removing the auxin-forming tip of the plant. Developing seeds produce auxin, which stimulates the formation of the fruit. There is only one naturally occurring auxin: indole-3-acetic acid, which is chemically similar to the amino acid tryptophan. However, many synthetic analogs have been developed, including the herbicides 2,4-D and 2,4,5-T.

3. Cytokinins promote growth by stimulating cell division. They have an effect on differentiation of plant cells. Leaves that turn yellow after being picked can be kept green longer by treatment with cytokinins. Chemically, they are derivatives of adenine.

4. Abscisic acid has the opposite effect of some of the other hormones: It inhibits growth and development. It is produced during water stress, causing the stomata to close and thereby inhibiting photosynthesis. It is involved in causing the seed embryo to become dormant so that it does not germinate prematurely. It also stimulates. A some processes, such as protein storage in seeds.

5. Ethylene is the simple hydrocarbon H2C=CH2. It stimulates maturation, promoting the ripening of fruit and the dropping of fruit, leaves, and flowers. Being a gas, it is released to the air by plants and ripening fruit. This explains the adage "one bad apple spoils the bunch,'' as ethylene from one fruit hastens ripening in the others. Plants also release it when injured. Ethylene is used commercially to stimulate ripening of fruit such as the tomato that were picked green so that they could be transported to market while firm and less liable to damage. Ethylene is active at air concentrations as low as 0.06 ppmv (parts per million by volume).

Plants also change their growth patterns in response to environmental stimuli, such as by light and gravity. The response to gravity is called geotropism. If a potted plant is placed on its side, the cells on the lower side of the stem will elongate, causing the stem to bend upward. Auxin seems to be involved. The turning of plants toward the light is called phototropism. Experiments have established that light coming from the side, especially blue light, causes auxin to migrate to the shadow side of a stem. This causes elongation on the shaded side, bending the plant toward the light.

Plants exhibit another behavior, familiar to those who raise houseplants, called photo-periodism, in which the length of the night controls when the plant flowers. Plants exhibit one of three photoperiodism behaviors: Short-day plants set flower only when the length of the night exceeds a critical period, which varies from plant to plant. Examples include some chrysanthemums, poinsettias, and strawberries. Short-day plants flower in the early spring or fall. Ragweed, for example, blooms in the fall and needs at least 9.5 hours of continuous darkness. Interestingly, a single flash of light in the middle of the night can fool a short-day plant and prevent its flowering. Long-day plants require a period of darkness less than some critical value and tend to flower in the summer. Spinach, lettuce, and some varieties of potato and wheat are long-day plants. Spinach will bloom only if there is less than 10 hours of darkness. In contrast to short-day plants, a flash of light during a long night can fool a long-day plant into flowering. Finally, there are day-neutral plants, such as cucumber, sunflower, rice, and corn, which are not controlled by photoperiod. Photoperiodism is controlled by a membrane-bound protein complex called phytochrome that acts as a detector for light. Plants also use phytochrome to detect if light is totally absent, such as if the plant is shaded by other plants or by a fallen log. In response, plants do not produce chlorophyll but instead, devote their energy to growing longer, seeking

TABLE 7.2 Elemental Plant Nutrients

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