Patterns in viability loss
Seeds lose their ability to germinate over a period of time. The histogram in Figure 2 shows numbers of seeds dying (the orange bars) against the length of time those seeds have spent in storage. Below the horizontal axis are descriptions of how the remaining seeds - those which have not yet died - germinate:
When first grown-out, there are no dead (i.e.: non-viable) seeds. While seeds have only been in storage for a short time, their germination, when taken out of storage, is fast and uniform. After this initial period, the number of seeds dying increases rapidly, reaches a maximum, and tails off after most of the seeds have died. Even among seeds that are still alive and able to germinate, the results become poorer as time goes by. The longer the seeds spend in storage, the greater the likelihood that germination tests will show sporadic or abnormal germination.
This same data can be converted into a graph plotting percentage of seeds germinating against time spent in storage, as we have done in Figure 3, below. In this case, you get an S-shaped seed survival curve. In the first few days, the seeds maintain most of their ability to germinate, then the loss of viability goes into a steeper decline. This decline slows back down again once the majority of seeds have already lost viability, with just the final few seeds still clinging on to viability. Eventually, none of the seeds in the seed lot will germinate.
Activity 1
Allow ten minutes for this activity.
Imagine what it would be like if instead of a smooth curve, the viability dropped in a straight line like this. To what extent would you be able to ensure that the seeds you pass on to users are viable?
Use the text box below to write down what your job would be like if seeds behaved like the graph in Figure 4.
When you are ready, press ‘reveal’ to see our comments.
Discussion
It would be very difficult! You would need to regenerate before viability drops from one hundred percent to zero. If you miss the turning point, your recipients might receive a seed lot with zero viability. They may not be able to breed from the samples you have sent out, and their experiments would be ruined. From your genebank’s point of view, there would be more risk of losing the genotype forever. The S-shaped curve, by contrast, gives warning that your stock may be losing viability, at a point when you can still do something about it.
How viability changes over time