration.
Therefore,the application of auxin is used in agriculture to prevent premature fruit drop.

(D) The chemical $2, 4-D$ ($2, 4-dichlorophenoxyacetic$ acid) is a synthetic auxin that acts as a herbicide.
It is widely used to control broad-leaved weeds and is also known as a defoliant,meaning it causes leaves to fall off from plants or trees.
Therefore,it is used to remove leaves from forest trees.

(A) The Avena curvature test is a classic bioassay developed by Frits Went in $1928$.
It is specifically used to detect and measure the biological activity of auxins,particularly Indole$-3-$Acetic Acid $(IAA)$.
In this experiment,agar blocks containing auxin are placed asymmetrically on the coleoptile tips of Avena sativa (oat) seedlings,causing the coleoptile to curve due to differential growth.

(B) Auxins,such as $IAA$ (Indole$-3-$acetic acid),play a dual role in the process of abscission.
In the early stages of development,auxins prevent the formation of the abscission layer,thereby preventing the premature drop of young leaves and fruits.
Conversely,in older or mature leaves and fruits,auxins promote the process of abscission.
Since the Assertion describes the inhibitory role in early stages and the Reason describes the promoting role in mature stages,both statements are correct,but the Reason does not explain why auxins prevent drop in early stages. Thus,the Reason is not the correct explanation of the Assertion.

(A) The plant hormone,$Auxin$,is distributed more on the side away from the unilateral illumination,causing cells to grow faster in the darker side. This differential growth causes the plant to bend toward the light,a phenomenon known as phototropism. If the distribution were equal,the plant would grow straight upwards. Neurotransmitters are chemicals secreted by multicellular animals and are used in transmitting impulses in the nervous system,not in plant growth responses.

(A) Auxins are a class of plant hormones that play a crucial role in plant growth and development. The primary physiological effects include:
$1$. Apical Dominance: Auxins produced in the apical bud inhibit the growth of lateral buds,ensuring the plant grows vertically.
$2$. Cell Elongation: They promote the elongation of stems and coleoptiles by increasing cell wall plasticity.
$3$. Root Initiation: Auxins are widely used in horticulture to induce root formation in stem cuttings.
$4$. Fruit Development: They promote fruit growth and prevent premature fruit drop (e.g.,in apples).
$5$. Weed Control: Synthetic auxins like $2,4-D$ are used as herbicides to kill broad-leaved weeds without affecting monocot crops.

(N/A) Auxin (Greek 'auxein': to grow) was initially isolated from human urine,but later its presence was also found in plants,proving it to be the first $PGR$ (Plant Growth Regulator) ever known.
The primary natural plant auxin is chemically known as Indole-$3$-Acetic Acid $(IAA)$.
The term 'auxin' is also applied to other natural and synthetic compounds having various growth-regulating properties.
Production of auxin generally takes place in the growing apices of stems and roots,from where they migrate to the site of action.
Types of Auxin:
$(a)$ Natural Auxins: These occur naturally in plants and fungi,e.g.,Indole Acetic Acid $(IAA)$ and Indole Butyric Acid $(IBA)$.
$(b)$ Synthetic Auxins: These are prepared from synthetic compounds that cause several responses similar to $IAA$,e.g.,Naphthalene Acetic Acid $(NAA)$ and $2,4$-Dichlorophenoxyacetic acid $(2,4-D)$.
Functions of Auxins:
$(a)$ Cell Elongation: Auxin stimulates the elongation of cells in shoots.
$(b)$ Initiation of Roots: In contrast to stems,higher concentrations of auxin inhibit shoot elongation but initiate lateral root formation.
$(c)$ Inhibition of Abscission: Natural auxins delay the abscission of young fruits and leaves and are used to control pre-harvest fruit drop.
$(d)$ Apical Dominance: The presence of auxin in higher concentrations in the shoot apex promotes apical dominance,where the apical bud inhibits the growth of lateral buds. Lateral buds only develop into branches when the apical bud is removed (decapitation).
$(e)$ Promotes Flowering: Auxin helps in promoting flowering in plants like pineapple and litchi.
$(f)$ Parthenocarpy: Auxins are used to induce parthenocarpy in unpollinated pistils,which increases market value.
$(g)$ Metabolism: Application of auxin can enhance metabolism due to the mobilization of plant resources.
$(h)$ Eradication of Weeds: Auxins are used as weedicides. For example,$2,4-D$ is widely used to kill dicotyledonous weeds without affecting mature monocotyledonous plants.
$(i)$ Cell Division: Besides cell elongation,auxin may also be active in cell division.

