Plant Physiology (March 13) - NEET MCQ

• 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.

The term auxin has been derived from the Greek word “auxein” which means “to grow”. It is known to be the first plant hormone that was discovered.

• Auxins are a group of compounds that include indole-3-acetic acid (IAA), and other natural and synthetic compounds that have certain growth-regulating properties. They are one of the various plant growth regulators(PGRs). They are generally produced by the growing apices of the stems and roots; from where they migrate to the other regions of their action. Auxins can be natural or synthetic. Indole Acetic Acid(IAA) and indole butyric acid (IBA) are some natural auxins that have been isolated from plants.
• Some synthetic auxins include NAA (naphthalene acetic acid) and 2, 4-D (2, 4-dichlorophenoxyacetic) which are used for various purposes.

Functions:

• Physiological:
  • Cell Elongation: Auxin promotes elongation and growth of stems and roots and enlargement of fruits by overall enlargement of cells in all directions,
  • Apical Dominance: In many plants, the apical bud grows while the lower axillary buds are suppressed. This is an effect of the auxin(IAA) of the axillary bud. This can be proven but removing the apical bud which then results in the growth of lateral buds.
  • Control abscission layer: Auxin inhibits the abscission of young leaves and fruits in normal amounts. An abscission layer is produced only when the auxin content is reduced below the minimum level which ultimately leads to the fall of ripe fruits.

There are many other uses:

• Rooting: Auxin stimulates root formation in woody stem cuttings.
• Flowering: The dilute solutions of synthetic auxins are sprayed on fruits like litchi and pineapple which induce flowering in them. However, in most plants, flowering will be inhibited by spraying a high concentration of auxins.
• Parthenocarpy: Auxins in diluted form can be used to produce parthenocarpic or seedless fruits. It has been proven that naturally parthenocarpic fruits like bananas and grapes have higher internal production of auxins.
• Weedicides: Spraying synthetic auxins like 2,4-D is a procedure commonly used as weedicides. They are used to remove many broad-leaved weeds or dicotyledonous weeds while sparing the monocot plants.
• Except for auxin, there are four other plant hormones called gibberellins, cytokinin, abscisic acid, and ethylene. Together they are called Plant Growth Regulators (PGRs). Some of them cause the plants to grow, while others like abscisic acid promote senescence. i.e. promote falling of leaves and fruits. Ethylene is the only gaseous hormone.

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.

• 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).