How Do Plant Growth Regulators Influence Plant Development?
Do you ever wonder how plants grow? What factors are responsible for their growth or what chemicals assist them in the process? If the answer to these questions is yes, then Vedantu is here to answer them. Understanding this topic will not only satisfy your curiosity but will also help you in scoring good marks in botany.
Light, water, minerals, oxygen along with other nutrients are the things that plants need for their growth and development. Apart from these, plants also need certain organic compounds to signal, regulate and control their growth. These are known as the Plant Growth Regulators or Hormones or Phytohormones.
Plant growth regulators are the chemical substances that control the aspects of growth and development within the plants completely. They are also known as plant growth hormones or phytohormones.
Plant growth hormones are those organic compounds that are either produced naturally within the plants or are synthesized in laboratories. They control and modify physiological processes like the growth, development and movement of plants.
Plant growth regulators can be broadly classified into two categories namely-
Plant growth promoters
Plant growth inhibitors
Auxins, Gibberellins and Cytokinins are grouped into Plant growth promoters while Abscisic acid and Ethylene are grouped into Plant growth inhibitors. Ethylene can be grouped either into the promoters or into the plant inhibitors.
Types of Plant Growth
Primary and Secondary Growth
The meristematic cells present at the root and shoot apiece segregate mitotically and increase the length of the plant body. This is known as primary growth. Secondary growth is referred to as the increase in the diameter of the plant body by the division of the secondary meristem.
Unlimited Growth
When the plant continually grows from the germination stage to death, it is called unlimited growth.
Limited Growth
In this stage, the plant parts stop growing after acquiring a certain size.
Vegetative Growth
It involves the production of stems, leaves and branches except for the flowers.
Reproductive Growth
When flowering occurs, that type of growth stage is called reproductive growth.
What are the Factors affecting Plant Growth?
Four major factors affect the growth of plants. They are:
Light
Plants are autotrophs which means they require light for manufacturing their food. Limited light or the absence of it greatly affects the growth of the plant. The intensity of light, quality of light and light duration influence the movement of stomata, chlorophyll synthesis, photosynthesis, and various other physiological factors. Light also helps in the process of flowering and fruiting. During winters when the days are short, the growth of the plants is retarded.
Water
We know that plants cannot survive without water and around 90% of the plant body comprises water. Plants become deadly in the absence of water and die. Water present in the soil is absorbed by the plant which absorbs and transports the nutrients along with it to keep it hydrated.
Temperature
Plant growth is greatly influenced by temperatures. High temperature helps speed up transpiration, photosynthesis and germination processes. Low temperature is considered harmful for the plants as it slows down their growth.
Nutrients
Plants need proper nourishment for their growth and development. Soil nutrients are divided into macronutrients and micronutrients. Nitrogen, potassium, calcium, magnesium, sulphur and phosphorus are some of the most important macronutrients that are required by plants. The deficiency of these nutrients in plants makes them prone to several diseases. Even if a single nutrient is lacking, it results in stunted growth of the plant.
Discovery of Plant Growth Regulators
The discovery of major plant growth regulators began with Charles Darwin and his son Francis Darwin. They observed the growth of coleoptiles of canary grass towards the light source known as Phototropism. By following a series of experiments, they concluded that the presence of a spreading substance helps in the growth of the canary grass towards the light. The transmittable substance is known as the Auxin. A scientist named Miller discovered another growth-promoting substance named Kinetin which is now known as Cytokinins.
What are the Characteristics of Plant Growth Regulators?
Along with some chemical substances, plants also require water, oxygen, sunlight and nutrition to grow and develop well. These chemicals are called Plant Growth Regulators and are produced naturally by the plants themselves. They are also described as phytohormones, plant growth substances, or plant growth hormones as they can accelerate the growth of plants.
