This activity can be adapted for students in 4th – 9th grade.
Part 1: Students will answer the question “How does light affect germination and growth?”
This part consists of an experiment in which students will keep fava bean plants in two treatments: light and darkness. Students will compare germination and growth between these two treatments over a period of 2 to 3 weeks.
Part 2: Students will answer the question “How does light affect the nutritious quality of the plants?”
This part is an extension of Part 1. Students will use herbivorous insect pests, aphids, sometimes called green flies, to test how the nutritious level of the plants is affected by the light and darkness treatments. Aphid growth and development are influenced by the quality of their food. So, the aphids feeding on plants previously kept in darkness, which will be less nutritious, will grow and develop at a slower pace.
- Fava bean seeds
- 18 Plant pots and saucer plates (for underneath the pots)
- Compost (sufficient for all the pots)
- Cardboard boxes big enough to put the entire pot inside and leave space for the plant to grow. Keep in mind that plants inside the boxes will grow taller than the plants outside
- Ruler to measure plant height
- Aphids. Ideally, students should use aphids that are big and can feed on fava bean plants. These three aphid species are usually easy to get: pea aphids (Acyrthosiphon pisum), potato aphids (Macrosiphum euphorbiae) and green peach aphid (Myzus persicae).
- If this experiment is carried out in Spring or early Summer, aphids can be collected from outdoors. If this is not possible, companies or research institutions working with aphids could be contacted to supply aphids for the activity
- Paintbrushes to place the aphids carefully on the leaves (it will be explained how to take the aphids so they don´t die while transferring them between plants)
This is a long-term experiment that will take approximately 4 weeks. In the first 2-3 weeks, the students will do Part 1. The teacher should check the plants so they don’t die before Part 2 has started. If the plants are not doing well, Part 2 can be started earlier. The last week the students will do Part 2 which will last approximately 1 week or 10 days, depending on how fast the aphids grow and develop.
Before the experiment starts:
- The teacher will soak the fava beans overnight before the day the experiment starts.
The first day of the experiment:
- Fill the pots with compost and put the saucer plate underneath.
- Place one fava bean in each pot. The fava bean will be approximately 1 cm deep in the soil and covered with a bit of compost (do not press the compost).
- Water the pots so the compost is humid.
- Separate the plants in groups (see below).
- Assign which plants in each group will be in darkness and cover these with a cardboard box as seen in the picture.
Continuation of the experiment:
- Remember to water the plants regularly so the compost is moist. It’s OK that during watering, the plants in darkness receives a bit of light. This will not affect the experiment.
- Students will mark the day in which the plants start to germinate.
- Regularly, students will measure how tall plants are and the number of leaves that they have. Although measuring plants every 2-3 days is ideal, the regularity of the measurements will be determined by the teacher according to availability.
- Data will be collected on the data collection sheets provided here.
Transition from Part 1 to Part 2:
- Remove the boxes that were covering the plants and keep the plants in the same position. All plants are now exposed to light.
- Place a gravid (pregnant) female on each of the plants and leave her overnight to give birth to small aphids or nymphs (see below “How to distinguish a gravid female from a nymph”).
- The next day, the gravid females will be removed and only 2 nymphs will be left on each plant.
- At regular intervals, the students will observe the nymphs on each plant. They do not tend to move so much on the leaf.
- Students will record when these nymphs become gravid females. If one of the nymphs becomes a gravid female and the other one does not, remove the gravid female and leave just the nymph until it becomes a gravid female. When both nymphs living on the same plant have become gravid females the experiment on that plant is finished. This plant can be taken home by the student, planted in a garden, or thrown away.
- Ideally, the experiment should last until all nymphs become gravid females. Alternatively, the experiment could last until all nymphs in the plants previously treated with light become gravid females. If this is not possible, the experiment could finish when exactly half of the nymphs have become gravid females.
