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Plant Biology and
Production
Unit 3
Seed Germination, Growth,
and Development
Lesson 5
Examining the Roles of
Plant Growth Regulators
Interest Approach
Several days before the introduction
of this lesson prepare two identical
plants for an interest approach.
Select an upright growing potted
herbaceous plant such as a coleus
or tomato.
Lay one of the two plants on its side.
The plant on its side will curve
upward in response to gravity.
Interest Approach
Present the two plants to the class and
ask a series of questions.
Ask the students why the one plant is
curved.
Is there an advantage to the plant for
responding in this way?
Is there a term used to describe the plant
response?
What caused it to curve?
Terms
Abscisic acid
Apical dominance
Auxins
Cytokinins
Ethylene
Etiolation
Indoleacetic acid
(IAA)
Indolebutyric acid
(IBA)
Gibberellins
Gravitropism
Phototropism
Plant growth
regulators
Synthetic growth
regulators
Thigmotropism
Tropisms
Student Learning Objectives
Identify plant growth regulators
(PGR) and their functions.
Explain plant tropisms.
Discuss synthetic growth regulators.
Describe commercial uses of plant
growth regulators.
What are Plant Growth Regulators
and What are Their Functions?
I. Natural occurring or synthetic
chemicals that regulate plant growth
and development are plant growth
regulators.
What are Plant Growth Regulators
and What are Their Functions?
A. Plant growth regulators promote
growth, inhibit growth, and modify growth
and development.
Plants produce compounds to regulate
growth.
These natural chemicals are hormones.
B. Plants produce five different
hormones.
What are Plant Growth Regulators
and What are Their Functions?
1. Auxins consist of a group of hormones
that have a primary role in promoting cell
elongation.
The primary auxin is indoleacetic acid
(IAA).
Plants produce auxins in the apical
meristem.
They migrate down the stem moving
through one cell to the next rather than
being transported through the phloem or
xylem.
What are Plant Growth Regulators
and What are Their Functions?
Auxins have the greatest influence on
cells closest to the apical meristem.
The higher concentrations of auxins give
the apical meristem dominance over
growth below.
Apical dominance is a term used to
describe the apical bud’s dominance
over the lateral buds.
What are Plant Growth Regulators
and What are Their Functions?
2. Gibberellins induce cell elongation
and cell division.
They are produced in stem and root
apical meristems, seed embryos, and
young leaves.
Gibberellins play a key role in the
development of flowers and in the
production of enzymes during seed
germination.
What are Plant Growth Regulators
and What are Their Functions?
3. Cytokinins are hormones
responsible for cell division and
differentiation.
They are produced in the roots and
transported throughout the plant via
the xylem.
What are Plant Growth Regulators
and What are Their Functions?
Cytokinins encourage the growth of
lateral shoots.
They inhibit the branching of the
roots.
As an ingredient in tissue culture
medium, cytokinins are an important
promoter of cell division.
What are Plant Growth Regulators and
What are Their Functions?
4. Ethylene is a colorless gas that
speeds the aging of plant parts,
particularly fruit.
It is produced in stems, ripening fruit,
and dying leaves.
What are Plant Growth Regulators
and What are Their Functions?
5. Abscisic acid is a growth-inhibiting
hormone largely responsible for
seed dormancy.
It also causes stomates to close in
times of drought in an effort to
conserve water.
What are Plant Tropisms?
II. External stimuli and
environmental conditions cause
plants to respond in certain ways.
Plant responses to the external
stimuli are known as tropisms.
What are Plant Tropisms?
A. Phototropism is a plant’s
response to the source of light.
Auxins move down the shaded side
of the plant stem.
The presence of auxins causes the
cells on the shady side of the stem
to elongate more than cells than the
bright side of the stem.
What are Plant Tropisms?
The result is a stem that bends
towards the light.
Under low light or dark conditions
cells elongate on all sides of the
stem.
The result is an appearance of the
stem stretching.
Stretching due to low light is known
as etiolation.
What are Plant Tropisms?
B. Gravitropism also referred to as
geotropism is a plant response to
gravity.
The stems of plants laid on their side
curve upward.
In this scenario auxins settle to the
bottom side of the stem and cause
cells to elongate.
What are Plant Tropisms?
C. Thigmotropism is a response to
mechanical stimuli.
A good example is the tendrils of a
cucumber plant.
When the tendrils touch an object,
the response is to curl around that
object.
What are Synthetic Growth
Regulators?
III. Other human-made chemicals
that regulate plant growth are
referred to as synthetic growth
regulators.
A. Many synthetic growth regulators
have been developed to regulate
plant growth.
What are Synthetic Growth
Regulators?
1. Two common root-promoting
materials used in the propagation of
plants are naphthaleneacetic acid
(NAA) and indolebutyric acid (IBA).
2. Growth retardant chemicals inhibit
cell elongation and keep plants
compact. Many growth retardants
are available.
What are Synthetic Growth
Regulators?
3. Some synthetic growth regulators
serve as herbicides by altering
growth and metabolic processes of
weeds.
How are Plant Growth Regulators
Used Commercially?
IV. Plant growth regulators are used
in agricultural practices to improve
the quality and quantity of yields.
How are Plant Growth Regulators
Used Commercially?
A. Auxins in the forms of
indoleacetic acid (IAA) and
indolebutyric acid (IBA) are widely
used to speed the rooting of cuttings
in the horticulture industry.
Naphthaleneacetic acid (NAA) is
sprayed on apples to prevent preharvest drop of the fruit.
How are Plant Growth Regulators
Used Commercially?
B. Gibberellins are used to induce
flowering.
Some plants that respond by
flowering are carrots, endive,
cabbage, turnips, and
chrysanthemums.
How are Plant Growth Regulators
Used Commercially?
Gibberellins serve as growth
stimulants making plants, including
sugar cane, grow larger.
Gibberellins are used with some
plant species, such as grapes, to
produce larger fruit.
How are Plant Growth Regulators
Used Commercially?
C. Cytokinins have been shown to
extend the shelf life of lettuce.
Cytokinins are also an important
ingredient of tissue culture medium,
as they promote cell division.
How are Plant Growth Regulators
Used Commercially?
D. Ethylene is used in the ripening of
fruits before being placed on grocery
shelves.
How are Plant Growth Regulators
Used Commercially?
E. Many synthetic growth regulators
are used to regulate plant growth.
Some promote growth.
Others inhibit growth.
Growth retardants are widely used in
the horticulture industry to keep
plants compact.
Review / Summary
1. What are plant growth regulators
and what are their functions?
2. What are plant tropisms?
3. What are synthetic growth
regulators?
4. How are plant growth regulators
used commercially?