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Plant growth regulators usually refers to a set of nutrients present in plants in the form of chemicals which help in regulating the growth and the development of the plants. We are well aware of the fact that plants rely upon sunlight, air and water which are considered as extrinsic factors that play a major role in overall growth and development of plants. However, apart from these extrinsic factors, intrinsic factors such as plant growth regulators or hormones exhibit a vital role in the life cycle of the plants.
In this article, we will read about various plant growth regulators, their chemical names, characteristics and various other associated concepts.
Table of Content
| Table of Content |
Key Terms: Photosynthesis, Phytohormones, Hormones, Cell Division, Mitosis, Plants, Soil, Sunlight, Water, Life Cycle, Dicot
What are Plant Growth Regulators?
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Plants are an important part of our ecosystem which ensure the existence of life on earth. One of the most important sources of their growth is sunlight. The process of photosynthesis becomes possible only because of the presence of fresh air and light on earth. Apart from this, another most important source which is responsible for the growth of the plants is called as plant growth regulators. These are mainly composed of a group of hormones, also known as phytohormones. These hormones become an intrinsic element of the plants which consistently play an important role in regulating their growth and development. They are chemical substances produced naturally by plants.
The video below explains this:
Plant Growth Regulators Detailed Video Explanation:
Read More: Getting to know Plants: Parts, Classification, Life Cycle
Discovery of Plant Growth Regulators
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The most remarkable discovery in history has been associated with the findings of the transmittable substance in the plants which in turn advances its growth and development. This invention was the work of the scientist Charles Darwin and his son Francis Darwin. In one of his experiments, he acknowledged the growth of the coleoptile of the canary grass, that too, in the direction of the light. This recognition made him believe in the presence of such regulators which help in the growth of these plants. Later, this chemical substance contributing to growth was termed as auxin, however, even further, various other experiments discovered multiple types of plant growth regulators. For example, Gibberellins or gibberellic acid.
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Types of Plant Growth
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There are the following types of plant growth.
- Primary and Secondary Growth: The growth of a plant is termed primary when the same happens through the mitotic division of the meristematic cells which are present at the root and the shoot of the plants. Whereas, the secondary growth in a plant takes place through the division of the secondary meristem, which, in turn increases the diameter of the body of the plants.
- Unlimited Growth: The plant growth is termed as unlimited if there is a continuous increase in its length from the stage of germination to its entire life cycle until it dies.
- Limited Growth: This refers to a type of plant growth which is limited in nature. Such types of plants can only attain a certain height, then eventually stop growing.
- Vegetative Growth: This type of plant growth is not at all associated with the growth of flowers, however, only involves the growth and development of stem, leaves, etc.
- Reproductive Growth: This type of plant growth mainly deals with the growth of the flowers in a plant.
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Types of Plant Growth Regulators
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The plant growth regulators are mainly of two types, namely,
- Plant Growth Promoters
- Plant Growth Inhibitors
Plant Growth Promoters
The three naturally produced chemical substances which promote the growth of plants are:
- Auxins
- Gibberellins
- Cytokinin
Auxins: Auxins is one of the most important plant growth regulators discovered by Charles Darwin. The term ‘auxin’ has been derived from the Greek word, meaning to grow. Indole 3 Acetic acid (IAA) is one of the auxins which originate naturally at the tip of the stems and the roots of the plant whereas naphthalene acetic acid is a type of auxin which is manufactured synthetically in a laboratory.
Functions of Auxins: Some of the remarkable functions of auxins in plants are:
- Accompanies the cell division
- Propagate flowering
- Prevent the fruits from falling at an early stage
- Supports growth of fruits
- Supports natural detachment of the older leaves and fruits
- Used as herbicides for preventing dicot weeds
- Performs the functions of a fertiliser for plant growth
- Used in gardening for preventing the weed growth
- Utilised for plant propagation
- Assist in xylem differentiation
Read More: Growth Hormones
Gibberellins: Gibberellins are the plant hormones which are also known to be found in the form of the gibberellic acid. There are more than hundred types of gibberellins found within a plant. These are mainly extracted from the fungi and other existing organisms. It plays a significant role in plant growth and its dwarfing traits.
Functions of Gibberellins: Some of the functions exhibited by Gibberellins in plants are:
- Supports elongation and shape enrichment in the fruits
- Promotes expansion in leaves
- Accelerates the malting process in brewing industry
- Delay senescence in fruits
- Breaks buds
- Accelerates the sugarcane yields
- Increase crop yield
- Increases the rate of maturation in young conifers
- Facilitate early production of seeds
- Enhances the rate of bolting in cabbage
Read More: Difference between Auxins and Gibberellins
Cytokinin: Cytokinin is another type of hormone which is secreted by the plants and effectively influences the growth and development in the plants. These hormones are mostly grown in the roots and the shoots of the plants and are usually found in the regions of the cell division in the plants.
