Plant Hormones and Tropisms Lesson Plan PDF

# Lesson 23.2 Plant Hormones and Tropisms ## Key Questions - What roles do plant hormones play? - What are examples of environmental stimuli to which plants respond? - How do plants respond to seasonal changes? ## Vocabulary - hormone - target cell - receptor - auxin - apical dominance - tropism...

# Lesson 23.2 Plant Hormones and Tropisms ## Key Questions - What roles do plant hormones play? - What are examples of environmental stimuli to which plants respond? - How do plants respond to seasonal changes? ## Vocabulary - hormone - target cell - receptor - auxin - apical dominance - tropism - phototropism - thigmotropism - gravitropism - photoperiodism ## Reading Tool Compare the different vocabulary terms from this lesson in your Biology Foundations Workbook. Explain how they are similar or different in the way they support plants. Plants grow in response to environmental factors such as light, moisture, temperature, and gravity. But how do roots "know" to grow down, and how do stems "know" to grow up? How do the tissues of a plant determine the right time of year to produce flowers? Somehow, plant cells manage to act together as a single organism. ## Hormones Hormones are chemical signals that affect the growth, activity, and development of cells and tissues. In plants, hormones may act on the same cells in which they are made, or they may travel to different cells and tissues. Plant hormones serve as signals that control the development of cells, tissues, and organs. They also coordinate responses to the environment. These two functions fit together well, because plants respond to the environment mainly by changing their development. Cells affected by a particular hormone are called **target cells**. To respond to a hormone, a target cell must contain hormone receptors - usually proteins - to which hormone molecules bind. The response will depend on what kinds of receptors are present in the target cell. One kind of receptor might alter metabolism; a second might speed growth; a third might inhibit cell division. Thus, depending on the receptors present, a given hormone may cause a different response in roots than it does in stems or flowers, and the effects may change as cells add or remove receptors. Cells that do not contain receptors are generally unaffected by hormones. ### Auxins The first step in the discovery of plant hormones came more than a century ago, and was made by a scientist already familiar to you. In 1880, Charles Darwin and his son Francis published the results of a series of experiments exploring the mechanism behind a grass seedling's tendency to bend toward light as it grows. **Figure 23-16** A diagram of an experiment by the Darwins showing the effect of light on a plant seedling. The setup includes five scenarios: 1. Control: A normal seedling. 2. Tip Removed: The tip has been removed from the seedling. 3. Opaque Cap: An opaque cap is placed over the tip. 4. Clear Cap: A clear cap is placed over the tip. 5. Opaque Shield Over Base: An opaque shield is placed over the base, leaving the tip exposed. **The Darwins conducted controlled experiments to determine which region of a plant senses light. When they removed the seedling tip or placed an opaque cap over the tip, they observed no bending toward light. But when they placed a clear cap on the tip or an opaque shield around the base, they observed bending similar to that seen in the control.** **Control Variables:** What variable did the Darwins control for by comparing the results of seedlings treated with a clear cap versus no cap? **The results of their experiments, shown in Figure 23-16, suggested that the tip of the seedling somehow senses light. The Darwins hypothesized that the tip produces a substance that regulates cell growth.** More than forty years later, the regulatory substances produced by the tips of growing plants were identified and named auxins. **Auxins stimulate cell elongation and the growth of new roots, among other roles that they play. They are produced in the shoot apical meristem and transported to the rest of the plant.** ### Auxins and Cell Elongation One of the effects of auxins is to stimulate cell elongation. As the Darwins saw in their experiment, when light hits one side of the shoot, auxins collect in the shaded part of the shoot. This change in concentration stimulates cells on the dark side to lengthen. As a result, the shoot bends away from the shaded side and toward the light, as shown in Figure 23-17. **Figure 23-17** A diagram of a plant seedling bending towards the light source, demonstrating cell elongation on the shaded side due to a higher concentration of auxins. **Reading Check:** What are auxins? ### Apical Dominance **Figure 23-18** A diagram of two basil plants. The plant on the left has been allowed to grow naturally. The plant on the right has had its apical meristem pinched off. **Observe:** How are the two plants different? **Auxins also regulate cell division in meristems. As a stem grows in length, it produces lateral buds. As you may have noticed, the buds near the top of the plant grow more slowly than those near the base of a plant. The reason for this delay is that growth at the lateral buds is inhibited by auxins.** Because auxins move out from the apical meristem, the closer a bud is to the stem's tip, the more it is inhibited. This phenomenon is called **apical dominance**. **If you snip off the tip of a plant, these lateral buds begin to grow more quickly and the plant becomes bushier, as you can see in Figure 23-18.