Download Growth Movements - tropisms

Growth Movements - tropisms
 Plants may not be able to get up
and go, but they do move.
 Plants can change the angle of their leaves so
that the leaves track the movement of the sun
in the sky.
 Plants can bend their stems toward the light,
too, and their roots grow downward in
response to gravity.
 Some plants even have quick movements, like
those of the Venus flytrap, which enable them
to respond to the presence of predators and
prey.
Growth Movements - tropisms
 One type of plant movement –
a tropism – is a directional movement that
results from growth in a particular direction.
 These growth movements occur in response
to elongation of selected cells.
 Plants exhibit tropic growth in response to
many signals, including light, gravity,
chemicals, and temperature.
Growth Movements - tropisms
 Phototropism:
 If light shines on a plant from one side, the
plant shoot will exhibit positive phototropism,
and bend toward the light.
 If you’ve ever grown plants near a window,
you’ve probably seen evidence of this
particular tropism.
 Molecules called photoreceptors in the tips of
the plant shoots absorb blue light.
Growth Movements - tropisms
 Signal transduction occurs, leading
to a greater production of auxin on the dark
side of the shoot.
 Auxin promotes cell elongation, so the cells
on the dark side elongate faster than the cells
on the light side and the shoot bends toward
the light.
Growth Movements – tropisms
Growth Movements - tropisms
 Geotropism (gravitropism):
 Plant roots grow toward the pull of gravity, so
they show positive geotropism.
 Plant shoots, on the other hand grow away
from gravity, showing negative geotropism.
 You can test this by laying a plant on its side.
After awhile, the shoots will turn and grow
upward, while the roots will turn and grow
downward.
Growth Movements - tropisms
Growth Movements - tropisms
 A great deal of evidence supports
explanation for geotropism outlined by the
starch-statolith hypothesis.
 According to this hypothesis, plants sense the
pull of gravity on amyloplasts located in the
cells of the root cap and close to the vascular
bundles in shoots.
 The amyloplasts get pulled by gravity so that
they collect on one side of the cell that
contains them.
Growth Movements - tropisms
 Auxin is produced in response to the
pressure of the amyloplasts, and transported
to the side of the root or shoot where the
amyloplasts have accumulated.
 In shoots, auxin stimulates cell elongation, so
the side of the shoot where the amyloplasts
have accumulated grows faster, and the
shoot curves up.
Growth Movements - tropisms
 In roots, high concentrations of
auxin actually inhibit cell elongation, so the
side of the root with the amyloplasts grows
more slowly than the other side.
 The root curves downward.
Growth Movements - tropisms
 Thigmotropism:
 This tropism is in response to the plant
touching a thing.
 One of the most familiar examples of
thigmotropism is when a climbing plant, such
as a pea or a vine, curls a tendril around a
support.
 Because these plants curve toward the solid
surface, they exhibit positive thigmotropism.
Growth Movements - tropisms
 Roots, on the other hand, will curve
away from solid objects like the rocks
they encounter in the soil, so they exhibit
negative thigmotropism.
 The mechanisms for thigmotropism are still
being investigated, but some things are
known.
 Some extremely rapid curvature responses
result from changes in turgor pressure.
Growth Movements - tropisms
 Prolonged curvature results from
differential growth, where one side of the
shoot or root grows more rapidly than the
other.
 The touch signal is received by epidermal
cells, and then signal transduction results in
differential growth of the shoot or root.
 Unlike phototropism and gravitropism,
thigmotropism does not seem to be
controlled by auxin.
Growth Movements - tropisms
Turgor Movements
 Turgor movements occur in
response to changes in turgor pressure in
certain cells.
 Whereas growth movements take some time
because the plant actually has to grow,
turgor movements are very rapid.
 For example, some tendrils exhibit rapid
coiling around a solid support, positive
thigmotropism, due to turgor movements,
followed by continued curved growth due to
growth movements.
Turgor Movements
 During turgor movements, cells
rapidly lose their turgor pressure and
collapse, causing the plant to fold around
those cells.
 Here’s what scientists think is happening to
cause these responses in tendrils:
 1. Touch signals sense the pressure of a solid
object.
 2. Signal transduction occurs, causing cells near
the touch stimulus to export potassium ions
outside the cells, thus making the environment
hypertonic to the plant cells.
Turgor Movements
 3. Water leaves the plant cells,
following the potassium ions, and the cells
collapse, causing the plant to fold.
 In addition to curving tendrils, turgor
movements also occur in Mimosa pudica,
which is sometimes called the sensitive plant.
 If you run your fingers along the leaves of this
plant, they fold up immediately.
Turgor Movements
 At the base of each leaflet is a
swollen structure called a pulvinus.
 When you touch the leaves of the sensitive
plant, the signal is relayed to the pulvinus.
 Cells on one side of the pulvinus lose their
turgor pressure, causing the leaves to fold
together.