5.4.1 Control and coordination
Where is the plant hormone made?
- In the root tip
Which hormone and how does it control the responses of plants to light and gravity?
Auxin
- Unequal distributions of auxin cause unequal growth rates in plant roots & shoots
- Higher con of hormone causes faster growth in shoot but slower growth in roots
Why do plant produce hormones?
- To coordinate & control growth and responses to light (phototropism) & gravity (gravitropism or geotropism)
Describe how light affect the direction of growth of shoots. (4)
When shoot tip is exposed to light
- More auxin accumulates on shaded side
- This causes cells elongate faster on shaded side as it has a higher con of auxin
- Shoot grow towards light (phototropism)
- This is a +ve tropism (plant grows towards stimulus)
Explain how phototropism helps plant to survive. (3)
- Plant can get as much light as possible
- More photosynthesis takes place
- Allow more energy to be generated
Describe how gravity affect the direction of growth of roots & shoots. (4)
When shoot grow sideways
- More auxin on lower side due to gravity
- This causes cells elongate faster on lower side as it has a higher con of auxin
- Shoot bends upwards (-ve gravi/geotropism)
When root grow sideways
- More auxin on lower side due to gravity
- Extra auxin inhibits growth
- Cells on top elongate faster
- Root bends downwards (+ve gravi/geotropism)
Describe how moisture affect the direction of growth of roots & shoots. (4)
- Roots grow towards moisture (hydrotropism)
Explain how the hormone causes the appearance of the seedlings in Figure 7 to be different. You should refer to both seedlings in your answer. (3)
- Uneven distribution of hormone in A (more auxin at bottom)
- So top grows faster than bottom in A
- Even distribution of hormone in B
- Equal growth in B
What happen if shoot is removed?
- No auxin available
- Shoot stop growing
Practical
Control variables
- Type of plant
- Amount of water poured to Petri dish
Method
- Set up 3 Petri dishes containing cotton wool soaked in equal amounts of water.
- Put 10 mustard seeds in each dish.
- Measure the length from the base of the shoot to the tip. Write your measurements in your result table.
- Make accurate labelled drawings of the seed.
- Place one Petri dishes into position.
Inside the box with the lid closed in darkness
Inside the box which is cut one side of the box in partial light
On a windowsill in full sunlight
6. Put a lamp above the box which is cut to make sure some light is reaching inside the box. Don’t move the dish.
7. Water daily with equal amounts of water to each dish.
8. Measure length from base of shoot to tip, for at least 5 consecutive days.
9. Record your measurements in a result table, one for each light condition and calculate a mean.
5.4.2 Use of plant hormones (HT only)
Where are plant growth hormones used?
- In agriculture & horticulture
Why is auxin used?
- As weed killers
- As rooting powders
- To stimulate growth in tissue culture
How weed killers has an effect on biodiversity?
- Kills plants that some species rely on as food source
Why are ethene used in food industry?
- To reduce time of ripening of fruit during storage & transport
Why is gibberellin used?
- To promote seed germination / flowering
- Increase fruit size
- End seed dormancy