[Biology Class Notes] on Plant Growth Regulators Pdf for Exam

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 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- 

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

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.

When the plant continually grows from the germination stage to death, it is called unlimited growth.

In this stage, the plant parts stop growing after acquiring a certain size.

It involves the production of stems, leaves and branches except for the flowers.

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:

1. 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. 

2. 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. 

3. 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.

4. 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:

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:

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

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

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

Plan
t 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

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.

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 ‘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.