Plant growth regulators are organic compounds that control plant growth and development. The main types are auxins, gibberellins, cytokinins, abscisic acid, and ethylene. Auxins and gibberellins promote growth, while abscisic acid and ethylene inhibit growth. Plant growth regulators influence processes like cell division and elongation, flowering, fruit ripening, dormancy, and response to stress.

1. Plant Growth
Regulators

2. Plant Growth Regulators
Plants require light, water, oxygen, minerals and other
nutrients for their growth and development.
Apart from these external requirements, plants also depend
on certain organic compounds to signal, regulate and
control the growth of plants.
These are collectively called as Plant Growth Regulators or
Plant Growth Hormones.

3. There are different types of Plant Growth Regulators, which
are also referred to as phytohormones.
Let’s learn about them in more detail below.
Table of Contents
Plant Growth
Factors
Plant Growth Regulators
Characteristics
Types

4. Plant Growth and Development- An Introduction
 Plant growth regulators can be defined as chemical
substances that control all the aspects of growth and
development within plants.
 They are also referred to as plant growth hormones or
phytohormones.
 Plant growth hormones are organic compounds which are
either produced naturally within the plants or are synthesized
in laboratories.
 They profoundly control and modify the physiological
processes like the growth, development, and movement of
plants.

5. Based on their actions, plant growth regulators are broadly
classified into two major groups:
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.

6. Let us have a detailed look on the plant growth and
development before studying plant hormones or plant growth
regulators.
Types of Plant Growth Primary and Secondary Growth
The meristematic cells present at the root and shoot apices
divide 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.

7. Unlimited Growth
When the plant constantly grows from the germination
stage to death, it is called unlimited growth.
Limited Growth
In this stage, the plant parts stop growing after attaining a
certain size.
Vegetative Growth
It involves the production of stem, leaves, and branches,
except the flowers.
Reproductive Growth
Flowering occurs at this type of growth stage

8. Factors Affecting Plant Growth
There are four major factors that affect the growth of the plants.
They are:
Light
• Plants are autotrophs. 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 encourages flowering and fruiting.
• During winters when the days are short, the growth of the
plants is retarded.

9. Water
• Plants cannot survive without water.
• Around 90% of the plant body comprises water.
• Plants become stressed 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. Water
keeps the plant hydrated.
Temperature
• Plant growth is greatly influenced by temperatures.
• High temperatures speed up transpiration, photosynthesis,
and germination processes.
• Low temperatures, however, slow down the growth of the
plants.

10. Nutrients
Just like human beings, plants require proper nourishment for
their growth and development.
Soil nutrients are divided into macronutrients and micronutrients.
Nitrogen, potassium, calcium, magnesium, sulfur, and
phosphorus are the macronutrients required by the plants.
The micronutrients include iron, copper, etc.
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.

11. Discovery of Plant Growth Regulators
• initial steps of 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-phototropism.
• Followed by a series of experiments, they concluded the
presence of a transmittable substance that influences the
growth of canary grass towards the light.
• That transmittable substance was what we know as auxin
which was isolated later by F.W. Went.

12. Effect of Auxin on Plant Growth
• Later many scientists discovered and isolated different plant growth
regulators.
• Gibberellins or gibberellic acid was formerly found in uninfected rice
seedlings and was reported by E. Kurosawa.
• F. Skoog and Miller discovered another growth promoting substance named
kinetin, which is now known as cytokinins.
Characteristics of Plant Growth Regulators
• As the plants require oxygen, water, sunlight, and nutrition to grow and
develop, they do require certain chemical substances to manage their
growth and development.
• These chemicals substances are called Plant Growth Regulators and are
produced naturally by the plants itself.
• These are simple organic molecules having several chemical compositions.
• They are also described as phytohormones, plant growth substances, r plant
growth hormones.
• They can accelerate as well as retard the rate of growth in plants.

13. Plants growth hormones or plant growth regulators exhibit the
following characteristics:
1. Differentiation and elongation of cells.
2. Formation of leaves, flowers, and stems.
3. Wilting of leaves.
4. Ripening of fruit.
5. Seed dormancy, etc. Generally, there are five types of
plant hormones namely, auxin, gibberellins
(GAs), cytokinins, abscisic acid (ABA) and ethylene.
In addition to these, there are more derivative compounds,
both natural and synthetic, which also act as plant growth
regulators.

