Angle of the Dangle. Simulating the effect of gravity on plants. Overview. Biological and agricultural concepts

Transcription

1 -----"' Lesson1 ~ Angle of the Dangle Simulating the effect of gravity on plants Overview Plants respond to the force of gravity. However, because gravity is constant and can't be manipulated, it is difficult to experiment with gravity in the classroom. But one can create a centrifugal force in the classroom using a standard record turntable. A centrifugal force can simulate the effects of gravity on plants. Through this activity, students will examine the effect of gravity and centrifugal force on plant stems. Students will form hypotheses and test those ideas using Wisconsin Fast Plant seedlings, film canisters and a rotating turntable. Biological and agricultural concepts Plant growth and development Gravitropism Centrifugal force Gravitropism I 1-1

2 Teacher material The teachable moment Background Agriculture and biology teachers can use this activity to illustrate a plant science unit. Plants know the way to grow. But what controls them? Why do shoots grow up and roots grow down? What causes a house plant to bend toward the window, wheat to grow upwards after being flattened by a strong storm, or a pea tendril to curve around and cling to any object in its reach? When environmental stimuli cause a plant to bend in a specific direction the response is termed a tropism. Response to gravity is called gravitropism (or geotropism), response to light is called phototropism, and response to physical contact is called thigmotropism. A plant's cells elongate at a faster rate on one side of the stem than the other in response to the environmental stimuli. Exactly what triggers the increased rate of elongation or how it is mediated remains, despite centuries of intense interest and experimentation on the subject, a virtual mystery. Because tropic responses are so important to basic plant growth and development, agriculture, space exploration and many other fields, they are worth examining. Gravitropism is one of the most important aspects of plant growth. Gravity impacts both root and shoot growing tips, but in opposite directions. Roots grow toward gravity (positive gravitropism) and shoots grow away from gravity (negative gravitropism). A plant's perception as to the direction of gravity appears to depend on the settling of dense particles in organelles, called amyloplasts, located in specialized cells. When the plant is turned, the amyloplasts sink quickly toward the source of gravity, to the side of the cell that is currently "down." Curving or bending of the roots or stem results from asymmetric growth as the plant elongates. Picture two layers of cells sandwiched together. If only one layer elongates, the two layers will bend. 1-2 I Gravitropism

3 Teacher material Movement will be away from the side where elongation occurs. Elongation of plant cells is controlled by a plant growth hormone called auxin. The amount of auxin present appears to correlate with the distribution of the amyloplasts and gravity induces an auxin gradient in the root or shoot tip. It is not clear if the cell elongation is due to a redistribution of auxin within the plant, an increased sensitivity to auxin, or to some other mechanism. If the root and shoot responded in the same way to auxin concentrations, plants would grow in circles. Fortunately, auxin stimulates cell elongation in shoots, but inhibits it in roots. Consider gravitropism as it relates to conditions found in space. In order for humans to stay in space for prolonged periods of time, they must be able to grow their own food. Will this be possible? How will astronauts create an artificial"gravity" that initiates root and stem growth? Since space travel occurs in a microgravity environment, astronauts will need a gravitational-like force that simulates gravity. Could centrifugal force be used as a substitute for gravity in space? Centrifugal force is an outward force that acts upon a mass rotating about an axis. Centrifugal force is generally expressed in multiples of the force of gravity, or G's. Therefore, the force of gravity on the surface of the earth is, by definition, one G. For this experiment it is useful to measure the magnitude of the centrifugal force on the plant. The magnitude of the centrifugal force exerted on an object depends on its angular velocity and distance from the center of rotation. Relative centrifugal force (FR) is represented by the following equation. FR = 0.204v 2 /D where v =peripheral speed of the film can in meters/sec D = diameter of the circle of rotation in meters Students will need to measure the diameter of the circle of rotation (and divide by two to determine the radius), calculate the circumference (2m) and record revolutions per minute (rpm) of the turning table. To calculate v (in meters/sec), use the following equation: v =(_meters/rev)(. rev /min)(l min/60 sec) Gravitropism I 1-3