Structure of Cytokinins
Functions of Cytokinin: Some of the functions of Cytokinin in plants are:
- Produces new leaves
- Delay ageing in leaves
- Promotes cell division
- Overcome apical dominance
- Keeps flowers fresh
- Breaks the bud
- Shoot formation is facilitated
- Growth of lateral bud is positively influenced
- Chloroplast organelles are produced
- Regulates senescence
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Plant Growth Inhibitors
The two naturally produced chemical substances which are classified as plant growth inhibitors are:
- Abscisic Acid
- Ethylene
Abscisic Acid: Abscisic Acid is a type of growth inhibitor which is usually synthesised within the roots or stems of the plants. It also serves the function of the stress hormone and makes plants tolerant in nature. However, it is known to be produced in all the forms of life, be it human or animal or plants. They are often represented as ABA.
Functions of Abscisic acid: Some of the most significant functions of abscisic acid are:
- Makes plants stress tolerant
- Acts opposite to the gibberellic acid
- Regulates abscission
- Controls dormancy
- Accelerate seed development
- Closing of the stomata in epidermis is regulated
- Accompanies maturation
- Plant metabolism is slowed down
- Negatively impacts the seed germination
- Regulates food droppings from the trees.
Ethylene: Ethylene is an unsaturated hydrocarbon which is the simplest of all the inhibitors. Double covalent bonds of the carbon atoms are its main feature. It is also known as the simplest alkaline gas which is an important chemical that is also used in the industries. It is also associated with the counter response behaviour of the plants against the abiotic stress conditions.
Functions of Ethylene: Some of the most important functions of Ethylene are:
- Promotes sprouting in the potato tubers
- Accelerates flowering in mango fruits
- Accelerates root hair formation and its growth
- Accelerates fruit ripening (citrus fruits, etc)
- Enhance the rate of respiration
- Regulates senescence
- Influence ageing
- Horizontal growth of the seedlings is regulated
- Affects the swelling of the axis in dicot seedlings
- Accompanies the plants to expand their surface area for absorption
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Characteristics of Plant Growth Regulators
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- Plant growth regulators are the simple organic compounds which support the growth and development of the plants.
- The development, here, refers to a wider arena of support that is provided to the plants through these regulators.
- Some of these plant growth regulators help in making the plants stress tolerant, while others support multiple physiological processes like delayed ageing, preventing ripening and so on.
- Plant growth regulators are a group of hormones which are found in every plant in the form of various types of chemical substances, also known as phytohormones.
- However, they can also retard the growth of the plant in the extreme cases.
Factors affecting Plant Growth
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The most important external factors which contribute to overall growth and development of plants are:
- Sunlight: The importance of light is known by the fact that the growth of plants is limited during the winter season and accelerated during the summers. Plants being an autotroph require sunlight or artificial light to prepare their food through photosynthesis. Other functions like chlorophyll synthesis, stomata movement are also greatly influenced by the light.
- Water: Water is an important source of energy for the plants. Most of the body of plants comprises water, they even absorb the required amount of water from the soil and simultaneously, water also acts as an important medium of providing the plants with the nutrients.
- Temperature: Higher the temperature of the surroundings, higher will be the growth of the plants. The rate of germination decreases in the presence of lower temperatures, as a result, tropics experience more vegetation than the temperate regions.
- Nutrients: The growth of the plants is greatly influenced by the amount of macro or micro nutrients present in them. Multiple nutrients like that of calcium, phosphorus, iron, sulphur, magnesium, etc. are all required by the plants in definite quantities depending upon the type of soil that plant has been growing in.
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Things to Remember
- The phenomenon of the growth of any plant consistent with the direction of the source of light is known as phototropism. For example, the growth of the coleoptile of the canary grass in the direction of the light.
- E. Kurosawa was a scientist who discovered the presence of the Gibberellins or gibberellic acid, which is also a type of plant growth regulator. This was firstly found in the rice seedlings.
- All the physiological processes in any plant or say grass, is always regulated by the plant growth regulators. Apart from the sunlight, phytohormones constitute an important source of their development.
- Plant growth regulators are a type of organic compound, which are most of the time, naturally present in the plants.
- However, these chemical substances can be produced or synthesised in the laboratories as well, which means that the plant growth regulators are eligible to be produced artificially.
Sample Questions
Ques. Classify the different types of plant growth regulators. Explain briefly. (5 marks)
Ans. The plant growth regulators could be classified into two types as mentioned below:
- Plant Growth Promoters: Some of its types are mentioned below
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- Auxins: It was discovered by Charles Darwin. It helps in controlling weeds, propagating flowering and also prevents the fruits from falling at an early stage. There are also multiple types of auxins such as, Indole 3 Acetic acid (IAA) is one type of auxin which originates naturally at the tip of the stems and the roots of the plant. Whereas, another type of auxin is naphthalene acetic acid which is manufactured synthetically in a laboratory.
- Gibberellins: These are also found in the form of the gibberellic acid, at present are of more than hundred types. These are mainly extracted from the fungi and other existing organisms. It supports elongation and expansion in fruits and leaves respectively and also contributes in accelerating the malting process in the brewing industry.
