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Seedy practicals - resources for secondary teachers

Seeds are back in the revised GCSE specifications!

Resources

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Mini Seed Bank
Seed sources
Germination of seeds on one per cent agar plates
Seed germination studies
Mycorrhizal association demonstration
Measuring radicle length
Viewing root hairs
Extraction of AUXIN

Mini Seed Bank

Seeds, like other parts of a plant, are adapted to their ecological niche and are affected by environmental changes, whether these are chemical or physical. Seed experiments can be used to look at plant adaptations, climate change, conservation, genetic variation, selective breeding and affects of commercial herbicides.

A low-tech highly developed seed bank used by conservation organisations worldwide is available online, and from Kew Gardens and Wakehurst Visitor Centres at £24.99 +P&P.

The kit enables you to demonstrate the seed banking process in the classroom by collecting, processing, drying and storing seeds (including commercially purchased seeds), following the same principals of seed banks, such as the Millennium Seed Bank at Wakehurst, Ardingly.

Seed banking prolongs the viability of seeds. Under the cold, dry conditions of a seed bank, seeds can survive for hundreds and possibly thousands of years. Seeds are stored at -20°C at 15% relative humidity (RH) in the dark. Research has shown that for every one percent decrease in seed moisture content, the seed storage life doubles and for every 5.5°C (10°F) decrease in storage temperature the seed storage life doubles at temperatures above freezing. Light degrades the viability of stored seeds.

Buy a Mini Seed Bank for your class

Seed sources

Speedy seeds:

Obtained from Suttons. ‘Speedy Cress’ and ‘ Speedy Mustard,’ germinates within 36 hours and the radicle ( root) emerges within 48 hours.

Native seeds:

Can be collected from the wild e.g. Taraxicum officinalis (Dandelion), Myosotis sp.(Forgetmenot), Digitalis purpurea (Foxglove).
Can be purchased from native suppliers easily found on the web.

Take care handling seeds as some are toxic. Always use fine forceps and never use fingers.

Germination of seeds on one percent agar plates

Preparation:

Mix ten grams of agar (NOT nutrient agar) in one litre of distilled water.
Microwave to just below boiling to dissolve the agar. Stir intermittently.
For safety cover a double the volume plastic container with clingfilm. Take care, agar super critically heats and can boil over with a slight tap. Always use heat proof gloves, protective eye wear and clothing when handling.
Allow to cool to around 65°C.
Pour into clean petri dishes. Approximately 20ml per plate.
Leave to cool and replace lid.
Plates can be stored in a refrigerator for a few of days. Use within a week as production is not undertaken using aseptic conditions.

Seed sowing:

Label base.
Sow in rows for ease of recording. Four rows of five seeds are possible.
Use sellotape to secure the lid to the base. DO NOT seal totally around the perimeter as aerobic conditions required for seed germination.
Record either the number of germinating seeds (emergence of radicle), or radicle length.
Dispose of used petridishes in an autoclave or equivalent as fungal growth occurs on agar after seed sowing.
To slow down seed germination and radical development, place petri dishes into the fridge at between 3°C to 5°C.

Seed germination studies

Method:

Using forceps place five seeds, of one type in a row, on the surface of the agar. Repeat this with up to three more rows of five seeds. Add the date/time/initial, and label seed types using a marker pen and labelling around the perimeter of the dish. Seal with two pieces of tape - one each side of the dish.

Intra specific variation:

Sow one species of native seed (not plant bred) and look for variation of germination time. The variation in such controlled environment is intra specific variation of germination.

Inter specific variation:

Sow more than one species of seed and count the number of germinating seeds over a fixed time period. The variation in germination in a controlled environment between species is inter specific variation. That is variation between species.

Selective breeding demonstration:

Using a pack of supermarket mustard cress, determine what selective traits were bred to produce mustard cress packs ready to eat within seven days. Test the hypothesis by comparing speedy cress seed germination with wild cress seed germination over a week to ten days.

Mycorrhizal association demonstration

Mycorrhizal association between plants and fungi has been found to be far wider than originally believed. Taraxicum officinalis (Dandelion) seeds can be collected and plated out on the agar medium. Germination of dandelion seeds takes place in around seven to ten days. However, within two days, a halo of fungal mycelium can be viewed around each seed. This fungal growth could be the result of:

background contamination or
mycorrhizal association with the seed carrying its own fungal associate when dispersed.

Place wild (NOT purchased) seed on agar with forceps. Leave for 24-48 hours and view fungal ‘halos’ around each seed. Hold up to window or lighting and view fungal halo around seed.

This would be an ideal extended project investigation.

Scientific knowledge of seeds and mycorrhizal association is limited, so a definitive statement saying this is mycorrhizal association is not possible. However this leads into further meaningful investigations:

Sterilise the seeds with Milton tablets and compare growth with non sterilised seeds.
Collect seed from different populations of a single species and investigate whether it is one species of fungus, or a mixture/the same or different fungus between different populations/comparisons of fungi between different seed types.

Further information:

Organisations such as the Society Of General Microbiology, Microbiology in Schools Advisory Committee, British Mycology Societymay be able to assist in fungal identification.

Measuring radicle length

This is an ideal opportunity for students to choose appropriate measuring tools to give precise readings.

Measuring germinating radicles can be undertaken using laminated graph or OHT graph sheets of different square size. Students are able to record radicle length over a period of time without opening the plate lid, or removing and disturbing the seedlings from the plate, by tracing the radicle onto the film, and then using cotton/string to measure its length accurately.

Viewing root hairs

Root hairs increase the surface area for absorption of minerals and movement of water into the root by osmosis. They occur a short distance behind the root apex in an area called the piliferous zone.
Germinated seeds on agar plates allows the roots and hairs to be visualised undisturbed and can been seen intact.

Use a x4 magnification lens (x40 total magnification) without taking the lid off.
Observe the increase in surface area produced by root hairs.
Focus on tip of one root hair and focus down towards the root surface. By look carefully it is possible to observe one root hair emerging from one root surface cell.

Extraction of AUXIN

Auxin is a plant hormone produced in growing tips such as the root and shoot apical meristems. It causes cell elongation by increasing the production of cell wall. Auxin is broken down by exposure to light and this property plays a key role in plant tropism reactions.

Auxin can be extracted in water from Salix sp.(willow trees). Its affect on radical development can be undertaken.

Extraction method:

Harvest four long young willow stems (whips) of this years growth.
Remove leaves and cut into roughly ten centimetre lengths.
Place all the cuttings in 400ml of distilled water, making sure all the stems are immersed. A plastic ice cream type container is excellent for this.
Seal using a lid cover with tin foil to prevent the light from entering and breaking down auxin.
Leave for 12 to 24 hours.
Decant the supernatant containing the hormone extract.

The auxin extraction supernatant must be used within 24 hours as it will contain plant sugars and nutrients that will promote microbial growth when used.

The extract can be used (instead of the distilled water) to make up the one percent agar solution for seed plating (before heating).
Use agar made up in distilled water as the control.

Plate out ‘Speedy Crest’ seeds and:

Observe radical (root) development after 36 to 48 hours.
Dilutions can be made to investigate potency of auxin extraction.

Auxin produces thicker, stunted root growth, which is often contrary to expectations. At these high concentrations auxin is toxic and stunts root development NOT stimulating growth.

For safety, all plates should have the lids attached to base to prevent removal and disposed of using the method used for microbial plates.