Long-term storage of orthodox seeds
Moisture content (MC) and storage temperature are two main factors affecting seed longevity. For genebanks the international recommendation is to dry the seeds to 3-7% MC and storage at -18°C (FAO/IPGRI, 1994). However, seed longevity could be affected by MC and storage temperature individually or in combination. There is only limited empirical long-term data available on seed longevity. One example comes from collections at the United States Department of Agriculture’s National Center for Genetic Resources Preservation (USDA-NCGRP), summarising the viability data for 276 species after over 30 years of storage at 5 and -18°C (3.5-9.5% moisture content [MC], [Walters et al., 2005]). They concluded that seed of individual species have characteristic potential life spans or ageing tendencies. Additional information comes from an extensive study at the Millennium Seed Bank that revealed a proportion of collections losing some viability after 20 years storage at c. 5% MC and -20°C (Probert et al., 2009). The mechanistic basis of viability loss is the subject of extensive ongoing research. However, there is also an urgent need to generate data on optimum storage conditions over a wide range of MCs and temperatures, particularly in the sub-zero range. It has been suggested that ultra-dry storage can enable medium term (many years) storage at room temperature, whilst other data indicate the onset of desiccation stress.
Isotherm construction (i.e. the relationship between MC and relative humidity) provides a biophysical understanding of seed physiological responses with respect to the binding properties of water to macromolecules (Sun, 2000). We are producing seed isotherms for a range of species to support our long-term storage study. So far we have committed about 1500 seed samples for each of five species to long-term (>15 years) storage, over a MC range of 2 to 14% and temperatures from 20°C to -196°C. The species are: Arrhenatherum elatius, Carica papaya, Dactylorhiza fuchsii, Gossypium herbaceum and Trachycarpus fortunei. Based on a maximum of three years storage to date, there is little evidence of problems with ultra-drying, and seeds of all species showed high germination under most conditions. However, we noted a rapid decline in germinability in two species under some conditions, although not at -70°C and -196°C. We intend to double the scale of this experiment in the near future. Our objective is to identify optimum conditions for long-term seed storage.
Probert, R.J., Daws, M, I. & Hay, F.R. (2009). Ecological correlates of ex situ seed longevity: a comparative study on 195 species. Annals of Botany 104: 57-69.
Sun W.Q. (2000) Dielectric relaxation of water and water-plasticized biomolecules in relation to cellular water organization, cytoplasmic viscosity, and desiccation tolerance in recalcitrant seed tissues. Plant Physiology 124: 1203-1215.
Walters, C., Wheeler, L.M. & Grotenhuis, J.M. (2005) Longevity of seeds stored in a genebank: species characteristics. Seed Science Research 15: 1-20.
Project partners and collaborators
National Centre for Research and Extension
Institute of Dendrology, Polish Academy of Sciences
Living Collections Department, Kew
Key papers published since 2006:
Seaton, P.T., Marks, T.R., Perner, H., Jijon, C. & Pritchard, H.W. (2009) Orchid seed banking takes off. Proceedings of the Second Scientific Conference on Andean Orchids (eds.) A.M. Pridgeon and J.B. Suarez. pp. 173-183.
Probert, R.J, Daws, M, I. & Hay, F.R. (2009) Ecological correlates of ex situ seed longevity: a comparative study on 195 species. Annals of Botany 104: 57-69.
Li., D-Z. & Pritchard, H.W. (2009) The science and economics of ex situ plant conservation. Trends in Plant Science 14: 614-621.
Pritchard, H.W. (2007) Cryopreservation of desiccation-tolerant seeds. In Day, J.G. & Stacey, G. (eds.) Cryopreservation and freeze-drying protocols (Methods in molecular biology. No. 368). 2nd ed. Totowa, N.J.: Humana Press. pp. 185-201.
Conferences and Workshops attended:
Pritchard, H.W., Kranner, I. & Nadarajan, J. FROZEN PLANeT – The biobanking of plants. Cryo 2010 Bristol, UK, July 2010 (invited talk).
Nadarajan, J. & Pritchard, H.W. Millennium Seed Bank- Working together to save plants worldwide. International Society for Biological and Environmental Repositories (ISBER) 2010 Annual Meeting, Rotterdam, 11-14th May 2010 (Invited talk).
McGill, C.R., Park, M.J., Nadarajan, J., Williams, W.M., MacKay, B.R. & Outred, H.A. Seed storage and dormancy in Chatham Island forget-me not (Myosotidium hortensia (Decne) Baillon). Paper presented at 29th ISTA Seed Symposium, Cologne, Germany 16-22nd June 2010.