(N/A) The plant hormone found in human urine is $Indole-3-acetic \text{ acid } (IAA)$, which is the most common naturally occurring auxin in plants. $IAA$ was first isolated from human urine. It is a phytohormone that promotes cell elongation, root initiation, and apical dominance in plants. Its presence in human urine is attributed to the consumption of plant-based foods, as the human body does not synthesize $IAA$ endogenously.

(C) Parthenocarpy is the process of developing fruits without the process of fertilization or seed formation.
Therefore,the seedless varieties of economically important fruits such as orange,lemon,and watermelon are produced using this technique.
This technique involves inducing fruit formation by the application of plant growth hormones,specifically auxins and gibberellins.
By inducing parthenocarpy,we can eliminate seeds in fruits where seeds are considered a nuisance,thereby increasing their market value and consumer preference.

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

(A) $2, 4-D$ stands for $2, 4-dichlorophenoxyacetic$ acid.
It is a synthetic auxin.
It is widely used as a herbicide to kill dicotyledonous weeds without affecting mature monocotyledonous plants.
Therefore,it is commonly used in agriculture to clear fields of unwanted broad-leaved weeds.

(N/A) $(1)$ $PGRs$ stands for $\text{Plant Growth Regulators}$. These are small,simple molecules of diverse chemical composition that regulate plant growth and development.
$(2)$ $IAA$ stands for $\text{Indole-3-Acetic Acid}$. It is the most common naturally occurring auxin in plants,responsible for cell elongation and various developmental processes.

(N/A) $(1)$ $IBA$ stands for Indole$-3-$butyric acid. It is a synthetic auxin used to promote root initiation in plant cuttings.
$(2)$ $2,4-D$ stands for $2,4-$Dichlorophenoxyacetic acid. It is a synthetic auxin widely used as a selective herbicide to kill broad-leaved weeds.

(N/A) $(1)$ $NAA$ stands for $Naphthaleneacetic \text{ } acid$. It is a synthetic auxin used in agriculture for rooting and preventing fruit drop.
$(2)$ $IPA$ stands for $Indole-3-propionic \text{ } acid$. It is a naturally occurring auxin found in plants that promotes cell elongation and growth.

(C) The term $IAAL$ refers to $Indole-3-Acetic Acid$ conjugated with $Leucine$.
$IAAL$ is a conjugate of the plant hormone $Auxin$ $(IAA)$ with the amino acid $Leucine$.
Such conjugates are involved in the storage and transport of $Auxin$ in plants.

(N/A) Auxin precursors: These are substances that serve as the biological starting material for the synthesis of auxins. For example,tryptophan is a well-known precursor for the biosynthesis of indole$-3-$acetic acid $(IAA)$ in plants.
$(b)$ Antiauxins: These are chemical substances that inhibit or antagonize the physiological actions of auxins. Examples include $p$-chlorophenoxyisobutyric acid $(PCIB)$ and $2,3,5$-triiodobenzoic acid $(TIBA)$.
$(c)$ Synthetic auxins: These are artificially synthesized chemical compounds that mimic the physiological properties and effects of natural auxins. Examples include naphthalene acetic acid $(NAA)$ and $2,4$-dichlorophenoxyacetic acid $(2,4-D)$.

(A) The phenomenon where apical buds inhibit the growth of lateral buds is known as $Apical \text{ } Dominance$.
This is primarily caused by the plant hormone $Auxin$, which is synthesized in the apical meristem and transported downwards.
This inhibition can be overcome by two methods:
$1$. $Decapitation$: Removing the apical bud (trimming) eliminates the source of $Auxin$, allowing lateral buds to grow.
$2$. $Cytokinin \text{ } Application$: Applying $Cytokinins$ to the lateral buds promotes their growth, as $Cytokinins$ counteract the inhibitory effect of $Auxin$.