4 Teacher material In this activity, the student will investigate how the effects of centrifugal force can be used to elicit a gravitational-like response in young Fast Plant seedlings (radish will work, too). The control in the experiment, which examines only the force of gravity on the plants, should be examined and discussed thoroughly before examining the results from the centrifugal force experiment. Results from the control can help the students refine their predictions about the effects of the centrifugal force. Teacher management Preparation As an extension, the students can change the magnitude of the centrifugal force by placing the canister closer to the center of the tumtable (changing "D") or change the rpm of the turntable (changing "n"). They can then correlate the amount of bending with the amount of force. Students can work individually or in groups. Four film can chambers will be used to measure centrifugal force and one film can will be set up as a control to measure the effects of gravity alone. Each team of students will set up five film cans total. Three days prior to this lab, sow enough Fast Plant seeds to ensure that there will be at least 4 three-day-old seedlings per film can chamber available for students. Plant 50 percent more seeds than you will need (if you need 15 plants, plant 30 seeds). By lab day 1, secure the materials listed below. Activity time Ten to 15 minutes will be taken on day 1 for students to place four Fast Plant stems on wicks of one germination chamber. These controls will test the effects of bending due to gravity only. This can be done individually or in groups. Record necessary information on data sheet. On day 2, 20 to 50 minutes will be needed. Students should predict results for the control gravity chambers before looking inside. In addition to making predictions, students can be assigned to create a hypothesized mechanism to accompany their predictions. Students will also examine their chambers, participate in pooling the class data and discuss results. Students will then create four experimental chambers. Each chamber will have one wick and two plants. These chambers will then be taped onto a turntable of a record player. Record necessary information on data sheet. Day 3 will require 30 to 50 minutes. Students should predict results 1-4 I Gravitropism

5 Teacher material for the experimental chambers. They will then remove the chambers from the turntable, observe the location of the stems, measure the angle, record and discuss the results. Materials For students in groups of four: 12 three- to four-day-old Fast Plants 5 film cans waterproof pen or pencil l-inch piece of double-sided foam tape (if not available, masking tape will do) eye dropper or small pipette 5 paper towel wicks 25 ml of water protractor 1 paper towel sheet (use kitchen-type, not the brown school-type) Class as a whole will need: 1 turntable (with a 78 rpm setting) l-inch roll of labelling tape Sources of materials Fast Plant seeds are available from Carolina Biological Supply Co. You can purchase turntables from local second-hand stores at low cost. Black plastic 35 mm film containers can be obtained from local film processing or camera stores. All other materials should be available from your school. Tips and safety Plant the seeds on a Friday. Experimental days 1 and 3 then fall on a Monday and a Wednesday. Make sure that film cans are securely fastened to the turntable. The most difficult parts of this lab are: too much water in the film cans cause the plants to slide around, while too little water causes the plants to dry out. accurately marking the initial and final positions of the plants on the tape transferring the markings to the data sheets avoiding confusion about positive and negative angles and control versus experimental containers. Gravitropism I 1-5

6 Teacher material Optional: suspend a small fishing weight (1 \8 oz) tied to the end of a string in a clear film can. Tape this can to the turntable along with the four experimental chambers. When the turntable is rotating, the weight will swing to the outside of the chamber, demonstrating to students that a (centrifugal) force is acting on the plant stems when the turntable is on. Key terms Amyloplasts: clusters of starch grains enclosed with a membrane; located in the statocyte Auxin: growth-promoting hormone found in some plant cells; causes cells to elongate Centrifugal force: fictitious or pseudo-outward force that acts upon a mass rotating about an axis Gravitropism: the physiological response of a plant to gravity Gravity: the force of attraction between two objects, generally perceived as the force of attraction between the earth and bodies on or near its surface Shoot primordium: the growing tip in a stem; located at the point of attachment of the cotyledons Statocytes: gravity-sensitive cells of plant organs such as roots or stems References "Centrifugal Force." McGraw -Hill Encyclopedia of Science and Technology, 7th edition. Vol. 3: "Gravitation." McGraw -Hill Encyclopedia of Science and Technology, 7th edition. Vol. 8: "Gravitropism Revisited." Fast Plants Notes. Vol. 4, No.2. Spring "Plants Know the Way to Grow." Fast Plants Notes. Vol. 1, No.2. Evans, M.L., R. Moore, and K.H. Hasenstein "How roots respond to gravity." Scientific American 255 (December 1986): I Gravitropism

