Fighting pollution – how can duckweed help? – protocols

These practical protocols are designed for students doing an Extended Project Qualification, Advanced Higher investigation, or IB investigation in biology.

They are linked to our Fighting pollution – how can duckweed help? project starter.

Protocol: Studying growth and measuring its rate

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.

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? 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!

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.

Protocol: 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?

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?