14. Types of Plant Growth Regulators
Plant growth hormones or regulators are of the following types:
1. Plant Growth Promoters
2. Plant Growth Inhibitors
Plant Growth Promoters
Auxins
• The first phytohormone to be discovered is the Auxin and it
was discovered by the biologist Charles Darwin.
• Auxins are one of the most important plant hormones.
• The chief naturally occurring auxin is indole-3 acetic acid –
IAA and other related compounds.
• The term Auxin is derived from the Greek language meaning
to grow.

15. These plant growth regulators are generally produced at the points of stems and
roots from where they are transported to other parts of the plants.
Functions of Auxins
1. Promote flowering in plants
2. Used in the process of plant propagation.
3. Used by gardeners to keep lawns free from weeds.
4. Involved in the initiation of roots in stem cuttings.
5. Prevent dropping of fruits and leaves at early stages.
6. Control xylem differentiation and help in cell division.
7. Auxins are widely used as herbicides to kill dicot weeds.
8. Used for the production of fruit without preceding fertilization.
9. Promote natural detachment (abscission) of older leaves and fruits.
10. 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 may be removed.
11. These are produced by the apex of root and shoot.

16. Gibberellins
Gibberellins are an extensive chemical family based on the ent-
gibberellane 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.
They are acidic and are denoted as follows – GA1, GA2, GA3etc.
Functions of Gibberellins
1. Delay senescence in fruits.
2. Involved in leaf expansion.
3. Break bud and seed dormancy.
4. Promote bolting in cabbages and
beet.
5. Help fruits like apples to elongate
and improve their shape.

17. 6. Used by the brewing industry for increasing the speed on the
malting
process.
7. Used as the spraying agent to increase sugarcane yield by
lengthening of the stem.
8. Used to fasten the maturity period in young conifers and
promote early seed production
9. Helps in increasing the crop yield by increasing the height in
plants such
as sugarcane and increase the axis length in plants such as
grape stalks.
10. Gibberellins are acidic in nature.
11. It also delays senescence.

18. Cytokinins
These 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 a longer time.
5. Used in tissue culture to induce cell division in mature tissues.
6. Promote lateral shoot growth and adventitious shoot formation.
7. Promote nutrient mobilization which in turn helps delay leaf senescence.
8. Helps in delaying the process of ageing (senescence) in fresh leaf crops like
cabbage and lettuce.
9. Involved in the formation of new leaves and chloroplast organelles within
the plant cell.
10. Used to induce the development of shoot and roots along with auxin,
depending on the ratio.

19. Plant Growth Inhibitors
Abscisic acid
It is a growth inhibitor which was discovered in the 1960s. It was initially called
dormant. Later another compound abscisin-II was discovered and are
commonly called as abscisic acid.
This growth inhibitor is synthesized within the stem, leaves, fruits, and seeds of the
plant.
Abscisic acid mostly acts as an antagonist 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.

20. Ethylene
• Ethylene is a simple, gaseous plant
growth regulator, synthesis 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.
• Ethylene is used as both plant
growth promoters and plant
growth inhibitors.
• Ethylene is synthesized by the
ripening fruits and ageing tissues.

21. Functions of Ethylene
Ethylene is the most widely used plant growth regulator as it helps in
regulating
many physiological processes.
1. Induce flowering in the mango tree.
2. Promotes sprouting of potato tubers.
3. Breaks the dormancy of seeds and buds.
4. Enhances respiration rate during ripening of fruits.
5. Applied to rubber trees to stimulate the flow of latex.
6. Promotes abscission and senescence of both leaves and flowers.
7. Used to stimulate the ripening of fruits. For example, tomatoes and citrus
fruits.
8. Affects horizontal growth of seedlings and swelling of the axis in dicot
seedlings.
9. Increases root growth and root hair formation, therefore helping plants to
increase their absorption surface area.
Thus we see how important are the plant hormones or the plant growth
regulators