7 Teacher material Iversen, Tor-Henning. The roles of statoliths, auxin transport, and auxin metabolism in root geotropism. Univ. of Trondheim, The Royal Norwegian Society of Sciences and Letters, The Museum Suge, H. and Turkan, I. "Can plants normally produce seeds under microgravity in space?" Japanese Journal of Crop Science 60(3): Wilkins, Malcolm. "Guidance Systems," Chapter 7 in Plant Watching. Facts on File Publications: New York. pg Extensions 1. How does the magnitude of the centrifugal force affect the degree of bending? How can you change the magnitude of the centrifugal force? Consider graphing the amount of bending versus the magnitude of the force. 2. How does the length of time exposed to centrifugal forces affect the degree of bending? 3. Does temperature affect the degree of bending? Does it affect the bending differently for gravitationally-caused bending versus centrifugally-caused bending? 4. Which is stronger, the bending due to gravitational force, centrifugal force or light? 5. Does age of seedling affect degree of bending? 6. Do different plants respond to forces differently? Ideas for discussion 1. Describe how animals orient themselves to gravity. They have multicellular organs ("ears") with fluid-filled cavities. Particles in the cavity sink, stimulating sensory hairs that line the cavity. Electrical impulses are carried from the hairs to the brain, which returns signals to the organism's limbs to correct its orientation. 2. Describe the following experiment: Immature crustaceans such as crayfish accumulate fine sand grains in their ears (statocysts). Substitute iron filings for sand in the aquarium. When the crustacean matures, place a magnet over the top of the statocysts. The filing will rise to the top of the statocysts, causing the crustacean to flip upside down. Gravitropism I 1-7

8 Student material Angle of the Dangle Simulating the effect of gravity on plants Introduction You probably know that plants require light, water, carbon dioxide and nutrients to grow. Did you also know that plants need gravity (or a gravitational-like force) to grow and develop? And how is it that a plant's shoots grow up and roots grow down? When a seed germinates, it sends out an immature stem. The ideal position for that stem is vertical because the plant will need to position its leaves to receive maximum light and to begin making food through the process of photosynthesis. If a plant is moved from its vertical position, say it is blown to one side by the wind, it can correct its orientation. This response to gravity is called gravitropism. Plants respond to gravity by elongating the cells on one side of the stem faster than on the other. This causes the stem to bend. We don't know exactly what triggers the increased rate of elongation or what controls this response. Because tropic responses are an important aspect to basic plant growth and development, they impact agriculture, space exploration and many other fields. Though for centuries researchers have studied and experimented with tropic responses, basic questions still remain. What we do know is that plant stems respond to gravity by secreting a hormone called auxin. Auxin causes cells in the stem to elongate. If a plant's stem is vertically oriented, then the hormone is evenly distributed to the cells. If a stem is not vertically oriented, then hormone levels are increased to the cells that are closest to the gravitational force. The increased level of auxin causes the cells to elongate, which corrects the position of the stem back to vertical. Picture two layers of cells sandwiched together. If only one layer elongates, it forces the two layers to bend. 1-8 I Gravitropism

9 Student material How does an individual stem cell react to gravity? In 1900 it was discovered that the perception of the direction of gravity appears to depend on the settling of dense particles (organelles called amyloplasts) in specialized cells called statocytes. When the plant is turned, amyloplasts sink quickly toward the source of gravity, to the side of the cell that is currently "down." Current research indicates that perhaps amyloplasts possess an electrical charge that alters membrane permeability, therefore regulating the movement of auxin. The exact connection between auxins and amyloplasts, both of which are involved in gravitropic responses, remains a mystery. Can we simulate a gravity-like force? Many researchers use centrifugal force as a substitute for gravity in the lab. You will create a centrifugal force using a record player turntable. Centrifugal force is an outward force that acts upon a mass that rotates about an axis. For example, if you rode on a merry-go-round and did not hold on, what would happen? You would be thrown off! Centrifugal force is generally expressed in multiples of the force of gravity, or G's. Hence, the force of gravity on the surface of the earth is, by definition, one G. For this experiment it is useful to measure the magnitude of the centrifugal force on the plant. The magnitude of the centrifugal force exerted on an object depends on its angular velocity and distance from the center of rotation. Relative centrifugal force (FR) is represented by the following equation. FR = 0.204v 2 /D where v =peripheral speed of the film can in meters/sec D = diameter of the circle of rotation in meters You will need to measure the diameter of the circle of rotation (and divide by two to determine the radius), calculate the circumference (27tr) and record revolutions per minute (rpm) of the turning table. To calculate v (in meters/sec), use the following equation: v = (_ meters/rev)( rev/min)(l min/60 sec) Consider gravitropism as it relates to conditions found in space. In order for humans to stay in space for prolonged periods of time, they Gravitropism I 1-9

