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VII. Seed Storage

What is meant by seed storage?

In a genebank, seed storage is the preservation of seeds under controlled environmental conditions which will prolong the viability of the seeds for long periods. Two types of seed stores are used for collections of genetic resources, those holding base collections and those holding active collections. The temperature, relative humidities, seed moisture contents, containers and distribution arrangements vary between these stores.

Why are seeds stored?

Seeds must be stored in a way which maintains their viability for long periods. Seeds left at ambient temperatures and relative humidities will lose their viability quickly whilst seeds stored in conditions of low moisture content and temperature will retain their viability for longer periods. Accessions held in a genebank are valuable and represent plants which are no longer available or which are endangered in their natural environment. These seeds must be conserved in the genebank for use in plant breeding in the future.

When should seeds be placed into store?

As soon as the seed has matured on the plant the slow process of deterioration begins. Therefore the sooner that seeds are placed into store the better. In practical terms this means that seeds collected in the field should be quickly returned to genebanks, processed and placed into store as soon as the cleaning, drying and packaging processes are complete.

How many seeds of each accession should be stored?

The IBPGR Advisory Committee on Seed Storage has recommended that for materials showing little morphological variation (genetically homogeneous) 3000 seeds are acceptable, but 4000 seeds are preferred, to represent each accession. For materials showing a large amount of morphological variation (heterogeneous) an accession should consist of at least 4000 seeds, but 12000 seeds are preferred. These sample sizes are recommended as number of seed but many genebanks would prefer to work with weight. An inter-conversion of numbers and weights is possible from the thousand seed weight of any accession. A list of approximate thousand seed weights of some common species is given in Appendix 2 of Cromarty, Ellis and Roberts (1982).

How should seeds be stored?

The conditions which prolong viability during storage have been well defined for seeds which are tolerant of desiccation. Storage conditions have been recommended by the IBPGR Advisory Committee on Seed Storage. For base collections, seeds of between 3-7 % moisture content should be stored in sealed containers. Sub-zero temperatures are acceptable, but -18 °C or less is preferred. For active collections sealed storage of seeds dried to 7% moisture content or less is recommended at temperatures of less than 15 °C. Unsealed storage is not encouraged. In particular, it is not recommended in tropical areas.

Table of thousand seed weights of species in your genebank

Fill in this table to use for future reference:

Species

1000 seed weight

number of seeds/g

















STEP 1. CHECK THE NUMBER OF SEEDS IN THE ACCESSION

1. Weigh the seeds of each accession.

2. Convert the weight of seeds to seed number, using the thousand seed weight of each accession for an accurate conversion. An approximate conversion can be done on a species basis by using the approximate weights given in Appendix 2 of Cromarty, Ellis and Roberts (1982).

3. For accessions containing mixtures of genotypes, the sample size should be at least 4000 seeds. For genetically uniform accessions, the sample size should be at least 3000 seeds.

4. If the sample contains more than the required number of seeds, proceed to storage.

5. If the sample contains less than the required number of seeds, either proceed directly to regeneration (See Section X) or store the seeds temporarily in the genebank and regenerate at the earliest opportunity.

Notes and Examples

An approximate inter-conversion of seed number and weight can be done easily using the thousand seed weight.

Example for Sorghum (1000 seed weight = 17 g)

100 g contains: .

These are minimum sample sizes for the start of storage and if both space and seeds are available, more seeds should be held.

Regenerate as soon as possible. Seeds processed and stored under good conditions will not loose viability before regeneration.

Equipment
Coarse balance
STEP 2. DETERMINE WHERE THE SEEDS SHOULD BE LOCATED

1. Check the inventory data file of the genebank to find the next available space where a container can be located.

2. When seeds from the same regeneration cycle of the same accession are stored in several containers, keep all the containers of the accession together.

3. Make a list of where each accession will be placed.

Notes and Examples

The storage arrangements will vary among genebanks. The most important point is to know exactly where to locate each accession within the store.
Equipment
Racks
Trays, boxes or drawers
Coldroom or freezer
STEP 3. PLACE IN THE SEED STORE

1. Place the container into the seed store in the listed location.

STEP 4. ENTER THE DATA INTO THE DATA BASE

1. Fill in the data on the location and date of storage of each accession and each container into the data file.

2. Record the date of the next monitoring test for germination in the data file. This date will be determined by the curator after considering the viability and moisture content of the seeds, storage conditions and the IBPGR recommendations.

Notes and Examples

A code can be used to locate an accession within the store. Each unit can be identified by a number or letter in the store. The code can indicate the number or the letter of the freezer, store, rack, basket or drawer, etc.

Example:

A010201 could be used to indicate the location as:



Coldroom:

A



Rack:

01



Shelf:

02



Box:

01

Colour codes can also be a quick and easy way to locate accessions. A colour can be used for each rack, shelf or species. This both speeds up the work in the coldroom and makes it easy to spot errors. Owing to the very cold temperatures, the faster that one can locate accessions in the cold room the better.

Arrangement of your seed store

Draw a diagram of and explain the coding system used in your seed store:


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