Behavior of water inside seeds

In dry seeds, water is chemically bound to macromolecular surfaces or other biological interfaces by ionic bonding. There is little scope for this water to participate in aqueous reactions. The dark blue area of Figure 4 (below) shows how the amount of water locked into the seed in this way increases steeply when a seed is very dry, then remains relatively stable as more water enters the system.

The graph shows the flattened, horizontal S-shaped curve, or seed moisture isotherm, introduced in Figure three. The x axis is the equilibrated relative humidity (eRH) of the air. The y axis is the moisture content of the seeds. A flat, darkly shaded area at the bottom of the graph represents the water in a seed when moisture content is low (below three percent). In this area, most water is strongly bound to the molecules of the seeds.
Figure 4: strongly bound water

As the moisture content of the surrounding air increases, more water can enter the seed. But the way this water is held is likely to be different: weakly bound, clustered round large molecules in polar groups with low affinity to the large molecules. Although still associated with large molecules inside the seed, there is more scope for this water to break free in its H2O (molecular) form. The mid-blue area of Figure 5 (below) shows how the proportion of weakly bound water increases as more water becomes available, while the proportion of strongly bound water does not increase.

The graph shows the S-shaped curve, or seed moisture isotherm, introduced in Figures three and four. As before, the x axis is the equilibrated relative humidity (eRH) of the air. The y axis is the moisture content of the seeds. Above the flat, shaded area at the bottom, there is now a mid-tone shaded area, taking up more of the area under the S-shaped graph, and echoing its shape. This area represents water in a seed when its moisture content is medium (between three and twelve percent). Most water in this area is loosely bound to molecules inside the seeds.
Figure 5: strongly and weakly bound water

As the moisture content of the air increases further, the additional water that enters the seed no longer needs to be chemically bound within the seed tissue: it is free to move around, and forms water-to-water bonds between freely moving water (H2O) molecules. The light blue area in Figure 6 (below) shows changes in the amount of freely-moving water. This type of water is both able to support aqueous biochemical reactions, and to escape out into the surrounding air.

The graph shows the S-shaped curve, or seed moisture isotherm introduced in Figures three, four and five. As before, the x axis is the equilibrated relative humidity (eRH) of the air. The y axis is the moisture content of the seeds. The remaining area under the S-shaped curve is now filled by a lightly-shaded area, positioned above the other two shaded areas. It area echoes the shape of the S-shaped curve and fills it snugly. This lightly shaded area represents the water in a seed when its moisture content is high (between twelve and twenty-five percent). Since there is already water tightly and loosely bound to molecules inside the seeds, most water in this third area is able to move freely inside the seeds.
Figure 6: strongly bound, weakly bound and freely-moving water

This model of water binding is somewhat simplistic; in reality, binding does not follow such a rigid sequence. But it is helpful to understand how the water in the seed influences the processes that can occur inside.

Relative humidity of air and seeds

Biochemical reactions in seeds