Improving the identification, handling and storage of “difficult” seeds ...

2.2. BOTSWANA presentation
Botswana National Plant Genetic Resources Centre
Mr. Tlhaloganyo Ounce Ofentse
Germplasm storage facilities (question 5.3)
The material collected by the gene bank is stored in cold storage here in Botswana and duplicated in
other gene banks like the SPGRC and RBG Kew. The NPGRC has two types of cold storage: a cold
room operating at around –5 o C and 10 upright freezers operating at around –20 o C. Until the cold
room temperature can be reduced to –20 o C, it will be used for temporary storage and the freezers
will be used for long-term storage. Although the freezers are more expensive than the cold room,
they have been found to be easier to manage than the cold room. Filing in both cold rooms and the
freezers follows the same format. The cold room has been installed with static steel shelves for ease
of filing. Since the accessions are contained in foil bags, it can be difficult to file them in storage as
they are. To solve this, the foil bags are put into cardboard cartons before they are placed on
shelves. This makes documentation and filing easy as the freezers, the cartons and the foil bags are
all numbered. The foil bags are also tagged with the accession number of the accession they
contain.
Drying facilities/methods
The NPGRC has a walk-in dryer of about 16m 3 , installed with a sorption air dehumidifier and
temperature controlling system. Since there is plenty of space in the dryer, the collections are spread
in well-labelled jute bags and left to dry for several weeks. The dry room is set at 15ºC and 15%
RH.
Before acquiring a dry room, the NPGRC tried a number of drying techniques: drying under the
sun, using silica gel and oven-drying. Desiccators with silica gel were also tried, but the space was
too small to dry a number of accessions at a time. This meant a lot of silica gel and desiccators had
to be bought. The gene bank resorted to drying the seeds by spreading them under the shade until
they dry and moisture contents as low as 10% were obtained. This was not always possible since the
technique is totally weather dependent.
Measuring seed moisture status
The NPGRC has been using the traditional gravimetric method for measuring the moisture content.
This method proved to be difficult to sustain as it took time to produce results for each accession
and it wasted a lot of germplasm as it is destructive. Recently the gene bank adopted the technique
of measuring equilibrium relative humidity (eRH) as an alternative. The seed moisture content is
checked at storage and then periodically checked. The gene bank has not yet set the frequency at
which this has to be checked. It is recommended that the moisture test be conducted under dry
conditions as seeds are hygroscopic, preferably in the dry room.
Storage containers
Some factors that the NPGRC took into consideration in choosing containers were availability, cost,
durability, strength and usage of space. The aluminium foil bag was chosen as it is readily available,
strong, and occupies the volume of the contents. Heat sealing is cheap and the bags are re usable.
The main shortcoming of these bags is that they are not transparent. This would be good for
indirectly monitoring moisture levels using self indicating silica gel.
Viability monitoring (question 5.5)
The NPGRC carries out germination testing, as it is seen to be the most direct and reliable method
for determining the percentage of seeds in a seed lot that are viable. When there are enough seeds in
the seed lot, 200 seeds are used in germination tests. There are situations where as few as 50 seeds