Transcript Homeostatic Responses of Seedling Plants

Unit
Plant Science
Problem Area
Managing Plant Growth
Lesson
Homeostatic Responses of
Seedling Plants: The
Tropisms
Student Learning Objectives
1. Explain plant tropisms.
 2. Identify the different tropisms affecting
plant growth.
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Terms
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Aerotropism
Amyloplasts
Auxins
Chemotropism
Coleoptile
Electrotropism
Geomagnetotropism
Geotropism
Hydrotropism
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Negative tropism
Photropism
Positive tropism
Riboflavin
Statocytes
Thermotropism
Thigmotropism
Traumatropism
Tropism
Skototropism
What are plant tropisms?
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Plants demonstrate sensitivity to stimulation.
Environmental conditions and external stimuli
cause plant growth to be altered. Plant
responses to the external stimuli are known as
tropisms.
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A. A positive tropism is movement or growth
toward the stimulus
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B. A negative tropism is movement or growth
away from a stimulus
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C. Under ideal environmental conditions, the
external stimuli of light and gravity will equally
affect the shoot and root development allowing
the plant to grow vertically.
With equal stimuli, growth regulators are
produced in equal proportions allowing for the
cell elongation of the plant to occur at an equal
rate. A tropic response results in an imbalance of
growth regulators produced due to external
stimuli.
How do tropisms affect plant growth?
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Tropism—a growth response (toward or away
from) an external stimulus that determines the
direction of growth.
A. Phototropism—growth toward a light source
due to unequal cell elongation. Plants have at
least three different photo-receptor mechanisms
involving three separate groups of pigments.
These include phytochrome, which controls the
plants developmental sequence. Plant response
to lights depends on the exposure time and
intensity of light.
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Auxins, a plant hormone that promotes cell elongation
produced naturally by plants or synthetically, will
accumulate on the side of the stem in shade. Light
decreases the auxin sensitivity of cells on the lighted
side. Photoreceptors allow a growth response toward
light. Using monocotoleydons, research has shown the
coleoptile tip to contain auxin.
Riboflavin, a photoreceptor and yellow pigment, is also
found in the tip of the coleoptile, a protective sheath
surrounding the emerging shoot of grass seedlings.
Riboflavin is the catalyst which aides in the
transportation of the auxin away from the lighted side of
the stem. With the light source driving auxin to the
shaded side, the abundance of auxins will cause the
cells to elongate. Due to this elongation, the stem will
bend toward the light source.
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B. Geotropism (Gravitropism)—plant growth
response to gravity. Plants demonstrate positive
geotropism through their root development and
negative geotropism in shoot development.
Statocytes, specialized cells that function to control
the direction of growth by perceiving gravitational
forces and containing amyloplasts, are found in the
root cap and in the plant’s stem.
Amyloplasts, colorless subcellular particles
containing starch grains that influence the direction
of plant growth, due to gravity position themselves
on the lower side of the statocytes. When the root
cap is removed, response to gravitational forces is
negated.
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When centrifugal forces are applied, horizontal plant
growth is demonstrated. Stem nodes can also exhibit
differential growth to regain vertical position of the plant
stem when plants are knocked down by external forces.
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As soon as the root or shoot of a plant detects that it is
not in a vertical position, growth-regulating processes
restore it to a vertical course. In vertically positioned
shoots, auxin is equally distributed to the sub-apical
cells. In horizontal positions, auxin is found in greater
concentrations on the lower side of the shoot, thus
creating faster growth on the lower side and a return to
vertical growth.
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C. Thigmotropism—plant response to touch. Diverting
the direction of a plant, mechanical stimuli can cause
plant growth patterns to be altered. Coiling in vine plants
around an external object can also occur due to
thigmotropism. Example: Cucumber plant tendrils will
wrap around an object.
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D. Hydrotropism—plant response to water. Roots will
exhibit a positive tropic response in relation to higher
concentrations of water. While plant roots have been
shown to apparently seek out and grow toward water,
some plant physiologists today doubt that responses to
water are true tropisms. The response of roots to water
sources is not clearly understood.
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E. Thermotropism—plant response to
temperature. Weeds will demonstrate horizontal
growth in lower temperatures and vertical growth
with an increase in temperature.
F. Electrotropism is a response to electricity.
G. Chemotropism is a response to chemicals
H. Traumatropism a response to wounding
I. Aerotropism is a response to oxygen
J. Skototropism is a response to dark
K. Geomagnetotropism is a response to
magnetic fields
Review/Summary
What are plant tropisms?
 How do tropisms affect plant growth?
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