Test: Plant Growth & Development - 1 - NEET MCQ

Auxins (from Greek ‘auxein’ : to grow) was 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. They are generally produced by the growing apices of the stems and roots, from where they migrate to the regions of their action. Auxins like IAA and indole butyric acid (IBA) have been isolated from plants. NAA (naphthalene acetic acid) and 2, 4-D (2, 4-dichlorophenoxyacetic) are synthetic auxins. All these auxins have been used extensively in agricultural and horticultural practices.

Topic in NCERT: Auxins

Line in NCERT: "naa (naphthalene acetic acid) and 2, 4-d (2, 4-dichlorophenoxyacetic) are synthetic auxins."

• 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.

Topic in NCERT: Gibberellins

Line in NCERT: "gibberellic acid (ga) was one of the first gibberellins to be discovered and remains the most intensively studied form."

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.

Topic in NCERT: Ethylene

Line in NCERT: "ethylene is highly effective in fruit ripening."

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.

Topic in NCERT: Auxins

Line in NCERT: "auxins help to prevent fruit and leaf drop at early stages but promote the abscission of older mature leaves and fruits."

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.

Topic in NCERT: Gibberellins

Line in NCERT: "gibberellins also promotes bolting (internode elongation just prior to flowering) in beet, cabbages and many plants with rosette habit."

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.

Topic in NCERT: Plant growth regulators

Line in NCERT: "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.

Topic in NCERT: Plant growth regulators

Line in NCERT: "in most higher plants, the growing apical bud inhibits the growth of the lateral (axillary) buds, a phenomenon called apical dominance."

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

Topic in NCERT: Abscisic acid

Line in NCERT: "aba stimulates the closure of stomata and increases the tolerance of plants to various kinds of stresses."

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.

Topic in NCERT: Auxins

Line in NCERT: "they help to prevent fruit and leaf drop at early stages but promote the abscission of older mature leaves and fruits."

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.

Topic in NCERT: Plant growth regulators

Line in NCERT: "the pgrs can be broadly divided into two groups based on their functions in a living plant body. one group of pgrs are involved in growth promoting activities, such as cell division, cell enlargement, pattern formation, tropic growth, flowering, fruiting and seed formation. these are also called plant growth promoters, e.g., auxins, gibberellins and cytokinins."

• 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).

Topic in NCERT: Apical dominance

Line in NCERT: "in most higher plants, the growing apical bud inhibits the growth of the lateral (axillary) buds, a phenomenon called apical dominance."

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

Topic in NCERT: Growth

Line in NCERT: "growth can be defined as an irreversible permanent increase in size of an organ or its parts or even of an individual cell."

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.

Topic in NCERT: Plant growth and development

Line in NCERT: "many of the extrinsic factors such as temperature and light, control plant growth and development via pgr."

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.

Topic in NCERT: Cytokinins

Line in NCERT: "search for natural substances with cytokinin-like activities led to the isolation of zeatin from corn-kernels and coconut milk."

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:

Topic in NCERT: Phases of growth

Line in NCERT: "the constantly dividing cells, both at the root apex and the shoot apex, represent the meristematic phase of 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.

Topic in NCERT: Cytokinins

Line in NCERT: "cytokinins help overcome the apical dominance. they promote nutrient mobilisation which helps in the delay of leaf senescence."

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.

Topic in NCERT: Auxins

Line in NCERT: "auxins (from greek ‘auxein': to grow) was first isolated from human urine."

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. 

Topic in NCERT: The discovery of plant growth regulators

Line in NCERT: "the coleoptiles of canary grass responded to unilateral illumination by growing towards the light (phototropism)."