Plant Growth Regulators

How Does Plant Growth and Development Take Place? 

Light, water, minerals, oxygen along with other nutrients are the things which plants need for their growth and development. Apart from these, plants also need certain organic compounds to signal, regulate and control the growth of plants. 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 which are either produced naturally within the plants or are synthesized in laboratories.  They control and modify the 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 the 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 stem, leaves, and branches, except the flowers.

  • Reproductive Growth

When flowering occurs, that type of growth stage is called reproductive growth. 

What are the factors affecting plant growth? 

There are four major factors that affect the growth of the 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 influences 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 the 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 the temperatures. High temperatures help in the speed up transpiration, photosynthesis, and germination processes. Low temperatures are considered harmful for the plants as it slows down the growth of the plants.

Nutrients

Plants need proper nourishment for their growth and development. Soil nutrients are divided into macronutrients and micronutrients. Nitrogen, potassium, calcium, magnesium, sulfur, and phosphorus are some of the most important macronutrients which are required by the plants. 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 which 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 have the ability to accelerate 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

  • Plant Growth 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

  1. To promote flowering in plants

  2. It is used in the process of plant propagation.

  3. It is used by gardeners to keep lawns free from weeds.

  4. It is involved in the initiation of roots in stem cuttings.

  5. It prevents dropping of fruits and leaves at early stages.

  6. It is used for the production of fruit without preceding fertilization.

  7. Helps in the natural detachment of older leaves and fruits.

  8. 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 

  1. Help to delay senescence in fruits.

  2. They are involved in leaf expansion.

  3. Break bud and seed dormancy.

  4. Promote bolting in cabbages and beet.

  5. Help fruits to elongate and improvise their shape.

  6. They are used by the brewing industry for increasing the speed of the malting process.

  7. Used as the spraying agent to increase sugarcane yield by lengthening of the stem.

  8. 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 ageing of leaves.

Functions of Cytokinins

  1. Break bud and seed dormancy.

  2. Promotes the growth of the lateral bud.

  3. Promotes cell division and apical dominance.

  4. They are used to keep flowers fresh for maximum time.

  5. Used in tissue culture to induce cell division in mature tissues.

  6. Promote lateral shoot growth and adventitious shoot formation.

  7. Helps to delay the process of ageing (senescence) in fresh leaf crops like cabbage and lettuce.

  8. Involved in the formation of new leaves and chloroplast organelles within the plant cell.

  9. 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 which 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

  1. Stimulates closing of stomata in the epidermis.

  2. Helps in the development and maturation of seeds.

  3. Inhibits plant metabolism and seed germination.

  4. It is involved in regulating abscission and dormancy.

  5. It is widely used as a spraying agent on trees to regulate dropping of fruits.

  6. 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.

  • Induce flowering in the mango tree.

  • Promotes sprouting of potato tubers.

  • Breaks the dormancy of seeds and buds.

  • Enhances respiration rate during ripening of fruits.

  • Applied to rubber trees to start the flow of latex.

  • Promotes abscission and senescence of both leaves and flowers.

  • Used to stimulate the ripening of fruits. For example, tomatoes and citrus fruits.

  • Affects horizontal growth of seedlings and swelling of the axis in dicot seedlings.

  • Increases the absorption of plants by increase in the root and hair formation. 

Thus, it can be concluded that the plant hormones or the plant growth regulators play a vital role in the growth and development of plants.

FAQ (Frequently Asked Questions)

1. What are Plant Growth Regulators?

Light, water, minerals, oxygen along with other nutrients are the things which plants need for their growth and development. Apart from these, plants also need certain organic compounds to signal, regulate and control the growth of plants. These are known as the Plant Growth Regulators or Hormones or Phytohormones.

2. What are the types of plant growth?

1. Unlimited Growth 

2. Vegetable Growth 

3. Primary and Secondary Growth

4. Limited Growth 

 5. Reproductive Growth 


3. What are the functions of Gibberellins?

1. Help to delay senescence in fruits.

2. They are involved in leaf expansion.

3. Break bud and seed dormancy.

4. Promote bolting in cabbages and beet.

5. Help fruits to elongate and improvise their shape.

6. They are used by the brewing industry for increasing the speed of the malting process.

7. Used as the spraying agent to increase sugarcane yield by lengthening of the stem.

8. Used to increase the maturity period in young conifers and promote early seed production


4. What are the types of Plant Growth Regulators?

Plant growth hormones or regulators are of two types:

  • Plant Growth Promoters

  • Plant Growth Inhibitors

5. What are the functions of Ethylene?

Ethylene is the most widely used plant growth inducer as it helps in regulating many physiological processes.

  • Induce flowering in the mango tree.

  • Promotes sprouting of potato tubers.

  • Breaks the dormancy of seeds and buds.

  • Enhances respiration rate during ripening of fruits.

  • Applied to rubber trees to start the flow of latex.

  • Promotes abscission and senescence of both leaves and flowers.

  • Used to stimulate the ripening of fruits. For example, tomatoes and citrus fruits.

  • Affects horizontal growth of seedlings and swelling of the axis in dicot seedlings.

  • Increases the absorption of plants by increase in the root and hair formation.

6. What are the characteristics of plant growth hormones?

Ans. Plants 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