- In this lab you will prepare six cultures of duckweed each at a specific pH, observe the growth, and graph the data. Both biotic and abiotic factors influence the growth of individuals and populations. The pH of an aquatic environment is an abiotic factor which can dramatically influence the growth of plants. You will be able to determine the optimal pH for the growth of duckweed.
- Duckweed is a small common aquatic floating plant(see Figure 1) with leaf like lobes, and roots extending down into the water. Each lobe is actually considered a separate plant. You can collect duckweed from a local pond, or purchase it from a biological supply.
- The growth of the plants occurs by mitosis and can be quantified by counting the number of new buds found on the plant after a period of time. A population of plants establishing itself in a new area will exhibit the S-shaped curve that we see so many times in science. See Figure 2.
This growth curve consists of three phases:
-
1. Lag phase - A period of slow growth.
2. Exponential growth phase - A period of very rapid growth.
3. Equilibrium phase - The population maintains itself at a fixed level.50-mL graduated cylinder
*6 containers of culture media
- each with a specific pH (4, 5, 6, 7, 8, 9)
6 sterile petri dishes
duckweed
Marking pens
Inoculation loops
Wash bottle with distilled water
Light source
Graph paper- *Culture media can be made with dechlorinated tap water and a liquid fertilizer. The pH can be adjusted down (more acidic) with a dilute solution of phosphoric or sulfuric acid, and up (more basic) with a dilute solution of sodium hydroxide or potassium hydroxide. Try a 0.5 or 1.0 Molar concentration. Remember, it is easier to adjust the pH of a solution when you have a larger volume. Use test papers or a pH meter.
PROCEDURE:
If you are doing this as a class, different students or pairs of students can be assigned one or more pH values.
1. Take 6 sterile petri dishes and label each with your initials and one pH value starting with 4 for the first dish, continuing until the last dish is labeled 9. Label on the side edge of the dish.
2. To each petri dish, add 25 mL of the culture media at the given pH marked on the dishes. Remember: if you use the same graduated cylinder to measure the solutions, rinse it with several small rinses of distilled water after each use.
3. With an inoculating loop, transfer 15 duckweed plant lobes to each of the petri dishes. Each lobe is actually a plant, although you often consider a three-lobed structure a plant. It is important, for comparison, that each pH culture start with the same number of lobes. Record the total number of lobes for each pH in your data table for day 0.
4. Place your 6 labeled petri dishes on the counter under the light and allow to remain undisturbed for the duration of the experiment.
5. Observe the plants in the petri dishes each day and record the number of lobes in each dish on the data table. Remember: new lobes will be small, so look closely.
DATATABLE: Number of lobes counted for each pH. 
pH of solutions Day 4 5 6 7 8 9
0
1
2
3
4
5
6
7
8
9
10
11
12
13
14DATA ANALYSIS:
5. Graph your data for each pH level, from the data table, with the independent variable on the x axis (days) and the dependent variable on the y axis (number of lobes). Be sure to title each of the 6 graphs.
6. From your graphs, what is the optimal pH for the growth of duckweed?
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Optimal pH _______
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- Duckweed is a small common aquatic floating plant(see Figure 1) with leaf like lobes, and roots extending down into the water. Each lobe is actually considered a separate plant. You can collect duckweed from a local pond, or purchase it from a biological supply.
OBSERVING THE EFFECT OF pH
ON THE GROWTH OF DUCKWEED
INTRODUCTION: