key concept plant hormones guide plant growth and development

Plant Hormones and Responses

KEY CONCEPT Plant hormones guide plant growth and development.


Plant hormones regulate plant functions.

• Hormones are chemical messengers.– produced in one part of an organism – stimulates or suppresses activity in another part– Often have adaptive advantages


Five major groups of plant hormones

• Gibberellins• Ethylene• Cytokinins• Auxins• Abscisic acid


– ending seed dormancy – rapid growth of young

seedlings – rapid growth of some

flower stalks

• Gibberellins are plant hormones that produce dramatic increases in size.


– some fruits picked before they are ripe

– sprayed with ethylene to ripen when reach destination

– Promotes abscission- The detachment of leaves,

flowers, or fruits

• Ethylene causes the ripening of fruits.


– final stage in cell division – produced in growing roots, seeds, and fruits – involved in growth of side branches

• Cytokinins stimulate cytokinesis.


• Auxins lengthen plant cells in the growing tip. – stimulates growth of

primary stem – controls some forms of

tropism• A tropism is the movement of plant in response to an environmental stimulus.


• Abscisic acid– Promotes dormancy in plant buds, maintains

dormancy in seeds, and causes stomata to close


Tropisms

• A tropism is a response in which a plant grows either toward or away from an environmental stimulus


Plants can respond to light, touch, gravity, and seasonal changes.

• Phototropism is the tendency of a plant to grow toward light. – auxins build up on

shaded side of stem– cells on shaded

side lengthen – causes stem to

bend toward light


• Solar tracking, also called heliotropism, is the motion of leaves or flowers as they follow the sun’s movement across the sky.


– climbing plants and vines– plants that grow in direction of constant wind

• Thigmotropism is a plant’s response to touch-like stimuli.


– positive gravitropism is downward growth (roots)

– negative gravitropism is upward growth (shoots)

• Gravitropism is a plant’s response to Earth’s gravitational pull.


• Chemotropism– Plant growth that occurs in response t oa chemical is

called chemotropism– An example of chemotropism is the growth of a pollen

tube after a flower is pollinated.


Nastic Movements

• Thigmonastic movements– Occur in response to touch,

such as the closing of the leaf trap of a Venus’ flytrap around an insect.

• Nyctinastic movements– Occur in response to the daily

cycle of light and dark,such as the cyclical vertical and horizontal positioning of leaves in prayer plants.


– triggers some plants to flower– triggers fall colors/winter dormancy of deciduous trees

• Photoperiodism is a response to the changing lengths of day and night.

• Vernalization is the promotion of flowering by cold temperatures– Farmers often plant wheat seeds in the fall so that the seedlings can be

exposed to winter temperatures and will flower before summer droughts begin

key concept plant hormones guide plant growth and development

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