How Do Plant Hormones Influence Growth and Development?
For their growth and development, plants require sunlight, moisture, oxygen, and minerals. External factors are required. The growth and development of plants are regulated by a variety of intrinsic factors as well. We call these phytohormones. Plants produce and transmit these hormones in almost every part of the plant. Plant growth hormones may act synergistically or individually. Each hormone may play a complementary or antagonistic role. As well as extrinsic factors, hormones are involved in processes such as vernalization, phototropism, seeds germination, dormancy, and so forth. Controlled crop production is achieved through the exogenous application of plant hormones. Charles Darwin first observed phototropism within the coleoptiles of canary grass, and Frederick Going isolated auxin for the first time from the coleoptiles of oat seedlings.
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Which of the Following is a Plant Hormone?
A phytohormone is a chemical compound found in very low concentrations in plants. There are derivatives of indole (auxins), terpenes (Gibberellins), adenine (Cytokinins), carotenoids (Abscisic acid), and gases (Ethylene) and also examples of plant hormones.
What are the Two Types of Plant Hormones?
The growth and development activities of plants are controlled by hormones such as cell division, enlargement, flowering, seed formation, dormancy, and abscission.
There are two main types of plant hormones based on how they act:
Growth Promoters for Plants
Growth inhibitors for plants
Plant Hormones and Their Functions
Auxin Hormone
To grow is what auxin means. Agricultural practices and horticultural practices use auxins extensively. The majority of them reside at the growing apices of roots and stems before migrating to other parts of the plant.
Naturally: Indole-3-acetic acid (IAA), Indole butyric acid (IBA)
Synthetically: 2,4-D (2,4-Dichlorophenoxyacetic acid), NAA (Naphthalene acetic acid)
Functions:
Roots and stems lengthen as a result of cell division.
The growth of lateral buds is inhibited by apical dominance and IAA in apical buds.
It is inhibited by apical dominance and IAA in the apical buds that lateral buds grow.
Ensures that leaves, flowers, and fruits do not fall prematurely.
Gibberellins Hormone
Gibberellins are acidic compounds found in higher plants and fungi (GA1, GA2, GA3,....). There are more than 100 kinds of gibberellins (GA1, GA2, GA3,...) known.
Functions:
Bolting in rosette plants like cabbage, beet induces sudden elongation of the internodes just before flowering.
Delays senescence.
Induces parthenocarpy.
Elongates the stem and reverses dwarfism.
Cytokinins Hormone
Plants produce cytokinins when rapid cell division occurs, for example, at roots apices, shoot buds, young fruits, etc. The movement of cytokinins is basipetal and polar.
Naturally: Zeatin (corn kernels, coconut milk), isopentenyl adenine
Synthetically: Kinetin, benzyladenine, diphenylurea, thidiazuron
Functions:
In cultures, it promotes the growth of lateral and adventitious shoots and is used to initiate the growth of shoots
Assists in overcoming apical dominance caused by auxin
Leaf chloroplasts should be stimulated
Activates nutrients and delays leaf senescence
Abscisic Acid Function
Inhibiting plant growth regulates abscission and dormancy, affects plant metabolism, and increases plant tolerance to stress. It's also called the "stress hormone" because it increases plant tolerance to stress.
Functions:
Leaf abscission and fruit abscission caused by this plant
Inhibition of seed germination
Senescent leaves are induced
Do not affect seeds' dormancy, so it is useful for storing seeds
Ethylene Plant Hormone
It regulates many physiological processes and is one of the most widely used agricultural hormones, acting as both a growth promoter and an inhibitor. It is produced in gaseous form by ripening fruits and tissues during senescence.
Functions:
Fruits ripen more quickly when it is present
Leaves become less epinasty
Breaks dormancy of seeds and buds
Enhances the rapid extension of petioles and internodes
Besides the main 5 hormones, other hormones also affect the physiology of plants, such as brassinosteroids, salicylates, jasmonates, strigolactones, etc., are the other examples of plant hormones in the study of the application of plant hormones. In this way, plant hormones and their functions are described.
Did You Know
The oilseeds, pulses, and cereal crops each requires specific plant growth hormones.
By producing auxins and cytokinins as examples of plant hormones, plants can grow stronger, root and shoot growth can be promoted, and stress can be reduced.
FAQs on Plant Hormones: Types, Functions, and Importance
1. What are plant hormones, and why are they often called phytohormones?
Plant hormones are naturally occurring chemical compounds that regulate a plant's growth, development, and responses to the environment. They are also known as phytohormones ('phyto-' means 'plant') and act as signal molecules, controlling processes like cell division, flowering, and dormancy even in very low concentrations.
2. What are the five major types of plant hormones that students should know?
The five major classes of plant hormones are:
- Auxins: Primarily responsible for cell elongation and apical dominance.
- Gibberellins: Promote stem elongation, seed germination, and flowering.
- Cytokinins: Stimulate cell division and growth, especially in shoots and roots.
- Abscisic Acid (ABA): Acts as a growth inhibitor, managing stress responses and dormancy.
- Ethylene: A gaseous hormone that promotes fruit ripening and senescence (ageing).
3. What is the difference between a plant growth promoter and a plant growth inhibitor?
Plant hormones can be broadly categorised based on their primary function:
- Growth Promoters: These hormones, such as Auxins, Gibberellins, and Cytokinins, are involved in activities that promote growth, like cell division, cell enlargement, flowering, and fruiting.
- Growth Inhibitors: These hormones, like Abscisic Acid and Ethylene (in some roles), are involved in processes that slow or stop growth, such as dormancy, abscission (shedding of leaves/fruits), and responses to stress.
4. What is apical dominance and which hormone is primarily responsible for it?
Apical dominance is a phenomenon where the central, main stem of a plant grows more strongly than the lateral (side) stems. This is primarily caused by Auxins produced in the apical bud (the tip of the main stem). This hormone inhibits the growth of the lateral buds, ensuring the plant grows upwards towards light.
5. Why is Abscisic Acid (ABA) commonly known as the 'stress hormone' in plants?
Abscisic Acid (ABA) is called the stress hormone because its production increases significantly when a plant faces stressful environmental conditions, such as drought, high salinity, or extreme temperatures. ABA helps the plant conserve resources by triggering responses like the closure of stomata to reduce water loss.
6. How does the hormone Ethylene cause fruits to ripen?
Ethylene is a gaseous hormone that triggers and coordinates the ripening process in many fruits. It stimulates the production of enzymes that soften the fruit's cell walls, convert starches into sugars (making it sweeter), and break down chlorophyll to reveal other pigments (causing the colour to change). This is why one ripening fruit can accelerate the ripening of others nearby.
7. How are synthetic auxins used in agriculture?
Synthetic auxins have several important applications in modern agriculture and horticulture. For example, they are used to:
- Promote root formation in stem cuttings for propagation.
- Prevent the premature dropping of fruits and flowers.
- Act as selective herbicides (weedkillers), as they can disrupt the growth of broad-leaf weeds at high concentrations.
- Induce parthenocarpy, which is the development of fruit without fertilisation.
8. How do plant hormones work together? Can you provide an example?
Plant hormones rarely act in isolation; their effects depend on complex interactions. They can work together (synergistically) or in opposition (antagonistically). A classic example is the control of shoot and root development in tissue culture. The ratio of Auxin to Cytokinin determines the outcome: a high auxin-to-cytokinin ratio promotes root formation, while a high cytokinin-to-auxin ratio promotes shoot formation.
