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13.4.3 - Physiological Effects of Plant Growth Regulators

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Introduction to Auxins

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Teacher
Teacher

Today, we're going to learn about auxins. Auxins promote cell growth and elongation in plants. Can anyone tell me where auxins are mostly produced?

Student 1
Student 1

Are they produced in the roots and stems?

Teacher
Teacher

Great! They are primarily produced in the apical buds of the stems and roots, from where they migrate to promote growth in other areas. This is an example of how plants have localized growth regulation. Auxins also play a role in rooting in cuttings. Can you think of a way we use this in gardening?

Student 2
Student 2

Yeah! We apply auxins to help new plants grow roots from cuttings.

Teacher
Teacher

Exactly! Plus, they also help delay leaf drop until the plant is mature. Let's remember that with the acronym 'ARMS' - Auxins Root, Maintain, and Support growth. Can anyone tell me about apical dominance?

Student 3
Student 3

It's when the main shoot grows stronger than the side shoots, right?

Teacher
Teacher

Correct! Removing the shoot tips can help lateral buds grow, like in hedge-making. To sum up, auxins are crucial for growth and plant shape.

Understanding Gibberellins

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Teacher
Teacher

Now let's talk about gibberellins. They are key players in promoting stem elongation and breaking dormancy. Does anyone know how they are used in farming?

Student 4
Student 4

Are they used to make plants grow taller or bigger?

Teacher
Teacher

Exactly! Gibberellins help in the elongation of grape stalks and can also increase sugarcane yield significantly. Remember 'Gibb and Grow' for their role in growth promotion. What's another effect of gibberellins?

Student 1
Student 1

They can help in fruit ripening too, right?

Teacher
Teacher

Yes! They help in elongating fruits like apples for better shape and delayed senescence. This highlights the importance of timing in farming. Can anyone share their thoughts on why gibberellins can be beneficial?

Student 2
Student 2

They help harvest fruits at the right time for better yield!

Teacher
Teacher

Exactly! It's all about maximizing potential in agriculture.

Exploring Cytokinins

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Teacher
Teacher

Next, we have cytokinins. These are unique because they primarily promote cell division. Where do you think they are most active?

Student 3
Student 3

In the roots and young shoots?

Teacher
Teacher

Right again! Cytokinins are produced in root apices and help develop new leaves and shoots. Let's use 'C for Cell division' as a memory aid. Why is it important to promote shoot growth in plants?

Student 4
Student 4

So that they can capture more sunlight and grow efficiently?

Teacher
Teacher

Exactly! They also help delay leaf senescence, keeping plants green for longer. Can you think of how this might help farmers?

Student 1
Student 1

They can harvest healthier crops!

Teacher
Teacher

That's correct! Let's remember that cytokinins are vital for sustaining plant vitality.

Role of Ethylene

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Teacher
Teacher

Ethylene is our next topic, and it's unique because it's a gas. What processes does ethylene influence in plants?

Student 2
Student 2

It helps in fruit ripening and leaf drop, right?

Teacher
Teacher

Absolutely! It enhances the ripening process and can be used to synchronize fruit ripening in agriculture. Let's remember 'E for Exit' to recall that it causes leaves to drop. What does this tell us about plant aging?

Student 3
Student 3

It shows how plants manage energy and resources by dropping older leaves or fruits.

Teacher
Teacher

Exactly! Ethylene is also crucial when dealing with stress responses or adapting to the environment. This adaptability is essential for plant survival.

Understanding Abscisic Acid

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Teacher
Teacher

Last, we will discuss abscisic acid (ABA). Often called the stress hormone, what roles do you think ABA plays in plants?

Student 4
Student 4

It helps regulate water loss by closing stomata?

Teacher
Teacher

Correct! It plays a vital role in helping plants conserve water during drought. Let's use 'ABA for A Big Absorption' to remember its function. What else does ABA help with?

Student 1
Student 1

It inhibits seed germination, right?

Teacher
Teacher

Exactly! It helps seeds remain dormant until conditions are right. Remember, ABA is crucial for survival strategies in plants. Any thoughts on how this might help a plant during changing seasons?

Student 2
Student 2

It allows the plant to save resources and only grow when conditions are suitable.

Teacher
Teacher

Well said! Adapting to the environment is key in plant biology.

Introduction & Overview

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Quick Overview

This section discusses how various plant growth regulators (PGRs) influence plant growth, development, and physiological processes.

Standard

Plant growth regulators (PGRs) such as auxins, gibberellins, cytokinins, abscisic acid, and ethylene play significant roles in controlling various physiological effects in plants. Each group of PGRs has distinct functions, including promoting growth, triggering fruit ripening, and regulating responses to stress.

