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Lesson 8
Understanding Plant Growth Regulators
Next Generation Science/Common Core Standard Addressed.
 HS‐LS1‐2. Develop and use a model to illustrate the hierarchical
organization of interacting systems that provide specific functions
within multicellular organisms. [Clarification Statement: Emphasis is
on functions at the organism system level such as nutrient uptake,
water delivery, and organism movement in response to neural stimuli.
 HS‐LS1‐3. Plan and conduct an investigation to provide evidence that
feedback mechanisms maintain homeostasis. [Clarification Statement:
Examples of investigations could include heart rate response to
exercise, stomate response to moisture and temperature, and root
development in response to water levels.] [Assessment Boundary:
Assessment does not include the cellular processes involved in the
feedback mechanism.
Agriculture, Food, and Natural
Resource Standards Addressed
 PS.02.03.04.a. Identify and categorize the five groups of
naturally occurring plant hormones and synthetic plant
growth regulators.
Bell Work!
 Let’s talk for a moment about yourself. What are
some differences between your present body and
the one you had as a child?
 What kinds of chemicals were involved in those
changes?
 These chemicals are called hormones; for
example, estrogen, testosterone, adrenaline,
etc.
 What chemical changes so plants undergo?
 Is it possible to control changes in plant life
cycles? If yes, explain.
Bell Work Cont.!
Terms!
 Abscisic acid
 Apical dominance
 Auxins
 Phototropism
 Cytokinins
 Plant hormones
 Ethylene
 Thigmotropism
 Gibberellins
Interest Approach Continued
 Plants also experience
growth changes with the
help of hormones
 Sometimes these changes
are more dramatic than
what you experienced!
 Take a look at this plant
to the right which fell over
the night before. What
happened to the plant?
What Do Plant Growth Regulators
Do?
 Plant growth regulators are chemicals which
affect the plant in many complex ways
They can control such activities as cell
division and differentiation, root and shoot
growth, flowering and ripening
 Plant growth regulators which are made by the
plant are called plant hormones
They are moved around the plant in very
low concentrations
Some growth
regulators are
man-made, or
synthetic
They can be applied
to plants to obtain
a wide variety of
changes
These changes often
make the plant
better or more
saleable!
Which grapes would you rather
eat?
The ones on the left were treated
with gibberellic acid to make them
larger.
What Are the Functions of
Several Plant Hormones?
 Several hormones are made by tissues of the
plant
 Each hormone has many different jobs, thus
they are very much like human hormones
 There are five groups of plant regulators:
auxins, cytokinins, ethylene, gibberellins and
abscisic acid
Auxins
 This growth hormone is produced by the tip,
or apical meristem, of the stem
 It causes the stem cells to elongate and divide
 They also flow down from the tip of the stem,
preventing lateral buds from sprouting until
needed
 There are three major affects caused by
auxins on the plant
Effects of Auxin on Plants
This trunk
should be a
single stem;
however, the
terminal bud
was removed
forcing it to
branch.
 1. Auxin causes the
tip of the middle stem
to grow at a faster
rate
 This is known as apical
dominance
 Apical dominance is why
many conifers have a
pyramid shape
 It can be overcome by
cutting off the dominant
or terminal stem, losing
the source of auxin
 2. Auxins are
responsible for allowing
a plant stem to grow
toward the sun
 This is known as
phototropism
 Sunlight slowly breaks
down auxin; When the
side not exposed to the
sun grows faster, the
stem bends towards the
light
The left side of the plant was
exposed to the sun. The right
side had shade and grew
faster.
 3. Auxins allow a plant to
respond to the touch of a
person or other object
 This is known as
thigmotropism
 The repeated touch of an
object causes less auxin to
remain on that side of the
stem
 When the auxin side starts to
grow faster, the plant grows
towards the object and
ultimately wraps around it
Cytokinins
These are hormones
that are mostly
responsible for cell
division and
differentiation
They are produced in
In tissue culture, cell division or
root growth can be encouraged by the root tips in seeds
adjusting hormones in the agar. If
They tend to travel up
given higher levels of auxins in
the stem
the agar, roots are produced. If
given higher levels of cytokinins,
shoots multiply.
Ethylene
This is a gas that affects the plant
like a hormone
It is produced in ripening fruit and
dying plant material
It stimulates flowering in some
plants and causes other fruits and
flowers to ripen more quickly and
evenly
Effects of Ethylene
 Ethylene gas is why fruit
will ripen faster in a paper
bag, than on the counter
 The bag helps to
concentrate the gas in a
specific area
 Ethylene has a negative
effect on cut flowers &
foliage
 It causes them to age more
quickly, reducing their
useful life
In this experiment, two holly twigs
were placed under separate jars
for a week. At the same time, an
apple was placed in the second
jar. The ethylene given off by the
apple caused the holly to lose its
leaves and die.
Gibberellins
 These hormones cause the internode of
a stem to elongate and cell division to
occur
 They are produced in the stems, roots
and young leaves
 Gibberellins are commonly used on
commercially grown dessert grapes to
spread the fruits out and cause them
to be bigger
 Flowering of plants and the breaking
off of seed dormancy can also be
achieved by adding gibberellins
Effects of Gibberellins
The cabbage plants on
the left were grown on
their own
The plants on the right
were given gibberellic
acid once a week for
eight weeks
Notice the long stems and
flowers at the top of the
plants on the right
They are a result of this
hormone
Abscisic Acid
 This hormone
inhibits (prevents)
growth
 It is found in seeds
which are dormant
and in dying leaves
 It also appears to
help a plant
prepare its buds for
winter
The dormant bud on the left has a
high concentration of abscisic acid.
As spring nears, the acid level drops
and the bud begins to develop a new
shoot.
What are Several
Commercial Uses for Plant
Growth Regulators?
Synthetic growth regulators are very
useful for commercial plant crops
They can save money, time and can lead
to a better crop
There are at least three commercial uses
of regulators:
1. Growth regulators are routinely sprayed on
crops such as poinsettias, Easter lilies and
mums to reduce size and make a shorter,
bushier and more attractive plant
Products such as A-rest, B-nine, Cycocel and
Florel are commonly used
2. Growth regulators are commonly
used to help plants root more
completely
These are often sold as a powder
under the names Rootone and
Hormodin
3. Ethylene gas is used commercially
to ripen bananas once they get to
market and to induce flowering in
pineapple crops
Summary
 What is a plant growth regulator called
and what does it do?
 What is the difference between a plant
hormone and a synthetic hormone?
 Where in a plant is auxin produced?
 Explain how apical dominance works in
a plant.
 How is phototropism different from
thigmotropism?
Summary continued
 What do cytokinins control?
 How does ethylene effect plants?
 What does gibberellin cause in a plant?
 What does abscisic acid control?
 Name one way that hormones are used commercially to
control plants.
The End!