Plant growth regulators are chemical substances that control all aspects of plant growth and development. They are also known as plant growth hormones or phytohormones. There are different types of plant growth regulators that either promote or inhibit growth. The major plant growth promoters are auxins, gibberellins, and cytokinins, while the major inhibitors are abscisic acid and ethylene. Plant growth regulators were discovered through various experiments over time and play an important role in plant cell differentiation, elongation, flowering, fruit ripening, and seed dormancy.

1. Plant growth regulations

2. Plant Growth Regulations
• 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.
• There are different types of Plant Growth
Regulators, which are also referred to as
phytohormones

3. Plant Growth and Development
• 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.
• Based on their actions, plant growth regulators are
broadly classified into two major groups:

4. • 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.

5. 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.
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.

6. 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.
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.

7. 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.
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.

8. Discovery of Plant Growth Regulators
• Though it was serendipity, 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.

10. 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

11. 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, or plant
growth hormones. They can accelerate as well as retard
the rate of growth in plants.

12. Plants growth hormones or plant growth regulators
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. In addition to these,
there are more derivative compounds, both natural and
synthetic, which also act as plant growth regulators.