Download investigations with duckweed - Science and Plants for Schools

Science and Plants for Schools – Student Project Starter
Doubling and dabbling in duckweed
This is a project starter, suitable for Advanced Higher biology investigations or A-level extended
projects. Don’t forget to credit this resource in your bibliography by including the title, the website,
the web address and the date you accessed it.
Background information
Duckweeds look like simple plants: small, green, and with no flowers. But this
simplicity makes them ideal experimental organisms for labs, or for your own
investigations. Using duckweeds, you can investigate a variety of environmental
topics, including pollution, competition, and many more.
The duckweeds (Family Lemnacea) are a small and cosmopolitan group, found
from the sub-polar regions to the tropics. They all prefer ditches, ponds, lakes and
slow-flowing rivers. There are, surprisingly, only 22 species in the world and most
are very widespread. One can imagine them jet-setting from one continent to
another on the feet of birds. All have a few flattened, often rounded, disc-like
leaves (they are really leaf-like stems or thalli) floating on the surface. New leaves
bud off from these as the plant grows. Once there are half a dozen leaves the
plants break apart. Often there is an unbranched rootlet hanging below the leaves.
Some of these rootlets are photosynthetic. The table and diagrams describe some
duckweeds you may find in your locality, identified by their individual leaf size, leaf
shape and rootlet characteristics.
There are at least 5 different species in Britain. Lemna minor is undoubtedly the
most often encountered, but the smaller Lemna minuta has recently been
spreading rapidly in Britain. Spirodela polyrhiza is less common but much the
largest and most elegant.
Experiments with duckweeds
Investigating optimal growth conditions
Surprisingly fast rates of growth are achieved if plants are grown next to bright strip
lights at 25 C. Optimal rates are with more than 15 hours of light per day. They
need rich nutrients, plants grow well with about 50 grammes of potting compost to
the litre of water. Experiments can be conducted on the nutrient requirements, on
the effectiveness of different types of artificial light, its intensity, colour
(wavelength) and on total day length. Colour filters and an electric timer are useful
for such projects. Duckweed can be grown easily in a variety of containers such as
old yoghurt pots.
Studying growth and measuring its rate
Daily counts of the numbers of leaves will give a rapid estimate of growth.
Graphed, these will show an exponential (multiplicatory) rate. On a log plot there
should be a straight line, until factors become limiting. How fast a growth can you
achieve and how short a doubling time? Can you work out a mathematical formula
for calculating duckweed doubling time? If you cannot, challenge your Maths
department! The fastest duckweed I have recorded has a doubling time of 2.4
days. Can you beat it? Once an alien duckweed invaded a lake in Africa and
covered 200 square kilometres in 6 months...duckweeds can be fast plants.
A fraction of the duckweed can be collected every few days from the culture and
dried on a filter paper to estimate dry weight productivity (use the top of your lightbank as a drier). You need a fine balance. Duckweed weight gained, per unit area,
approaches yields comparable to cereals! Duckweeds are readily eaten by ducks.
In the summer the ducks certainly cannot keep pace with duckweed production.
What area of duck-pond does a duckweed-feeding duck need? Do goldfish eat
duckweed?
Species
Leaf size
Leaf character
Rootlets
The great duckweed
Spirodela polyrhiza
large 5 - 8 mm
shiny and circular, floating
many up to 30
mm long
The lesser duckweed
Lemna minor
small 1.5 - 4
mm
opaque and more or less
elliptical, floating
single to 15
mm long
The least duckweed
Lemna minuta
very small 1 2.5 mm
elliptical, floating
single to 8 mm
The ivy-leaf
duckweed Lemna
trisulca
elongated 5 to
15 mm
spear-shaped with opposite
branches, submerged and
translucent
single and
hooked
The gibbous
duckweed Lemna
gibba
very small 3 5 mm
ovate, convex below, fat and
bouyant
single to 6 mm
One way to record growth is to make regular counts of fronds and plot a graph.
Under good growing conditions, this will often show an exponential growth rate.
However, the opportunities for counting may be few, so a better method is to
calculate D. To calculate this, do a frond count at the start and at the end of any
period of time. Then use the formula below. It does not matter how many fronds
you start with or how fragmented the clusters are as long as the time interval
between any two counts is known.
n = the number of days over which growth is measured
Fo = the number of fronds at day 0
Fn = the number of fronds at day n
D = the doubling time of duckweed (in days)
D = {(nlog2) divided by (logFn - logFo)} days
As an example. if at day 0 there are 8 fronds and at day 4 there are 18 fronds,
then
D = {(4 x log2) divided by (log18 - log 8)}
= {(4 x 0.301) divided by (1.255 - 0.903)}
= 3.4 days
You may now ask your duckweed what conditions it likes most? Set up a series of
beakers with differing nutrient concentrations and find out which produces the
shortest doubling time. Start at one extreme with a rich fertiliser addition and dilute
it down by half with distilled water in a series of containers. Jam jars will do.
Illuminate the duckweed from above, either with a light bank or other good light
source. Put a little duckweed in each container and then do the initial count. One
week later a second count can be made and the value of the doubling time for
each can be calculated.
Assaying Environmental Pollutants
Once you have found the ideal nutrient and lighting conditions for your Lemna then
you can begin to assay environmental pollutants. Using the ideal nutrient solution,
make serial dilutions of environmental toxins such as weed killer or heavy metals
like copper. How is doubling time affected by the pollutants you investigate? Can
you relate your research to real life pollution incidents in farm ditches? Bioassays
of this kind are used by water authorities in assessing some pollution incidents.
Competition between species
Try growth rate races between different species.
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Do they compete best under the same or under different conditions?
If you can get 2 different species to grow well, you could let them fight it out in
a beaker.
Which species does best in the end?
Why did the winner win? What is the adaptive significance in the diversity of
root shapes and lengths?
Why are some duckweeds rare?