Submitted to: Journal of Experimental Botany
Publication Type: Peer reviewed journal
Publication Acceptance Date: 11/1/2006
Publication Date: 4/1/2007
Citation: Ballesteros, D., Walters, C.T. 2007. Water properties in fern spores: sorption characteristics relating to water affinity, glassy states and storage stability. Journal of Experimental Botany 58:1185-1196. Interpretive Summary: Reports from the scientific literature suggest that fern spores do not store well under conditions commonly used for seed storage even though they are tolerant of desiccation. Poor longevity of fern spores under conditions presumed optimum for seeds calls into question the general applicability of established storage regimens for diverse species and germplasm. Conditions that optimize moisture levels for seed storage are based on physical and chemical properties of the seed and their interaction with water. Here, the same properties are studied in fern spores to address whether water within fern spores behaves differently than water within seeds. These studies suggest that stable aqueous glasses are formed in fern spores; however, unlike most materials that form glasses, fern spores have low affinity for water. The low water affinity implies a very narrow range of water contents that is acceptable for fern spore storage. In contrast, pollen has high water affinity and tends to form relatively unstable aqueous glasses. These studies will allow us to investigate the relative contributions of water affinity and glass stabilization to longevity in storage. Ultimately these measurements will form the basis of predictive tests of how long germplasm can survive in seed banks.
Technical Abstract: Ex situ conservation of ferns may be accomplished by maintaining the viability of stored spores for many years. Storage conditions that maximize spore longevity can be inferred from an understanding of the behaviour of water within fern spores. Water sorption properties were measured in spores of five homosporeous species of ferns and compared with properties of pollen, seeds and fern leaf tissue. Isotherms were constructed at 5, 25 and 45 degrees C and analysed using different physicochemical models in order to quantify chemical affinity and energy of sorption of water in fern spores. Fern spores hydrate slowly but dry rapidly at ambient relative humidity (RH). Low Brunauer-Emmet-Teller (BET) monolayer values, few water binding sites according to the D’Arcy-Watt model, and limited solute-solvent compatibility according to the Flory-Huggins model suggest that fern spores have low affinity for water. Despite the low water affinity, fern spores demonstrate relatively high values of sorption enthalpy ('Hsorp). Parameters associated with biding sites and 'Hsorp decrease with increasing temperature, suggesting temperature- and hydration- dependent changes in volume of spore macromolecules. Collectively, these data may relate the degree to which cellular structures within fern spores are stabilized during drying and cooling. Water sorption properties within fern spores suggest that storage at subfreezing temperatures will give longevities comparable to those achieved with seeds. However, the window of optimum water contents for fern spores is very narrow and much lower than measured in seeds, making precise manipulation of water content imperative for achieving maximum longevity.