AP Biology

February 15, 2012

Objective Describe the effects of various

hormones on plant growth Investigate how a plant respond to

different stressors

Homework Hormones Free Response Essay

AP Biology

The Discovery of Plant Hormones Any response resulting in curvature of organs

toward or away from a stimulus is called a tropism

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

Phototropism

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

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

How would they design their experiments to test these hypothesis?

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AP Biology

Figure 39.5

Control

Light

Shaded side

Illuminatedside

Boysen-Jensen

Light

Light

Darwin and Darwin

Gelatin(permeable)

Mica(impermeable)

Tipremoved

Opaquecap

Trans-parentcap

Opaqueshield overcurvature

RESULTS

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

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Plant hormones Auxin cytokinins gibberellins abscisic acid ethylene and more…

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Auxin (IAA) Effects

controls cell division & differentiation

phototropism growth towards light asymmetrical distribution of auxin cells on darker side elongate faster

than cells on brighter side apical dominance

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Cytokinins Cytokinins are so named

because they stimulate cytokinesis (cell division)

Control of Cell Division and Differentiation

Cytokinins are produced in actively growing tissues such as roots, embryos, and fruits

Cytokinins work together with auxin to control cell division and differentiation

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AP Biology

Gibberellins Family of hormones

over 100 different gibberellins identified

Effects stem elongation fruit growth seed germination

plump grapes in grocery stores have been treated with gibberellin hormones while on the vine

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Abscisic acid (ABA) Effects

Closure of stomata during H2O crisis slows growth seed dormancy

high concentrations of abscisic acid germination only after ABA is inactivated or

leeched out

survival value: seed will germinate only under optimal conditions

light, temperature, moisture

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Ethylene Hormone gas released by plant cells Effects

fruit ripening leaf drop

Senescence (cell death) Abscission (drop)

One bad apple spoils the

whole bunch…

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Fruit ripening Adaptation

hard, tart fruit protects developing seed from herbivores

ripe, sweet, soft fruit attracts animals to disperse seed

Mechanism triggers ripening process

breakdown of cell wall softening

conversion of starch to sugar sweetening

positive feedback system ethylene triggers ripening ripening stimulates more ethylene production

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Etylene Develop a hypothesis about what will

happen to the bananas in the bags

Be sure to explain why you think it will happen that way.

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Apoptosis in plantsWhat is the evolutionary advantage of loss of leaves in autumn?

Many events in plants involve apoptosis response to hormones

ethylene auxin

death of annual plant after flowering senescence

differentiation of xylem vessels loss of cytoplasm

shedding of autumn leaves

AP Biology

Learning Check Identify the function and location of the

following plant hormones Auxin Cytokinins Gibberellins Abscisic acid Ethylene

AP Biology

Plant Stress Stress response You will be divided into groups. Each group will

research and explain how one of the following 1) causes stress to the plant, 2) effects a plants growth, and 3) how the plant over comes the stress

Gravity Herbivory Light Salt Drought Mechanical stimuili Flooding Heat Cold Disease

AP Biology 2007-2008

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