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Plant Response to the Environment

Today�s topics: •  Finish water transport

•  Phloem pushes water and sugars from sources to sinks

•  Plants are dynamic (but slow) •  Time-lapse videos of plant

movements

•  Plant hormones –  Auxin –  Gibberellin –  Cytokinin –  Ethylene –  ABA –  & others

10 November 2021

Movement in the Phloem

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2

1

1

2

3 4

Vessel (xylem)

Sieve tube (phloem)

Uptake of water

Unloading of sugar

Sink

Sucrose

H2O

H2O

Bul

k flo

w b

y ne

gativ

e pr

essu

re

H2O Sucrose

Bul

k flo

w b

y po

sitiv

e pr

essu

re

Osmotic pressure drives

bulk flow of water and

sugars

Source (leaf)

Active transport of sucrose

How do plants respond to the environment?

http://i.imgur.com/B9M7Wck.jpg

Link to �Plants in Motion� Videos http://plantsinmotion.bio.indiana.edu/plantmotion/starthere.html

(from another textbook) Auxin

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Figure 39.5

Control

Light

Shaded side

Illuminated side

Boysen-Jensen

Light

Light

Darwin and Darwin

Gelatin (permeable)

Mica (impermeable)

Tip removed

Opaque cap

Trans- parent cap

Opaque shield over curvature

RESULTS

Control

RESULTS

Excised tip on agar cube

Growth-promoting chemical diffuses into agar cube

Control (agar cube lacking chemical)

Offset cubes

Auxin causes shoot cells to

expand

Auxin and Cell Expansion

Acid-growth hypothesis

Figure 39.7

Cell wall before auxin activates proton pumps

Cellulose microfibrils

Nucleus Vacuoles PLANT CELL WALL

Cellulose microfibril Cross-linking polysaccharide

Loosening of cell wall, enabling elongation

Auxin increases activity of proton pumps.

H+

H+

H+

Low pH activates expansins (red).

H+ H+

Elon

gatio

n

H+

H+ H+

H+

H+

H+

H+

H+

Polysaccharides are cleaved by cell wall-loosening enzymes (purple), loosening the microfibrils (brown).

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Auxin Transport

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Gravitropism

http://botit.botany.wisc.edu/Interface/web-lessons/Physiology/pos_gravitropism.html

Effects depends on dosage and tissue Herbicide 2,4D is an Auxin Rooting powder is also an Auxin

Often, the Ratio

of hormones is important

http://users.ugent.be/~pdebergh/pri/pri4et07.htm

Auxin mg/l

Cytokinin mg/l

(a) Apical bud intact (not shown in photo)

(b) Apical bud removed

(c) Auxin added to decapitated stem

Axillary buds

Lateral branches

�Stump� after removal of apical bud

Auxin promotes apical dominance

Auxins and Cytokinins interact Auxins and Cytokinins interact

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Ethylene is a ripening hormone

One bad apple CAN spoil the whole bunch . . .

Figure 39.14

(a) ein mutant (b) ctr mutant

ctr mutant ein mutant

Leaf abscision is controlled by the ratio of ethylene to auxin

0.5 mm

Protective layer

Stem Petiole

Abscission layer

ABA is a dormancy hormone

Seed Dormancy

Cold Tolerance

ABA Drought stress in roots causes stomata to

close