This is a project starter, suitable for Advanced Higher biology investigations or
A-level extended project s. Don’t forget to credit this resource in your bibliography by including the title, the website, the web address and the date you accessed it.
Have you thought why a ripe banana can speed up the ripening process when placed amongst some green tomatoes? What is the invisible signal that passes between these fruits? And how does potassium permanganate (or certain other mundane chemicals) prevent or delay the effect of this invisible agent?
The answer (which you probably know) lies in the activities of ethene, a colourless gas. Ethene, also known as ethylene, is a plant growth substance.
It can be produced by almost all parts of higher plants, though the rate of production is highest where cell division occurs. Its production is increased during leaf fall, flower senescence and fruit ripening. In addition, stress factors such as wounding, flooding, chilling, disease, high temperatures and drought seem to induce ethene synthesis.
Ethene has a deceptively simple structure and is lighter than air in physiological conditions - so it diffuses rapidly amongst the air spaces of a plant, and between plants. Perhaps it is these properties that make it such a powerful signal molecule. Certainly plants are sensitive to it, and responses occur at only 1 part ethene per million of air. Yet the concentration of ethene in the tissues of a ripening apple is 2500 times greater!
A bioassay for measuring ethene concentrations was developed in 1901, using the so-called triple response of etiolated pea seedlings. Other bioassays have used the effects of ethene on root hair formation or leaf abscission.
The ethene acts at the genetic level, activating regulatory genes called ethene-responsive elements (EREs), which in turn activate and repress other genes which lead to the effects seen in the plant.
All fruits that ripen in response to ethene show a characteristic rise in respiratory rate before the ripening phase, called a climacteric. Some fruits such as apples, bananas, avocados, figs, mangoes, peaches, pears, persimmons, plums and tomatoes show a sharp peak in ethene production just before the respiratory burst, and these are called climacteric fruits. Other non-climacteric fruits, such as grapes, strawberries, cherries and citrus fruits,
An investigation from Science & Plants for Schools, www.saps.org.uk/students

do not show these peaks of ethene production and respiratory activity.
Climacteric fruits can be induced to ripen by the application of ethene, and this is something that has been exploited by commercial suppliers in management of their fruit stores - so that the fruit is ripened at just the right time for sale.
For some time it has been known that if you add ethanol to the water in which cut flowers are standing, this causes a delay in petal abscission. Normally, ethene promotes petal abscission. It is now thought that the action of ethanol with the cut flowers is due to the toxic effects of the ethanal (acetaldehyde) made from the ethanol, at a key step (ACC synthase) in the synthesis of ethene. (Ref: Is Acetaldehyde the causal agent in the retardation of carnation flower senescence by ethanol? Podd LA van Staden J (1999) Journal of Plant
Physiology 154: 351-354)
Some suggestions for experiments:

Detecting the release of ethene from ripening fruits

Accelerating and delaying leaf fall in cut flowers
You can probably devise others - for example, exploring the effects of different sorts of packaging, with and without potassium permanganate in the container.
Ethene is an important plant growth substance, and plays an essential role in the triggering of ripening in many fruits. The precise nature of the ethene receptor on plant cells and the molecular basis of its action is not fully understood, but research is currently being carried out with mutants defective in this particular control system. The production of ethene occurs during a short part of the ripening process.
Presence of ethene can be detected by a relatively simple biological assay, using germinating seedlings.
The assay
This technique is based on certain responses made by young seedlings, grown in the dark, then exposed to ethene. The morphology of the seedlings treated with ethene changes noticeably, compared to controls.
For example, the ethene-treated seedlings

are shorter, with decreased length of hypocotyl and root

are fatter, with increased girth of hypocotyl
An investigation from Science & Plants for Schools, www.saps.org.uk/students


tend to respond less to gravity (diatropism)

tend to have more extensive and longer root hairs
The technique uses 2-day old seedlings. To detect ethene released from fruit, batches of seedlings are exposed to the fruit for one day. The morphology of exposed and control seedlings is then compared after two further days of growth.
The assay method can be used to determine the period of ethene production by ripening fruit. However, in order to study the production of ethene by some fruit on a daily basis, some forward planning is needed, because each day the fruit needs to be provided with a fresh batch of 2-day old seedlings
So, if you wish to follow the production of ethene by a ripening banana over a
5-day period, you need to sow seeds 2 days before the first seedlings were to be exposed, and then every day for the next 4 days. This will provide the supply of fresh seedlings required. The period of maximum ethene production is likely to last for less than a day.
Seeds of any reasonably robust dicotyledonous species could be used - promising results have been achieved with fenugreek seeds (Trigonella foenum-graecum). Avoid very large seeds (such as broad bean) as they simply take too long to develop sufficiently for the experiment.

