Chemical coordination in plants

Question 1

What is directional stimuli? examples?

Answer 1

Stimuli that act in a particular direction such as light and gravity.

Question 2

What is a tropism?

Answer 2

The growth response of a plant to a directional stimulus.

Question 3

Difference between a positive/negative tropism?

Answer 3

If the growth response is towards the direction of the stimulus, it is a positive tropism. If it is away from the direction of the stimulus it is a negative tropism.

Question 4

What is a response to gravity called?

Answer 4

geotropism

Question 5

what is a response to light called?

Answer 5

phototropism

Question 6

What is a response to water called?

Answer 6

hydrotropism

Question 7

Response of shoots/roots to phototropism?

Answer 7

Shoots grow towards light source (positive phototropism)

With the roots most species show no response but in some they grow away from the light (negative phototropism)

Question 8

Response of shoots/roots to geotropism?

Answer 8

Shoots grow away from the direction of gravity (negative geotropism)
Roots grow towards the direction of gravity (positive geotropism)

Question 9

Response of shoots/roots to hydrotropism?

Answer 9

Shoots have no response to water

In some species the roots grow towards water (positive hydrotropism)

Question 10

What are auxins?

Answer 10

Plant hormones which control the growth at the tips of shoots and roots. They move through the plant in solution.

Question 11

Where is auxin produced?

Answer 11

In the tip of the coleoptile and diffuses backwards to stimulate the cell elongation process which occurs in the cells just behind the tip. They are involved in phototropisms and geotropisms.

Question 12

What happens when one side of the shoot is exposed to light

Answer 12

The light causes more hormone to accumulate on the side thats in the shade than the side thats in the light. This makes the cells elongate faster on the shaded side so the shoot grows towards the light.

Question 13

What happens when you turn a shoot on its side?

Answer 13

Shoots are negatively geotropic because when a shoot is growing sideways, gravity produces an uneven distribution of auxin in the tip, with more auxin on the lower side.
The lower side grows faster, bending the shoot upwards.

Question 14

What happens when a root is growing sideways

Answer 14

A root growing sideways will also have more auxin on the lower side but in a root extra auxin inhibits growth. This means the cells on top elongate faster and the root bends downwards. h

Question 15

An experiment to test which part of the coleoptile is sensitive to light:

Answer 15

• place 5 oat seeds in 3 petri dishes and soak them overnight in water then leave them to germinate on wet cotton wool.
• when the coleoptiles have grown to a length of 10-15 mm cut 2 mm from the tip of the first coleoptile, cover the tips of the second in aluminium foil, leave the third group as a control.
• grow the seeds in a box with a hole cut in the side to allow a unidirectional light source to enter. Leave them for 24-48 hours until measurable growth has taken place.
• find the new mean length and calculate the percentage increase in length. Record the change in direction if any too.