Gearing up for germination

The image shows a seedling emerging from the soil.

  

In viable seeds, all sorts of chemical reactions take place in preparation for germination. It is instructive to map the different types of reactions onto three different regions of the isotherm, as we have done in Figure 8 (below):

The graph shows the three regions of water activity introduced in figure seven. As before, the x axis is the equilibrated relative humidity (eRH). The y axis is the moisture content of the seeds. The graph is divided by two vertical, dotted lines. Three curves show the types of enzyme activity that can take place in the three different regions. In “Region I”, eRH is low and the water in seeds is mainly tightly-bound. Here, the main chemical reactions going on inside the seeds are lipid oxidation. “Region II”, where eRH is between eighteen and eighty percent and there is an increasing amount of weakly-bound and freely-moving water. Here, lipid oxidation drops off then builds up again as eRH increases. Two new types of reaction also get established: non-enzymatic reactions (browning), and enzymatic reactions. In “Region III”, the eRH is between eighty and ninety-nine percent and there is plenty of weakly-bound and freely-moving water. Here enzyme activity increases steeply while lipid oxidation and non-enzymatic reactions are also proceeding at quite high rates in preparation for germination.
Figure 8: relative reaction rates by isotherm region

Figure 8 shows the types of reactions that can occur in food products. It is known as a ‘food stability diagram’, and is relevant to us, since many food products are derived from seeds such as grain. In region I, where aqueous water is scarce, reactions include lipid oxidation, which does not require a lot of free water. Lipid oxidation falls off in region II, in favor of enzymatic and non-enzymatic reactions, which require more water. These reactions reach their highest rates in region III. This enzymatic activity allows the seed to carry out important metabolic processes including respiration, repairing aging-induced damage and preparing for germination.

Biochemical reactions in seeds

Imbibition