Plant hormones

A student completes a series of experiments to investigate the role of auxin in apical dominance.

The experimental procedures are shown in the table.

What are the expected results of the investigation?

A Lateral buds will grow in all the experiments.

B Lateral buds will grow in experiments 2 and 3.

C Lateral buds will grow in experiments 2, 3 and 4.

D Lateral buds will not grow in any of the experiments.

Your answer

Describe the role of plant hormones in the control of seed germination.

* Scientists investigate the effect of gibberellin on stem length in young soybean plants.

This is the method they use:

• Select young plants with 4 pairs of leaves.

• Apply gibberellin solution to the leaves of 8 young plants.

• Select 8 more young plants as a control group.

• Allow young plants to grow until they develop 7 pairs of leaves.

• Measure the distance between the fifth and sixth pairs of leaves in each young plant. This is called the internode distance.

• Calculate the mean and standard deviation of the internode distance for each group.

Their results are shown in the table.

Here are the critical values for t.

The scientists used Student’s t-test to compare the mean internode distances in the treated and control groups.

Their calculated value for t was 4.05.

The scientists concluded that gibberellin caused a significant increase in stem length in young soybean plants.

Suggest a null hypothesis for the investigation and evaluate this conclusion.

Extra answer space if required.

Soybeans are important food crops.

Farmers often grow soybeans with the plants close together but this causes the stems to grow too long so that the plants fall over, which reduces the crop yield.

A scientist suggested that spraying soybean plants with a chemical that inhibits gibberellin production could increase yields.

Explain the scientist’s suggestion.

Some students investigate the effect of IAA (auxin) concentration on the growth of roots and shoots in mustard seedlings.

This is the method that they use with 8 different concentrations of IAA:

• Place a filter paper on a bed of cotton wool in the Petri dish and soak it with IAA solution.

• Place 10 seeds in a horizontal line on the damp filter paper.

• Cover with a lid that has a grid marked on it, as shown in the diagram.

• Repeat this process with each different concentration of IAA.

• Add distilled water to one Petri dish in place of IAA solution.

• Place the Petri dishes vertically in a warm dark place for 2 to 3 days until the seeds have germinated and the roots and shoots have started to grow.

• Measure the length of the shoots and roots by counting the number of grid squares the shoot or root covers.

Explain why the Petri dishes were placed vertically and in the dark.

Explain the purpose of the Petri dish containing distilled water instead of IAA solution.

One of the students recorded the result shown in Table 17.1.

Table 17.1

Calculate the percentage change in mean root length in test solution compared with water.

Give your answer to 2 significant figures.

One student’s results are shown in Table 17.2.

The results have been rounded to the nearest 5%.

Positive values represent increased growth and negative values represent decreased growth.

The concentration of IAA was measured in parts per million (ppm).

Table 17.2

Plot a graph of the data in Table 17.2.

The students were given an IAA solution of concentration 100 ppm.

Outline a procedure the students could use to prepare the 8 test solutions required for the experiment.

Describe two precautions the students should take to make sure the concentrations of the solutions they prepare are accurate.

Suggest why the range of IAA concentrations could be described as logarithmic.

A student wrote a method for taking a cutting to clone a plant:

• Select a stem with many flowers and leaves.

• Make a slanting cut in the stem, below some leaves.

• Dip the cut stem in rooting powder.

• Plant the cutting in watered compost.

Describe and explain how the student’s procedure could be improved.

The student investigated the effect of auxin concentration on the growth of shoots.

The student applied different concentrations of auxin to the apical shoot and the lateral shoots.

The student measured the percentage of growth stimulation or inhibition compared to normal.

Normal growth was represented by 0%.

The student’s results are shown in the graph.

Use the graph to estimate the auxin concentration at which inhibition of lateral shoots is 100%.

Auxin concentration =........ mol dm^–3

The student identified a possible anomaly in their results: the data point for the apical shoot receiving 10^–6 mol dm^–3 of auxin.

State what the student could do to determine whether this data point was an anomaly.

Using the graph, describe the conclusions that can be drawn about the role of different auxin concentrations in the control of apical dominance.

Another student plans to investigate the effect of gibberellin concentration on the rate of stem elongation in the pea plant, Pisum sativum.

Suggest appropriate units for the dependent variable in this investigation.

* The student has access to standard laboratory equipment and planting materials.

Outline a method that the student could use to investigate the effect of gibberellin concentration on stem elongation in P. sativum.

In your answer, you should include details of an appropriate statistical test for this investigation.

Additional answer space if required.

A friend of the student had an apple tree in their garden. The friend asked the student if there was a way to ripen the fruit on the apple tree more quickly so that it would be ready to eat within a few days.

The student gave this advice:

‘You should spray the tree with ethene.’

Evaluate whether acting on the student’s advice would produce fruit that was ready to eat within a few days.

Students investigated the effect of light on the growth of garden cress seedlings.

• A total of 120 seedlings were divided into 2 groups of 60.

• Group A was grown in darkness for 2 days.

• Group B was grown for 1 day in darkness and then for 1 day in white light using the set‑up shown in Fig. 3.1.

The results of the students’ experiment are shown in Tables 3.1 and 3.2.

* Describe and explain the results shown in Tables 3.1 and 3.2.

Additional answer space if required.

The students wanted to test whether there was a significant difference between the stem lengths of the seedlings in Group A and the seedlings in Group B.

State the name of the most appropriate statistical test for the students to use.

Justify your choice of statistical test given in part (ii).

Table 3.2 records the direction of growth as:

• away from light

• neither away from nor towards light

• towards light.

The students used the chi‑squared test to determine whether the direction of root growth was significantly different from their expectations.

Their null hypothesis was:

There is no difference between the expected direction of root growth and the observed direction of root growth.

The calculated chi‑squared value was 8.10.

The students compared their chi‑squared value of 8.10 to the values in Table 3.3.

What can the students conclude about their results based on a chi‑squared value of 8.10?

The solution in which the seedlings were grown contained various dissolved ions.

The table below lists two of the functions of these ions in the seedlings.

Complete the table below with the formula of each ion.

The students also investigated the effect of plant hormone concentration on root growth.

State the name of a plant hormone that would be expected to affect root growth.

In the investigation, the students controlled light, temperature and mineral concentration.

State one other factor that the students should have controlled in this investigation.

The following passage outlines the process of phototropism in plants:

Auxin is synthesised in cells at the........ of the shoot. Auxin causes

the cells to........ on one side, so the stem bends.

Scientists originally thought auxin was........ by light but this was

disproved by the fact that plants growing in the dark and plants growing in

unilateral light had........ auxin levels.

Which option, A to D, is the correct sequence of missing words?

A meristem, shorten, destroyed, different

B tip, elongate, destroyed, similar

C meristem, shorten, synthesised, raised

D tip, elongate, synthesised, similar

Your answer

A scientist tested a plant suffering from water stress. The plant was found to have high levels of abscisic acid (ABA) in its tissues.

Which of the following statements, A to D, explains this observation?

A ABA causes fruit ripening

B ABA prevents leaf drop

C ABA causes phototropism

D ABA stimulates stomatal closing

Your answer

Many trees drop their leaves in the autumn.

Which of the following plant hormones is/are thought to be involved in the control of leaf drop?

1 auxin 2 ethene 3 gibberellin

A 1, 2 and 3

B only 1 and 2

C only 2 and 3

D only 1

Your answer

Gibberellin causes stem elongation in plants.

Fig. 17.1 shows the effect of gibberellin on cabbage plants.

Gibberellin causes an increase in the distance between the leaves on the stem, which is known as the internodal length.

Explain why gibberellin is classed as a plant hormone.

A scientist carried out an investigation into the effect of gibberellin on cabbage plants.

The scientist applied a range of volumes of gibberellin and measured the rate of increase of internodal length over 30 days.

Table 17 shows the scientist’s results.

Plot the results from Table 17 as a suitable graph.

Gibberellin causes an increase in internodal length.

State one other role of gibberellin in plants.

A student carried out chemical tests on cabbage leaves to investigate which molecules were present.

The student’s method was as follows:

Name a type of food molecule that the student will not be able to detect using these chemical tests.

The table below is a summary of some of the student’s findings.

Complete the table by writing in the missing observations and conclusions.

The student then used a colorimeter to measure the absorbance of the contents of Tube 2.

Explain how the use of a colorimeter could improve the student’s conclusion.

Plants are capable of synthesising a variety of molecules from the products of the light-independent stage of photosynthesis.

Fig 22.1 summarises these processes.

Identify the molecules represented by the letters A, B, C and D in Fig. 22.1

Identify the following variables from the scientist’s method:

independent variable

dependent variable

one control variable

Identify one variable that was not controlled in the scientist’s method.

Fig. 22.2 is a graph of the scientist’s results.

Describe and explain what these results show about photosynthesis in P. pusillus.

The light-independent stage of photosynthesis used to be referred to as the ‘dark reaction’.

Explain why this is both an accurate and an inaccurate way to describe the light-independent stage.

Name the enzyme responsible for fixing CO₂ in the light-independent stage.

The scientist then investigated the effect of auxin on P. pusillus stems.

The growing tips of stems were removed and the stems were placed in solutions containing different concentrations of auxin.

The scientist analysed the results and determined the following relationship:

The higher the concentration of auxin in the solution, the fewer side shoots grew on the P. pusillus stems.

Explain why this relationship occurs in P. pusillus stems.

Give two examples of the commercial uses of auxin.