OxfordAQA International AS and A-level Biology: Plant growth substances, tropisms, ethene and ABA (9610)

This guide covers OxfordAQA International AS and A-level Biology (9610), Version 5.1, section 3.4.5 (control systems in plants: 3.4.5.1 to 3.4.5.3) and Required practical 10. The specification places this content in Unit 4 (Control), an International A-level only unit. Revise it with the revision notes, then test yourself with the practice questions. Use the course hub to see neighbouring topics, tick items off on the printable checklist, and check your starting point with a free diagnostic.

Scope at a glance

Section	Skills needed
3.4.5.1 Principles	Say what plant growth substances are, where they act and what they regulate
3.4.5.2 Auxins and tropisms	Define tropisms; explain light and gravity responses of roots and shoots using how IAA (indoleacetic acid) at different concentrations changes cell elongation
3.4.5.3 Ethene	Explain the role of ethene in ripening; explain how its control lets climacteric fruits such as bananas be picked green and ripened after shipping
3.4.5.3 Abscisic acid (ABA)	Explain how ABA closes stomata in water-stressed plants, including the movement of potassium and chloride ions
Required practical 10	Test how one chosen factor alters which way a root or shoot grows

3.4.5.1 Plant growth substances as signal molecules

Plant growth substances are chemical signals made inside plants, and they work at tiny concentrations. Some regulate targeted cells near their site of production. Others move to other tissues and regulate processes there. The specification names three kinds of process they control:

• growth that follows the direction of a stimulus
• fruit development (ripening)
• closure of stomata in response to drought.

Plants have no nervous system, so these chemical signals do the coordinating. The three you need are IAA (an auxin), ethene and ABA. A signal only affects a cell that can detect it, so a given substance can act on one tissue and do nothing to another.

Because concentrations are so low, small changes matter. The key idea in this unit is that one substance can produce different effects at different concentrations, or in different tissues.

3.4.5.2 Auxins and tropisms

A tropism is a growth response to a directional stimulus. It helps the roots and shoots of flowering plants stay where conditions suit them. Growth towards the stimulus is positive; growth away is negative.

Response	Stimulus	Shoot	Root
Phototropism	Light arriving from a single direction	Positive (grows towards light)	Not covered in this unit
Gravitropism	Gravity	Negative (grows upwards)	Positive (grows downwards)

The advantage is easy to state. A shoot that grows towards light can carry its leaves to where light is available for photosynthesis. A root that grows downwards reaches soil water and mineral ions and anchors the plant.

How IAA produces bending

IAA affects cell elongation. Bending happens when the two sides of a root or shoot receive different concentrations of IAA and so elongate by different amounts. The side that elongates more becomes the outside of the curve.

The effect depends on the organ:

• In shoots, a higher concentration of IAA stimulates cell elongation.
• In roots, a higher concentration of IAA inhibits cell elongation.

Light and shoots

When light shines from one side, IAA is distributed unequally, so the shaded flank holds more IAA than the illuminated flank. Cells on the shaded side elongate more, the shoot curves towards the light, and the response is positive phototropism.

Gravity and shoots

When a shoot lies horizontally, more IAA collects on the lower side. Lower-side cells lengthen more than upper-side cells, so the shoot tip turns up. This is negative gravitropism.

Gravity and roots

When a root lies horizontally, more IAA again collects on the lower side. Here the higher concentration inhibits elongation. Cells on the upper side elongate more than those on the lower side, so the root curves downwards. This is positive gravitropism.

The same hormone, with more of it on the lower side, bends a shoot up and a root down. Always state which side has more IAA, then what that concentration does in that organ, then which side elongates more.

Worked example 1: unequal IAA in a shoot

A shoot receives light from the right. The IAA concentration on the shaded side is 0.62 arbitrary units and on the lit side is 0.38 units. Calculate how much greater the shaded-side concentration is, as a percentage of the lit-side value, and explain the result.

difference = 0.62 - 0.38 = 0.24

percentage = 0.24 / 0.38 × 100 = 63.2 %

The shaded side has 63 % more IAA. A higher IAA concentration stimulates elongation in a shoot, so the shaded cells elongate more than the lit cells. The shoot bends towards the light.

Worked example 2: IAA and root growth

Root segments were grown for 48 hours in agar containing different IAA concentrations. Mean growth was measured.

IAA (µmol dm⁻³)	0	0.0005	0.005	0.05	0.5
Mean root growth (mm)	14.0	15.4	12.6	8.4	4.2

Percentage change from the control (no IAA) is (growth - 14.0) / 14.0 × 100:

IAA (µmol dm⁻³)	0.0005	0.005	0.05	0.5
Change (%)	+10.0	-10.0	-40.0	-70.0

Description: the lowest concentration slightly stimulated root growth, but from 0.005 upwards growth was inhibited, and the inhibition increased with concentration. This supports the idea that the higher IAA concentration on the lower side of a horizontal root inhibits elongation there.

