Phototropism. Classroom Experiments Developed by YES I Can! Science & Space Center Houston 2007

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1 Phototropism Classroom Experiments Developed by YES I Can! Science & Space Center Houston Background Long duration space flights and journeys to other planets will create some very large challenges. To ensure that human space travelers have sufficient food and oxygen to maintain their health and well being will require considerable ingenuity. Scientists suspect that green plants may be an essential part of the solution to the problems that arise during very long space missions. Green plants produce food and oxygen and consume carbon dioxide, all of which are important for humans. Plants however have their own requirements that are essential for their survival. These requirements must be well understood before plants can be integrated into mission plans for future astronauts. The activities in this unit will help reveal important information about the successful growth of green plants; important information required for mission planning! Green plants, those which use sunlight and chlorophyll to photosynthesize water and carbon dioxide into oxygen and a simple carbohydrate, are genetically programmed to grow towards the brightest source of light. They also tend to align their leaves so that the maximum area of the leaf is exposed to the light. The response of a plant to a source of light is known as phototropism, and we describe such plants as being phototropic. The investigations here will help reveal details of how plants and light interact. 2 Investigations Two interesting experiments can be performed to observe the phototropic effect on the growth of radish seedlings. In one experiment we will invoke the photosynthetic response by placing germinating radish seeds in a location in which the primary light source comes from a well defined direction. 1

2 In the second experiment we will suppress the phototropic response by slowly rotating the seeds so that they cannot determine any well defined direction towards the primary source of light. 3 Materials 1. Radish seeds or other rapidly germinating seeds. 2. Two light-weight disposable plastic containers. 3. Small battery operated clock mechanism. (for investigation section 5) 4. Several small germination peat pots. (for investigation section 5) 5. A small plywood base, approximately 20cm by 20cm. (for investigation section 5) 4 Invoking the Photosynthetic Response 4.1 Objective To observe the growth of seedlings when the direction of the primary light source is well defined. 4.2 Setup This experiment is quite easy to perform. Simply germinate some radish seedlings in small peat pots in a warm location The Basic Experiment Place the pots in a location in which the primary source of light comes from a well defined direction. The far side of a classroom, well away from bright windows, is an ideal location. In this way the light from the windows will come from a distinct and well defined direction. Figure 1: Figure (a) shows the germination module in a small box. The only light that can reach the seedlings is from the open side of the box. In figure (b) the germination module has been slightly extracted from the box to show the phototropic response of the seedlings. 2

3 4.2.2 The Modified Experiment You may modify this experiment by placing the germination module in a cardboard box painted flat black on the inside. Cover the open end with an optical filter made of a clear transparent sheet of coloured plastic or mylar. Try each of the three primary colours, red, yellow and blue. Figure 2: Cover each of the boxes with a coloured plastic or mylar filter so that the seedlings only receive light of a single colour. Figure 3: The germination module with blue light filter covering the opening. Allow the seedlings to grow for several weeks and keep a daily journal record of their growth pattern. Also see section 6 for details. Record your observations. See section 7 for details. 3

4 5 Suppressing the Photosynthetic Response 5.1 Objective To observe the growth of seedlings when the direction of the primary light source is ambiguous. 5.2 Setup This experiment requires that you mount a very light plastic bowl on the minute shaft of a small quartz clock so that it rotates once per hour. This will require that you punch a small hole exactly in the centre of the bowl so that it balances as perfectly as possible. The hands of the clock can be removed and discarded but the retaining nut and washer will be used to secure the bowl onto the minute shaft. For details refer to figure 4 and figure 5 below. Figure 4: Setting up rotating bowl for seed gemination. Two nested clear plastic bowls mounted on a simple quartz battery clock which rotates once per hour. Figure 5: Photographic details of the mounting scheme for the plastic bowls 4

5 Since the outer bowl has a hole at the mounting point on the clock, a second bowl should be nested inside it to provide a waterproof germination module. The inner germination module can be prepared with the peat pots prior to placing it in the outer (rotating) bowl. It is important to arrange the peat pots as symmetrically as possible in the germination module so that the centre of balance is as close to the geometric centre of the bowl as possible. Figure 6: Radish seedlings deprived of a definite direction to a source of light tend to grow straight up. After nesting the germination module into the outer bowl, one or more paperclips can by used to adjust the balance if necessary. Figure 7: The system will rotate once per hour (1 rph) and will make the direction of the light source seem ambiguous to the seedlings 5

6 Set the clock in motion and water the seedlings as required, being careful not to upset the balance of the system. A spray bottle works well for controlling the amount of water added to the peat pots while they are in the rotating germination module. Record your observations. See section 7 for details. 6 Your Journal Keep a detailed daily journal/diary of the experiment. It should contain every detail that you can possibly think of about the experiment. It should contain details of every observation, every problem, every change (or lack of change), every question and thought that occurs to you during the experiment, and every idea that you can think of that relates to modifying or improving the experiment. Include lots of sketches and diagrams in your journal to document everything that you observe. Digital photos can be pasted into your journal to facilitate recording day to day changes or to supplement written explanations. TIP: Spiral bound note books with graph paper pages make excellent journals. 7 Observations 1. What changes occur during the development of the seedlings? 2. Each day measure (to the nearest millimetre) the length of each seeding and plot a graph of length as a function of time. (If the seedlings are not straight you will have to approximate their length by holding a ruler along side their stems. Do not handle, or in any way disturb, the leaves or stems of the seedlings) 8 Discussion Green plants are uniquely designed to use the energy in sunlight to convert water and carbon dioxide into carbohydrates (food). For long duration space travel it might be more advantageous to grow at least some of the food required for the journey. Investigate photosynthesis. What is the minimum (threshold) amount of light required for plants to thrive? Is it the same for all plants? Is it the same for all phases of a plant s growth? Does the light threshold depend upon the duration, the colour (wavelength) or intensity of the light? Investigate the photosynthetic efficiency of various crops (in joules per hectare) Select one which might be well suited for growing in space. Justify your choice. 6

7 Investigate the difference in the available solar energy that reaches the surface of Mars compared to the surface of the Earth. Relate this to crop growth and crop yield. Referring to your journal notes from these experiments, suggest ways to improve the design. What additional questions have arisen as a result of these experiments? Design and perform experiments to address these questions. 7