Detailed

Physiological Effects of Plant Growth Regulators

Plant growth regulators (PGRs) are essential molecules that significantly influence plant growth and development. This section outlines the physiological effects of the five major categories of PGRs:

  1. Auxins: These are primarily responsible for promoting cell elongation, initiating rooting in stem cuttings, controlling apical dominance, and preventing premature leaf and fruit drop. Natural auxins like indole-3-acetic acid (IAA) are produced in developing plant tissues, while synthetic varieties like NAA and 2,4-D are used in agriculture to promote rooting and act as herbicides.
  2. Gibberellins: Gibberellins, such as Gibberellic acid (GA), enhance growth by promoting stem elongation, fruit shape improvement, delaying fruit senescence, and executing bolting in rosette plants. They are used in agriculture to promote growth in crops like sugarcane and grapevines.
  3. Cytokinins: These compounds promote cell division and influence the growth of shoots and leaves. They help in nutrient mobilization, overcoming apical dominance, and delaying leaf senescence. Cytokinins are found in younger plant tissues.
  4. Ethylene: Ethylene is a gaseous PGR that promotes fruit ripening, leaf abscission, and adaptation to stress conditions. It also contributes to processes like horizontal growth of seedlings and rapid internode elongation in certain aquatic plants.
  5. Abscisic Acid (ABA): Initially recognized for its role in regulating abscission and dormancy, ABA is known as the stress hormone as it helps plants cope with various environmental stresses. It inhibits seed germination and signaling processes essential for dormancy.

Overall, PGRs have varied roles that can be synergistic or antagonistic, impacting plant behavior in response to intrinsic and extrinsic stimuli.

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Audio Book

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Introduction to Auxins

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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.

Detailed Explanation

Auxins are a group of plant hormones that promote growth. They are mainly found where plants grow, such as at the tips of stems and roots. The name 'auxin' comes from the Greek word for 'to grow'. They play a crucial role in plant development by regulating various growth processes. Natural auxins like IAA are produced in plants, while synthetic versions like NAA are created by humans for agricultural use.

Examples & Analogies

Think of auxins like the fuel that powers a race car. Just like fuel is vital for the car to go fast, auxins are essential for plants to grow properly. Whether it’s growing tall like a tree or spreading out like a bush, auxins ensure plants have the right ‘fuel’ to develop.

Functions of Auxins

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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. In most higher plants, the growing apical bud inhibits the growth of the lateral (axillary) buds, a phenomenon called apical dominance. Removal of shoot tips (decapitation) usually results in the growth of lateral buds.

Detailed Explanation

Auxins serve various purposes in plant growth. They are crucial for helping stem cuttings develop roots, allowing gardeners to propagate plants easily. Auxins also encourage flowering, which is essential for fruit production. They can prevent young fruits and leaves from dropping prematurely but help older ones to fall off when they are ready. Apical dominance refers to the phenomenon where the main growth tip of the plant inhibits the growth of side branches, a trait that can be altered by trimming the tips.

Examples & Analogies

Imagine you have a boss at work who oversees a team. If the boss is present, the team members may not take the initiative to lead projects themselves. This is similar to how the apical bud, filled with auxins, can keep side buds from growing. When the boss (or apical bud) is removed, the team (or lateral buds) is free to step up and take charge!

Gibberellins and Their Effects

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Gibberellins are another kind of promotory PGR. There are more than 100 gibberellins reported from widely different organisms such as fungi and higher plants. They are denoted as GA1, GA2, GA3 and so on. Gibberellic acid (GA3) was one of the first gibberellins to be discovered and remains the most intensively studied form. All GAs are acidic. They produce a wide range of physiological responses in the plants.

Detailed Explanation

Gibberellins (GAs) are plant hormones that promote growth and development. They are derived from various organisms, including fungi and plants. One well-known gibberellin is Gibberellic acid (GA3), which has been extensively studied for its effects on plant growth. Gibberellins influence many aspects of plant development, including stem elongation and fruit growth.

Examples & Analogies

Think of gibberellins as the motivational coach of a sports team. Just as a coach encourages players to reach their full potential, gibberellins help plants grow taller and stronger. When plants receive gibberellins, like a player getting a pep talk, they can stretch out and flourish, leading to larger fruits and more robust stems.

Cytokinins and Their Role

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Cytokinins have specific effects on cytokinesis, and were discovered as kinetin (a modified form of adenine, a purine) from the autoclaved herring sperm DNA. Kinetin does not occur naturally in plants. It helps to produce new leaves, chloroplasts in leaves, lateral shoot growth and adventitious shoot formation. Cytokinins help overcome the apical dominance and promote nutrient mobilisation.

Detailed Explanation

Cytokinins are hormones that promote cell division and help plants grow in specific ways. They were first discovered from herring sperm but are also found in corn and coconut milk. Cytokinins aid in forming new leaves and aiding lateral growth, effectively allowing for a fuller plant experience. They can counteract apical dominance, enabling side branches to grow.

Examples & Analogies

Imagine cytokinins as a helpful team member in a group project who encourages everyone to contribute their ideas. Just as this team member inspires others to take action, cytokinins stimulate side branches to grow, allowing the plant to become bushier and healthier by utilizing its resources more effectively.

Ethylene's Functions

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Ethylene is a simple gaseous PGR. It is synthesised in large amounts by tissues undergoing senescence and ripening fruits. Influences of ethylene on plants include horizontal growth of seedlings, swelling of the axis and apical hook formation in dicot seedlings. Ethylene promotes senescence and abscission of plant organs especially of leaves and flowers.

