Test: Plant Growth & Development - 1 - NEET MCQ

Please note that there are 2 sets of information given in the question. The first set (options a-e) mentions the names of compounds. The second set (options 1-4) mentions the combinations of the first set of options.

• Auxin refers to a group of plant hormones that regulate plant growth.
• Based on their origin, auxins can be divided into:
  • Natural Auxins - They occur naturally in the plant and affect physiological functions. E.g. - IAA, IBA.
  • Artificial Auxins - These are synthetically-derived chemical compounds that resemble IAA in biological activity. E.g. - NAA.
• Artificial auxins are widely used in horticultural practices, as weedicides, rooting agents, and defoliating agents.

• IPA
  • Indole-3-pyruvic acid (IPA) is an intermediate in the IAA biosynthesis pathway.
  • Hence, it is not an artificial auxin.
• PAA
  • Phenylacetic acid (PAA) is a natural auxin.
• NAA
  • Naphthalene acetic acid (NAA) is an artificial auxin that is used as a rooting agent.
• 2,4,5-T
  • 2,4,5 - Trochlorophenoxyacetic acid is an artificial auxin that is used as a herbicide.
• 2,4-D
  • 2,4 - Dichlorophenoxy acetic acid (2,4-D) is an artificial auxin that is widely used as a herbicide.

Thus the answer will be (c), (d), and (e).

I.A.A. is auxin. The precursor of auxin is tryptophan and Zn.

Presumptive pathways of IAA biosynthesis from tryptophan:

• Gibberellic acid is a Terpene.
• All known gibberellins are diterpenoid acids that are synthesized by the terpenoid pathway in plastids and then modified in the endoplasmic reticulum and cytosol until they reach their biologically-active form.
• All gibberellins are derived via the ent-gibberellane skeleton, but are synthesized via ent-kaurene.

The following are plant growth regulators or phytohormones that control various developmental as well as physiological functions of a plant:

Auxin:

• It helps in cell elongation, growth, and differentiation.
• It is present mainly at the root and shoots apices.
• It is responsible for the phenomenon of apical dominance, where the apical buds inhibit the growth of lateral buds.
• It results in increasing the height of a plant due to apical dominance.
• It can also initiate root growth in stem cuttings.
• It may also promote flowering and prevent early fruit drop.

Cytokinin:

• It is mainly associated with cell division and is present in regions of rapid cell division.
• It helps to overcome apical dominance and promotes lateral shoot growth.

Ethylene:

• It is a gaseous phytohormone produced in the tissues undergoing senescence.
• It helps in fruit ripening and other senescence-related activities.
• It breaks seed and bud dormancy to initiate germination in some species.
• It also induces leaf epinasty under stress conditions.
• The triple response of ethylene includes reducing the rate of elongation, increasing lateral expansion, and swelling of the region below the hook in seedlings.

Abscisic acid:

• As the name suggests, it helps in abscission and dormancy.
• It inhibits plant growth, metabolism, and seed germination.
• It also plays an important role in the closure of stomata under stress conditions.

The plant hormone auxin is used for agriculture and horticulture.

• During the process of plant propagation, auxin is used to initiate rooting in stem cutting.
• Auxin performs the role of promoting flowering in the plant.
• Abscission of older mature fruits and leaves is regulated by the hormone auxin.
• It also prevents leaves and fruits from dropping at the early stage. The growing apical buds in most higher plants inhibit the growth of auxiliary buds. This phenomenon is called apical dominance.

Therefore, the correct answer is option D.

Stem elongation is a kind of growth of stem.

Auxin and Gibberellin are known as growth hormones of plants. Therefore, stem elongation is affected by hormones Auxin and Gibberellin.

Auxins are generally produced by the growing apices of the stems and roots, from where they migrate to the regions of their action. In most higher plants, the growing apical bud inhibits the growth of the lateral (axillary) buds, a phenomenon called apical dominance.

The apical bud produces an auxin hormone that inhibits the growth of the lateral buds further down on the stem towards the axillary bud.

2,4-dichlorophenoxyacetic acid is a herbicide class, which is a synthetic auxin. Hence it is not a naturally occurring plant hormone.

It is absorbed through the leaves and is translocated to the plant meristems resulting in stem curling, leaf withering, and finally plant death.

Abscisic acid (ABA) induces the closure of plant stomata, helping plants to conserve water during drought conditions. This role as a stress hormone showcases ABA's importance in plant responses to environmental stress. An additional fact is that ABA also plays a crucial role in seed dormancy, ensuring seeds only germinate under favorable conditions

Due to a reduction in photosynthesis, this may trigger the abscission of leaves (separation of leaves) and this abscission layer is produced when auxin content falls below a minimum causing leaf or other plant part to fall off.

• A chemical substance produced in one part of body and regulates the activity of other parts is called as hormones.
• Plant hormones are produced in the stem, bud and root tips and control the plant growth, formation of flowers, stems, leaves, development, and ripening of fruit, etc.
• Auxins are specialized hormones which acts on the tip.

Thus, the correct answer is option B.

IAA is actually a type of Auxin hormone so it helps in nodule formation.

