Introduction: (Initial Observation)
So here is the problem with which you are faced.
You have someone who appears to have found that plants don’t grow as well (or die) under some types of light and he wants to know why, so he asks you and wants you to do a research and come up with information that he can use to prevent such problems in future. So first you have to decide what parameters can you truly manipulate or measure. What do we mean by “different types of light”? Well incandescent and fluorescent lights are different, but how? They are different in heat output, colors of light and light intensity.
Because of the distance between the light source and the plant, heat can not be a very influential factor in this case. However the light color and the light intensity could be affecting the the plant growth.
In this project you will perform experiments to determine the effect of light intensity and light color on plant growth.
Find out about what you want to investigate. Read books, magazines or ask professionals who might know in order to learn about the effect or area of study. Keep track of where you got your information from
Image in the right shows a plant cell.
Vacuole is a single membrane, containing water, food, or metabolic waste.
Chloroplasts contain chlorophyll that is needed for photosynthesis.
OK, so let’s say you think that infrared light is something to test (Why?). Do you shine it on the plant and measure its growth? Would you really see a measurable difference in plant growth in 2 hours? (Hint – No!). Would this experiment answer your question? You already has evidence that the plants don’t grow as well in a certain light condition, you wants to know WHY! Showing that plants grow better under different colors of light will not answer the questions anyway.
Is there a parameter that is correlated with growth that you could measure? Growth requires energy. So this should make you think about metabolic rate and cellular respiration. What process that is dependent on light and that only plants (and algae and cyanobacteria) do provides “food” and energy?
You have to come up with a hypothesis in the truest sense. What do we mean by that? Hypotheses are explanations for phenomena – What is the mechanism or cause for what is observed. Look at the concept map below. It shows a variety of relationships and posses questions about the process you should be investigating.
You should be testing some idea of why the plants do not grow well under different color lights not whether they grow better under different color light and you can’t test growth but must measure something that contributes to growth.
You now know that light is required for the process of plant growth, and that the light interacts with the chlorophyll (the green pigment inside chloroplasts). But which colors of light are most important? Will a green plant grow in just any light? If a pigment is blue in color, what does that tell you about the colors of light that must be absorbed by the pigment? An artist could tell you the answer in a flash. But let’s try to find out through an experiment.
Something to do …
What colors are absorbed most by chlorophyll?
To answer this question, you must first find some way to shine various colors of light onto a green plant and observe how the process of plant growth is affected. First think about the process itself. You would expect to have oxygen produced, and the more the process is going on, the more oxygen is produced. Similarly, glucose is made and usually stored as starch. Also, carbon dioxide and water are consumed. Now, what is easiest to detect? You have already observed bubbles of oxygen. Perhaps you can do the experiment with an aquatic plant and just count the bubbles.
But how will you get colors of light? You may remember that Isaac Newton used a triangular prism to break light into its colors. You could use a prism to break light into its colors and shine that light onto an aquatic plant just below the surface of water. Where you see bubbles, you can infer that oxygen is produced.
Try to design and carry out such an experiment. What kinds of problems did you encounter? What colors of light were absorbed the most? Which ones the least?
In 1833 the German botanist Theodore Engelmann did this experiment, but in a somewhat different way. Engelmann used an alga, Spirogyra, that has long spiral chloroplasts. He placed the alga on a microscope slide with aerophilic (“air-loving”) bacteria, which move toward regions of greatest concentration of oxygen. Using a prism to disperse the light, Engelmann illuminated different parts of the alga with different colors. He thought that the plant would produce the most oxygen where light of a given color was absorbed the most. He would therefore see the most bacteria in that region.
Engelmann observed that the bacteria did indeed concentrate in the region where red light fell, and also in the region where blue light fell. If you remove red and blue light from white light (that is, if those colors get absorbed), what color is left? Mainly green! And that’s the color that gets reflected to our eyes from the leaves of living plants.
Absorbance: The ability to take up electromagnetic radiation by objects; different wavelengths of the visible light spectrum have different absorbances and appear as different colors.
Absorption Spectrum: The particular wavelengths of light that are absorbed by objects, e.g. pigment molecules, measured by a spectrophotometer.
Action Spectrum: Rate of activity in relation to wavelength of light, e.g. photosynthesis most active in blue and red parts of the visible spectrum
Chlorophyll a: Primary photosynthetic pigment in all organisms except bacteria; absorbs red and orange (600-700 nm) and blue and violet (400-500 nm).