Worked example 3: reading a curvature experiment

Six seedlings were lit from one side for 24 hours. Curvature towards the light, in degrees, was 34, 41, 38, 45, 36 and 40.

total = 234 and mean = 234 / 6 = 39.0 degrees

The range is 34° to 45°. Quote both: the mean shows the typical bend and the range shows the spread.

Required practical 10

The specification requires a practical on how a chosen factor alters which way a root or shoot grows. The A2 papers test your knowledge of the procedure, evaluation of the techniques and interpretation of specimen results. A suitable design:

1. Choose the variable. Light direction (shoots) and gravity (roots or shoots) are the obvious ones.
2. Grow seedlings of the same species, age and size, under the same temperature and moisture.
3. Expose the test group to the stimulus from one direction only. Keep a control group in even light, or turned slowly on a clinostat if you are testing gravity.
4. Measure the angle of curvature, or the direction of growth, using a protractor on the seedling or on a photograph, and repeat the method identically each time.
5. Use many seedlings, then calculate a mean and the range for each group.

Controlled variables include species, age, temperature, water supply and light intensity (when light is not the variable). A limitation to expect is that the seedlings grow at slightly different rates, so repeats and a large sample matter.

3.4.5.3 Ethene and fruit ripening

Ethene is a plant growth substance that plays a role in fruit ripening. Ripening fruit produce ethene, and ethene stimulates the changes of ripening. Because it is a gas, it can pass from one fruit to its neighbours, so a ripe fruit in a bowl can hasten the ripening of fruit beside it.

Because ethene can be managed, climacteric fruits such as bananas are harvested while green and ripened artificially once shipped. Green fruit is firmer and less easily damaged in transport. Growers and shippers can then limit ethene while the fruit travels, and expose the fruit to ethene at the destination so that it ripens together, ready for sale. Typical methods of limiting ripening during transport include cool storage and removing ethene from the store atmosphere. These are examples of how control is achieved; the specification statement is simply that control of ethene allows green picking and artificial ripening.

Worked example 4: effect of ethene on ripening time

Green bananas in air took 8 days to reach full ripeness. A matched batch exposed to ethene took 3 days.

reduction = 8 - 3 = 5 days

percentage reduction = 5 / 8 × 100 = 62.5 %

Ethene reduced the time to ripen by 62.5 %. State the comparison and the evidence; do not just say “it was faster”.

3.4.5.3 Abscisic acid and stomatal closure

When a plant is water-stressed because its roots lack enough water, abscisic acid (ABA) closes the stomata. ABA stimulates the transport of potassium ions and chloride ions out of the guard cells. This raises the water potential in the guard cells. Water then leaves the guard cells and they shrink, so the stomata close.

Put it in order:

1. The plant is water-stressed because its roots lack enough water.
2. ABA acts on the guard cells.
3. K⁺ and Cl⁻ ions move out of the guard cells.
4. Guard-cell water potential increases, and water moves out.
5. The guard cells shrink and the stomata close.

Closed stomata reduce water loss by transpiration. They also reduce carbon dioxide uptake, which is the cost of the response. For transpiration and stomatal control in the whole plant, see mass transport in plants.

Worked example 5: stomatal aperture

Before ABA treatment the mean stomatal aperture was 8.4 µm. Thirty minutes after treatment it was 2.1 µm.

decrease = 8.4 - 2.1 = 6.3 um

percentage decrease = 6.3 / 8.4 × 100 = 75.0 %

mean rate of closure = 6.3 / 30 = 0.21 um per minute

Common errors

• Writing that IAA “makes the shoot bend” without saying that the two sides elongate by different amounts.
• Saying a higher IAA concentration stimulates elongation in roots. In roots, a higher concentration inhibits it.
• Describing phototropism as the shoot “wanting” light. Give the mechanism: unequal IAA, unequal elongation.
• Saying ethene ripens fruit “by making it sweet”. State that it is a signal that stimulates ripening.
• Saying ABA makes guard cells lose water first. The ions leave first, which makes the guard-cell water potential rise, and then water leaves.
• Saying water potential falls when ions leave. The specification states that it rises.
• Quoting a percentage change without the base value. Divide by the control or the original.

Next steps

Condense this into notes with the revision notes, then answer the practice questions. For exam technique across the course, see the exam preparation page.

Official syllabus

OxfordAQA International AS and A-level Biology (9610), Version 5.1, for International A-level exams May/June 2018 onwards, published by OxfordAQA. Sections 3.4.5.1 to 3.4.5.3 and Required practical 10.