5.4 Plant Hormones

Describe the effect phototropism has on the growth of plant roots.

In the late 1800s, Charles Darwin performed an experiment where he grew maize seedlings in the dark, in a horizontal orientation.
Half of the seedlings had the root tip removed (A), the other half of the seedlings were left intact (B).
The results of his investigation are shown in Figure 2.

Figure 2

Explain what these results demonstrate for each experiment.

Figure 3 represents a plant shoot growing, it is illuminated from the side.

Figure 3

Draw an annotated diagram showing accumulation of auxins in response to light and how the shoot tip will grow.

Which method of movement most likely explains how auxins travel to other parts of the plant shoot or root?

Higher Tier Only

In UK supermarkets, bananas are usually in perfect condition even though they have been imported via ships from countries with tropical climates.

Describe how plant hormones can be used to ensure bananas arrive in perfect condition.

Higher Tier Only

Read the following two statements:

.

Evaluate the use of artificial auxins.

Higher Tier Only

A company called SME agrochemicals were trailing a novel plant hormone selective herbicide in order to be able to register the product for commercial use.
The agrochemical, named SME roundup ®, showed broad spectrum efficacy against dicot weeds in cereal crops.
During a trial, winter wheat was grown alongside the broad leafed 'Yellow hawkweed' Hieracium pratense over four testing plots of 1 m² in size and different concentrations of SME roundup were applied.
The only data captured was the number of weeds, two weeks post application. The results, are shown in Table 1.

Table 1

Calculate the total percentage efficacy for each application rate.

Use the following equation:

Higher Tier Only

During early lab-based trials, SME Agrochemicals suggested that the recommended application rate for the herbicide SME roundup ®  should be 1000 mg/m²
Explain why this was an incorrect assumption (Use the information from part a to support your answer).

Higher Tier Only

SME Roundup ® is marketed as a selective herbicide that can be used for cereal crops such as wheat and barley.

Explain how this product acts as a selective herbicide

Higher Tier Only

Suggest how the trials could be improved.

Higher Tier Only

In addition to auxins, gibberellins, and ethene, there are a number of other groups of plant hormones that control many aspects of plant growth.
When plant roots sense a water shortage in the soil, the plant hormone abscisic acid is released and binds to receptor proteins in guard cells. 
Suggest how abscisic acid may help plants in drought conditions. 

A student wanted to investigate the mechanism of phototropism in plants. They set up their investigation as shown in Figure 4.

Figure 4

In experiment A - the shoot tip was left untreated (as a control).

In experiment B - 2mm was removed from the growing end of the shoot tip.

In experiment C - The shoot tips were covered with caps of aluminium foil.

The shoot tips were initially measured and left for 48 hours next to a unidirectional light source.

Predict what differences in growth occurred in each experiment after 48 hours.

Explain why the plant shoots grow differently in experiments B and C.

A group of students investigated control and coordination of growth in plants.

They planted a bean seed in a box filled with moist soil. After the seed germinated, the box was turned onto its side and placed in a dark room.

Figure 1 illustrates their experiment.

Figure 1

Describe the expected appearance of the seed 3 days later.

Explain the role of auxins in coordinating the response of seeds in this experiment.

Suggest a suitable control for the investigation detailed in 2(a) and explain why it is necessary.

When a seed germinates, the developing root grows downwards towards gravity and the developing shoot grows away from gravity.

These effects are controlled plant hormones called auxins.

Name this type of plant response to gravity.

Suggest why this growth response in developing roots and shoots is important.

Higher Tier Only

Another important plant hormone is ethene.

Ethene is the most widely produced organic compound in the world. It is used in the manufacture of plastics and by the food industry.

Explain why ethene is widely used in the food industry.

A group of students wanted to investigate gravitropism in bean seedling roots.

Figure 1 shows part of the set-up of their experiment.

Figure 1

This is part of the method they planned to use:

• Measure the length of the root of 5 bean seedlings
• Pin the 5 bean seedlings to a cork mat as shown in Apparatus A
• Place apparatus A in a dark cupboard for 3 days
• After 3 days, measure the length of the root in each seedling and make a drawing of each seedling

The students' teacher told them that their method was incomplete and that they needed to set up a control experiment (Apparatus B) for comparison.

Describe how a suitable control could be set up so that the students could improve their investigation.

Why was it important that the seedlings in the experiment were placed into a dark room?

The students carried out their experiment using 10 bean seedlings split across two sets of apparatus; Apparatus A and Apparatus B (the control).

The results of the student’s investigation are shown in Table 1 below. The table is incomplete.

Complete the table to calculate the mean change in length for the bean seedlings in Apparatus A and Apparatus B.

Table 1

Figure 2 shows the appearance of the seedlings at the end of the 3 days.

Figure 2

A plant hormone, made in the tip of the root, diffuses from the tip into the tissues of the root.

Explain how the hormone causes the appearance of the seedlings in Figure 2. 

You should refer to both seedlings A and B in your answer.

Higher Tier Only

Plant hormones are used in horticulture to control plant growth.

Draw one line from each plant hormone to its correct use.

A group of students were given a number of young plant shoots and some powdered auxin.

Describe how the students could use these items to demonstrate how auxins cause shoots to grow towards a light stimulus.

Figure 1 shows a developing seedling.

Figure 1

Identify which letter represents the region which shows positive geotropism and explain why this is important for a developing seedling.

Higher Tier Only

Describe three commercial uses of gibberellins for the food industry.