(A) To get a carpet-like grass,lawns are mowed regularly because mowing causes the removal of the shoot apex (decapitation).
In many plants,the apical bud produces auxins which inhibit the growth of lateral buds,a phenomenon known as apical dominance.
By removing the shoot apex through mowing,the source of auxin is eliminated,which releases the lateral buds from inhibition and promotes their growth.
This results in a denser,bushier growth of grass,giving the lawn a carpet-like appearance.

(N/A) The diagram represents the shoot apical meristem of a dicotyledonous plant.
$(b)$ If we remove part $1$ (the shoot apical meristem),the phenomenon of apical dominance is lost. This leads to the inhibition of the apical bud and promotes the growth of axillary buds,causing the plant to become bushier.
Labeling:
$1 \rightarrow$ Shoot apical meristem
$2 \rightarrow$ Leaf primordium

(N/A) Parthenocarpy is the process in which fruits develop without the process of fertilisation,resulting in seedless fruits.
Plant hormones such as auxins and gibberellins are used to induce parthenocarpy artificially in plants like grapes,papaya,and tomatoes.

(A) The broad-leaved dicot weeds can be killed and eliminated by the spray of the weedicide $2,4-D$ ($2,4$-Dichlorophenoxyacetic acid).
$2,4-D$ is a synthetic auxin that acts as a selective herbicide.
It specifically targets and kills broad-leaved dicot plants while leaving monocot plants (like lawn grasses) unaffected.
Therefore,spraying $2,4-D$ is an efficient method for a gardener to clear weeds from a lawn.

(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 caused by the hormone $Auxin$,which is synthesized at the shoot tips.
$Auxin$ inhibits the growth of lateral buds,thereby promoting the vertical growth of the plant.

(B) Normally,water moves from a region of higher water potential to a region of lower water potential (osmosis). In a hypertonic solution,the external environment has a lower water potential than the cell,which usually leads to water loss (exosmosis).
However,$Auxin$ treated cells exhibit an increase in metabolic activity,specifically respiration.
This increased respiration provides the necessary metabolic energy required for the active absorption of water against the concentration gradient,allowing the cells to absorb water even from a hypertonic solution.

(D) Zinc $(Zn^{2+})$ acts as an essential cofactor for various enzymes,including those involved in the synthesis of the plant growth hormone auxin (indole$-3-$acetic acid).
Specifically,zinc is required for the synthesis of tryptophan,which is the precursor for auxin biosynthesis.
Therefore,a deficiency in zinc leads to a reduction in the levels of auxin in plants,resulting in stunted growth.
Ferredoxin synthesis requires iron $(Fe)$,not zinc.

(A) Phototropism is the growth or movement of a plant part in response to a light stimulus.
Specifically,positive phototropism refers to the growth of plant organs,such as stems,towards the light source.
Charles Darwin and his son Francis Darwin observed that the coleoptiles of canary grass respond to unilateral illumination by bending and growing towards the light source.

(D) Auxins are plant hormones that play a crucial role in various growth processes.
$1.$ Auxin promotes cell elongation,particularly in the cells located just below the apical meristem of the shoot.
$2.$ It is involved in the regulation of cell division,especially in the cambium.
$3.$ Auxin also plays a significant role in cell differentiation,such as in the formation of vascular tissues.
Therefore,all three processes ($I, II,$ and $III$) are functions of auxin.

(A) Auxins are a class of plant hormones that primarily induce cell elongation in stems and coleoptiles. $IAA$ (Indole$-3-$acetic acid) is the most common naturally occurring auxin. Auxins are generally acidic in nature and play a crucial role in phototropism and apical dominance.

(A) $Auxin$ concentration increases in the shaded area,meaning $Auxins$ accumulate on the side opposite to the light source.
Increased $Auxin$ concentration stimulates cell elongation in the shoot.
Because of this,the shaded side grows faster than the side exposed to light.
This differential growth causes the shoot to bend towards the light,a phenomenon known as phototropism.

(C) The Cholodny-Went theory explains the mechanism of phototropism in plants.
It states that when a plant is exposed to unilateral light,auxin $(IAA)$ is redistributed to the shaded side of the stem.
This higher concentration of auxin on the shaded side causes faster cell elongation compared to the illuminated side.
Consequently,the plant stem bends towards the light source,a phenomenon known as phototropism.

