Why does water activity matter?

Water activity is important because water is required to promote the aqueous (water-based) reactions that are essential to the seed’s metabolism. The type of reactions that can take place depends on the amount ‘free water’ inside the seeds. If the moisture content is high, reactions can take place readily within the seed, but in the context of a genebank, rates of spoilage also tend to be higher. If the moisture content is low, this reduces the scope for chemical reactions to go on within the seed, meaning the seeds can be stored for longer.

Most seeds are hygroscopic (they readily take up and retain moisture). Water will move in and out of the seed in order to reach equilibrium with the humidity of the surrounding air. If you consider the seed and the air around it as a system, this system will tend towards equilibrium between water in the air and water in the seed, as Figure 2 shows.

The image shows a seed reaching equilibrium with the air around it. The left side of the image represents what happens when there is a wet seed in dry air. More water molecules move out of the seed and into the air than in the opposite direction. The right hand side represents what happens when there is a dry seed in wet air. More water molecules move from the air into the seed than the opposite direction. The central image shows a seed that has reached equilibrium with the surrounding air. Here, an equal number of water molecules move in and out of the seed.
Figure 2: equilibrium between a seed and the air around it.

This is important because seeds behave in different ways, depending on their moisture content. We will look at the underlying mechanisms later, but let’s first think about visible behavior of the seeds. Table 1 summarises how seeds are likely to behave under conditions of low, medium and high moisture content:

  

Low moisture content Assuming temperature is constant, seeds with low moisture content survive for a long time because their metabolism is barely ticking over. They will not germinate.
Medium moisture content As the seed’s moisture content increases, rates of many aging reactions increase, resulting in damage to nucleic acids, proteins and lipids.
High moisture content Seeds with high moisture content are fully metabolically active. They are able to respire if oxygen is available, and to gear up for germination, including the repair of damaged macromolecules. If environmental conditions are appropriate, they will show their viability. In a storage setting, relative rates of spoilage may be higher, because of oxidation and the growth of yeasts and bacteria.

  

Table 1: changes in seed behavior with moisture content

At different stages in a genebank’s operations, it is desirable for seeds to behave in different ways. During storage, minimal metabolism is important, whereas during regeneration, the seeds must be metabolically active. To this extent, controlling the moisture content of seeds is key.

Alternatives to the gravimetric method

Reaching equilibrium