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Indoleacetic acid is produced in actively growing shoot tips and developing seeds, and it is involved in elongation. Before a cell can elongate, the cell wall must become less rigid so that it can expand. Indoleacetic acid triggers an increase in the plasticity, or stretcha-bility, of cell walls, which allows elongation to occur.

Synthetic Auxins

Auxins have a variety of possible effects, so they are used for several purposes in gardening, commercial agriculture, and scientific research. Chemists have synthesized several inexpensive compounds similar in structure to IAA. Synthetic auxins, such as naphthalene (NAF-thuh-LEEN) acetic (uh-SEET-ik) acid, or NAA, are used to promote root formation on stem and leaf cuttings, as shown in Figure 31-2 on the next page. Such vegetative propagation of genetically identical plants is useful in agriculture and in laboratory research.

figure 31-2

(a) This stem cutting from a coleus plant, Coleus hybridus, is growing roots in pure water. (b) This coleus stem cutting has been treated with a synthetic auxin— naphthalene acetic acid (NAA). Roots form more rapidly when the stem is treated with NAA. Both cuttings were taken at the same time.

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 some other fruits—such as 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— they cause fruit to drop or prevent 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.

Naphthalene acetic acid is used to prevent the undesirable sprouting of stems from the base of ornamental trees. Recall that 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 intact. The inhibition of lateral buds by the presence of a shoot tip is called apical dominance. If the shoot tip 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. NAA is used commercially to prevent buds from sprouting on potato tubers during storage.

Another important synthetic auxin is 2,4-D, which is a weedkiller. At certain concentrations, it kills dicots, such as dandelions, without injuring monocots, such as lawn grasses and cereal crops. Given our dependence on cereals for food, 2,4-D has been important to agriculture. Agent Orange, a mixture of 2,4-D and another auxin, was used to defoliate jungles in the Vietnam War. A nonauxin contaminant in Agent Orange is thought to have caused health problems in many people who were exposed to it.

figure 31-2

(a) This stem cutting from a coleus plant, Coleus hybridus, is growing roots in pure water. (b) This coleus stem cutting has been treated with a synthetic auxin— naphthalene acetic acid (NAA). Roots form more rapidly when the stem is treated with NAA. Both cuttings were taken at the same time.

In the 1920s, scientists in Japan discovered that a substance produced by fungi of the genus Gibberella caused fungus-infected rice plants to grow abnormally tall. This type of hormone, named gibberellin (JIB-uh-REL-uhn), was later found to be produced in small quantities by plants themselves.

figure 31-3

These bird's nest ferns, Asplendium nidus, are the same age. The fern in (b), however, has been treated with gibberellin, which stimulates the leaves to grow larger.

Gibberellins have many effects on a plant, but they primarily stimulate elongation growth. Spraying some plants with gibberellins may cause them to grow larger than normal, as shown in Figure 31-3.

Like auxins, gibberellins are hormones that have important commercial applications. Many seedless grapes, such as those shown in Figure 31-4, are sprayed with gibberellins to increase the size of the fruit. Beer makers use gibberellins to increase the alcohol content of beer by increasing the amount of starch converted to sugar in the brewing process. Gibberellins are also used to treat seeds, because they break seed dormancy and promote uniform germination.

figure 31-4

Almost all of the raisins produced in California are made from Thompson Seedless grapes. Normal Thompson Seedless grapes are smaller than those treated with a gibberellin hormone. The grapes on the left were treated with a gibberellin. As you can see, the addition of a gibberellin to grapes stimulates the grapes to grow larger.

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