2(j) Coordination and Response in flowering plants

syllabus: 2.83-2.85

Stimulus

A change in the environment

Typical response to stimuli in plants

Change (the direction of) growth: Plants are able to grow in response to certain stimuli

→ Tropism

Tropism

A growth response of a plant to a directional stimuli.

2 Types of Tropism in Plants

• Phototropism

• Geotropism

Phototropism

= Growth response towards or away from the direction of the light source

(positive = towards light, negative = away from light)

• Shoots grow upwards, towards light, showing a positive phototropic response.

→This is to ensure the leaves can absorb light for photosynthesis.

• Roots grow downwards into the soil, away from light, showing a negative phototropic response

→ This is to anchor the plant and absorb water and minerals from the soil.

Geotropism

= Growth response towards or away from the direction of gravity

• Shoots grow upwards, away from gravity, showing a negative geotropic response.

→This is to ensure the leaves can absorb light for photosynthesis.

• Roots grow downwards into the soil, towards gravity, showing a positive geotropic response.

This is to anchor the pant and absorb minerals and water from the soil.

Auxin

• Plant growth regulator(plant hormone)

• coordinate & control directional growth responses such as phototropisms and geotropism

• causes cell elongation

• the more auxin present, the faster the cells elongate and the more the shoot grows.

• made in the tips of growing shoots.

• It then diffuses downwards into the region just below the tip, where cells can elongate and divide

Auxin in even sunlight

When light shines evenly on the shoot,

• Auxin is distributed evenly.

• Both sides grow at the same rate, and the growth of the shoot is straight.

Describe the growth response when the plant is exposed to light on one side. (include role of auxin)

1. Auxin(plant growth regulator/like human hormone) moves to the shaded side of the shoot

2. It causes the cells to elongate more on the shaded side.

3. The shoot will bend and grow towards the light source.

4. Therefore, the plant is able to absorb more sunlight for more photosynthesis.

5. The shoots show a positive phototropism response.

Describe the growth response in horizontal shoots.(include auxin)

1. Auxin moves to the lower side of the shoot.

2. It causes the cells to elongate and grow more/faster on the lower side.

3. This makes the shoot bend and grow away from the ground.

4. Therefore, the plant is able to absorb more sunlight for more photosynthesis.

5. The shoots show a positive phototropic response.