Phycobilin: Water-soluble photosynthetic pigment
Solvent/Solute: A chemical in which others dissolve, forming a solution
Transmittance: The fraction of radiant energy that passes through a substance [syn: transmission]
Visible Light Spectrum: The small range of the electromagnetic spectrum that human eyes perceive as light. Ranges from about 400 to 700 nm, corresponding to blue through red light
Wavelength: The distance moved by a photon during a complete vibration; dependent on the energy of the photon; higher energy photons have shorter wavelengths.
What do you want to find out? Write a statement that describes what you want to do. Use your observations and questions to write the statement.
The purpose of this investigation is to find out how different color lights affect the plant growth.
When you think you know what variables may be involved, think about ways to change one at a time. If you change more than one at a time, you will not know what variable is causing your observation. Sometimes variables are linked and work together to cause something. At first, try to choose variables that you think act independently of each other.
Controlled variables are:
- Light intensity
- amount and type of water and nutrients
- rate of CO2 in the test environment
Independent variable also known as manipulated variable is the color of the light.
Dependent variable is the plant growth (height, dry weight, size or number of leaves.)
Since the amount of consumed CO2 and produced oxygen are correlated with the plant growth, you may also choose one of those as dependent variable.
Based on your gathered information, make an educated guess about what types of things affect the system you are working with. Identifying variables is necessary before you can make a hypothesis.
Some Bad Hypotheses
- Light makes plants grow.
Yes… but How? Why?
- Different colored lights make plants grow differently.
Too general. Which color(s) of lights? What is “different” growth?
- Red light affects photosynthesis.
Again, too general. How does red light affect photosynthesis? Why?
- Plants grow best in bright light.
How do you define bright light? How do you define the “best” growth?
- Plants grow good in green and blue light.
How will you test “good” growth?
- Plants on an average will grow an extra 2 inches in blue light than red light.
How will you test this?
Some Good Hypothesis
- Blue and black color lights that have lower wave lengths can accelerate the photochemical reactions of the plant (or photosynthesis) resulting the absorption of more CO2 from the environment and production of more oxygen and glucose that is ultimately converted to starch and cellulose.
- White color that is closer to natural light and contains many different wave lengths can provide more light energy to the plant resulting a higher rate of photosynthesis and plant growth.
Design an experiment to test each hypothesis. Make a step-by-step list of what you will do to answer each question. This list is called an experimental procedure. For an experiment to give answers you can trust, it must have a “control.” A control is an additional experimental trial or run. It is a separate experiment, done exactly like the others. The only difference is that no experimental variables are changed. A control is a neutral “reference point” for comparison that allows you to see what changing a variable does by comparing it to not changing anything. Dependable controls are sometimes very hard to develop. They can be the hardest part of a project. Without a control you cannot be sure that changing the variable causes your observations. A series of experiments that includes a control is called a “controlled experiment.”
Light is one of the most important environmental factors affecting plant growth. Light quality is a limiting factor in the growth of the plant. Chemical reactions and physiological reactions are controlled by the color of light.
You would select plants of the same species and alter the light color for those plants. Any potted plant would be acceptable for conducting this experiment.
Visible light consists of different colors having wavelengths of different ranges. Each wavelength of light causes chemical and thermal responses in plants that influence various phases of growth.
Plants should be exposed to a diversity of light colors. You may use color light bulbs as your light source. You may also use natural light and use color films or color glass or color plastic to filter the light so only one color will pass through.
To determine affects of the light on growth factors several observations should be made; length of internodes, leaf width and length, quantity and quality of flower production and quantity of leaves should be considered. Plant quality should also be a consideration. This would include strength of stems, color of the leaves and flowers, tissue damage and plant survival.
- 24 Tomato Plants
- White, Blue, Green, red and Black Light Bulbs (Black light is near UV or Ultra Violet)
- Water and Watering Apparatus
- 5 fish tanks or boxes
- 5 lamps (desk lamps or any similar light fixture)
- Electrical Plug and Electrical Resource
- Black Construction Paper
- 15 Small flower pots
- 5 Timer For Lights (You may be able to use one timer for 5 lights. For more details read the packaging or ask the seller)
- About 25 Cups of soil
You may change the material and procedures as you need. For example you may decide to experiment with a different light source or different plant.
- Cover each fish tank completely, using the black construction paper and tape, so no other light can come inside.
- Label each fish tank a different light color: black blue green, red or white.
- Put 1.5 cups of soil in each of the 15flowerpots.
- Label each flowerpot a different light color: black blue green, red or white.
- For every color label each flowerpot a different letter: a b c. For example, Black A, Black B and Black C.
- Plant two tomato plants into each labeled flowerpot.
- Measure each tomato plant. If you are planting seeds, you need to wait a few days before starting the light experiment and doing your first measurement.
- Place three flowerpots in each fish tank, according to their labels. Ex. Black A, Black B, and Black C are grouped together in the same fish tank.
