Some definitions

The amount of water inside a seed is its moisture content: seeds containing a lot of water are more metabolically active than dry seeds. There are various ways to measure moisture content, but they are often destructive.

The fresh weight of a seed sample is easy to measure, but it varies with moisture content. The dry weight of a seed sample is an estimate of the sample’s weight when all the water has been removed.

Another measure of seed moisture status is water activity (a_w). It measures the availability of ‘free water’ inside the seeds, and therefore the type of reactions, including damaging ones, that can occur.

Relative humidity is a measure of how much water vapor there is in the air, compared to the maximum amount of water vapor the air can theoretically hold at the same temperature (the saturation point).

Once seeds have reach equilibrium with the air around them, they reach equilibrium relative humidity. Seed equilibrium relative humidity is approximately equal to the water activity (a_w) multiplied by 100.

As you increase the relative humidity, water enters the seed. As you decrease the relative humidity, water leaves the seed. These responses can be plotted as an isotherm, showing the relationship between seed moisture content and equilibrium relative humidity at any given temperature; a relationship that controls whether seeds gain or lose moisture.

Dry seeds take up moisture in a process called imbibition. As long as there is no hard seed coat acting as an impermeable barrier, they can take water up very quickly.

What has happened to a seed in the past makes a difference to how seeds take up water at any given time. For instance, a dry seed takes up water slightly differently from a moist seed losing water. This is called hysteresis.