** This is because the apical meristem - the source of the growth-inhibiting auxins - has been eliminated. ### Cytokinins Cytokinins are plant hormones that are produced in growing roots and in developing fruits and seeds. Cytokinins stimulate cell division, interact with auxins to help to balance root and shoot growth, and stimulate regeneration of tissues damaged by injury. Cytokinins also delay the aging of leaves and play important roles in the early stages of plant growth. Cytokinins often produce effects opposite to those of auxins. For example, root tips make cytokinins and send them to shoots; shoot tips make auxins and send them to roots. This exchange of signals keeps root and shoot growth in balance. Auxins stimulate the initiation of new roots, and they inhibit the initiation and growth of new shoot tips. Cytokinins do just the opposite. So if a tree is cut down, the stump will often make new shoots because auxins have been removed and cytokinins accumulate near the cut. ### Ethylene One of the most interesting plant hormones, ethylene, is actually a gas. Fruit tissues release small amounts of the hormone ethylene, stimulating fruits to ripen. Ethylene also plays a role in causing plants to seal off and drop organs that are no longer needed. For example, petals drop after flowers have been pollinated, leaves drop in autumn, and fruits drop after they ripen. Ethylene signals cells at the base of the structure to seal off from the rest of the plant by depositing waterproof materials in their walls. ### Gibberellins For years, farmers in Japan knew of a disease that weakened rice plants by causing them to grow unusually tall. The plants would flop over and fail to produce a high yield of rice grain. Farmers called the disease the "foolish seedling" disease. In 1926, Japanese biologist Eiichi Kurosawa discovered that a fungus, Gibberella fujikuroi, caused this extraordinary growth. His experiments showed that the fungus produced a growth-promoting substance. In fact, the chemical produced by the fungus mimicked hormones produced naturally by plants. These hormones, called gibberellins, stimulate growth and may cause dramatic increases in size, particularly in stems and fruits. An example of the effect of gibberellins is shown in Figure 23-19. **Figure 23-19:** A picture of two cabbage plants, one is normal sized, the other is abnormally large. **Gibberellin hormones can cause incredible growth spurts, like those seen in the cabbage plants on the right.** ### Abscisic acid Gibberellins also interact with another hormone, abscisic acid, to control seed dormancy. Abscisic acid inhibits cell division and halts growth. Recall that seed dormancy allows the embryo to rest until conditions are right for growth. When seed development is complete, abscisic acid stops the seed's growth and shifts the embryo into a dormant state. The embryo rests until environmental events shift the balance of hormones. Such events may include a strong spring rain that washes abscisic acid away. (Gibberellins do not wash away as easily.) Without the opposing effect of abscisic acid, the gibberellins can signal germination. **Reading Check:** Summarize. Describe the roles of cytokinins, ethylene, gibberellins, and abscisic acid in plants. ## Analyzing Data **Auxins and Plant Growth** This graph shows the results of experiments in which carrot cells were grown in the presence of varying concentrations of auxins. The blue line shows the effects on root growth. The red line shows the effects on stem growth. 1. Analyze Graphs: At what auxin concentration are the stems stimulated to grow the most? 2. Analyze Graphs: How is the growth of the roots affected by the auxin concentration at which stems grow the most? 3. Apply Scientific Reasoning: If you were a carrot farmer, what concentration of auxin should you apply to your fields to produce the largest carrot roots? **Effects of Hormone Concentration on Plant Growth** A graph that shows the effects of increasing auxin concentration on plant growth. The graph uses a logarithmic scale for auxin concentration. The vertical axis shows growth, with a positive value indicating promotion and a negative value indicating inhibition. The graph shows two lines, one for stems and one for roots. **Tropisms and Rapid Movements** Like other living things, plants move to respond to the environment. Many plant movements are slow, but some are so fast that even animals cannot keep up with them. ### Tropisms Plant sensors that detect environmental stimuli signal elongating organs to reorient their growth. These growth responses are called tropisms. Plants respond to environmental stimuli such as light, touch, and gravity. These three tropisms are shown in Figure 23-20. **Figure 23-20** An illustration showing three plant tropisms: phototropism, thigmotropism, and gravitropism. **Build Vocabulary:** The word tropism comes from a Greek word that means "turning." ### Light The tendency of a plant to grow toward a light source is called phototropism. This response can be so quick that young seedlings reorient themselves in a matter of hours. Recall that changes in auxin concentration are responsible for phototropism. Experiments have shown that auxins migrate toward shaded tissue, possibly due to changes in membrane permeability in response to light. ### Touch Some plants even respond to touch, a process called thigmotropism. Vines and climbing plants exhibit thigmotropism when they encounter an object, such as a tree or trellis, and wrap around it. Other plants, such as grape vines, have extra growths called tendrils that emerge near the base of the leaf and wrap tightly around any object they encounter. ### Gravity Auxins also affect gravitropism, the response of a plant