The experiment will be organized in blocks or experimental units. Every block will be the same, i.e. will contain the same number of plants and an equal number of plants will receive light or darkness treatment.
Experimental units (aka blocks):
- There will be 3 blocks. Each block will have an assigned number that will not change through the experiment.
- A block is made of 6 plants: 3 of these plants will be outside the box (light treatment) and 3 of these plants will be inside the box (darkness treatment).
- The figure below is an example of how to design the experimental layout:
- The position of each plant in the block must be randomized. This is important because if different locations have slightly different conditions, this will not affect the results of the experiment.
- The blue surface on the figure could be for instance a bench next to a window in the classroom.
- Each rectangle represents a block or experimental unit. Blocks could be one next to the other as in the figure but if this is not possible they can be placed further apart from each other.
- The circles represent plants.
- The numbers inside the circles represent the treatment that each plant should receive: 1-3 plants will be kept with light and 4-6 plants will be kept in darkness.
- If the teacher or the students want to randomize the position of the plants in the block themselves, here is an easy tutorial.
How to distinguish a gravid female from a nymph?
In early spring and summer, aphids reproduce asexually (see “Aphid life cycle” below). This means that aphids do not need to mate. Adult females are gravid and give birth to nymphs as seen in these pictures. We can distinguish gravid adult females from nymphs as adults are much bigger than the nymphs. Additionally, If a big aphid (adult gravid female) is found amongst little aphids (nymphs) we can be sure that this is a gravid female. These gravid females will be used to start the experiments in Part 2.
Aphid life cycle
The life cycle of aphids has a sexual and an asexual phase. In the sexual phase, mating between female and male aphids is required to lay eggs. These eggs will be left in protected areas in plants to overwinter. Then, when spring arrives and the weather is suitable for aphid growth, aphids will hatch from the eggs. These aphids will reproduce asexually, meaning that they will all be females who do not need to mate to give birth to new aphids. For more information you can check this video from SciShow:
How to pick the aphids without damaging them?
Aphids are sap-sucking insects and for feeding, they use a mouthpart called a stylet. If aphids are not manipulated with care the stylet can be broken and the aphids die. So, to avoid this we need to first touch a bit the aphids with the paintbrush. When the aphids feel this light touch they stop eating and remove the stylet from the leaf. At this moment is when they can be transferred to the new leaf.
Start by explaining what photosynthesis is and what is the role of light for the photosynthesis. Explain how plants take carbon dioxide from the atmosphere and with the energy of light transform it into organic compounds releasing oxygen back to the atmosphere. This could have been explained in a previous lesson as well.
Once the students hold the knowledge to understand how important light is for plants, start a discussion asking:
- What do you think would happen if plants did not receive light?
- Plants will not be able to get carbon dioxide from the atmosphere to accumulate as organic compounds. What consequences could this have for growth?
- Do you think plants will grow more or less when they don´t receive light?
- How do you think this can affect herbivorous insects that feed on plants?
- Do you think herbivorous insects will prefer plants that have grown in light or darkness? Or do you think they will not mind?
When the experiment is finished, students should pool all the data they collected on a single datasheet. This will enable cooperation between all the students.
From the data, the students can calculate:
- Average time until the fava beans start to germinate (in days).
- Average height of the plants at each of the times this was measured. If plants were measured every 2 days, then we will have an average number for every 2 days.
- Average number of leaves at each of the time points this was measured.
- Average time (in days) for the nymphs to become gravid females.
With these calculations, students will make the following graphs:
- Bar graph with one bar representing the average time until germination for plants kept with light and another bar with plants kept in darkness. A similar graph for the time until aphid nymphs become gravid females. Example of the graph:
- A line graph for the averages obtained from height and number of leaves. Example of graph:
This experiment was designed by biologist Pilar Morera Margarit from Easy Peasy Science.
A great follow up reading assignment is our scientific article about a similar botanical experiment, adapted from the peer-reviewed journal PLOS One.