Calculating p50

The measure of longevity is p50: the time taken for the seed lot viability to fall to 50%. Since p50 will vary with every seed lot, let’s start by looking at what happens in a fictional seed survival experiment:

The image shows a pile of seeds in a fictional seed lot, and what happens to them on day two and six of storage. The ones that have died during storage are coloured black. By day two, four seeds have died, and by day six a total of twenty two seeds have died.

Table 1 (below) shows the full set of data. Note that the third column of the table shows the number of seeds dying between sample times, rather than the cumulative number of non-viable seeds:

Time elapsed (days) Viability (% germination) Number of additional non-viable seeds
0 100 1
2 99 3
4 96 6
6 90 12
8 78 18
10 60 23
12 37 19
14 18 10
16 8 5
18 3 2
20 2 1
22 0

Table 1: results of viability testing

In order to calculate p50, you simply plot these data as a seed survival curve. Then read across from the point the germination on the vertical axis is 50%, to the time this has taken on the horizontal axis. It will look something like Figure 5 (below).

The image shows how to measure longevity (p50), from a seed survival curve, like the one described in Figure three. As before, the x axis is the passage of time. The y axis is the percentage of seeds that will germinate. You can measure longevity by drawing a horizontal line from the y axis to meet the S-shaped seed survival curve, then dropping a vertical line from that point to the x axis (time spent in storage). This shows the storage time when fifty percent of the seeds have lost their viability.
Figure 5: measuring p50 from a seed survival curve

Whether you are dealing with species whose survival is measured in days, years or decades, this measurement, p50, is useful to compare the longevity of different seed lots. But it is also possible to work out the time taken for 75% of the seeds to lose viability (p75), the time taken for 85% of the seeds to lose viability (p85), or any other percentage. They are all worked out in a similar way.

Patterns in viability loss

Viability thresholds