Biology Workbook For Dummies
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Plant cells communicate with one another via messengers called hormones, chemical signals produced by one type of cell that travel to target cells and cause changes in their growth or development. Plant hormones control many familiar plant behaviors.

Six categories of hormones control plant growth and development:

  • Auxins stimulate the elongation of cells in the plant stem and trigger phototropism (the growth of plants toward light). If a plant receives equal light on all sides, its stem grows straight. If light is uneven, auxin moves toward the plant's darker side and causes cells on that side to lengthen.

    This may seem backward, but when the shady side of the stem grows, the stem, in its crookedness, actually bends toward the light. This action keeps the leaves positioned toward the light so photosynthesis can continue.

    Auxin also inhibits buds on the sides of plants, called lateral buds, from growing into branches. Auxin is produced by the apical meristem, a region of dividing cells at the tip of the main branch.

    Because it inhibits lateral buds, auxin establishes apical dominance; that is, growth of the main shoot is favored over growth of the side shoots. As the main shoot's tip gets farther away from side shoots, there's less auxin in the neighborhood, and side shoots can begin to grow. (If you've ever pinched off the tip of a plant to encourage side shoots to grow and make your plant bushier, you've already seen how apical dominance works.)

  • Gibberellins promote both cell division and cell elongation, causing shoots to elongate so that plants can grow taller and leaves can grow bigger. They also signal buds and seeds to begin growing in the spring, and they promote flowering.

  • Cytokinins stimulate cell division, promote leaf expansion, and slow down the aging of leaves. Florists actually use them to help make cut flowers last longer.

  • Abscisic acid inhibits cell growth and can help prevent water loss by triggering stomata to close. Plant nurseries use abscisic acid to keep plants dormant during shipping.

  • Ethylene stimulates the ripening of fruit and signals deciduous trees to drop their leaves in the fall. Fruit growers use ethylene to partially ripen fruit for sale.

  • Brassinosteroids control many aspects of plant growth and development. They're extremely powerful and can affect the concentrations of other plant hormones, promoting or inhibiting plant growth depending on the stage of development. They can counteract the effects of abscisic acid, acting to promote flowering, seed germination, and the opening of stomata.

    Scientists are very interested in brassinosteroids because they have the ability to increase photosynthesis, an ability that may come in handy in agriculture!

Using plant hormones to affect crop plants doesn't always work out in your favor. You've already seen this first hand if you've ever bought a big, beautiful tomato at the grocery store, then taken it home only to discover that it doesn't have much flavor. That tomato was probably picked before it was ripe and then treated with ethylene to make it "ripen" and turn red. (Unripe tomatoes are firm, so they ship better than ripe tomatoes.)

But even though ethylene can trigger some parts of ripening, like softening a fruit after it has been picked, fruit that's picked unripe doesn't taste as good as fruit that has ripened on the plant. That's because some of the flavor-making processes that occur in fruits only happen while the fruits are still on the plant. A fairly recent solution to the firm versus tasty issue is the sale of "tomatoes on the vine." These tomatoes are picked before they are ripe and firm (good for shipping), but left attached to their stems so that their flavor can continue to develop after they begin to ripen in response to the ethylene.

In the practice questions, use the following terms to indicate which plant hormone is triggering the growth or behavior of the plant.

a. Auxin

b. Cytokinin

c. Gibberellin

d. Abscisic acid

e. Ethylene

f. Brassinosteroids

  1. A seed germinates in the spring.

  2. It's autumn, and apples are ripening on the trees.

  3. It's autumn, and leaves are turning red and gold.

  4. It's summer, and a bud opens to reveal a beautiful rose.

  5. It's spring, and bright green leaves begin growing out of the buds on trees.

  6. A houseplant on your windowsill leans its stems toward the window.

  7. You pinch off the tip of one of your houseplants to encourage the plant to grow bushier instead of taller.

The following are the answers to the practice questions.

  1. The answer is c. Gibberellin.

    Gibberellin promotes seed germination. Germination can also be enhanced by f. Brassinosteroids.

  2. The answer is e. Ethylene.

    Ethylene triggers the ripening of fruit like apples.

  3. The answer is e. Ethylene.

    Ethylene triggers the senescence (aging) of leaves.

  4. The answer is c. Gibberellin.

    Gibberellin promotes flowering. Flowering can also be enhanced by f. Brassinosteroids.

  5. The answer is b. Cytokinin.

    Cytokinin promotes leaf expansion.

  6. The answer is a. Auxin.

    Auxin triggers phototropism, causing plants to bend toward the light.

  7. The answer is a. Auxin.

    Auxin establishes apical dominance. By pinching off the main shoot's tip, you stop the production of auxin and encourage the side shoots to grow.

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