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

rapid growth of young seedlings rapid growth of some flower stalks Gibberellins are plant hormones that produce dramatic increases in size. ending seed dormancy rapid growth of young seedlings rapid growth of some flower stalks In the 1920s, scientists in Japan discovered that a substance produced by fungi of the genus Gibberella caused fungus-infected rice plants to grow abnormally tall. This type of hormone, named gibberellin, was later found to be produced in small quantities by plants themselves. Gibberellins have many effects on a plant, but they primarily stimulate elongation growth. Spraying some plants with gibberellins may cause them to grow larger than normal. Gibberellins are hormones that have important commercial applications. Many seedless grapes are sprayed with gibberellins to increase the size of the fruit. Beer makers use gibberellins to increase the alcohol content of beer by increasing the amount of starch converted to sugar in the brewing process. Gibberellins are also used to treat seeds, because they break seed dormancy and promote uniform germination.

Ethylene causes the ripening of fruits. some fruits picked before they are ripe sprayed with ethylene to ripen when reach destination Promotes abscission The detachment of leaves, flowers, or fruits

Cytokinins stimulate cytokinesis. final stage in cell division produced in growing roots, seeds, and fruits involved in growth of side branches Cytokinins promote cell division in plants. They are produced in the developing shoots, roots, fruits, and seeds of a plant. Cytokinins are very important in the culturing of plant tissues in the laboratory. A high ratio of auxins to cytokinins in a tissue culture medium stimulates root formation. A low ratio promotes shoot formation. Cytokinins are also used to promote lateral bud growth of flower crops.

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 Abscisic acid, or ABA, generally inhibits other hormones. It was originally thought to promote abscission, hence its name. Botanists now think that ethylene is the main abscission hormone. Abscisic acid helps to bring about dormancy in a plant’s buds and maintains dormancy in its seeds. Abscisic acid causes a plant’s stomata to close in response to drought. Water-stressed leaves produce large amounts of ABA. It is too costly to synthesize ABA for agricultural use.

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.

Thigmotropism is a plant’s response to touch-like stimuli. climbing plants and vines plants that grow in direction of constant wind

Gravitropism is a plant’s response to Earth’s gravitational pull. positive gravitropism is downward growth (roots) negative gravitropism is upward growth (shoots)

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.

Photoperiodism is a response to the changing lengths of day and night. triggers some plants to flower triggers fall colors/winter dormancy of deciduous trees 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