Plant growth hormones exhibit the following characteristics:
Differentiation and elongation of cells
Formation of leaves, flowers and stems
Wilting of leaves
Ripening of fruit
Seed dormancy, etc
Generally, there are five types of plant hormones namely: auxin, gibberellins (GAs), cytokinins, abscisic acid (ABA) and ethylene.
Types of Plant Growth Regulators
Plant growth hormones or regulators are of two types:
Plant Growth Promoters- Auxin, Gibberellins, Cytokinins
Plant Growth Inhibitors- Abscisic Acid, Ethylene
This distinction is based on the role that they play. Hormones that help in the growth of the plants are known as promoters whereas hormones that restrict the process of growth are known as inhibitors.
Plant Growth Promoters
Auxins
The first phytohormone that was discovered is the Auxin discovered by the biologist Charles Darwin. Auxins play a very important role as a plant hormone. The chief naturally occurring auxin is indole-3 acetic acid – IAA and other related compounds. The term Auxin is derived from the Greek language which means ‘To grow’.
These plant growth hormones are generally produced at the points of stems and roots from where they are transported to other parts of the plants. These plant hormones involve both natural and synthetic sources. Indole-3-acetic acid and indole butyric acid are obtained from natural plant sources, whereas naphthalene acetic acid and 2, 4-dichlorophenoxyacetic acid are produced from synthetic sources.
Functions of Auxins
It promotes flowering in plants.
It is used in the process of plant propagation.
It is used by gardeners to keep lawns free from weeds.
It is involved in the initiation of roots in stem cuttings.
It prevents the dropping of fruits and leaves at early stages.
It is used for the production of fruit without preceding fertilization.
It helps in the natural detachment of older leaves and fruits.
Apical dominance may occur in which the growth of lateral buds is inhibited by the growth of apical buds. In such cases, the shoot caps should be removed.
Gibberellins
Gibberellins are an extensive chemical family based on the ent-gibberellin structure. The first gibberellin to be discovered was gibberellic acid. Now there are more than 100 types of gibberellins and are mainly obtained from a variety of organisms from fungi to higher plants.
Functions of Gibberellins
They help to delay senescence in fruits.
They are involved in leaf expansion.
They break bud and seed dormancy.
They promote bolting in cabbages and beet.
They help fruits to elongate and improvise their shape.
They are used by the brewing industry for increasing the speed of the malting process.
They are used as the spraying agent to increase sugarcane yield by lengthening the stem.
They are used to increase the maturity period in young conifers and promote early seed production.
Cytokinins
Cytokinins are produced in the regions where cell division occurs, mostly in the roots and shoots. They help in the production of new leaves, lateral shoot growth, chloroplasts in leaves etc. They help in overcoming apical dominance and delay the ageing of leaves.
Functions of Cytokinins
They break bud and seed dormancy.
It promotes the growth of the lateral bud.
It promotes cell division and apical dominance.
They are used to keep flowers fresh for a maximum time.
They are used in tissue culture to induce cell division in mature tissues.
They promote lateral shoot growth and adventitious shoot formation.
It helps to delay the process of ageing (senescence) in fresh leaf crops like cabbage and lettuce.
They are involved in the formation of new leaves and chloroplast organelles within the plant cell.
They are used to induce the development of shoot and roots along with auxin, depending on the ratio.
Plant Growth Inhibitors
Abscisic Acid
It is a growth inhibitor that was discovered in the 1960s. This growth inhibitor is processed within the stem, leaves, fruits and seeds of the plant. Abscisic acid mostly acts as an enemy to Gibberellic acid. It is also known as the stress hormone as it helps by increasing the tolerance of plants to different kinds of stress.
Functions of Abscisic Acid
It stimulates the closing of stomata in the epidermis
It helps in the development and maturation of seeds
It inhibits plant metabolism and seed germination.
It is involved in regulating abscission and dormancy.
It is widely used as a spraying agent on trees to regulate the dropping of fruits.
It induces dormancy in seeds and helps in withstanding desiccation and other unfavourable growth factors.
