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Plant Hormones 

Controls of growth, development

and movement

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Processes in growth 

Cell division.

Cell enlargment.

Cell differentiation.

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Primary growth

Apical meristem

Leaf primordia

Forming axillary bud

Ground meristem

protoderm 

procambium 

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Secondary growth

xylem 

Phloem withbands of fibers 

Vascular cambium 

Ray parenchyma 

cork  

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Seed Germination 

Scarification

mechanical

chemical

heat

Mobilization of reserves 

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Mobilization of reserves 

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Movement in Plants 

Turgor movements (changes in turgor 

 pressure in selected cells)

Growth movements (elongation of selected

cells in response to stimulus)

 phototropism

geotropism

thigmotropism 

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Turgor movement Mimosa pudicaL. (sensitive plant) 

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Pulvinus of Mimosa pudica 

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Growth movement

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Phototropism

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Geotropism 

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Statoliths

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Thigmotropism

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Hormones 

Signal molecules produced at specific

locations.

Occur in low concentrations.

Cause altered processes in target cells at

other locations. 

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Plant growth substances

Artificially synthesized substances

which produce hormone like

responses

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General plant hormones

Auxins

Gibberellins

Cytokinins

Abscisic acid

Ethylene

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Auxin 

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Auxin associated with phototropism - early experiments

demonstrate tip as receptor.

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Demonstration of transported chemical

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Auxin 

Discovered as substance associated with

 phototropic response.

Occurs in very low concentrations.

Isolated from human urine, (40mg 33 gals-1)

In coleoptiles (1g 20,000 tons-1)

Differential response depending on dose. 

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Loosening of cell wall 

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Polar transport of Auxin 

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Additional responses to auxin 

abscission - loss of leaves

flower initiation

sex determination

fruit development

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Control of abscission by auxin 

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Gibberellin

Di d i i i i h F li h

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Discovered in association with Foolish

disease of rice (Gibberella fujikuroi)

infected uninfected 

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Effects of Gibberellins 

General cell elongation.

Breaking of dormancy.

Promotion of flowering.

Transport is non-polar, bidirectional

 producing general responses.

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Cytokinins 

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Function of cytokinins 

Promotes cell division.

Morphogenesis.

Lateral bud development.

Delay of senescence.

Stomatal opening.

Rapid transport in xylem stream.

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Abscisic acid 

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Functions of abscisic acid 

General growth inhibitor.

Causes stomatal closure.

Readily translocated.

Produced in response to stress. 

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Ethylene 

H H

\ /C = C

/ \

H H

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Functions of ethylene 

Gaseous in form.

Rapid diffusion.

Affects adjacent individuals.

Fruit ripening.

Senescence and abscission.

Interference with auxin transport. Initiation of stem elongation and bud

development.

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Action and Interaction of 

Hormones

Constitutive enzymes - always present but

level of hormone can affect level.

Adaptive enzymes - formed or activated as

a result of the presence of a hormone.

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Signal-transduction pathways

in plants

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Interaction of cytokinin and auxin

in tobacco callus tissue