Osmosis: The Molecular Theory. Larry Howlett

Seed Storage Macro Model

      Seed stored in a hermetic container is shown in figure 5. As noted, the seed exchanges heat and moisture with the atmosphere within the hermetic container but only exchanges heat between the hermetic container and the external atmosphere. We look at seed stored in a hermetic container from a macro viewpoint. The macro model data is shown in table 1.

      Initial properties of the seed are values S1 through S3. The corresponding initial properties of the atmosphere are taken equilibration chart figure 2, values (A1, A2). Additional properties of the atmosphere within the hermetic container are found from psychrometric charts and tables. A short version of a psychrometric chart is shown in figure 3. From the properties of the seed and the atmosphere within the container, and assuming one cubic foot of seed and air within the hermetic container, we calculate that the total mass of seed and water within the seed is much greater than the total mass of air and water vapor in the air. (values S4-S6 & A5-A8, table 1)

      If we raise the storage temperature to 90ºF, The atmosphere properties (A9 – A13) show that the maximum moisture content in the air within the hermetic container is much less than the total water content of the seed. For practical purposes, the seed water content will remain constant at the initial value of 15%. We note that this is agrees with the experimental data.

      Hermetic Seed Storage Vapor Pressure

      We next examine storage in hermetic container using the seed vapor pressure model. This model is shown in figure 4. The properties of seed and atmosphere using the vapor pressure model are shown in table 2.

      The initial seed properties from figure 4 and atmosphere properties are: (S1 – S4 & A1 – A5) Note that we used the seed vapor pressure to determine the atmosphere relative humidity from psychrometric data

      Next we define a one cubic foot volume of seed and one cubic foot volume of atmosphere within the within the hermetic container. As in our macro model for hermetic containers, the mass of seed and seed moisture content is much greater that the mass of dry air and moisture in the air. Data is shown as (S5 – S7 & A6 – A9)

      We now raise the storage temperature to 90°F. From figure 4, we find that the seed vapor pressure rises to 0.47 psia from psychrometric data, the atmosphere with in the hermetic container must be at 67% relative humidity to for equilibrium between the seed and its local atmosphere. At 90ºF and 0.47 psia vapor pressure, the final data is (S8 – S10 & A10 – A14).

      Again, we find that the total moisture in the local atmosphere is very small compared to the total moisture in the seed. Note that the total mass of dry air does not change within the hermetic container. The total moisture content of the air at 90 F is very small. Therefore, the seed moisture content must remain practically constant. We reach the same conclusion. Changing storage temperature and the resulting changed moisture content of the air has a very small effect on the moisture content of the seed. Thus, the change in equilibrium conditions from 50°F to 90°F storing temperature creates a negligible change in moisture content of the air and moisture content in the seed.

      Seed Storage Envelope

      From the seed model, (figure 4), we select temperature and seed moisture content as dependent variables. With these two variables, we propose the storage condition map shown in figure 6. Viability of stored seeds is improved by controlling moisture content and temperature as shown on the graph. Introducing an independent thermodynamic model for a seed reduces the complexity in defining ideal seed storage conditions by removing the reference to the storage atmosphere relative humidity.

      Acknowledgements: The author thanks DEKALB Genetics / Monsanto for the opportunity to work on seed storage systems, Christine Walters for her willingness to supply papers as needed and his family and friends for their support.

      References

      1.Roberts, E.H., Predicting Storage Life of Seeds, Seed Science & Technology, 1, 499-514, 1973

      2.Ellis, RH, & Roberts EH; Improved Equations of the Prediction of Seed Longevity; Annals of Botany, 45, 13-30, 1980

      3.Ellis, RH and Roberts EH, The Influence of Temperature and Moisture on Seed Viability Period in Barley, Annals of Botany, 45, 31-37, 1980.

      4.Walters, C., Understanding the Mechanisms and Kinetics of Seed Aging, Seed Science Research, 8, 223-244, 1980.

      5.Bewley, JD and Black M.; Seeds. Physiology of Development and Germination. (2^nd edition) New York, Plenum Press, 1994.

      6.Fundamentals Handbook, American Society of Heating , Refrigerating and Air Conditioning Engineers, 1997

      7.Walters, C., Kameswara Rao, N., Xiaorong Hu; Optimizing See Water Content to Improve Longevity in ex situ Genebanks , Seed Science Research; 8, Supplement No. 1, 15-22; 1998.

      8.Xiaorong Hu, Yunlan Zhang, Chenglian Hu, Mei Tao and Shuping Chen; A Comparison Of Methods For Drying Seed: Vacuum Freeze-Drier Versus Silica Gel; Seed Science Research, 8, Supplement No. 1, 29-33; 1998.

      9.Walters, C., Water Activity: Bad Habits Die Hard, Cryo-Letters 19,265-266, 1998.

      10.Mead, A and Gray, D; Prediction Of Seed Longevity; A Modification Of The Shape Of The Ellis And Roberts Seed Survival Curves. Seed Science Research, Volume 9, No.1, March 1999