Plant Responses to Internal and External Signals

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Tropisms are often caused by hormones.

Slide 15

In the late 1800s, Charles Darwin and his son Francis conducted experiments on phototropism, a plant’s response to light.

They observed that a grass seedling could bend toward light only if the tip of the coleoptile was present.

They postulated that a signal was transmitted from the tip to the elongating region.

Slide 16

Signaling and Phototropism

RESULTS

Control

Light

Light

Phototropic response only when tip is illuminated

Illuminated

side of

coleoptile

Shaded

side of

coleoptile

Tip

removed

Light

Tip covered

by opaque

cap

Tip

covered

by trans-

parent

cap

Site of

curvature

covered by

opaque

shield

Phototropic response when tip separated by permeable barrier,

but not with impermeable barrier

Tip separated

by gelatin

(permeable)

Tip separated

by mica

(impermeable)

Slide 17

What part of a grass coleoptile senses light, and how is the signal transmitted?

RESULTS

Control

Light

Illuminated

side of

coleoptile

Shaded

side of

coleoptile

Slide 18

RESULTS

Light

Tip

removed

Phototropic response only when tip is illuminated

Tip covered

by opaque

cap

Tip

covered

by trans-

parent

cap

Site of

curvature

covered by

opaque

shield

Slide 19

In 1913, Peter Boysen-Jensen demonstrated that the signal was a mobile chemical substance.

In 1926, Frits Went extracted the chemical messenger for phototropism, auxin, by modifying earlier experiments.

Slide 20

RESULTS

Light

Boysen-Jensen: phototropic response when tip is separated

by permeable barrier, but not with impermeable barrier

Tip separated

by gelatin

(permeable)

Tip separated

by mica

(impermeable)

Slide 21

Question: Does asymmetric distribution of a growth-promoting chemical cause a coleoptile to grow toward the light?

Excised tip placed

on agar cube

RESULTS

Growth-promoting

chemical diffuses

into agar cube

Agar cube

with chemical

stimulates growth

Offset cubes

cause curvature

Control

(agar cube

lacking

chemical)

has no

effect

Control

Slide 22