Detailed Explanation

Ethylene is a gaseous plant hormone involved in several growth processes, particularly in aging and ripening. This hormone is produced in high quantities as plants age or when fruits ripen. Ethylene encourages various growth behaviors such as horizontal growth and can trigger the separation of parts of the plant (like leaves and flowers) when they are ready to fall.

Examples & Analogies

Think of ethylene as the ‘grandparent’ of a family. As the grandparents age, they often stay in one place, encouraging the whole family to gather and reminisce. In plants, when ethylene is present, it influences the plant parts to separate and age gracefully, similar to how family gatherings can mark the passage of time in a gentle way that’s memorable.

The Role of Abscisic Acid

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As mentioned earlier, abscisic acid (ABA) was discovered for its role in regulating abscission and dormancy. It acts as a general plant growth inhibitor and an inhibitor of plant metabolism. ABA inhibits seed germination. ABA stimulates the closure of stomata and increases the tolerance of plants to various kinds of stresses. Therefore, it is also called the stress hormone.

Detailed Explanation

Abscisic Acid (ABA) is a plant hormone that primarily acts as an inhibitor of growth. It plays a key role in ensuring seeds do not germinate prematurely, instead promoting dormancy until conditions are favorable. ABA is also crucial for helping plants manage stress, contributing to stomatal closure which minimizes water loss.

Examples & Analogies

You can think of ABA as the cautious friend who advises against making hasty decisions. Just as a cautious friend might suggest waiting for the right moment to act, ABA prevents seeds from sprouting until the conditions are right, ensuring plants aren’t exposed to unnecessary risks. It's like the friend who only allows you to leave the house when the weather is perfect!

Synergistic and Antagonistic Roles of PGRs

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We may summarise that for any and every phase of growth, differentiation and development of plants, one or the other PGR has some role to play. Such roles could be complimentary or antagonistic. Similarly, there are a number of events in the life of a plant where more than one PGR interact to affect that event, e.g., dormancy in seeds/buds, abscission, senescence, apical dominance, etc.

Detailed Explanation

Plant growth regulators (PGRs) play vital roles during every phase of plant development, either supporting or contrasting each other's effects. For instance, while one growth regulator may promote growth, another might inhibit it, balancing the plant's development. Additionally, many plant events involve interactions between multiple PGRs, illustrating that plant processes are complex and interconnected.

Examples & Analogies

Think of PGRs as a team of diverse chefs preparing a meal together. Each chef has a unique set of skills – some enhance flavors while others might tone them down. Just like the flavors combine to create a well-balanced meal, PGRs work together in plants to ensure everything grows in harmony, whether promoting growth or ensuring the right balance in development.

Definitions & Key Concepts

Learn essential terms and foundational ideas that form the basis of the topic.

Key Concepts

  • Auxins: Promote elongation and influence a plant's growth pattern.

  • Gibberellins: Enhance growth and promote fruit development.

  • Cytokinins: Stimulate cell division and growth in shoots.

  • Ethylene: Gaseous hormone regulating ripening and leaf drop.

  • Abscisic Acid: Hormone that plays a key role in stress responses and seed dormancy.

Examples & Real-Life Applications

See how the concepts apply in real-world scenarios to understand their practical implications.

Examples

  • Auxins are used in gardening to root stem cuttings.

  • Gibberellins help grapes grow larger on the vine.

  • Cytokinins are used to extend shelf life of vegetables.

  • Ethylene accelerates fruit ripening, seen in bananas and apples.

  • Abscisic acid helps plants manage drought conditions.

Memory Aids

Use mnemonics, acronyms, or visual cues to help remember key information more easily.

🎵 Rhymes Time

  • Auxins help plants grow tall, gibberellins make fruits enthrall.

📖 Fascinating Stories

  • Imagine a gardener using auxins to grow rooted cuttings, while gibberellins make their grapes plump and luscious. Ethylene works behind the scenes, ripening fruits that share sweet, delicious dreams.

🧠 Other Memory Gems

  • Remember 'AGEE' for Auxins, Gibberellins, Ethylene, and Ethylene as key PGRs.

🎯 Super Acronyms

ABCs for ABA (Abscisic Acid)

  • A: - Abscission
  • B: - Bud Dormancy
  • C: - Conservation under stress.

Flash Cards

Review key concepts with flashcards.

Glossary of Terms

Review the Definitions for terms.

  • Term: Auxin

    Definition:

    A plant hormone that promotes elongation and growth in stems and roots.

  • Term: Gibberellins

    Definition:

    Group of plant hormones that promote stem elongation, fruit ripening, and seed germination.

  • Term: Cytokinins

    Definition:

    Plant hormones that promote cell division and growth in shoots.

  • Term: Ethylene

    Definition:

    A gaseous hormone that influences fruit ripening and leaf abscission.

  • Term: Abscisic Acid (ABA)

    Definition:

    A plant hormone that inhibits growth, promotes seed dormancy and closes stomata.