Transcript Horticulture Science

Horticulture Science
Lesson 13
Understanding Plant Growth
Regulators
Interest Approach
Start a discussion on the obvious effects of human hormones.
Ask what the students know about estrogen, testosterone,
adrenaline, etc. Ask students to think about differences
between little boys and men and between little girls and
women. Reinforce the concepts that human hormones are very
potent, have a lot of individual jobs, and are found in
extremely small amounts. Now ask about plant hormones. Note
the silence. Tell the students that plants do indeed have
hormones and that they may cause even more dramatic
changes in plants than humans experience.
Student Learning Objectives
•Describe the work of plant growth
regulators.
•Compare the functions of plant
hormones.
•Examine commercial uses for
plant growth regulators.
Terms
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abscisic acid
abscission layer
apical dominance
auxins
cytokinins
ethylene
etiolation
gibberellins
Terms
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gravitropism
growth retardants
phototropism
plant growth regulators
plant hormones
rooting compounds
synthetic growth regulators
thigmotropism
tropism
What do plant growth
regulators do?
• Plant growth regulators are chemicals
that affect the plant in many complex
ways.
• They can control such activities as cell
division and differentiation, root and shoot
growth, flowering, and ripening.
What do plant growth
regulators do?
• Naturally occurring chemicals that
regulate all growth and development of
plants are called plant hormones.
• Hormones are moved around the plant in
extremely low concentrations.
• Some plant growth regulators are manmade synthetic growth regulators.
• These synthetic growth regulators can be
applied to plants to get a myriad of
changes.
What are the functions of several
plant hormones?
• Several hormones are made by tissues of
the plant.
• Each plant hormone has many different
jobs.
– Auxins are growth hormones that are
produced by the tip or apical meristem of
the stem.
• Auxins cause stem cells to elongate and
divide.
• They also flow down from the tip of the
stem, preventing lateral buds from
sprouting.
What are the functions of several
plant hormones?
• Auxins set up a situation in which the
middle stem grows at a greater rate,
suppressing the side growth.
• This is called apical dominance.
• Apical dominance is why many conifers
are pyramidal in shape.
• Apical dominance can be overcome by
simply cutting off the dominant stem,
losing the source of the auxin.
What are the functions of several
plant hormones?
• Auxins are responsible for a response called
phototropism.
• Phototropism is a response by a plant to grow
toward a light source while tropism is a
growth response to an external stimulus.
• Sunlight slowly breaks down auxins.
• When the side with more auxins
grows faster, the stem starts to bend
toward the sun.
• The earth’s gravity elicits a plant
response known as gravitropism; in
this situation, a plant placed on its
side will still result in downward root
growth and upward stem growth.
What are the functions of several
plant hormones?
• Auxins are involved in a behavior called
thigmotropism.
• Thigmotropism is a plant’s response to a
hard object.
• The repeated touch of an
object causes less auxin to
remain on that side of the
stem.
• When the auxin side starts
to grow at a greater rate, the
plant grows toward the plant.
• This continues to happen until the plant is
actually wound around the object.
What are the functions of several
plant hormones?
• Cytokinins are hormones that are mostly
responsible for cell division and
differentiation.
• Cytokinins are produced in the root tips
and in seeds.
• They tend to travel up the stem.
What are the functions of several
plant hormones?
• Ethylene is a gas that affects the plant
like a hormone.
• Ethylene is produced by ripening fruit and
dying plant materials.
• Ethylene stimulates flowering in some
plants and causes other fruits and flowers
to ripen more quickly and evenly.
What are the functions of several
plant hormones?
• It is also associated with the abscission, or
dropping, of leaves. (The abscission layer
is a specialized layer of cells that extends
across the petiole near its attachment to the
stem.)
– Ethylene gas is why fruit will ripen faster in a
paper bag than out on a counter.
• The bag concentrates the ethylene gas.
– Ethylene has a negative effect on cut flowers.
• It causes them to age more quickly, reducing
their useful life.
What are the functions of several
plant hormones?
• Gibberellins are hormones that cause
internodal elongation and cell division.
• Gibberellins are produced in 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.
What are the functions of several
plant hormones?
• However, if light levels are low,
gibberellins cause the internodes to
lengthen.
• Stretching of internodes raises leaves to a
position where they can capture light.
• Plants grown in low light
or darkness develop a
severe case of stretching
known as etiolation.
What are the functions of several
plant hormones?
• Abscisic acid is a hormone that inhibits
growth.
• It is found in seeds that are dormant and
in dying leaves.
• It also appears to help a plant prepare its
buds for the winter.
What are several commercial uses
for plant growth regulators?
• Plant growth regulators are very useful for
commercial plant crops.
• They can save money and time while
leading to a better crop.
• These are some commercial uses for
growth regulators.
What are several commercial uses
for plant growth regulators?
• Growth regulators (growth retardants)
are routinely sprayed on crops such as
poinsettias, Easter lilies, and
chrysanthemums to make shorter,
bushier, and more attractive plants.
• Products such as A-rest, B-nine, Cycocel,
and Florel are commonly used.
What are several commercial uses
for plant growth regulators?
• Synthetic auxins (rooting compounds)
are used in the horticulture industry to
promote rooting of cuttings.
• Cuttings are treated with synthetic growth
regulators to increase the number of
cuttings that form roots, to speed rooting,
to increase the number and quality of
roots, and to increase the uniformity of
the roots.
What are several commercial uses
for plant growth regulators?
• The two synthetic root-promoting
materials most widely used are
naphthaleneacetic acid (NAA) and
indolebutyric acid (IBA).
• IBA is regarded as the best material for
general use.
• It can be used with a wide variety of
plants and is nontoxic over a wide range
of concentrations.
• These materials are available in a liquid
formulation or mixed with talc.
• Two commercially available products are
Rootone and Hormodin.
What are several commercial uses
for plant growth regulators?
• Ethylene gas is used commercially to ripen
bananas once they get to market and to
induce flowering in pineapple crops.
Review/Summary
•What do plant growth
regulators do?
•What are the functions of
several plant hormones?
•What are several commercial
uses for plant growth
regulators?