- Cut out a circle of the construction paper on the top of the fish tank big enough for the lamp to fit, so that the light can shine threw the fish tank and only the fish tank. There should be at least 2 feet distance from the lamp to the plant.
- Place each lamp with the colored light according to the labeled plants. Ex. Black light would go on top of the fish tank with the Black A, Black B, and Black C flowerpots.
- Set lamp timer to turn on light for 12 hours and off for 12 hours each day.
- Water each plant with 20 ml of water when all of the plants are dry to the touch. Remember to water all the plants on the same day and at the same time.
- Measure each plant to the end of its stem every three days for six weeks.
- At the end of the six week period take the measuring results and put them into a chart.
- Which ever plant, out of the plants placed under the blue, black, green or white lights, has the highest growth percentage shows which color of light offers a better growing capability for tomato plants.
A Different Method and a totally different experiment
The main challenge in this project is to decide how are we going to test the rate of photosynthesis? In previous experiment we let the plant grow for a few weeks and then measured the height of plants. Height represents plant growth but is not enough in many cases. Maybe in addition to height we could also count the number of leaves or measure the area of the leaves. We can also dry the plants and weight them to measure the dry weight as an indication of plant growth. Some of the other methods that we can think of are:
- Photosynthesis absorbs carbon dioxide. If we perform our tests in a closed container, we can measure and use the rate of carbon dioxide reduction as the rate of photosynthesis. But a carbon dioxide monitoring tool is about $500 that seems too expensive for a science project test.
- Photosynthesis produces oxygen. If we perform our tests in a closed container, we can measure and use the rate of oxygen production as the rate of photosynthesis. However, there is no tool that easily monitors and shows the rate of oxygen in the air. We could possibly try to burn a candle inside the container and estimate the oxygen amount based on the burning time of candle, but this does not produce a reliable result.
- Photosynthesis produces organic material that form the body of the plant. In other words growth of the plant is the direct result of photosynthesis. So we can measure the weight of dry plant and use it as a product of photosynthesis. This requires a longer period of experiments. In order to produce enough organic material (plant body) and weight it, you need to continue each experiment at least 30 days. Otherwise the amount of weight increase will not be enough to provide you with reliable results.
- Final way that is a quick and inexpensive method is using a water plant. Oxygen produced by a water plant can be gathered under a glass tube such as a test tube and can simply be measured. Following is the detail:
In this experiment we will observe evidence of photosynthesis in a water plant. We will assemble the equipment needed to measure the rate of photosynthesis in elodea (or any other water plant). We count or collect bubbles of oxygen gas given off by elodea to determine the rate of photosynthesis. We will then change the conditions of photosynthesis by altering light intensity, and determine the effects on the photosynthesis rate.
Finally we prepare a graph of the collected data and analyze it.
- Pond weed like Elodea or Lagarosiphonelodea (water plant)
- Lamp (40 watt)
- Test tube or measuring cylinder
- Razor blade (single-edge)
- Dechlorinated water (room temperature)
- Sodium bicarbonate powder (baking soda)
- Clock or timer
- Metal stand with rod or test tube rack
- Metric ruler
PART A. Setting Up the Experiment
1. Obtain a sprig of Elodea. Remove several leaves from around the cut end of
the stem. Slice off a portion of the stem at an angle and lightly crush the
cut end of the stem.
2. Place the plant into the test tube, stem end up, filled with water.
3. Secure the test tube to a metal stand with tape or place the test tube in a
test tube rack.
PART B. Running the Experiment
1. Place a 40 watt lamp 5 cm from the plant. After one minute, count and
record the number of oxygen bubbles rising from the cut end of the stem.
Count bubbles for five minutes. If bubbles fail to appear, cut off more of
the stem and recrush.
2. Run a second five-minute trial. Record and average your results.
3. Move the lamp so it is 20 cm from the plant. After one minute count and
record bubbles for two five-minutes trials. Again, average and record your
4. Add a pinch of sodium bicarbonate powder to the test tube. Place the lamp
5 cm from the test tube. After one minute, record bubbles for two five-
minute trials. Average and record your results.
5. Prepare a graph of your results. Use the average number of bubbles for the
vertical axis. Use the type of environmental condition for the horizontal
Instead of counting bubbles we can collect bubbles under a test tube or measuring cylinder, so we can measure the volume of oxygen. You just need to place the plant in a larger clear container and use a funnel to direct the oxygen into the test tube. (Note that you need to fill up the test tube with water, use your thumb to close the mouth of the test tube and turn it over into water. No bubbles must be in the tube at the start of your experiment.
Use the result of your experiments to answer these questions:
1. How does this investigation demonstrate that plants give off oxygen during
photosynthesis? Explain your answer based on your observations.
