Ethene - a plant growth substance with the key to ripening . . . in fruits and vegetables

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, 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)

Your hotlinks to practical protocols and other suggestions to explore

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.

Further Links

1. Catalytic generators - a commercial site concerning the role of ethylene in postharvest handling of fruit for sale
   
   
2. Plant hormone introduction page and gateway

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