Plant Tropisms
Introduction
Plants are anchored into the ground and may seem incapable of responding to favorable or unfavorable conditions in their surroundings. How do plants become anchored? Why do roots grow downward into the soil? Can plants respond to changes in their surroundings?
Concepts
Tropism
- Gravitropism
- Phototropism
Background
Directed movements resulting from external stimuli are commonly referred to as tropisms in plants. The main shoots of most plants tend to develop vertically. If a box is placed over a plant growing vertically and a hole is cut to admit light from one side, the tip of the plant stem will begin to bend toward the light in a relatively short time. If the box is later removed, a compensatory bend develops, causing the tip to grow vertically again. Such a growth movement toward light is called a positive phototropism. The shoot tips of most plants are positively phototropic, while roots are either insensitive to light or negatively phototropic. Growth responses to the stimulus of gravity are called gravitropisms. The primary roots of plants are positively gravitropic while shoots forming the main axis of plants are negatively gravitropic.
Many interesting experiments have been performed trying to elucidate the internal mechanisms that cause these tropic responses. It is clearly a complex interaction of various plant hormones and other cellular components. We know that plant auxins are produced in the growing tip of the stem. If the growing tip of the stem is covered or removed, the tip will not bend toward the light. It is also known that auxin promotes the elongation of cells. For some time it was believed that stem tips bend toward the light because auxin is destroyed or inactivated on the sun-exposed side of the stem. This would leave more growth-promoting hormone on the side away from the light, causing the cells there to elongate more and produce a bend. Careful experiments, however, have shown that stem tips growing in non-directional light have the same total amount of auxin present as do stem tips from the same species receiving bright light from one side. Other experiments indicate that the auxin migrates away from the light, accumulating in greater amounts on the opposite side and promoting greater elongation of cells on the “dark” side. Apparently, an active transport system enables the auxin to migrate against a diffusion gradient.
Gravitropisms are equally as complex to explain. There is evidence that plant organs perceive gravity through the movement of large starch grains in special cells in the root cap. The starch grains, called statoliths, will, within a few minutes, begin to float or tumble down until they come to rest on the side of the cells closest to the gravity stimulus. In roots, the cells on the side opposite the stimulus begin elongating very quickly bringing about a downward bend of the root. The opposite reaction seems to occur in the stem. Some other researchers have suggested that mitochondria or Golgi bodies also respond to gravity. Precisely how the gravitropic mechanism is coordinated in and between cells is not yet entirely understood.
Materials
Part I Water, tap, 3–5 mL Corn seeds, pre-soaked, 4 Filter paper Modeling clay, small dab Paper towels, 2 Petri dish Pipet Tape, clear Wax pencil or marker Part II Water, tap, 5–10 mL Dark closet or lightproof box Greenhouse or other lighted area Pipet Pots, 4 Potting soil, to fill 4 pots Radish seeds, pre-soaked, 40 Watering tray (shared) Wax pencil or marker
Safety Precautions
Plant materials used during laboratory work should never be consumed. Seeds are often treated with mold inhibitors and other chemicals. Wash hands thoroughly with soap and water before leaving the laboratory.
Procedure
Part I.
{10326_Procedure_Figure_1_Experimental setup}
- Select four pre-soaked corn seeds and place them on the bottom of an empty Petri dish. Place the four seeds with their pointed ends toward the center of the Petri dish and evenly spaced so that each seed points toward the center from an imaginary north, south, east and west direction. See Figure 1 for a view of the setup.
- Soak a piece of filter paper in tap water and then carefully place it over the top of the corn seeds without disturbing their orientation or position.
- Pack the dish tightly with crumpled, damp paper towels so that when the cover is placed on the dish the seeds cannot move. When the dish is set on its edge, the corn seeds should stay in place and be visible through the bottom of the dish.
- When the seeds are in perfect position and will not move, place the cover on the dish, and seal it shut with clear tape.
- Stand the Petri dish on edge with one seed in the top position. Mark the dish with the word “top.” Use a small dab of modeling clay to fashion a foot to hold the dish in a vertical position. The setup should look like Figure 1 with one seed germinating in the “normal” position, two germinating in a horizontal position and one “upside down.”
- Place the dish in its vertical position in a safe place as directed by your instructor.
- Let the seeds germinate for three days and make observations as they grow.
- At the end of three days, note the direction in which the roots and stems have grown, and complete Part I of the Plant Tropisms Worksheet.
Part II.
- Fill four pots with potting soil to approximately ½" below the rim of the pots. Gently, but firmly, pack the potting soil in the pots. Water the soil thoroughly before doing the planting. This can be done over a sink or a watering tray.
- Use a wax pencil or marker to number the pots 1 through 4. Label the pots appropriately with other key information (e.g., name, class, date).
- Plant 10, pre-soaked radish seeds in each pot by gently pressing the seeds, one at a time, down into the potting soil. Do not press any deeper than 12 mm. Spread the seeds as evenly as possible over the entire top surface of the pot.
- Place the pots in a watering tray in a well-lit area (next to a sunny window, in a greenhouse, or under artificial growing lights).
- Monitor the germinating radish seeds over the next three days. Water the seedlings each day to keep the surface of the soil moist. Be sure there is approximately 1 cm of water in the bottom of the watering tray each day. Also make sure to check the moisture on the seedlings before any weekends during the experiment.
- Within 2–3 days of planting, the young germinating plants should be poking up through the surface of the potting soil.
- Five days after planting (or when the seedlings are at least one inch tall), treat the four pots as follows:
Pot 1: Continue to grow in light undisturbed—pot upright Pot 2: Continue to grow in light—pot turned on its side Pot 3: Place in complete darkness—pot upright Pot 4: Place in complete darkness—pot turned on its side
Be sure that the plants placed in the dark receive absolutely no light!
- Each day for the next several days, check the growth pattern of the plants in each pot. For the plants in the dark, check them very quickly and then return them to their dark environment. Allow plants to remain in their specific treatment conditions for the amount of time specified by your instructor.
- Record your summary observations on Part II of the Plant Tropisms Worksheet when directed to do so by your instructor. Answer the questions on Part II of the worksheet.
- The seeds may be planted in a garden or greenhouse upon completion of the lab work or disposed of as directed by your instructor.
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