Pre-storage factors

Everything that happens to seeds before they go into storage can affect their longevity. One reason for seeds failing to germinate when they come out of storage could be that they were immature when they first arrived at the genebank. Figure 3 (below) shows the changes that normally take place when seeds mature on the parent plant, while they are still in the field.

The graph summarises the changes in dry weight and moisture content of developing seeds. The x axis is time from flowering. There are two y axes: one is seed dry weight, the other is percentage moisture content. Between twelve and forty days after harvesting, the dry weight of the developing seed increases, then flattens off at the stage known as “mass maturity”. The moisture content, by contrast, continues to drop until about fifty-six days, beyond “mass maturity”, when the dry weight has already stabilised.
Figure 3: seed development

A very young seed is effectively a tiny, globular mass of cells in a bag of water. Figure 3 shows the changes that follow, as it develops into a mature seed. The brown curve shows how dry weight increases, as the seed fills with carbohydrates, lipids, proteins and so on. The blue curve shows how moisture content decreases over the same period.

The large molecules build up until the point we call ‘mass maturity’, where the brown curve flattens. At this point, there ceases to be a vascular connection between the parent plant and the developing seed. No more large molecules can enter the seed. Whereas the blue curve shows how moisture content continues to decline steeply, even after mass maturity.

It should come as no great surprise that if you harvest immature seeds, you’ll have a hard job convincing them to germinate. However, as a genebank scientist, the germination of freshly harvested seeds is not the only thing you need to think about. You also need to be sure your seeds will germinate after being dried and stored for many years at low temperature!

The graph shows the viability of seeds harvested between twelve and fifty-six days after flowering. The x axis is time from flowering. The y axis is percentage germination. There are three curves: fresh, dry and stored seeds. Fresh seeds show an S-shaped improvement in germination. They reach eighty-five percent viability at the stage called “mass maturity” (around forty days after flowering). Seeds which have been dried during development also show an S-shaped improvement, but they do not reach eighty-five percent viability until forty-five days after flowering, some five days after mass maturity. Seeds which are dried then put into storage only achieve optimal viability at around fifty days after flowering, in other words about twelve days after mass maturity. This is the time when in nature, seeds would disperse from the parent plant.
Figure 4: effect of seed development on germination

Figure 4 (above) superimposes the results of germination tests over the development graph shown in Figure 3. The green curve shows how germination improves for fresh seeds the longer after flowering they are harvested. These fresh seeds are already close to their maximum capacity for germination when they reach mass maturity.

By contrast, the orange curve shows what happens when the seeds are air-dried before attempting to germinate them. For these seeds, harvesting at mass maturity does not yield optimal germination rates – their longevity will improve if you wait and harvest later than mass maturity.

The purple plot shows longevity when seeds are both dried and kept in storage: these seeds are even less likely to germinate if harvested at mass maturity. They only come into their own very late in the maturing process, at around the time when the wild plant naturally disperses its seeds. This is less common in domesticated crops, where shattering resistance is a common trait, meaning that the seeds remain on the plant until they are harvested.

Activity 1

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Take another look at Figure 4 above. Based on this evidence, at what point would you advise a colleague to harvest the seeds of this plant, in order to maximize longevity in storage? Use the text box to write down your prediction.

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Discussion

Your colleague would do well to harvest these seeds after the vascular connection with the parent plant has severed, and the moisture content has reduced as much as environmental conditions in the field permit. This is why gene bankers are advised to harvest wild species as close as possible to natural seed dispersal, because that is when their longevity in storage is likely to be at its greatest.

Development of a soybean seed

Seed dispersal mechanisms