Auxins, gibberellins, and cytokinins can indeed act synergistically in promoting plant growth. These hormones work together to regulate various aspects of growth and development, such as cell division, elongation, and differentiation. This synergy allows plants to adapt their growth in response to environmental conditions.

An interesting fact is that while these hormones can promote growth, they can also interact with other hormones like abscisic acid and ethylene, which can inhibit growth under certain conditions, demonstrating the complex regulation of plant development.

• Apical dominance: In many plants, the apical bud grows and the lower axillary buds are suppressed. Removal of apical bud results in the growth of lower buds.
• The auxin (IAA) of the terminal bud inhibits the growth of lateral buds. This phenomenon is known as apical dominance.
• This property of auxins has found use in agriculture. Sprouting of lateral buds (eyes) of the potato tuber is checked by applying synthetic auxin (NAA).

Parthenocarpy (literally meaning "virgin fruit") is the natural or artificially induced production of fruit without fertilization of ovules, which makes the fruit seedless. 

It may also produce apparently seedless fruit, but the seeds are actually aborted while they are still small. 

Most of the auxins in plants are believed to be produced by apical shoot meristem.
This auxin then suppresses the growth of lateral buds, the phenomenon is called as apical dominance.
Auxins like IAA and indole butyric acid (IBA) have been isolated from plants.

Hence, option C is correct.

Different concentrations of IAA appear to have differing effects on root growth.
These effects may vary from one species to another but generally, at low concentrations, IAA stimulates root growth while at higher concentrations it inhibits root growth.

As the most important site of auxin synthesis is shoot tips, they frequently migrate in the downward direction. They are generally produced by the growing apices of the stems and roots, from where they migrate to the regions of their action.

Hence, option A is correct.

Auxin causes growth of the coleoptile of wheat, maize, and other cereals. The tip of the coleoptile produces auxin. Therefore, coleoptile shows rapid growth.
When the tip is removed, the growth of the coleoptile stump stops. Auxin causes growth of the coleoptile by cell elongation. It also causes growth in stem by cell elongation.

In plants, growth can refer to change in size, it is restricted to meristematic regions of the plant and cannot be reversed.

Turgor pressure is the water pressure maintained by osmosis.
It is also known as the driving force for cell extension, any general theory of plant growth requires quantitative information on the relationship between steady irreversible growth rate and turgor pressure.
At constant turgor pressure, growth is irreversible.

The climatic factors include rainfall and water, light, temperature, relative humidity, air, and wind. They are abiotic components, including topography of  the soil and the environmental factors that influence plant growth and development.

trans-zeatin riboside is the most abundant type of cytokinin found in coconut water (Table 2). trans-zeatin is normally used to induce plantlet regeneration from callus in plant tissue culture. Based on experimental data, trans-zeatin plays a key role in the G₂-M transition of tobacco cells.

Phases Of Plant Growth

1. Meristematic Phase

• The cells in the root and shoot apex of a plant are constantly dividing. They represent the meristematic phase of growth.
• The cells in these regions have large nuclei and are rich in protoplasm and their cell walls are thin and contain cellulose.

2. Elongation Phase

• The cells in the zone just after the meristematic region represent the phase of elongation.
• The characteristics of cells in this zone are cell enlargement, increased vacuole formation and new cell wall deposition.

3. Maturation Phase

• Just close to the phase of elongation, but away from the apex lies the phase of maturation.
• The cells in this region reach their maximum size with respect to their protoplasm and cell wall thickening.

Phases of Plant Growth:

• Cytokinin does not help in promoting leaf senescence.
• Cytokinin production slows down the process of senescence. However, the accumulation of sugars during senescence, due to the breakdown of accumulated starch or the preferential export of N₂ from the leaf, can block the effect of cytokinin, especially in low light.
• Ethylene is regarded as a multifunctional phytohormone that regulates both growth, and senescence.
• It promotes or inhibits growth and senescence processes depending on its concentration, timing of application, and the plant species.

Auxins were first isolated from human urine. The term auxin is applied to the indole-3-acetic acid (IAA), and to other natural and synthetic compounds having certain growth-regulating properties.

Cytokinins have specific effects on cytokinesis and were discovered as kinetin from the autoclaved herring sperm DNA.
Kinetin does not occur naturally in plants. Search for natural substances with cytokinin-like activities led to the isolation of zeatin from corn kernels (corn germ oil) and coconut milk.

Hence option C is correct.

Zinc is an essential micronutrient required for the growth and development of plants. It is involved in the synthesis of indoleacetic acid (IAA), which is a natural auxin.

The function of zinc is to activate the enzyme tryptophan synthetase which is responsible for auxin biosynthesis from tryptophan.

They help to initiate rooting in stem cuttings, an application widely used for plant propagation. Auxins promote flowering e.g. in pineapples. They help to prevent fruit and leaf drop at early stages but promote the abscission of older mature leaves and fruits

For a plant organ to bend in response to a stimulus, differential growth of cells on either side of the organ is required.

Example: For the stem of a plant to bend toward a light source, cells on the shaded side of the stem near the shoot tip must elongate faster than cells on the lighted side.