Materials Included In Kit

Additional Materials Required

Safety Precautions

Plant materials (or any other materials) used during laboratory investigations should never be consumed. Seeds are often treated with mold inhibitors and other chemicals. Remind students to wash hands thoroughly with soap and water before leaving the laboratory.

Disposal

Seedlings may be disposed of in the trash according to Flinn Suggested Disposal Method #26a.

Teacher Tips

• Enough materials are provided in this kit for 30 students working in pairs or for 15 groups of students. The initial laboratory activity can be set up in 20–30 minutes. Observations and watering will require about 5 minutes a day until the final observations are recorded.

• Both lettuce and radish seeds sprout easily and quickly if kept warm and moist. Results from this lab are highly predicable. The seeds will sprout in several days and the full effect will be evident in 4–5 days.
• This topic tends to be of high interest to students and is also a good opportunity for students to design and conduct their own experiments as an extension to this laboratory. Experiments to show allelopathy usually require simple techniques and simple equipment. Encourage students to design controlled experiments. Then provide the time and equipment for students to conduct the experiments. When all experiments are completed, hold an allelopathy seminar to share results.
  
  The following list of common plants are known to have alleleopathic “toxins.” This list might be a good starting point for student experimental designs.
  
  Bleeding Heart 
  Calla Lily 
  Chrysanthemum 
  Crocus
  Daffodil 
  Hyacinth 
  Iris 
  Lily of the Valley 
  Magnolia 
  Monkshood 
  Morning Glory 
  Oleander 
  Pines
  Poppies
  Ragweed
  Rhododendron
  Rhubarb
  Sage 
  Sweat Pea
  Tulip
  Walnuts
  Willow
  Wisteria 
  Yew 
  
  What effects do extracts from these plant have on the growth of other plants? How might a landscape architect benefit from this knowledge? How about a florist? What happens if the lettuce seeds are given a head start?—will the radishes still inhibit the growth later?

Sample Data

Answers to Questions

1. Do radish and lettuce have an allelopathic relationship?
   

   It would appear from the results that radish seeds somehow affect the germination/sprouting of lettuce seeds.
   

2. Hypothesize what the mechanism of allelopathy might be between radish and lettuce?
   

   Perhaps a chemical found or produced by the radish sprouts inhibits the sprouting of the lettuce seeds.
   

3. Design an experiment that might demonstrate the mechanism working between radish and lettuce seeds.
   

   Extract chemicals from radish seeds and test them on lettuce seeds. Observe effects on the sprouting of the lettuce seeds.

Introduction

Have you ever noticed that plants do not seem to grow under pine trees? What is the reason for this? Can plants defend their own space and prevent others from growing in their area?

Concepts

• Allelopathy

• Toxicity

Background

Allelopathy is a process by which certain plants release chemicals which inhibit the growth of other plants. One of the best known examples is the black walnut, Juglans nigra, which contains a chemical called juglone, mostly in its roots, which is toxic to many species of plants. Tomatoes and many other vegetables suffer “walnut wilt” and die as soon as their roots come in contact with the roots of a walnut tree. Walnut chemicals cause allergic reactions in some people and may be toxic to some animals.

The exact mechanisms of how allelopathy works varies and is not well understood. There are most likely many cases of allelopathic effects of one plant upon another that have not yet been identified. In nature, by inhibiting potential competitors, allelopathic plants might enjoy an advantage in the struggle for growing space, soil nutrients, water and sunlight.

Materials

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 after handling seeds and other plant material.

Procedure

1. Make paper towel disks at least five layers thick to fit inside the bottom of three Petri dishes. To do this, trace the bottom of a Petri dish to draw a circular pattern on the paper towel and cut inside the pattern line to create disks just slightly smaller than the inside of the Petri dish bottom.
2. Place a five-layer paper towel disk in the bottom of each of the three Petri dishes.
3. Label the three Petri dishes as follows:

“L” (lettuce)
“R” (radish)
“L/R” (lettuce and radish)

4. Saturate the paper towel disks with water in all three Petri dishes. Try to use the same volume of water in each Petri dish.
5. Evenly space approximately 25 lettuce seeds in the bottom of the dish labeled “L.” Evenly space 25 radish seeds in the bottom of the dish labeled “R.” In the dish labeled “L/R” place equal numbers of lettuce and radish seeds (12–13 seeds each), interspersing them throughout the dish.
6. Replace the covers on the dishes and place the Petri dishes in a warm spot. A growth chamber, plant stand, greenhouse or other controlled environment is ideal. Otherwise, find a window sill or other location that will not get much below room temperature.
7. Examine the dishes daily. Add water to each dish as necessary to keep the bottom saturated with water. Notice which seeds have germinated and which seeds have not.
8. When the majority of seeds in both the “R” and “L” dishes have germinated and when the sprouts are nearly as tall as the Petri dish, record observations on the Allelopathy Worksheet. Draw a sketch of the seedlings and their numbers in the circles provided on the worksheet. Complete the Post-Lab Questions on the worksheet.
9. Dispose of the seedlings as directed by your instructor.

Student Worksheet PDF

10329_Student1.pdf