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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
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The average population on day 13 for the treatment group was 172.5 individuals and 307 individuals for the control group. Our results show that the group which received less light had a lower population count and exhibited a lower overall per capita growth rate. The average population count for duckweed in the treatment groups was lower than that in the control groups for each day data was recorded (Table 1). By day fourteen, the average population size of the control group reached 307 individuals, nearly twice that of the treatment group (Table 1).
Table I: Population count for duckweed in the presence/absence of a screen over a period of fourteen days
Day Average population count of Duckweed in the absence of a screen Average population count of Duckweed in the presence of a screen
0 20.5 19.5
5 53 45
7 85 65
11 185.5 138
13 307 172.5

Using the experimental data, we calculated the per capita population growth rate of Lemna minor using the equation r = 1/t∙ln(Nt/N0). The results showed that duckweed grown in the presence of a screen exhibited a lower per capita growth rate than duckweed grown in the absence of a screen (Table 2).

Table II: Calculated per-capita population growth rate (r) values for each group over a fourteen day period. All values measured in day-1
Day Duckweed grown in the absence of screen (1) Duckweed grown in the absence of screen (2) Duckweed grown in the presence of
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