Ethylene
Ethylene is a simple, gaseous plant growth regulator, synthesised by most of the plant organs including ripening fruits and ageing tissues. It is an unsaturated hydrocarbon having double covalent bonds between and adjacent to carbon atoms.
Functions of Ethylene
Ethylene is the most widely used plant growth inducer as it helps in regulating many physiological processes.
It induces flowering in the mango tree.
It promotes the sprouting of potato tubers.
It breaks the dormancy of seeds and buds.
It enhances the respiration rate during the ripening of fruits.
It is applied to rubber trees to start the flow of latex.
It promotes abscission and senescence of both leaves and flowers.
It is used to stimulate the ripening of fruits. For example, tomatoes and citrus fruits.
It affects the horizontal growth of seedlings and swelling of the axis in dicot seedlings.
It increases the absorption of plants by increasing the root and hair formation.
Thus, it can be concluded that plant hormones or plant growth regulators play a vital role in the growth and development of plants.
Fun Facts about Plant Growth Regulators
Gibberellin is used in the cultivation of fruits to help unfertilised fruits such as pears and apples to mature fully.
The concentration of Cytokinins is highest in the youngest part of the plant.
Supplying Cytokinins results in faster flower formation.
Apart from being responsible for the ripening of fruits, ethylene causes leaves to shed.
Accumulation of ethylene around roots can lead to leaf chlorosis, stem thickening and leaves bending towards the stem. This makes plants more susceptible to diseases.
Abscisic acid is responsible for the closing of stomata during water stress.
Flowering in plants can be manipulated with the help of a plant growth regulator named florigen.
Conclusion
Having gone through this topic you have learnt about plant hormones which regulate their growth and also you have understood the topic of factors affecting their growth.
After reading this topic from Vedantu's website you will be able to answer questions related to this topic not just in your board exams but also in various medical entrance exams.
FAQs on Plant Growth Regulators: Functions, Types & Uses
1. What are Plant Growth Regulators (PGRs)?
Plant Growth Regulators, also known as phytohormones or plant hormones, are small organic compounds that regulate, signal, and control the growth and development of plants. Unlike nutrients, they are required in very small quantities but have a profound impact on physiological processes like cell division, elongation, flowering, and fruiting. For more details, you can explore the Full Form of PGR.
2. What are the five major types of plant hormones as per the CBSE Class 11 syllabus?
The five major types of plant hormones, or growth regulators, are broadly classified based on their functions. These are:
Auxins: Primarily involved in cell elongation and apical dominance.
Gibberellins: Promote stem elongation, seed germination, and bolting.
Cytokinins: Essential for cell division and overcoming apical dominance.
Ethylene: A gaseous hormone known for promoting fruit ripening and senescence.
Abscisic Acid (ABA): A growth inhibitor that plays a key role in dormancy and stress response.
You can learn more about these chemical messengers in the detailed topic on Plant Hormones.
3. What are the primary functions of auxins in a plant?
Auxins are a crucial class of plant hormones with several key functions:
Apical Dominance: They promote the growth of the main central stem while inhibiting the growth of lateral buds.
Cell Elongation: They stimulate the elongation of cells in shoots, which is a primary mechanism of plant growth.
Root Initiation: Auxins are widely used in horticulture to stimulate root formation in stem cuttings.
Tropic Movements: They are responsible for phototropism (growth towards light) and gravitropism (growth in response to gravity).
For an in-depth look, see the guide on Auxin.
4. Which plant hormone is responsible for cell division and where is it primarily synthesised?
The plant hormones primarily responsible for promoting cell division (cytokinesis) are the Cytokinins. They are synthesised in regions where rapid cell division occurs, such as in the root apical meristems, developing shoot buds, and young fruits. From the roots, they are transported upwards through the xylem to other parts of the plant to stimulate growth. Discover more about their role in Cytokinins.