(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 perception of light.
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 auxin) that causes the bending of the entire coleoptile towards the light source.
Therefore,$A$ is the tip and $B$ is the bending.

(C) Auxin concentration increases in the shaded area (the side opposite to the light source).
Increased auxin concentrations stimulate cell elongation in the shoot.
Therefore,the shaded side shows more growth compared to the side facing the light.
This differential growth causes the shoot to bend towards the light,a phenomenon known as phototropism.

(D) The most common auxin is Indole Acetic Acid $(IAA)$,which is the principal naturally occurring auxin in all higher plants.
It performs many physiological functions in plants,such as cell elongation,apical dominance,and root initiation.

(A) Auxins are plant hormones that play a crucial role in root initiation. When shoot cuttings are treated with auxin,it stimulates the formation of adventitious roots at the base of the cutting. This property is widely utilized in horticulture for the vegetative propagation of plants.

(D) Auxins are plant hormones that perform several physiological functions.
$1$. They promote cell enlargement and cell division.
$2$. They stimulate the activity of the vascular cambium,which is essential for secondary growth.
$3$. They are responsible for apical dominance,where the terminal bud suppresses the growth of lateral buds.
$4$. Therefore,all the listed processes are promoted by auxins.

(D) Auxins are plant hormones that promote cell elongation,apical dominance,and tropic movements (like phototropism). They are also used to induce parthenocarpy in plants such as tomatoes,apples,and cucumbers.
Bolting,which refers to the internode elongation just prior to flowering,is primarily induced by gibberellins,not auxins. Therefore,bolting is not an influence of auxin.

(B) Charles Darwin and his son Francis Darwin conducted experiments on the coleoptiles of canary grass $(Phalaris \text{ canariensis})$.
They observed that the coleoptiles bent towards a light source, a phenomenon known as phototropism.
They concluded that the tip of the coleoptile is the site of perception of light and that some transmissible influence (later identified as auxin) is produced in the apical part, which causes the bending of the plant towards the light.

(A) Auxin (derived from the Greek word 'auxein',which means 'to grow') was the first plant hormone to be isolated from human urine.
Kogl and Haagen-Smit $(1931)$ isolated three chemicals from human urine and named them as auxin.

(D) Auxins are widely used as herbicides in agriculture.
$2,4-D$ ($2$,$4$-Dichlorophenoxyacetic acid) is a synthetic auxin that is specifically used to kill broad-leaved dicotyledonous weeds.
It does not affect mature monocotyledonous plants,making it an effective selective herbicide for crops like wheat and maize.

(D) Auxin is synthesized primarily in the growing apical regions of the plant,such as the shoot and root apices. From these sites of production,it is transported to the regions where it exerts its physiological effects,such as promoting cell elongation. This transport is typically polar (often basipetal) and is an active process,not simple diffusion or osmosis.

(C) Auxins,such as $IAA$ (Indole$-3-$acetic acid) and $IBA$ (Indole$-3-$butyric acid) are natural auxins,while $NAA$ (Naphthalene acetic acid) and $2,4-D$ ($2$,$4$-dichlorophenoxyacetic acid) are synthetic auxins.
These plant growth regulators are widely used in agriculture for crop management and in horticulture for practices like rooting of cuttings,preventing premature fruit drop,and promoting flowering.

(A) Nyctinastic or sleep movements are primarily caused by changes in turgor pressure within the pulvinus cells,which is triggered by the alternation of day and night. These movements are independent of auxin concentration. In contrast,the movement of roots towards soil (geotropism),the movement of shoots towards light (phototropism),and the tracking of the sun by sunflowers (heliotropism) are all growth-related movements mediated by the differential distribution of auxin.

(D) $Rhizobium$ is a nitrogen-fixing bacterium that inhabits the root nodules in leguminous crops.
This bacterium induces the production of the plant hormone $IAA$ (Indole$-3-$acetic acid),which is a type of auxin.
$IAA$ is known to stimulate cell division and expansion,thereby facilitating the formation of root nodules in legume plants.