Checking germination

Most of the species in Kew’s Millennium Seed Bank (MSB) have never been germinated in a laboratory before. The conditions needed for germination vary considerably between different species and even between different populations of the same species. MSBP scientists use knowledge about the plant such as its ecology and life cycle as well as climate information to predict the best conditions for germination and any pre-treatment that may be required to overcome seed dormancy.

The viability of MSB seed-lots is assessed about one month after they are placed into the -20°C cold store to check that they are able to withstand the freezing process. Once the seeds are in long-term storage, germination is re-tested approximately every ten years. For wild species this testing can take many weeks or even months.

Seeds are kept at an appropriate temperature in an incubator where it is light for 8 hours then dark for 16 (Image: RBG Kew)

A container of seeds for testing is removed from the cold room and allowed to warm up for one day in the adjacent drying room. The number of seeds removed for testing depends on the number of dormancy-breaking treatments and the size of the sample. Normally 20 seeds are used for each treatment, though for very small collections, as few as 10 seeds may be used. Seed-lots with less than 500 seeds are usually left untested. It is hard to draw firm conclusions from very small germination tests but they do provide an indication of the success of dormancy-breaking treatments and the general level of viability. The seeds are then sown onto 10 gl-1 water agar and kept at an appropriate temperature in an incubator where it is light for 8 hours then dark for 16.

Germination can be identified by the protrusion of a radicle (Image: RBG Kew)

Each week the seeds are checked, and where germination is identified (usually by a 1-2 mm protrusion of radicle) the germinated seeds removed and discarded. The tests are carried out in a clean air cabinet, to minimise the risks of inhalation of fungal spores produced by any mould on the seeds.

When we decide that germination has stopped, the test is ended. Visual inspection and a cut test is used to determine whether the remaining seeds are full, empty or mouldy. If the seeds are excessively mouldy but filled, a loss in viability may be indicated, particularly if they are very soft.

The germination pass level for the MSB collections is 75% (after empty and damaged seeds are taken into account). Failures may be regenerated, discarded or kept, dependent upon the rarity of the material, the level of germination and the potential ease of regeneration. Re-collection may be considered for some seeds if possible. Collections are also monitored for moisture status.