Biochemical reactions in seeds

So far, you have learned that water is held within a seed in different ways, depending on the amount of water already within that seed. This in turn can affect the type of metabolic reactions that can go on inside the seed. By plotting the isotherm, you can see these as predictable, real-world differences.

Look at Figure 7 (below), which shows three distinct ‘regions’ along the x-axis of the isotherm. These are not just lines on a graph. They relate to observable differences in metabolic reactions that go on inside the seeds. Metabolism is limited when eRH is less than 15%: we call this ‘region I’ of the isotherm. More reactions can take place at an eRH between 15% and about 80%: we call this ‘region II’. Metabolic reactions inside seeds ramp up again in ‘region III’, when eRH rises above 80%.

The graph shows the S-shaped curve or seed moisture isotherm, introduced in Figures three, four, five and six. As before, the x axis is the equilibrated relative humidity (eRH) of the air. The y axis is the moisture content of the seeds. As before, shaded areas under the graph shows the nature of bonds between water and molecules within the seeds (strongly-bound, weakly-bound and freely-moving). What’s new in this graph is that it is broken down into regions along the y axis as well. A vertical dotted line is ruled at eighteen percent. This indicates the area where eRH is below eighteen percent. Most of the area under the curve here is darkly shaded, indicating that the water in seeds is mainly tightly-bound, with a little weakly-bound and freely-moving water. This is called “Region I”. Another vertical dotted line is ruled at eighty percent creating a new area between eighteen and eighty percent eRH. This is called “Region II”. Here, the colour of shading is more equally distributed. There is still strongly-bound water in the seeds, but there is also an increasing amount of weakly-bound and freely-moving water. The other side of the second vertical dotted line marks out the region where eRH is between eighty and one hundred percent. In this region, there is comparatively more weakly-bound and freely-moving water, which now appears more plentiful than the strongly-bound water. This is “Region III”.
Figure 7: isotherm regions

Activity 1

Allow 5 minutes for this activity

Examine Figure 7 carefully, then try to match the three regions to the most relevant descriptions of the behavior of water for each. There will be more than one interpretation associated with each region. Try to do the exercise yourself first, before looking at our response.

Regions:

  • Region I
  • Region II
  • Region III

  

Descriptions:

  • a. moisture content increases rapidly at first, then tails off as eRH increases
  • b. more water is freely available than in any other region
  • c. water inside seeds may be strongly bound, loosely bound or free H2O
  • d. most water is strongly bound inside seeds
  • e. increase of moisture content accelerates as eRH increases
  • f. moisture content increases slowly as eRH increases
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Discussion

In region I, moisture content and eRH are both low. Most water inside seeds is strongly bound, so there is little water available for aqueous reactions. In region II, moisture content flattens out while eRH continues to increase. Inside seeds, there is weakly bound water and some freely circulating water, which increases the scope for certain types of metabolic reactions. In region III, relatively high quantities of freely available water allow ever more water-based reactions to take place.

Region I

a. moisture content increases rapidly at first, then tails off as eRH increases

d. most water is strongly bound inside seeds

Region II

c. water inside seeds may be strongly bound, loosely bound or free H2O

f. moisture content increases slowly as eRH increases

Region III

b. more water is freely available than in any other region

e. increase of moisture content accelerates as eRH increases

Behavior of water inside seeds

Gearing up for germination