Plant hormones and tropisms (RP8)

Plants respond to stimuli such as light and gravity.

(a) Name the plant hormone that causes plant shoots to grow towards light. [1]

(b) Choose the correct word from the box to complete each sentence.

The growth of a shoot towards light is called ____________.

The growth of a root downwards in response to gravity is called ____________. [2]

(c) Give one reason why it is useful for a plant's roots to grow downwards into the soil. [1]

The diagram shows a young plant shoot. Light is shining on the shoot from one side only.

(a) After a few hours, which side of the shoot, X or Y, will contain the higher concentration of auxin? [1]

(b) After two days the shoot has bent towards the light.

Explain how auxin causes the shoot to bend towards the light. [3]

(c) Explain why it is an advantage to the plant for its shoot to grow towards the light. [1]

A student placed a germinating seed on its side in a dark box, so that its young root and shoot were horizontal, as shown in the diagram.

The box was kept in the dark for two days.

(a) Why was the box kept in the dark? [1]

(b) Auxin collects on the lower side of both the root and the shoot. After two days the root had curved downwards.

Explain why the root curved downwards. [3]

(c) The shoot curved upwards, even though auxin had also collected on its lower side.

Explain why. [2]

A student investigated the effect of light on the growth of newly germinated cress seedlings.

• Dish A was placed in a box with a slit in one side, so that light shone on the seedlings from one side only.
• Dish B was placed in a box with no slit, so the seedlings were in the dark.

After 5 days the student measured the length of six seedlings from each dish. Some of the seedlings in dish A were curved, so the student laid a piece of thread along each shoot and then measured the thread with a ruler.

(a) Identify the anomalous result in the table. [1]

(b) Calculate the mean length of the seedlings in dish A. Do not include the anomalous result. [2]

(c) Explain why the student measured the curved seedlings with a thread instead of holding a ruler straight against them. [1]

(d) Give two variables the student should have kept the same for both dishes. [2]

A student wants to investigate how gravity affects the direction of growth of the roots of newly germinated bean seedlings.

The student has: bean seeds that have just germinated (each with a short, straight root), damp cotton wool, Petri dishes with lids, pins, a ruler and a dark cupboard.

Describe a method the student could use. Your method should give valid results. [6]

A scientist set up four young oat shoots, A, B, C and D. Light shone on all of them from the left.

• A was left untreated.
• B had its tip cut off.
• C had its tip covered with a cap that light cannot pass through.
• D had the part below the tip covered with a sleeve that light cannot pass through. The tip was not covered.

(a) Which two shoots would you expect to bend towards the light after two days? [1]

(b) What do the results for shoots C and D suggest about which part of the shoot detects light? Explain your answer. [2]

(c) Shoot B did not bend and grew very little.

Suggest an explanation for this result. [2]

Plant hormones are used in agriculture and horticulture.

(a) Complete the table. Choose from: auxin, ethene, gibberellin. Each hormone may be used once, more than once or not at all. [4]

(b) A gardener dips the cut end of a stem cutting into rooting powder before planting it.

Explain how this helps the cutting to grow into a new plant. [2]

Bananas are picked while they are still green and unripe, and then shipped to the UK. The journey takes about 14 days.

A company tested a filter that removes a gas from the air in shipping containers. Green bananas were stored at the same temperature in two containers, one with the filter and one without. The table shows the percentage of bananas that were ripe.

(a) Name the plant hormone that the filter removes from the air. [1]

(b) How many times greater was the percentage of ripe bananas without the filter than with the filter on day 8? [1]

(c) Use the data to explain why the company should fit the filter to its shipping containers. [2]

(d) When the bananas arrive in the UK they are put into a sealed room and the hormone is added to the air.

Suggest two reasons why. [2]

Some lawn weedkillers contain a synthetic auxin. When the weedkiller is sprayed on a lawn, broad-leaved weeds such as dandelions die, but the grass survives.

(a) Suggest an explanation for why the weedkiller kills the dandelions but not the grass. [3]

(b) The label on the weedkiller says:

Add 25 cm³ of concentrate to 5 dm³ of water. This treats 50 m² of lawn.

A gardener has a rectangular lawn that is 8 m long and 12 m wide.

Calculate the volume of concentrate the gardener needs to treat the whole lawn. [2]

(c) Suggest one reason why the gardener should not use more concentrate than the label says. [1]

Seeds of a wild plant are dormant: most of them do not germinate for many months, even in good conditions.

A scientist soaked 50 seeds in a gibberellin solution and 50 seeds in water only. All the seeds were then kept in the same warm, moist conditions. The graph shows the results.

(a) Give the percentage of gibberellin-treated seeds that had germinated by day 6. [1]

(b) Calculate the mean rate of germination of the gibberellin-treated seeds between day 2 and day 6.

Give your answer in percentage of seeds per day. [2]

(c) Calculate the number of seeds soaked in water only that had not germinated by day 14. [1]

(d) A farmer plans to sow a large field with these seeds. Use the results to suggest two advantages of treating the seeds with gibberellin first. [2]