10 Student material must be able to grow their own food. Will this be possible? How will astronauts create an artificial "gravity'' that initiates root and stem growth? Since space travel occurs in a microgravity environment, astronauts will need a gravitational-like force that simulates gravity. Could centrifugal force be used as a substitute for gravity in space? Materials 12 three- to four-day-old Fast Plants 5 film cans waterproof pen or pencil l-inch piece of double-sided foam tape (if not available, masking tape will do) eye dropper or small pipette 5 paper towel wicks 25 ml of water protractor 1 paper towel sheet Class as a whole will need: 1 turntable (33, 45 or 78 rpm) 1 roll of labelling tape Procedure Dayl 1. Cut a piece of paper towel to produce small wick strips that are approximately 4.5 em long and 1 em wide. 2. Pre-moisten four wicks with several drops of water and place them along the inner sides of the film can so that there is one wick each on the top, bottom and both sides of the can when it is placed on its pedestal. Each wick can be slid in and out of the can by gently pushing or pulling it with the sharp tip of a pen or pencil. 3. Cut eight three-day Fast Plants seedlings at soil level leaving the small stem (hypocotyl) and seed leaves (cotyledons) intact. Stick two of these seedlings onto each wick by placing the cotyledons against the wick. The water on the wick should hold the seedlings in place. If the plant is reluctant to stick, you may need to add an additional drop of water on the wick. 4. Add a couple of drops of water to the bottom of the film can when all of the plants are in place and put a lid on the can. Make sure the ends of the wicks don't protrude out of the can. The extra drops of 1-10 I Gravitropism

11 Student material water in the can should keep the air in the can moist. This will prevent the wicks from drying out. Paper Towel Wick Double-Stick Foam Tape Film Can.:::=:::~~=~- Top 5. Place the chamber in a warm (but not hot) location. A Fast Plants light bank works well. After 2 to 4 hours, gently take the lid off and observe the orientation of the hypocotyls. Continue your observations for the next five to seven days. Keep your eye out for new growth of your seedlings. Day 1 Question. What do you think will happen to the stems? Given what you know about plants, explain why you predict this. Day2 1. Examine your chamber from yesterday. Which way did each if the stems bend? What happened? With a protractor, measure the angle of the stems. How many degrees did the stem bend? Fill in the day 2 data sheet. 2. Label four film cans as El, E2, E3 or E4 (E is for experimental.) 3. Set up the four chambers as you did in steps 1-4 on Day 1, except put one wick with two seedlings on the lid of a can. Seedlings Gravitropism I 1-11

12 Student material 4. With a protractor, measure the angle of the stem of the seedlings. 5. Press the cans down over the lid, closing the chamber. 6. Attach the experimental chambers to the turntable with labelling tape, so that the stems are perpendicular to the surface of the turn table and the cotyledons are facing down. Mark an x on the side of the chamber that faces the center of the turntable. 7. Set the turntable to 78 rpm and turn it on. Record the date and time on the data sheet provided. Day 2 Question or hypothesis from students: What do you think will happen to the stems on the turntable? Explain your prediction. Day3 1. Turn the turntable off. Record the date and time on the Student Data Sheet. 2. Take the cans off the turntable and open them. 3. Using a protractor in the same orientation as you used for the original measurement, measure the angle of the stems. Record the angles I Gravitropism

13 Data sheet Student name: Day 1: Directions: Record the information requested in the blanks provided. Diameter of circular path =--- rpm setting on turntable = Relative force expressed in G's = Beginning date of exposure to centrifugal force = Beginning time of exposure to centrifugal force = Ending date of exposure to centrifugal force = Ending time of exposure to centrifugal force = Total duration of exposure to centrifugal force = Day 2: Results from control chamber Directions: Draw in the stems showing how they bent or did not bend. Seedling # Degree of bending Day 3: Results from turntable experiment Angle of change in plant subjects Container or plant Degree of change Average degree of change in the control plants: Average degree of change in the experimental plants: Gravitropism I 1-13

14 Student material Results and discussion 1. Compare and contrast the effects of gravity and centrifugal force on the plants in the germination chambers. 2. Did the plants in the chambers respond to gravity? Explain. 3. Stems respond to light after they break the soil's surface. Is it possible for roots, which grow in darkness, to respond to light? How could root studies in space prove your answer? 4. How could you test whether certain plant cells have structures that respond to gravitational-like forces? 5. Calculate the relative force at 78 rpm. How does it compare to the relative force you calculated? 6. You are given the opportunity to do one experiment in space. This is your once-in-a-lifetime opportunity to clear up questions about gravitropism. You do not want to waste this opportunity. Design an experiment. Include your hypothesis I Gravitropism