Plant hormones and Responses What is a hormone? A hormone is a chemical messenger produced in one part of an organism that stimulates or suppresses the activity of cells in another part. Plant hormones are divided into groups based on their functions and chemical properties.

1.Gibberellins: Produce dramatic increases in size. Involved in ending seed dormancy, starting germination and 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 naturally produced by fruit.

3. Cytokinins: Stimulate cytokinesis – final stage of cell division. Produced in growing roots and developing seeds and fruit. Slow down the aging process in some plant organs. When florists dip the leaves of flowers in cytokinin solution, they stay green much longer.

4. Auxins: Involved in the lengthening of plant cells produced In 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 branch This 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 an environmental stimulus is called a tropism

6.1 Plants can respond to light, touch, gravity and 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 turn bends in the direction of the light 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 gravitropism The 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 together when touched. Venus fly trap uses this Response to catch a meal

e. Response to change in length of day or night. This is called photoperiodism. Poinsettias Changing colors of leaves during the Fall