5. Why is Abscisic Acid (ABA) often called the “stress hormone”?
Abscisic Acid (ABA) is known as the “stress hormone” because its concentration in plant tissues increases significantly when the plant is exposed to stressful environmental conditions. Its primary functions during stress include:
Stomatal Closure: ABA signals the guard cells in leaves to close the stomata, which reduces water loss through transpiration during a drought.
Inducing Dormancy: It inhibits seed germination and bud growth, helping the plant survive unfavourable conditions like winter or dry periods.
Promoting Senescence: It accelerates the ageing process in leaves, allowing the plant to shed them and conserve resources.
These actions help the plant tolerate and survive various stresses, as detailed in the NCERT Solutions for Class 11 Biology Ch 13.
6. How can ethylene act as both a growth promoter and an inhibitor?
Ethylene has a dual role, making it unique among plant hormones. It can be a promoter or an inhibitor depending on the plant tissue, its concentration, and the developmental stage.
As a Growth Promoter: Ethylene is famous for initiating and synchronising fruit ripening in many species (e.g., bananas, tomatoes). It also promotes the sprouting of potato tubers and can induce flowering in plants like mangoes and pineapples.
As a Growth Inhibitor: It strongly promotes senescence (ageing) and abscission (shedding) of leaves, flowers, and fruits. It also inhibits stem elongation in some conditions, causing the stem to swell.
This context-dependent function makes Ethylene a versatile regulator of a plant's life cycle.
7. What is the key difference between growth-promoting and growth-inhibiting hormones?
The primary difference lies in their overall effect on plant activities.
Growth-promoting regulators, such as Auxins, Gibberellins, and Cytokinins, are involved in processes that lead to growth. This includes cell division, cell enlargement, pattern formation, flowering, fruiting, and seed formation.
In contrast, growth-inhibiting regulators like Abscisic Acid and Ethylene are largely involved in responses to stress and processes like dormancy and abscission (shedding of plant parts). They play a crucial role in helping plants survive unfavourable conditions. Many growth inhibitors are vital for a plant's lifecycle.
8. What is apical dominance and which plant hormone is responsible for it?
Apical dominance is a phenomenon where the central, main stem of a plant grows more strongly than the lateral (side) stems. The main growth hormone responsible for this is Auxin. It is produced in the apical bud (at the very tip of the main stem) and flows downwards, inhibiting the growth of the lateral buds. If the apical bud is removed, the inhibitory effect ceases, and the lateral buds begin to grow, making the plant bushier. This is a common practice in gardening and tea plucking.
9. How do synthetic auxins provide benefits in agriculture and horticulture?
Synthetic auxins are artificially created compounds that mimic the effects of natural auxins and are widely used in agriculture. Examples include Indole-butyric acid (IBA) and Naphthalene acetic acid (NAA). Their key applications include:
Rooting Agent: They are applied to stem cuttings to induce root formation, which is crucial for vegetative propagation.
Herbicide: At high concentrations, synthetic auxins like 2,4-D are used as selective herbicides to kill broadleaf weeds without harming monocot crops like cereals.
Preventing Fruit Drop: They are sprayed on fruit trees to prevent the premature dropping of young fruits and flowers.
Promoting Flowering: In some plants, like pineapples, they can be used to induce uniform flowering.
The usage of compounds like NAA in plant growth has significantly improved agricultural productivity.
10. What is the role of gibberellins in the malting process for the brewing industry?
In the brewing industry, the malting process involves the controlled germination of barley grains to activate enzymes that convert starches into fermentable sugars. Gibberellins (specifically GA) play a vital role here. They are sprayed on the barley seeds to:
Break Dormancy: They overcome any residual seed dormancy, ensuring uniform germination.
Speed up the Process: They promote the synthesis of hydrolytic enzymes like α-amylase, which breaks down the starch in the endosperm into simple sugars much faster. This significantly shortens the time required for malting, making the process more efficient and predictable.
This is a prime example of the commercial application of Gibberellins.
