The Tropisms

1
The Tropisms

• Meg Barker

2
Tropisms

• Tropism- growth response from an external
  stimulus that determines the direction of
  movement
• Can be or -

3
Phototropism

• Caused by elongation (from auxin) of cells on
  shaded side of the tip
• 3 possibilities- a)Light ?the auxin sensitivity
• of cells on lit side b) light destroys auxin
• c) light drives auxin to shaded side of
• growing tip.
• Tested by Winslow Briggs
• after work found by
• Frits Went
• Phototropins 1 and 2

4
Phototropism

• Auxin moves laterally toward shaded side at tip
  of plant.
• - Auxin then moves to elongation zone, where
  cells elongate.
• B/C of accelerated growth on
• shaded side, and slowed
• growth on lit side,
• curve begins toward light.

5
Gravitropism

• Response to gravity.
• Seedling will grow roots downward ()
• and shoot grow upward (-).
• Shoots ? auxin concentration on low
• side stimulates rapid cell expansion
• on that side upward curve.
• Roots ? auxin on lower side inhibits
• cell expansion, thus downward bend
• of root b/c cells expand rapidly
• on the upper side.

6
Gravitropism

• Ca plays impt role in response in shoots and
  roots.
• -shown to move toward upper surface of shoots
  before it curves up, and toward lower surface of
  roots before it curves downward.
• Perceive gravity by-
• -Coleoptiles and shoots, cells form layer around
  vascular tissue roots cells are in root cap
  (central column).
• Cytokinins also may have response of roots to
  gravity. Produced in rootcap uniformly, but when
  horizontal becomes distributed asymmetrically and
  accumulates in lower side of cap.

7
Amyloplasts

• When plants horizontal, amyloplasts are near
  transverse wall of vertically growing roots.
• They slide downward and come to previously
  vertically oriented walls.
• After several hours roots curve downwards and
  amyloplasts return to previous position.
• These play role of statoliths (gravity sensors).

8
Thigmotropism

• Response to contact with a solid object.
• Navigate, wrap around objects (tendrils)
• Can be rapid response, maybe lt1 hour
• Cells touching support shorten and cells
• on other side elongate.
• Tendrils have memory tactile stimulation
• -Peas can coil w/in 2 hours or rubbing if
• In light, but if in dark for 3 days wont coil.