Plants Hormones

Plants Hormones

Definition

1- Plant hormones are chemical signals that affect the plant's ability to respond to its invironment.

2- Hormones are organic compounds that are effective at tiny or very low concentration.

3- They are usually synthesized in one part of plant and are transported to another location.

4- They interact with specific tissues to cause physiological responses ( Division, elongation, development and differentiation).

Experiments on how plants turn toward light led to the discovery of a plant hormone:

Darwin's Experiment

1- When plant tips were removed, plants did not grow toward light.

2- When plant tips were covered with an opaque cap, they did not grow toward light.

3- When plant tips were covered with a clear tip, they did grow toward light.

Jensen's Experiment

1- When a gelatin block that allowed chemical diffusion was placed below the shoot tip, plants grew toward light.

2- When a mica block that prevented chemical diffusion was placed below the shoot tip, plants did not grow toward light.

Through 2 experiments we should know

Phototropism

-- Phototropism is a phenomenon by which plants grow toward a light source.

-- Phototropism occurs when the cells on the dark side of a plant stem elongate faster than those on the light side.

-- Charles Darwin and his son Francis conducted experiments that showed that the shoot tips of plants controlled their ability to grow toward light.

-- Peter Boysen-Jensen later conducted experiments that showed that chemical signals produced in shoot tips were responsible for phototropism.

Experiment 3

Went isolated the chemical hormone responsible for phototropism

–   Plant tips were placed on an agar block to allow the chemical signal molecules to diffuse from the plant tip to the agar.

–   When agar blocks containing chemical signals were centered on the ends of “decapitated” plants, they grew straight.

–   When agar blocks were offset to one side of the “decapitated” plants, they bent away from the side with the agar block.

–   Went concluded that a chemical produced in the shoot tip was transferred down through the plant, and high concentration of that chemical increased cell elongation on the dark side of the plant.

v The chemical signal responsible for phototropism is a                          hormone that Went called auxin.