2. How does the rate of photosynthesis change when the light source is moved
from a distance of 5 cm to 20 cm?
3. How does the rate of photosynthesis change when sodium bicarbonate is added
to the water?
Plants use green pigments called chlorophylls to trap light energy. The
chlorophylls give a plant its green color. Inside the cells that have
chloroplasts, the light energy is used to make a simple sugar called glucose.
The process by which plants use light energy to make glucose is called
During this process of sugar production, carbon dioxide combines with water to
form glucose and oxygen is released. Oxygen that is produced in photosynthesis
is given off as a gas. If a lot of oxygen is being given off, photosynthesis is
occurring rapidly. If little oxygen is being given off, photosynthesis is
occurring slowly. The amount of trapped light energy and the amount of carbon
dioxide available affects the rate of photosynthesis.
The purpose of adding sodium bicarbonate powder to the water increases the
amount of carbon dioxide in the water.
This investigation can be performed with water plants grown in many parts of
the world, except regions that have permanent ice.
You can use the same procedures with different color lights to see how different color lights may affect the release of oxygen produced by photosynthesis action.
Materials and Equipment:
Materials for tomato plant growth under different color lights
- 24 Tomato Plants
- White, Blue, Green, and Black Light Bulbs
- Water and Watering Apparatus
- 4 fish tanks
- 4 lamps
- Electrical Plug and Electrical Resource
- Black Construction Paper
- 12 Small flower pots
- Timer For Lights
- About 18 Cups of soil
Results of Experiment (Observation):
Experiments are often done in series. A series of experiments can be done by changing one variable a different amount each time. A series of experiments is made up of separate experimental “runs.” During each run you make a measurement of how much the variable affected the system under study. For each run, a different amount of change in the variable is used. This produces a different amount of response in the system. You measure this response, or record data, in a table for this purpose. This is considered “raw data” since it has not been processed or interpreted yet. When raw data gets processed mathematically, for example, it becomes results.
Make daily observations and water the plants as needed. Measure the plant height every 3 to 5 days and record the results in a table like this.
Plant height in different dates
By taking average plant height for each color light you can simplify the above results table to a new table like this:
|Color of the light||Average plant height|
* When taking average, you may choose to take the average of all daily observations or just the last day for the 3 test samples exposed to each color light.
Make a graph:
Use the above results table and make a bar graph to visually present your final results. Your bar graph may have one vertical bar for each color light you test. The height of each bar will represent the plant height or plant growth under one specific color.
You will need to calculate the average height of plants in each group.
Summary of Results:
Summarize what happened. This can be in the form of a table of processed numerical data, or graphs. It could also be a written statement of what occurred during experiments.
It is from calculations using recorded data that tables and graphs are made. Studying tables and graphs, we can see trends that tell us how different variables cause our observations. Based on these trends, we can draw conclusions about the system under study. These conclusions help us confirm or deny our original hypothesis. Often, mathematical equations can be made from graphs. These equations allow us to predict how a change will affect the system without the need to do additional experiments. Advanced levels of experimental science rely heavily on graphical and mathematical analysis of data. At this level, science becomes even more interesting and powerful.
Using the trends in your experimental data and your experimental observations, try to answer your original questions. Is your hypothesis correct? Now is the time to pull together what happened, and assess the experiments you did.
Related Questions & Answers:
What you have learned may allow you to answer other questions. Many questions are related. Several new questions may have occurred to you while doing experiments. You may now be able to understand or verify things that you discovered when gathering information for the project. Questions lead to more questions, which lead to additional hypothesis that need to be tested.
If you did not observe anything different than what happened with your control, the variable you changed may not affect the system you are investigating. If you did not observe a consistent, reproducible trend in your series of experimental runs there may be experimental errors affecting your results. The first thing to check is how you are making your measurements. Is the measurement method questionable or unreliable? Maybe you are reading a scale incorrectly, or maybe the measuring instrument is working erratically.
If you determine that experimental errors are influencing your results, carefully rethink the design of your experiments. Review each step of the procedure to find sources of potential errors. If possible, have a scientist review the procedure with you. Sometimes the designer of an experiment can miss the obvious.
Following images shows the experiment of testing the effects of different color lights on the growth of radish seedlings. Experiment is performed in Petri-dishes. Colored cellophane is used as a light filter. Filter paper is used to distribute moisture.
The Petri Dish Method for Germinating and Growing Fast Plant Seeds
Under Experimental Conditions
The seeds easily adhere to the wet filter paper that is placed in the top of the petri dish. The covered petri dish is then placed, almost vertical, in the bottom of a 2 liter soda bottle, which is filled with water or an experimental solution like 0.1% gibberellin.