The influence of elevated zinc concentration on population growth rate of Lemna minor
Alonsyus van Windhopper III
201306666
Biology 203 Laboratory, Monday p.m.
7 November 20xx
Submitted to Mr. Jonas Johnson
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Abstract
High concentrations of metals in a plant’s environment generally inhibit plant growth and
cause metabolic disorders. While some plants can develop tolerance to increased metal content,
there is still a threshold limit which causes the plant to die when exceeded. In a previous study
carried out by ecology students at St. Francis Xavier University, it was hypothesized that zinc
from pennies placed in Lemna minor cultures inhibited normal population growth. Our
experiment was designed to test this theory by using the same control cultures as the original
study on five replicates of duckweed, and by using a growth medium containing 0.9 mg/L of zinc
sulphate on another five replicates. Subsequent growth of the two groups was monitored for two
weeks and a t-test was used to compare population growth during this time. The two populations
were not significantly different, meaning that the increased zinc concentration had no adverse
effects on duckweed growth.
Introduction
In certain environments, plants are exposed to high levels of heavy metals as a result of
either nature or human interference. While some metals, such as zinc, are essential for plant
growth in small quantities, most plants are unable to tolerate highly metallic environments. High
concentrations of heavy metals can remain in soil for long periods of time, potentially causing
metabolic disorders in plants, as well as inhibiting their growth (Peralta et al. 2000). In mustard
plants, for example, exposure to zinc caused deposits to accumulate throughout the roots and
stems. These plants also had lower water content, and exhibited serious damage to leaf and root
tissues. (Maruthi-Sridhar et al. 2004).
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Some plant species can develop a tolerance for metallic environments after prolonged
exposure. Matthews et al. (2004) found that two of three wetland species studied (Eriophorum
angustifolium and Juncus effusus) could survive zinc concentrations up to 1000 µmol L-1. The
other species studied (Juncus articulatus) could not tolerate the elevated metal concentration, so
it appears that zinc tolerance is not characteristic of all plants..
Our study was designed to test whether toxicity has an effect on the population growth of
duckweed (Lemna minor). A previous study in the St. Francis Xavier University ecology
laboratory led us to hypothesize that the presence of zinc (from pennies placed in growth
medium to control algae) inhibits normal duckweed population growth.
Materials and Methods
Duckweed was obtained from a pond in the Antigonish area and kept in the laboratory
under 430-Watt high pressure sodium lamps prior to and throughout the experiment. We
prepared two growth cultures, the control, as outlined in Table 1 of the Introductory Ecology
manual, (Taylor, 20xx, p. 17) and the treatment, which contained five times the amount of zinc
sulphate (0.9 mg/L) as the control. Ten beakers were prepared, five containing 90 mL of the
control medium and the other five containing 90 mL of the treatment medium. A penny (dated
prior to 1997, containing 0.25% zinc) was placed in each beaker. We then transferred 25
duckweed thalli into each of the ten beakers and placed them on a tray under the high intensity
lamp. Beakers were inspected every day for two weeks and the number of thalli in each beaker
was recorded. Distilled water was added as the growth medium evaporated to maintain the
90 mL volume of solution in each beaker. When all the data were obtained, we used Microsoft
Excel to carry out calculations, and we used a t-test to determine whether there was a significant
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difference between the number of plants in the replicates containing zinc and the number of
plants in the control condition. (Taylor 20xx)
Results
Figure 1 shows the mean number of thalli with a natural logarithmic transformation,
which should yield an approximately linear graph. Using Microsoft Excel, the intrinsic growth
rate r (slope of the graph) was calculated for both conditions. The r-value for the control group
(low zinc) was 0.1083345 and the r-value for the treatment group (high zinc concentration) was
0.105838. The critical t-value for the data set is 1.70 (using 28 degrees of freedom and p=0.05)
and the t-value found using the t-test is 0.0037. There is not a significant difference in growth
rate between the two duckweed populations. This result does not support our hypothesis.
Discussion
Our hypothesis was that duckweed growth would be inhibited in the presence of zinc, but
our results show that this is not necessarily true. The increased zinc concentration did not
decrease the L. minor population growth rate and the growth of both populations was more or
less identical. This could indicate that zinc concentrations do not affect duckweed, or, as
Matthews et al. (2004) found in their wetland plant study, that duckweed can develop a tolerance
to high concentrations of zinc in the environment.
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Figure 1: Mean population size of Lemna minor at high and low zinc concentrations over a two
week period. The mean number of thalli (plotted along the y-axis) has undergone a natural
logarithmic transformation.
There should be few sources of error in an experiment like this, since most confounding
variables known to affect population growth were monitored (light intensity, population density,
nutrient levels). These factors should not have influenced the population growth, which is why it
is difficult to explain why our hypothesis was not supported. If sufficient time were available, it
might be beneficial to repeat the experiment by varying only the penny composition in the
beakers (as this is the factor that initially led us to believe zinc toxicity had a negative effect on
growth). It might also be beneficial to repeat the experiment with a treatment containing a
higher concentration of zinc (greater than the 0.9 mg/L used in this experiment). The results of
this experiment show that the presence of zinc is not an inhibiting factor on the growth of
duckweed.
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References
Matthews, D.J., Moran, B.M. and Otte, M.L. 2004. Zinc tolerance, uptake, and accumulation in
the wetland plants Eriophorum angustifolium, Juncus effusus, and Juncus articulatus.
Wetlands, 24(4): 859-869.
Maruthi-Sridhar, B.B., et al. 2004. Anatomical changes due to uptake and accumulation of Zn
and Cd in Indian mustard (Brassica juncea). Environmental and Experimental Botany,
54: 131-141.
Peralta, J.R., et al. 2000. Study of the effects of heavy metals on seed germination and plant
growth on alfalfa plant (Medicago sativa) grown in solid media.
http://www.engg.ksu.edu/HSRC/00Proceed/gardeal.pdf (5 November 20xx).
Taylor, B.R. 20xx. Introductory ecology: Laboratory manual 20xx. St. Francis Xavier
University, Antigonish, NS, Canada.
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