OSMOSIS 6, SUMMER 1993

 

Contents

Bioassays with duckweed
Noticeboard (Zum Zum or Buzz Buzz)
Your Questions Answered
The response of seedlings to light
Brassica gigantea

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Bioassays with duckweed

In the last issue of Osmosis we introduced you to the great variety of English duckweeds. The most common and vigorous species is the lesser duckweed, Lemna minor. It is eaten by ducks and fish but its growth rate is truly phenomenal. Each leaf-like frond can grow new fronds until the cluster falls apart at the 3 to 8 frond stage. These fronds are easily counted by eye without a lens. Grown in culture a doubling time of less than three days may be achieved. 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 or interrupted by other classes, 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. 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.

Stephen Tomkins

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Noticeboard

"Zum zum" or "Buzz buzz"?

Do you use beesticks for cross-pollinating rapid-cycling brassicas (and other plants) as recommended in our Teachers' Guide and Student Sheet? If so, encourage your students to do some quiet buzzing when they are pollinating.

At recent workshops teachers have told us about the words which are used in other languages to describe the noise made by bees.

Why not develop this into a multi-cultural, cross-curricular topic? You could combine it with a drama activity in which your pupils design and make costumes for the various parts of the flower - anthers, petals, stigma, etc - and, of course, for the bee. We find that ping-pong balls covered with 'velcro' and painted yellow make good pollen grains - and they stick well on to wool, so a woolly hat makes a good 'stigma'.

What noise do bees make throughout the world?

1. gungunane	A. Hindi
2. buzz buzz	B. German
3. ong ong	C. Sinhalese
4. mulaka	D. Romanian
5. zum zum	E. Malayalam
6. bhin bhin	F. Afrikaans
7. biz biz	G. Urdu
8. bhinbhanana	H. Marathi
9. zoem zoem	I. Garo
10. gumu gumu	J. English

Key: 1 H; 2 J; 3 I; 4 E; 5 B; 6 A; 7 D; 8 G; 9 F; 10 C.

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Your questions answered

Q. Can rapid-cycling brassicas be used for teaching genetics?

• Genetics is one of the most exciting topics which students can tackle with rapid-cycling brassicas and we are starting to get some interesting feedback from schools on this subject. For example, Jane Preston who teaches at Hitchin Girls' School, writes: "We have crossed rosette with elongated internode and collected plenty of seed. We sowed 156 seeds and the F1 generation produced almost equal numbers of rosette and intermediate height plants - the same height as wild type previously grown." Rosette and elongated internode are said to be self-incompatible but Jane Preston's students removed the stamens from plants chosen as female parents as an added precaution.

  Has anyone else done this cross and, if so, what results did you obtain? Please write and let us know.

Q. What about root tip squashes?

• John Ward of St Vincent Sixth Form College, Gosport writes: "We have tried root tip squashes with fast plants but these turned out to be very disappointing - the main problem being the tiny size of the nuclei. We were unable to see any stages in mitosis with any certainty." However, he has had great success with pollen tube growth and his students made a video of this using time-lapse photography.

Q. Are other growing systems suitable for rapid-cycling brassicas?

• Mark Smith of Leeds Grammar School writes: "Whilst the standard technique for growing rapid-cycling brassicas in polystyrene minipots is effective, filling the pots with growing medium and setting up the wicks can be messy and fiddly. The use of cellulose plugs from commercial plant culture techniques offers a fast, clean and flexible alternative. The plugs are small cylinders of cellulose fibre (2 cm height and 1 cm diameter) which will draw up a nutrient solution so wicks are not needed. Seeds can be placed singly or in groups on the top surface and, after germination, roots grow down into the plug. Plant and plug become an easily transferable unit.

  Suitable sized cellulose plugs can be obtained from Baumgartner Papiers SA, Churchdown, Bordyke, Tonbridge, Kent TN9 1NR, UK (Tel: 0732 350407). They are supplied as 'Sorborods' and are packed in clear plastic trays of around 100. The trays come with lids and are intended as plant culture chambers. Four weeks are needed for dispatch (in UK). The trays can be flooded with complete nutrient solution to about 1 cm depth for batch growing of plants prior to distribution to pupils for investigations. Alternatively, single plugs fit neatly into specimen tubes, or film cans, for individual plants. For easy handling, the lid of the tray can be used as a base to group a whole series of specimen tubes or mini-beakers, and the base becomes the lid. Evaporation of the nutrient solution is therefore reduced until the plants reach lid height. Such trays can be placed under a light-bank in the normal way. Plugs with established seedlings can be planted into a solid growing medium at any stage.

Q. When using pelleted fertiliser to grow rapid-cycling brassicas I find that the resulting seedlings sometimes do not grow well. Despite good germination they often show yellowish patches on the cotyledons and appear to stop growing after about 4 days. If development does continue the plants are stunted and yellowed. What is going wrong?

• Barry Meatyard of the SAPS Unit at Warwick University says: "This problem is probably due to the use of fertiliser pellets which are past their 'sell by' date. Pelleted fertilisers such as 'Osmacote' and 'Ficote' are designed to release their nutrients over a period of time which often extends for several weeks. However, if the pellets are stored in damp air the binding agent may break down. When the pellet is then exposed to damp compost or soil the nutrients are released immediately, providing an 'overdose'. We understand that new legislation will be introduced shortly to put a 'use by' date on packs of fertiliser pellets. A solution (literally) to this problem is to use a liquid feed in the reservoir rather than pellets in the 4-pots. Good uniform results have been obtained with general purpose 'Algoflash' (NPK 7:7:7) fed at the rate of 3 cm³ per litre, but there is much scope for experiment here to find the optimum concentration."

We are pleased to recommend a booklet called Investigations with rapid-cycling brassicas and other plants, by Colin Wood-Robinson and Dianne Cook. It is published by the Centre for Studies in Science and Mathematics Education, University of Leeds, Leeds, LS2 9JT, UK. The booklet, which costs £6 incl p & p (UK price only), contains worksheets from Colin's highly successful plant science workshops for teachers and postgraduate students. These INSET activities provide plenty of ideas for investigations with plants.

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Brassica gigantea

1 Use the template (printed from this link) to cut out the shapes from a squeezy kitchen mop sponge. Insert the shoot/root into the hole in the cotyledons. Wet the sponge then squeeze out the excess water.

2. Fold the sponge in half	3. Fold in half again
4. Fold the radicle (root) round the edge and bind very tightly with thick cord or boot lace. Leave to dry for 2 days in a warm place.	5. When completely dry, remove the cord. The 'seed' is now ready.

6 Put the 'seed' in a clear glass bowl of water and watch germination take place before your eyes.

Questions

What happens to the seed just after you put it in the bowl of water?

Which part of the seedling emerges first, the radicle (root) or the plumule (shoot)?

What conditions does a seed need in order to germinate?

Which part of the seed is missing from our model?