In 1901, Dimitry Neljubow, a Russian student at the St. Petersburg Botanical Institute, discovered that pea seedlings grown in the dark in a laboratory showed reduced stem elongation, increased swelling of stems, and abnormal horizontal growth. When the seedlings were placed outside in fresh air, however, they resumed normal growth. It was determined that the simple gas ethylene, present from gas lamps in the laboratory, had produced the abnormal growth.
In 1934, R. Gane discovered that ethylene was produced naturally by fruits, although it had been known for some time that the ripening of green fruits could be accelerated artificially by placing them in ethylene. It is now known that ethylene is produced not only by fruits but also by flowers, seeds, leaves, and even roots. Several fungi and a few bacteria are also known to produce it, and its regulatory effects make it a hormone in the broad sense of the word. Ethylene is produced from the amino acid methionine; oxygen is required for its formation.
The production of ethylene by plant tissues varies considerably under different conditions. A surge of ethylene lasting for several hours becomes evident after various tissues, including those of fruits, are bruised or cut, and applications of auxin can cause an increase in ethylene production of two to ten times. As pea seeds germinate, the seedlings produce a surge of ethylene when they meet interference with their growth through the soil. This apparently causes the stem tip to form a tighter crook, which may aid the seedling in pushing to the surface.
When a storm stirs up a green field or animals pass across it, plants respond to these and other mechanical stresses with an increase in the production of fibers and col-lenchyma tissue. Cell elongation is also inhibited, resulting in shorter, sturdier plants. The responses, called thigmomor-phogenesis, are under the control of genes that are activated by touch. Several substances, including enzymes, ethylene, and a protein called calmodulin, are involved. Calmodulin, which may play a role in several plant growth responses, constitutes up to 2% of a plasma membrane and is activated when it binds to calcium. The calmodulin-calcium complex activates enzymes in membranes, which then activate or deactivate other enzymes, and plays a role in several kinds of plant reactions to stimuli, including thigmomorphogenesis.
Ethylene apparently can trigger its own production. If minute amounts are introduced to the tissues that produce the gas, a tremendous response by the tissues often results. These tissues may then produce so much ethylene that the part concerned can be adversely affected. Flowers, for example, may fade in the presence of excessive amounts, and leaves may abscise (Fig. 11.5).
In ancient China, growers used to ripen fruits in rooms where incense was being burned, and citrus growers used to ripen their fruits in rooms equipped with a kerosene stove. In houses where gas heating is used, occupants often experience great difficulty in growing house plants, and greenhouse heaters using such fuel create a dilemma for owners attempting to promote plant growth.
In the days before electric street lights became commonplace, gas lights were used, and in some cities in Germany, leaves fell from the shade trees if they were located near a gas line that leaked. In all of these instances, minute amounts of ethylene gas resulting from the fuel combustion or leakage brought about the results, both good and bad. In fact, as little as 1 part of ethylene per 10 million parts of air may be sufficient to trigger responses.
Today, commercial use of ethylene is extensive. It has been used for many years to ripen harvested green fruits, such as bananas, mangoes, and honeydew melons, and to cause citrus fruits to color up before marketing. Since ethylene production almost ceases in the absence of oxygen, apple and other fruit growers have found that if they place unripe fruit in sealed warehouses after harvest, pump out the air, reduce temperatures to just above freezing, and replace the air with inert nitrogen gas or carbon dioxide, the fruit will remain metabolically inactive for long periods. The growers can then remove the fruit in batches throughout the year, add as little as one part per million of ethylene, and have ripe fruit at any time there is a demand. This is why apples are always available in supermarkets, even though harvesting is usually confined to a few weeks in the fall.
Fruits that respond to ethylene usually have a major increase in respiration just before ripening occurs. The increase in ethylene production at that time is often up to 100 times greater than it was a day or two earlier. The accompanying major increase in respiration is called a climacteric, and fruits that exhibit such phenomena are called climacteric fruits. Some fruits, such as grapes, are noncli-macteric and do not respond in this way to ethylene.
Afew growers still use natural ethylene to ripen pears and peaches by wrapping each fruit individually in tissue paper. The paper retards the escape of ethylene from the fruit and hastens ripening. "Resting" potato tubers will sprout following brief applications of ethylene, and seeds may be stimulated to germinate if given a short exposure to the gas just before sprouting, although treatment after sprouting inhibits growth.
Ethylene is used in Hawaii to promote flowering in pineapples, and it causes members of the Pumpkin Family (Cucurbitaceae) to produce more female flowers and thus more fruit. Many trees in nurseries are grown in containers and are usually crowded. Consequently, they are often tall and spindly, but applications of small amounts of ethylene to the trunks while they are enclosed in plastic tubes causes a marked thickening, making the trees sturdier and less likely to break.
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