II. Seed Cleaning

What is seed cleaning?

This is the removal of any debris or low quality, infested or infected seeds and seeds of different species (weeds) that are foreign to the sample.

Why is seed cleaning important?

The cost of maintaining an accession in a genebank is high and space is limited. Debris and damaged seeds can spread infection. Therefore only good quality viable seeds should occupy space in the store and damaged or non-viable seeds should be destroyed to prevent the spread of infection. Each accession should be represented in the genebank by a minimum number of clean seeds. FOR INFORMATION ON SAMPLE SIZE, SEE SECTION VII.

When should seed cleaning be carried out?

Seeds should be cleaned immediately after registration or harvest. If the seeds are very moist and cleaning is done by machines, it may be necessary to dry the seeds within the fruits to within 12 - 16% moisture content before cleaning to prevent damage. Seeds donated by other institutes often arrive already cleaned.

How should cleaning be done?

Cleaning should be done in a way that causes the least damage to the sample and does not waste good seeds. The following procedures can be used as a guide.

STEP 1. EXAMINE EACH SAMPLE TO DETERMINE THE STATE OF THE MATERIAL PROVIDED

1. Treat each accession separately.

2. Empty the contents of each packet into a labelled tray large enough for the material to be spread out.

3. Check whether the seeds are in dry fruits or fleshy fruits or have already been cleaned.

STEP 2. CLEAN THE SEEDS OF ACCOMPANYING DEBRIS

Seeds from dry fruits

1. Check that the seeds are dry enough to be cleaned without damage because moist seeds (above 16% moisture content) can be damaged during cleaning.

2. If the samples are very moist, place the container with the fruits in the drying room, if available, or in a shady place to dry.

3. When it is predicted that the seed moisture content is in the range of 12 - 16%, the seeds can be safely cleaned.

4. Clean the seeds from the fruits, preferably by hand.

5. Small threshing machines can be used for some crops, provided that they do not damage the seeds. Clean threshing machines between each accession to prevent mixing of seeds from different accessions. Mechanical cleaning is of less value in multi-crop genebanks.

6. Remove debris from the seeds by the use of gradated sieves.

7. Pieces of debris of a similar weight and size as the seeds will have to be removed by hand.

8. Separate dry lightweight material from the seeds by winnowing, blowing gently on the seeds or by use of a seed blower.

9. Return the cleaned seeds to the tray ready for inspection.

Notes and Examples

Seeds which are moist can be bruised and damaged during extraction. Seeds which are too dry are brittle and can easily crack. Cleaning by hand is gentler and will do less damage to the seeds.

With small samples it is more convenient and quicker to clean by hand because of the time needed to clean machinery between accessions. The flat bamboo baskets used in many countries are ideal for cleaning and winnowing.

Small containers and trays
Set of sieves
Bamboo baskets
Paper packets
Threshing machine
Seed blower
Spatulas

1. Some species have fruits which irritate the skin, e.g. Solanaceae, and gloves should be worn when handling them.

2. Open the fruit using a knife but take care not to cut through the seeds.

3. Extract the seeds from each fruit and place in a tray.

4. Remove any juice or flesh adhering to the seeds using clean cold water and a small brush to remove tenacious flesh.

5. If mucilage surrounds the seeds and cannot be removed by washing in clean water, gently rub the wet seeds with coarse sand and then wash off the sand and mucilage in a sieve. It is also possible to dry the seeds and then rub the dry mucilage off, but the seeds must be well separated, to prevent their sticking together during drying, on a surface to which they will also not stick.

6. Do not allow seeds to remain in the water too long because they will absorb moisture which can cause the start of germination and then cause a loss of viability when the seeds are dried.

7. Dry the seeds in a thin layer, preferably one seed thick, on cotton towelling or paper in the drying room, if available, or in a cool shady place.

Notes and Examples

Use your knowledge of the morphology of the fruit and the position of the seeds to ensure that seeds are not damaged by cutting.

It is often extremely difficult to remove flesh from small seeds. It may be easier to dry the seeds and rub off the flesh when dry.

Make sure that the paper or cotton towelling used is clean to prevent contamination and avoid affecting the subsequent germination.

Plastic bowls
Nail brush
Absorbent paper
Paper packets
Spatulas

1. If the seeds are infested with insects or infected with fungi, isolate the sample and dry the infested seeds in sealed containers with silica gel to prevent the spread of the insect.

2. It is a general rule that drying seeds to a low moisture content will prevent further growth of fungi and development of further reproductive cycles of insects, but it will not kill them.

3. Storage at sub-zero temperatures will kill the adult insects but not fungi. Larva and eggs are more difficult to kill.

4. If seeds are to be stored above 0°C, place in a freezer for about 7 days to kill the insects before continuing with the normal packaging and storage procedures.

Notes and Examples

The use of low temperatures prevents fungal growth, but the spores remain viable during storage and can begin to grow again when the seeds are sown or germinated. To avoid this, fungicides can be used at planting but should not be used during storage because some chemicals can reduce seed longevity over long periods.

Effective treatment to kill insects without the use of insecticides which may damage seeds during storage is also possible. Seed drying prevents further development and sub-zero temperatures for about 7 days will kill the insects and larvae. Seeds must be dry or viability will be lost by treatment at these temperatures. Large seed or seeds packaged in bulk may take longer to treat to ensure that the centres of the seeds have been exposed to the low temperature.

Freezer

1. Spread the seeds on a flat well lit surface of contrasting colour. An illuminated table can be used if available.

2. Examine the seeds carefully for physical damage and empty seeds.

3. Discard any visually damaged, shrivelled or empty seeds.

4. Empty seeds may be difficult to detect unless the seeds are dissected or tested with special X-ray equipment to determine the percentage of empty seeds (Ch. 8, Ellis, Hong and Roberts, 1985).

5. Empty seeds are lighter than full ones and can be separated by use of the seed blower.

Notes and Examples

Take care that lightweight full seeds are not discarded during this process. If in doubt dissect some of lighter seeds taken from the debris to check whether they contain embryos.

Seed blower
Light table
Scalpels
Forceps
Spatulas

1. Destroy any waste material to prevent spread of the disease or insects to other material.

Notes and Examples

If possible burn the waste material to ensure complete destruction.

Table of standard cleaning procedures for use in your genebank

Fill in this table to use for your future reference:

Species	Method