1.3 | Stimuli and Response in Plants

Plant response is divided into two:

1. Tropism- plant-directed response to stimulation. The action is slow

   • Positive tropism - towards stimulation

   • Negative tropism - stay away from stimuli.

2. Nastic movement - reactions do not depend on the direction of the stimulus. faster action

TROPISM

There are 4 types of tropism: -

1. Phototropism - the response to light.

2. Hydropropism - reaction to water.

3. Geotropism - the response to gravity

4. Tigmotropism - a reaction to touch

phototropism (light)

• Positive phototropism

  • The shoots grow toward the light

  • Important - helps the shoots get light for photosynthesis

• Negative phototropism

  • The roots grow away from light

hydroTropism (water)

• Positive hydrotropism

  • The root grows toward the water

  • Important- helps the roots get water and mineral salts from the soil

  • Positive hydrotropism is stronger than positive geotropism

• Negative hydrotropism

  • The shoots grow away from the water

geotropism (gravity)

• Positive geotropism

  • The root grows toward the water

  • It is important to help the roots penetrate the soil to get water and the roots can hold the soil strong

  • Positive hydrotropism is stronger than positive geotropism

• Negative geotropism

  • The shoots grow in opposite directions to gravity.

tigmotropism (touch)

• Positive tigmotropism

  • An adhesive or stem is attached to any object or plant in contact with it.

  • Important- help plants grow vertically for light and support.

• Negative tigmotropism

  • The root will avoid any objects that prevent it from getting water.

nastic movement

• Plants respond to touch, light and heat.

• The direction of the reaction does not depend on the direction of the stimulus

• Examples of nasty movements:

• The leaves of the Venus fly when the insects settle on it.

• Mimosa pudica plants are closed when touched.

• Important - protect plants from injury & get food.