Monitoring seed viability
A germination test is the most reliable way to measure seed viability. It also provides valuable information that can be used in the future to turn the seeds into plants for reintroduction, restoration or research.
Seeds germinating on an agar plate [Image: RBG Kew]
Most of the species conserved by the MSBP 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 collections is assessed about one month after they are placed into the -20°C cold store to establish the 'initial viability' of the collection. Subsequently, collections are re-tested every five or ten years depending on whether they are expected to be short- or long-lived. For wild species germination tests can take many weeks or even months.
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 50 seeds are used for each treatment, though for very small collections, as few as 20 or even 10 seeds may be used. Seeds are sown into Petri dishes containing agar and then incubated at an appropriate temperature, depending on the local climate, where the collection came from and the time of year that germination would probably occur in the wild.
Each week the seeds are checked, and germinated seeds are removed, recorded and discarded. The tests are checked in a clean air cabinet, to minimise the risks of inhalation of fungal spores produced by any mould on the seeds.
When 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. Excessively mouldy but filled seeds are an indication that seed viability has declined.
Statistical tests built into the seed bank database are used to check re-test results when they are entered into the database to see if viability has declined since the last test. This information assists the management of collections by informing re-rest intervals and by signalling that viability is approaching the MSBP viability standard (85% of initial viability). Decisions about whether to make a new collection or undertake regeneration are taken at this point.
The presence of seed dormancy means that sometimes we are unable to get all viable seeds to germinate. Thus the MSBP germination standard (75%) is lower than our viability standard. Collections are given a ‘pass’ if the lower 95% binomial confidence interval on the germination percentage is above 75%.
Germination test results are ‘accepted’ if there is no statistical difference between the number of seeds that germinate and the number of germinated seeds + the number of fresh seeds remaining at the end of a test as determined by a ‘cut test’. If the number of germinated seeds is significantly lower, then experiments will be carried out to investigate the dormancy problem if the collection is rated as high priority.
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