Unit A2-8
Horticultural Science
Horticulture CD
Problem Area 2
Plant Anatomy and Physiology
Lesson 8
Understanding Plant Growth
Regulators
Interest Approach
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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?
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These chemicals are called hormones; for
example, estrogen, testosterone,
adrenaline, etc.
Interest Approach Continued
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Plants also experience
growth changes with the
help of hormones
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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?
Courtesy of McGraw Hill Publishers
Vocabulary
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Abscisic acid
Apical dominance
Auxins
Cytokinins
Ethylene
Gibberellins
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Phototropism
Plant hormones
Thigmotropism
Student Objectives
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1. Describe the work of plant growth
regulators
2. Explain the functions of several plant
hormones
3. List several commercial uses for
plant growth regulators
What Do Plant Growth
Regulators Do?
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Plant growth regulators are chemicals
which affect the plant in many complex
ways
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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
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They are moved around the plant in very
low concentrations
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Some growth
regulators are manmade, or synthetic
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They can be applied to
plants to obtain a wide
variety of changes
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These changes often
make the plant better or
more saleable!
Courtesy of Wm. C. Brown Publishers
Which grapes would you rather
eat?
The ones on the right were treated
with gibberellic acid to make them
larger.
What Are the Functions of
Several Plant Hormones?
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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
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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.
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1. Auxin causes the tip
of the middle stem to
grow at a faster rate
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Courtesy of McGraw Hill Publishers
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
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2. Auxins are
responsible for allowing
a plant stem to grow
toward the sun
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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
Courtesy of McGraw Hill Publishers
The left side of the plant was
exposed to the sun. The right
side had shade and grew
faster.
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3. Auxins allow a plant to
respond to the touch of a
person or other object
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Courtesy of McGraw Hill Publishers
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
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In tissue culture, cell division or
root growth can be encouraged by 
adjusting hormones in the agar. If
given higher levels of auxins in
the agar, roots are produced. If
given higher levels of cytokinins,
shoots multiply.
These are hormones that
are mostly responsible for
cell division and
differentiation
They are produced in the
root tips in seeds
They tend to travel up the
stem
Courtesy of McGraw Hill Publishers
Ethylene
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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
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Ethylene gas is why fruit
will ripen faster in a paper
bag, than on the counter
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The bag helps to concentrate
the gas in a specific area
Ethylene has a negative
effect on cut flowers &
foliages
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It causes them to age more
quickly, reducing their useful
life
Courtesy of McGraw Hill Publishers
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
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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
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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
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Notice the long stems and
flowers at the top of the plants
on the right
They are a result of this
hormone
Courtesy of McGraw Hill Publishers
Abscisic Acid
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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
Courtesy of Wm. C. Brown Publishers
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?
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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:
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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
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Products such as A-rest, B-nine, Cycocel and Florel
are commonly used
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2. Growth regulators are commonly used
to help plants root more completely
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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
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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
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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.