Effect of the Phytohormone Ethylene on Fruit Ripening

Aim

To compare the rate of ripening of unripe fruit kept in the presence of the phytohormone ethylene, released by a ripe banana, with the rate of ripening of similar fruit kept without it.

Variables

• Manipulated variable: the presence or absence of an ethylene source, such as a ripe banana, in a sealed container with unripe fruit.
• Responding variable: the degree and rate of ripening of the test fruit, assessed from changes in colour and firmness over the days of the investigation.
• Controlled variables: the type, size, and initial ripeness of the fruit used, the size of the container, whether both containers are sealed in the same way, the temperature and location of storage, and the total duration of the investigation.

Materials and apparatus

• Unripe fruit of the same type and similar size, such as green bananas or unripe tomatoes
• A ripe banana, used as a natural source of ethylene
• Two identical airtight containers or plastic bags
• A colour reference chart, or a camera and notebook for recording observations
• A calendar

Procedure

1. Select unripe fruit samples of similar size and ripeness and divide them equally between two identical containers.
2. Place a ripe banana into one container, which becomes the test set-up, and seal it.
3. Seal the second container, the control, without adding any ethylene source.
4. Store both containers under the same conditions, at the same location and temperature.
5. Observe and record the colour and firmness of the fruit in each container at the same time each day.
6. Compare the degree of ripening of the fruit in the test container with the control container over the following days.

Expected results

The fruit in the container with the ripe banana is expected to ripen faster and more fully than the fruit in the control container, as illustrated in Table 1.

Conclusion

The phytohormone ethylene speeds up fruit ripening, since the fruit exposed to it changes colour and softens faster than the fruit kept without an ethylene source. This supports the aim of the investigation and shows that ethylene released by a ripe fruit can affect the ripening of nearby unripe fruit of the same or different types.

Common mistakes

Paper 3-style questions

Question 1 (making a hypothesis). State a suitable hypothesis for this investigation.

Model answer. Fruit kept in a sealed container with a ripe banana, a source of ethylene, ripens faster than identical fruit kept in a sealed container without one. This links the manipulated variable, the presence of an ethylene source, to the responding variable, the rate of ripening.

Question 2 (controlling variables). Identify the manipulated and responding variables, and name one variable that must be controlled.

Model answer. The manipulated variable is the presence or absence of an ethylene source; the responding variable is the degree and rate of ripening, seen as colour change and softening. One controlled variable is the type, size and initial ripeness of the fruit, so any difference is due to the ethylene alone.

Question 3 (tabulating and displaying data). Explain how you would make ripening measurable and record it in a table.

Model answer. Define ripeness operationally using a numbered colour-and-firmness scale, for example 1 for green and firm up to 5 for fully coloured and soft. Record the daily score for each container in a table, then plot ripeness score against day as a line graph, because day is a continuous variable, with one line per container.

Question 4 (making an inference). The control fruit also ripens slightly by day 7. What can you infer from this?

Model answer. You can infer that fruit produces its own ethylene as it ripens, so it ripens slowly even without an added source. The ripe banana therefore speeds up ripening rather than being the only cause of it.

Safety

• Do not taste or eat any of the fruit used in the investigation, as it has been handled and stored only for observation.
• Wash your hands after handling the fruit and the containers.
• If any fruit develops mould, seal it in a bag before disposal and avoid touching or breathing in the mould.
• Handle the containers carefully to avoid spills, and keep the set-up out of direct sunlight so the containers do not overheat.

Source:SRC-DSKP-EN

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