to gravity. For reasons still not understood, auxins migrate to the lower sides of horizontal roots and stems. In horizontal stems, the migration causes the stem to bend upright. In horizontal roots, however, the migration causes roots to bend downward. ### Interactivity Explore how photoperiodism affects plants. ### Rapid Movements Some plant responses are so rapid that it would be a mistake to call them tropisms. One example of a rapid response is what happens if you touch a leaf of the Mimosa pudica, appropriately called the "sensitive plant." Within only two or three seconds of being touched, its two leaflets fold together completely. The carnivorous Venus flytrap, shown in Figure 23-21, also demonstrates a rapid response. When an insect lands on a flytrap's leaf, it triggers sensory cells on the inside of the leaf, sending electrical signals from cell to cell. A combination of changes in osmotic pressure and cell wall expansion interact, snapping the leaf shut so quickly that the insect is trapped inside - and digested. **Figure 23-21:** Two photographs showing: 1. A Venus flytrap with its leaves open. 2. A Venus flytrap with its leaves closed around an insect. **Reading Check:** Integrate Information. How do tropisms help plants survive in their environments? ## Response to Seasons "To every thing there is a season." Nowhere is this more evident than in the regular cycles of plant growth. Year after year, some plants flower in the spring, others in the summer, and still others in the fall. Plants such as chrysanthemums and poinsettias flower when days are short and are therefore called short-day plants. Plants such as spinach and irises flower when days are long and are therefore known as long-day plants. ### Photoperiod and Flowering How do all these plants manage to time their flowering so precisely in response to the environment? In the early 1920s, scientists discovered that tobacco plants flower according to their photoperiod, the number of hours of light and darkness they receive. Additional research showed that many other plants also respond to changing photoperiods, a response called photoperiodism. This type of response is summarized in Figure 23-22. Photoperiodism is a major factor in the timing of seasonal activities such as flowering and growth. **Figure 23-22:** A diagram depicting the effect of photoperiod on flowering in short-day plants and long-day plants. There are three columns showing the effects of three different photoperiods: 1. Long Day: This column shows that long-day plants flower when exposed to a short period of darkness. 2. Short Day: This column shows that short-day plants flower only when exposed to an extended period of darkness. 3. Interrupted Night: This column shows that long-day plants also flower if a brief period of light interrupts the darkness, essentially dividing one long night into two short nights. **Reading Check:** Form an Opinion. Are "short-day plant" and "long-day plant" the best names for categorizing these plants, or would it be better to name plants after their responses to night length? Explain your reasoning. It was later discovered that a plant pigment called phytochrome (FYT oh krohm) is responsible for plant responses to photoperiod. Phytochrome absorbs red light and activates a number of signaling pathways within plant cells. By mechanisms that are still not understood completely, plants respond to regular changes in these pathways. These changes determine the patterns of a variety of plant responses. ## Winter Dormancy Phytochrome also regulates the changes in activity that prepare many plants for dormancy as winter approaches. Recall that dormancy is the period during which an organism's growth and activity decrease or stop. As cold weather approaches, many plants prepare by turning off photosynthetic pathways, transporting materials from leaves to roots, and sealing off leaves from the rest of the plant. ### Leaf Loss In temperate regions, many flowering plants lose their leaves during the colder months. At summer's end, the phytochrome in leaves absorbs less light as days shorten and nights become longer. Auxin production drops, but the production of ethylene increases. This triggers a series of events that gradually shut down the leaf. As chlorophyll breaks down, other pigments - including yellow carotenoids and red anthocyanins - become visible, producing the beautiful colors of autumn. ### Changes to Meristems Hormones also produce changes in apical meristems. Instead of continuing to produce leaves, meristems produce thick, waxy scales that form a protective layer around new leaf buds. Enclosed in its coat of scales, a terminal bud can survive the coldest winter days, as shown in Figure 23-23. At the onset of winter, xylem and phloem tissues pump themselves full of ions and organic compounds. The resulting solution acts like antifreeze in a car, preventing the tree's sap from freezing. This is one of several mechanisms plants use to survive the bitter cold. **Figure 23-23:** A microscopic image of a plant bud covered in protective scales. **LESSON 23.2 Review** ## Key Questions 1. Describe how hormones contribute to homeostasis. 2. Describe three examples of tropisms in plants. 3. Summarize plant responses to seasonal changes. ## Critical Thinking 4. Plan an Investigation: How could a garden-store owner determine what light conditions are needed for a particular flowering plant to bloom? 5. Apply Scientific Reasoning: Review what you learned about evolution by natural selection in Chapter 17. Then, using what you know about natural selection, describe how plant adaptations for dormancy may have developed over time.