Sow the seeds (minimum of 10) on moist filter paper in shallow dishes
(e.g. petri dish base). The roots and shoots should then be clearly seen, and easy to measure.

Keep all the seeds in a light-proof cupboard - after sowing, during exposure, and during the subsequent two days of post-exposure development.

Place the ripening fruit in a plastic box (e.g. 2 litre ice-cream box) with a dish of seedlings and seal it with a plastic lid. (The ethene gas diffuses very easily, so it is important to seal the container.) Set up a control box, containing another dish of seedlings, but no fruit. Place the sealed boxes in the same light-proof cupboard.

After every 12 or 24 hours, remove the dishes of seedlings in both boxes and replace them with fresh ones (grown to the same age as the original ones). Allow the earlier pair of dishes to develop further for 24 hours, before making any measurements.

Make quick records of the contents of the dishes by taking photographs, or lay the seeds on a sheet of acetate and photocopy them against a background of graph paper (to provide a scale). Photos give a useful record, but may also provide an opportunity to make
An investigation from Science & Plants for Schools, www.saps.org.uk/students

measurements later, directly off the computer screen, if a digital camera is used.
The control measurements are important, as they provide a baseline comparison for measuring the influence of ethene on exposed seedlings. It might be useful to express the changes in the seedlings exposed to ethene, as a percentage change from the controls.
Premature senescence of cut-flowers exposed to ethene is another phenomenon that can be studied in this manner.
The transient beauty of cut flowers inside homes and offices has long been appreciated. Surely, no one can fail to be saddened by their inevitable decline, when the flowers shrivel and drop their petals (petal abscission). In the wild, of course, petal abscission occurs at the time that the fruit develops - the plant no longer needs to attract pollinating animals, the pollinators have done their job (see hotlinks below), and it is too early for the fruit to be dispersed as the seeds within will be immature.
A number of methods are used to delay petal abscission artificially, including the use of chemicals. Indeed, many flower shops provide a sachet of chemicals intended to promote the ‘health’ of the flower (or delay senescence) for as long as possible.
Ethene plays a major role in petal abscission, and at least some of the chemicals that delay the process do so by hindering its production or the response of plant tissues to it.
For instance, it has been known for some time, that adding ethanol to the water in which cut flowers are standing, causes a delay in petal abscission. It is now thought that this is due to the toxic effects of the ethanal
(acetaldehyde), made from the ethanol at a key step (ACC synthase) in the synthesis of ethene (which normally promotes petal abscission).
Some suggested investigations
1. If you place cut flowers of similar age in containers with water containing additional chemicals (pure, or sachets from florists), it should be possible to investigate some of the following.

Whether a particular chemical has an accelerating / delaying effect

Whether there is a dose / response relationship between the amount of chemical and the delay in petal abscission
An investigation from Science & Plants for Schools, www.saps.org.uk/students


Whether other environmental factors, such as light or temperature, are important in affecting the rate of senescence
2. If you place cut flowers of similar age in containers with water, and then place these in sealed containers (such as tupperware) with, and without, a ripening climacteric fruit, you could do some further investigations.

Whether the influence of ethene can be detected

Whether the age of the fruit inside the container affects your observations

Whether the period of exposure matters

Whether the presence of a fruit plus a chemical which absorbs ethene
(e.g. potassium permanganate) has an effect
Other techniques described on the SAPS site could be used to investigate the changes in the colours of the petals that occur during the process of petal senescence.
For further information, try this reference:
"Is acetaldehyde the causal agent in the retardation of carnation flower senescence by ethanol?" Podd LA van Staden J (1999) Journal of Plant
Physiology 154: 351-354
Useful Links
Superb series of slides on pollination - http://bhort.bh.cornell.edu/hort243/pollen/sld 001.htm
An investigation from Science & Plants for Schools, www.saps.org.uk/students