Abscisic acid, or ABA, generally inhibits other hormones, such as the auxin IAA. It was originally thought to promote abscission, hence its name. Botanists now know that ethylene in the main abscission hormone. ABA helps to bring about dormancy in a plant’s buds and maintains dormancy in its seeds. ABA causes the closure of a plant’s stomata in response to drought. Water stressed leaves produce large amounts of ABA, which triggers potassium ions to be transported out of the guard cells. This causes stomata to close, and water is held in the leaf. It is too costly to synthesize ABA for commercial agriculture use.
Category: 4. Groups of Hormones
Cytokinins promote cell division in plants. Produced in the developing shoots, roots, fruits, and seeds of a plant, cytokinins are very important in the culturing of plant tissues in the laboratory. A high ratio of auxins to cytokinins in a tissue-culture medium stimulates root formation. A low ratio promotes shoot formation. Cytokinins are also used to promote lateral bud growth in flowering plants.
The hormone ethylene is responsible for the ripening of fruits. Unlike the other four classes of plant hormones, ethylene is a gas at room temperature. Ethylene gas diffuses easily through the air from one plant to another. The saying “One bad apple spoils the barrel” has its basis in the effects of ethylene gas. One rotting apple will produce ethylene gas, which stimulates nearby apples to ripen and eventually spoil because of over-ripening.
Ethylene is usually applied in a solution of ethephon, a synthetic chemical that breaks down and releases ethylene gas. It is used to ripen bananas, honeydew melons, and tomatoes. Oranges, lemons, and grapefruits often remain green when they are ripe. Although the fruit tastes good, consumers often will not buy them, because oranges are supposed to be orange, right? The application of ethylene to green citrus fruit causes the development of desirable citrus colors, such as orange and yellow. In some plant species, ethylene promotes abscission, which is the detachment of leaves, flowers, or fruits from a plant. Cherries and walnuts are harvested with mechanical tree shakers. Ethylene treatment increases the number of fruits that fall to the ground when the trees are shaken. Leaf abscission is also an adaptive advantage for the plant. Dead, damaged or infected leaves drop to the ground rather than shading healthy leaves or spreading disease. The plant can minimize water loss in the winter when the water in the plant is often frozen.
In the 1920s, scientists in Japan discovered that a substance produced by the fungus Gibberella caused fungus-infected plants to grow abnormally tall. The substance, named gibberellin, was later found to be produced in small quantities by plants themselves. It has many effects on a plant but primarily stimulates elongation growth. Spraying a plant with gibberellins will usually cause the plant to grow to a larger than expected height, i.e. greater than normal.
Like auxins, gibberellins are a class of hormones that have important commercial applications. Almost all seedless grapes are sprayed with gibberellins to increase the size of the fruit and the distance between fruits on the stems. Beer makers use gibberellins to increase the alcohol content of beer by increasing the amount of sugar produced in the malting process. Gibberellins are also used to treat seeds of some food crops because they will break seed dormancy and promote uniform germination.
Chemists have synthesized several inexpensive compounds similar in structure to IAA. Synthetic auxins, like naphthalene acetic acid, of NAA, are used extensively to promote root formation on stem and leaf cuttings. Gardeners often spray auxins on tomato plants to increase the number of fruits on each plant. When NAA is sprayed on young fruits of apple and olive trees, some of the fruits drop off so that the remaining fruits grow larger. When NAA is sprayed directly on maturing fruits, such as apples, pears, and citrus fruits, several weeks before they are ready to be picked; NAA prevents the fruits from dropping off the trees before they are mature. The fact that auxins can have opposite effects, causing fruit to drop or preventing fruit from dropping, illustrates an important point. The effects of a hormone on a plant often depend on the stage of the plant’s development.
NAA is used to prevent the undesirable sprouting of stems from the base of ornamental trees. As previously discussed, stems contain a lateral bud at the base of each leaf. In many stems, these buds fail to sprout as long as the plant’s shoot tip is still intact. The inhibition of lateral buds by the presence of the shoot tip is called apical dominance. If the shoot tip of a plant is removed, the lateral buds begin to grow. If IAA or NAA is applied to the cut tip of the stem, the lateral buds remain dormant. This adaptation is manipulated to cultivate beautiful ornamental trees. NAA is used commercially to prevent buds from sprouting on potatoes during storage.
Another important synthetic auxin is 2,4-D, which is an herbicide, or weed killer. It selectively kills dicots, such as dandelions and pigweed, without injuring monocots, such as lawn grasses and cereal crops. Given our major dependence on cereals for food; 2,4-D has been of great value to agriculture. A mixture of 2, 4-D, and another auxin, called Agent Orange, was used to destroy foliage in the jungles of Vietnam. A non-auxin contaminant in Agent Orange has caused severe health problems in many people who were exposed to it.
Auxins are hormones involved in plant-cell elongation, apical dominance, and rooting. A well known natural auxin is indoleacetic acid or IAA which is produced in the apical meristem of the shoot. Developing seeds produce IAA, which stimulates the development of fleshy fruit. For example, the removal of seeds from a strawberry prevents the fruit from enlarging. The application of IAA after removing the seeds causes the fruit to enlarge normally. IAA is produced in actively growing shoot tips and developing fruit, and it is involved in elongation. Before a cell can elongate, the cell wall must become less rigid so that it can expand. IAA triggers an increase in the plasticity, or stretchability, of cell walls, allowing elongation to occur.
Groups of Hormones
Plant hormones are chemical messengers that affect a plant’s ability to respond to its environment. Hormones are organic compounds that are effective at very low concentrations; they are usually synthesized in one part of the plant and are transported to another location. They interact with specific target tissues to cause physiological responses, such as growth or fruit ripening. Each response is often the result of two or more hormones acting together.
Because hormones stimulate or inhibit plant growth, many botanists also refer to them as plant growth regulators. Many hormones can be synthesized in the laboratory, increasing the quantity of hormones available for commercial applications. Botanists recognize five major groups of hormones: auxins, gibberellins, ethylene, cytokinins, and abscisic acid.