- Cytokinin: Cytokinin is another type of plant growth promoter usually found in the regions of the cell division. It delays ageing, promotes growth, promotes cell division, regulates senescence and performs other important functions.
- Plant Growth Inhibitors: Some of its types are mentioned below
- Abscisic Acid: It is a type of growth inhibitor which is usually synthesised within the roots or stems of the plants. It also serves the function of the stress hormone and makes the plants tolerant. It helps in regulating abscission, controlling dormancy and also stimulates the seed development.
- Ethylene: It is an unsaturated hydrocarbon which is the simplest of all the inhibitors. Double covalent bonds of the carbon atoms are its main feature. It promotes sprouting in the potato tubers and simultaneously accelerates flowering in the mango fruits.
Ques. What are the features of primary growth in plants? Give examples.(2 marks)
Ans. The primary growth of a plant is associated with the process of mitosis. As a result, primary growth takes place when the same happens through the mitotic division of the meristematic cells which are present at the root and the shoot of the plants, thus, majorly causing the growth of the root and shoot only. Intercalary meristem and apical meristem are known to play a major role in the primary growth of the plants. Elongation of the plants in terms of its stem is the best example of primary growth in plants.
Ques. What are the plant growth regulators? Mention other factors affecting plant growth. (5 marks)
Ans. Plant growth regulators are the most important source which is responsible for the growth of the plants. These regulators mainly comprise the group of hormones, also known as phytohormones. These hormones become an intrinsic element of the plants which consistently play an important role in regulating their growth and development. Plant growth regulators are a type of organic compound, which are most of the time, naturally present in the plants. However, these chemical substances can be produced or synthesised in the laboratories as well. Apart from these regulators, there are also other factors which affects the plant growth, such as:
- Sunlight promotes photosynthesis and the synthesis of chlorophyll as plants are autotrophs.
- Water helps in transporting nutrients and stimulates growth as 90% of the plant’s body comprises water.
- Temperature influences the process of germination, most of the time, the growth of the plant is slowed down due to extreme temperature.
- Soil type like laterite, alluvial, black soil, etc affects the development of the plants as soil is the main source of nutrients in the plants.
Ques. How is auxin different from gibberellins? Discuss. (4 marks)
Ans. Auxins is a very first and most important plant growth regulator, discovered by Charles Darwin. The term ‘auxin’ has been derived from the Greek word, meaning to grow. Indole 3 Acetic acid (IAA) is one of the auxins which originate naturally at the tip of the stems and the roots of the plant. Whereas, naphthalene acetic acid is a type of auxin which is manufactured synthetically in a laboratory. Controlling weeds, propagating flowering, and preventing the fruits from falling at an early stage are some of its main functions. However, it also supports the growth of fruits and also regulates the natural detachment of the older leaves and fruits.
On the other hand, Gibberellins are mainly extracted from the fungi and other existing organisms. This can also be found in the form of the gibberellic acid, at present are of more than hundred types. Its main functions involve accelerating the sugarcane yields and supporting the elongation and expansion in fruits and leaves respectively. Accelerating the malting process in the brewing industry and delaying senescence in fruits are some other important functions.
Ques. Explain the main role of plant growth regulators. (4 marks)
Ans. Plant growth regulators are considered as an organic compound, which are most of the time, naturally present in the plants. These regulators mainly comprise of the phytohormones which remain an intrinsic element of the plants and are known for consistently playing an important role in regulating their growth and development. All the physiological processes in any plant or say grass, is always regulated by the plant growth regulators. Some of their main roles or objectives are mentioned below.
- Some of these plant growth regulators help in making the plants stress tolerant, while others support multiple physiological processes like delayed ageing, preventing ripening and so on.
- Plant growth regulators are a group of hormones which are found in every plant in the form of various types of chemical substances which can also retard the growth of the plants in the extreme cases.
Ques. What is the significance of ethylene? Discuss briefly. (2 marks)
Ans. Ethylene is an unsaturated hydrocarbon which is the simplest of all the inhibitors. Double covalent bonds of the carbon atoms are its main feature. It can be classified as both the plant promoter and plant inhibitor. It has multiple characteristics which makes it different from other hormones, such as, it directly contributes in promoting the sprouting in the potato tubers, it effectively accelerates flowering in mango fruits, raises the rate of root hair formation and also enhances the rate of respiration.
Ques. How does Cytokinin and the abscisic acid affect plant growth? (3 marks)
Ans. Cytokinin is a type of plant growth promoter and it is mostly grown in the roots and the shoots of the plants. Usually, these cytokinins are found in the regions of the cell division in the plants. It has several impacts over the plants which accelerates its growth and development, such as, it helps in producing the new leaves and also contributes in delaying the ageing in the leaves. Simultaneously, cytokinin also influences cell division and plays a great role in overcoming apical dominance.
Whereas, the abscisic acid is a type of growth inhibitor which acts opposite to the gibberellic acid and is usually synthesised within the roots or stems of the plants. It also serves the function of the stress hormone and makes the plants stress tolerant. It also helps in regulating the abscission and dormancy.
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