Water activity as a more reliable method than moisture content applied to pollen and seeds moisture management

Pollen and Seed moisture management, as a key factor influencing storability, is a continual challenge for gene conservation. Moisture content (MC) procedure is commonly used to assess the degree of humidity of forest pollen and seeds to be stored. The range of optimal moisture content for storage of seeds and pollen has been defined for numerous species both by know-how and conducting tests. However, this method is not always reliable because it does not take into account the chemical properties of water remaining in dried materials; this occurs because moisture content in organic matters is the consequence of two main factors : The Equilibrium Relative Humidity (ERH) of air nearby them and the proportion and ability of hygrophilous constituents, like starch or proteins, to absorb and retain water. Consequently, a given moisture content may result from different combinations of HRE and proportion of hygrophilous constituents. Forest seeds and pollen characteristics are weakly predictable, this is mainly due to a high level of genetic diversity which is sought after for forest reproductive material and non-or partly controlled growing conditions of trees. On the contrary, the concept of activity of water (Aw) quantifies the vapour pressure generated by the moisture present in hygroscopic products like organic substances. It is known as the chemical potential of water. Aw has been widely used for decades by food industry because it is very well-known that reactivity and depreciating ability of hydrolytic reactions, enzymes activity, lipid oxidation or biotic agents growth are dependent on Aw rather than on MC. Consequently, Aw is more consistent to define stability moisture for organic materials preservation. In addition, Aw assessment is a quick and non destructive test, it requires only an hygrometer fitted with a special attachment to enclose samples. The relationship between Aw and MC can be pictured by the sorption/desorption isotherm. Different models are available to analyse sorption isotherms in order to define water activity values of greater stability. The relation between Aw and MC is rather stable for homogeneous organic materials like sugars or flours, this allows to use equally MC or Aw for moisture management. On the contrary, the experimental construction of sorption isotherms of different pollen and seed lots demonstrated a large variability for MC and Aw relationship among different seed or pollen lots for a given species. These results confirmed the interest of water activity assessment versus moisture content evaluation. Optimal Aw values for pollen and seeds storage are given according to sorption isotherms analyse and bibliographic references. Aw assessment is now profitably used as a routine by the two main French forest seed dealers and state seed orchard managers for both seed and pollen moisture management.