The Duckweed Experiment: Effects of Lowering Light Intensity on the Rate of Per Capita Growth of Lemna Minor

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Abstract

Duckweed is a small aquatic plant that is able to grow rapidly, making it the ideal specimen for our experiment. It is hypothesized that altering the amount of light received by duckweed will alter its photosynthetic rate. It is predicted that a lower light intensity will lower the rate of growth in duckweed.

Two treatment groups were covered with a screen in order to reduce light intensity. Both groups were kept under a controlled light source for fourteen days and plant counts were taken at regular intervals. The ravg for the experimental group was 0.1613 and the ravg for the control group was 0.2047. The results indicated that our predictions were correct; duckweed that received less light exhibited a lower rate of growth. For those interested in harvesting duckweed, future studies can focus in determining the amount of light needed for optimal growth.

Introduction

Plants are able to convert light energy into chemical energy through the process of photosynthesis (Campbell & Reese, 2005). This process is dependent on both abiotic and biotic factors. Since plants are autotrophs, the most vital are abiotic factors such as light, temperature, wind, water, and atmospheric gases (Campbell & Reese, 2005). The plant of interest in this study is Lemna minor. Duckweed is a small aquatic watering plant that inhabits fresh water environments and exists in clusters of 3-4 leaves that are approximately 2-4 mm in size (Galt et al, 2005). Duckweed was chosen for this experiment because it can be manipulated easily within a laboratory setting.

This experiment was designed to test the hypothesis that altering the amount of light received by the Duckweed will alter its photosynthetic rate. Plants that receive more light are predicted to exhibit a higher rate of per capita growth because light is a necessary to carry out the process of photosynthesis. Plants that receive more light are also predicted to reach their carrying capacity sooner because of their increased rate of growth. The carrying capacity is defined as the maximum number of individuals that can sustain life given the limited resources of the environment (Campbell & Reese, 2005). Materials and Methods

In this experiment, four plastic cups were used. These plastic cups represented two control groups and two treatment groups. These four cups were filled with 200 ml of a culture growth medium consisting of a 0.075 gram∙liter-1 concentration of Stern’s Miracle-Gro 15-30-15 Plant Nutrients in distilled water. Added to each of the cups were approximately 20 duckweed individuals as the initial population. The fill line was also marked in order to accurately refill the cups with the culture medium as plant uptake and evaporation occurred.

The two treatment groups were covered by screens with approximately 1 mm sized holes. The screens served as a filter which would lower the light received by the treatment groups without interfering with gas exchange. Two groups were not covered by screens in order to serve as controls. These four cups were then placed under Chroma-50 full-spectrum fluorescent lamps and kept at a constant light intensity of 54.16 micromole of photons/square meter/second measured with the Li-Cor ML250 Quantum Light Meter. The photoperiod was 12 hours for each day. The cups were kept under the light for fourteen days, with data recorded on days 0, 5,7,11, and 13. Data recorded on these days included the population count, root length, cluster size, and leaf size. Once the data was recorded, it was analyzed for the per capita growth rate using the equation: r = (1/t) ln (Nt/N0)

In this equation, r is the intrinsic rate of growth. Nt represents the number of plants counted at time t, N0 represents the initial number of individuals within the population, and t represents the time (in days, for our experiment). Results

The data obtained from the experiment included the population count for both the treatment and control groups over...
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