9.3 Growth in Plants

The diagram below illustrates a theory of auxin action known as the acid growth hypothesis. In the acid growth hypothesis plant cell wall rigidity is reduced, allowing plant cells to expand and therefore elongate.

Suggest how activated expansin proteins could reduce the rigidity of plant cell walls.

An investigation was carried out into the effect of a respiratory inhibitor on the elongation of wheat seedling shoots. The results of the study are shown in the graph below.

Identify, with a reason, the condition that functions as an experimental control.

[2]

[3]

Suggest an explanation for the effect of the respiratory inhibitor in the graph in part b).

It is thought that auxin activates proton transporters by increasing expression of a gene from a group of genes known as the SAUR genes.

Identify one other gene where expression is thought to be influenced by auxin. Note that you do not need to name the gene itself.

[1]

Outline the role of the proteins for which the gene identified in part i) codes. 

[2]

The graph below shows the effect of increasing auxin concentration on the growth of plant roots and shoots. Note that auxin is also known as IAA.

Describe the effect of increasing auxin concentration on the growth of plant shoots shown in the graph.

Use the graph in part a) to suggest a possible auxin concentration for the following parts of a plant. Note that your value readings should be given to the nearest accurate value on the scale provided.

The shaded side of a shoot.

[1]

The lower side of a root.

[1]

The upper side of a root.

[1]

As implied by the questions in part b) there is often an auxin concentration gradient across a plant stem or root.

Explain how light falling on one side of a plant shoot can generate an auxin gradient across that shoot.

In addition to regulating shoot and root elongation, auxin is involved with other aspects of plant growth. The image below shows the appearance of a plant that has been influenced by auxin.

Explain how auxin and any other relevant plant hormones can cause the plant appearance shown in the image.

Current knowledge of plant tropisms has been gained from multiple experiments carried out on plant seedlings. For example, early experiments showed that a plant growth influence (now known as auxin) was produced in the growing tip of seedlings. The image below illustrates three more experiments. Note that gelatin is a permeable material while mica is impermeable.

Identify three control variables that would be required to ensure valid results from all of the experiments shown. 

Predict, with a reason, the seedling growth that would be seen in each of the following experiments shown in part a):

Experiment 1

[1]

Experiment 2

[1]

Experiment 3A

[1]

State two possible conclusions about phototropism that can be drawn from the experiments shown in part a).

A fourth experiment was set up as shown below.

Predict the growth of the seedling in this experiment.

[1]

Identify one additional conclusion that can be drawn from this experiment that can not be drawn from experiments 1-3 in part a).

[1]

One mark is available for clarity of communication throughout this question.

The image below is a micrograph showing a cross-section through part of a plant. The areas of dark stain represent the presence of cell nuclei.

Annotate the micrograph below:

Compare and contrast animal and plant hormones.

Indole-3-acetic acid (IAA) is a common auxin that affects the growth of plants. An investigation was done to determine the effect of different concentrations of IAA on the growth of young roots. The results of this investigation are shown in the graph.

State the conclusion that can be drawn from these results.

Contrast the root growth observed at IAA concentrations of 0.03 μM and 0.3 μM.

Calculate the rate of root growth per hour when roots are not exposed to IAA from the graph in part a).

Explain the importance of gravitropism in roots.

The image below shows the tip of a root that was placed in a horizontal position, as well as the changes taking place inside the root.

State the function of statoliths in a root.

Identify the substance labelled X in the image in part a)

Explain the link between the presence of statoliths and the distribution of substance X shown in the image in part a).

A group of students investigated the effect of removing the root tip on the gravitropic response of a radicle.

The students had two groups of plants, each containing five seedlings, that were germinated at the same time and under the same conditions. Group 1 had the end 2 mm of the root tip removed, while Group 2 was left intact. All ten seedlings were laid flat and the position of each root tip was marked on a sheet of paper that was placed beneath the seedlings. After five hours the new position of the root tips were marked on the paper and the angle of bending was measured. An upward-bending root was recorded as a negative value and a downward-bending root was recorded as a positive value.

The results are displayed in the table.

Suggest an explanation for the results shown in the table above.

In an investigation to test the effect of auxin on shoot growth in seedlings, three different experiments were set up:

   Group A = Shoot with the tip removed.

   Group B = Shoot has been covered in a light proof container.

   Group C = Shoot was grown under a directional light source.

Contrast the growth that would be seen in group A and group B.

Explain how directional light in group C from the experiment in part a) will affect gene expression in the shoot.

Explain how being unable to respond to light stimuli would lead to reduced growth in a plant.

One mark is available for clarity of communication throughout this question.

Describe the process of plant micropropagation.

Outline some of the commercial and environmental benefits of micropropagation.

Explain how microarrays are used in genomics to increase our understanding of plant hormones and their effect on gene expression.