Science and Plants for Schools – Student Project Starter




Дата канвертавання22.04.2016
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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 light-bank 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.


  • 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?


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