Plant hormones and Responses

What is a hormone?

A hormone is a chemical messenger produced in one partof an organism that stimulates or suppresses the activityof cells in another part.

Plant hormones are divided into groups based on theirfunctions and chemical properties.

1. Gibberellins:• Produce dramatic increases in size.• Involved in ending seed dormancy, starting germinationand promoting rapid growth of young seedlings.

Grape growers often spray their vines with GA (gibberellin solution) which makes the grapes grow larger.

2. Ethylene:

A plant hormone that causes ripening and is naturallyproduced by fruit.

3. Cytokinins:

Stimulate cytokinesis – final stage of cell division.Produced in growing roots and developing seedsand fruit.

Slow down the aging process in some plant organs.

When florists dip the leaves of flowers in cytokininsolution, they stay green much longer.

4. Auxins:

Involved in the lengthening of plant cells producedIn the apical meristem.

Auxins stimulate the growth of primary meristem,preventing the growth of new branches.

Example: Gardeners can cut off the tip from a branchThis encourages the growth of side branches.

6. Living things respond to stimuli in their environment

Stimulus (stimuli pl.)- a signal to which an organism responds.

Response- how an organism reacts to a stimulus.

The movement of a plant in response to anenvironmental stimulus is called a tropism

6.1 Plants can respond to light, touch, gravityand seasonal changes.

a. Response to light:The tendency of a plant to grow toward light is called phototropism.

Auxins stimulate the lengthening of cells.

Auxins build up on the shaded side of the stem,causing the cells to lengthen

The stem in turnbends in the direction of thelight source.

b. Response to touch:

This is called thigmotropism for example climbing plants and vines.

c. Response to gravity:

Called gravitropism.When a seed germinates, the root grows down into the soil = positive gravitropismThe stem grows away from the soil surface and away from gravity = negative gravitropism

d. Rapid response: Adaptations that help protect plants from predators.

Example – Mimosa leaves fold togetherwhen touched.

Venus fly trap uses this Response to catch a meal

e. Response to change in length of dayor night.

This is called photoperiodism.

Poinsettias

Changing colors of leaves during the Fall