Storage of seed at low temperature is the main ex situ conservation method employed in genebanks. Of the total reported 6.1 million germplasm accessions maintained worldwide, almost 90% are stored as seed in approximately 1300 genebanks, according to the Report on the State of the World's Plant Genetic Resources for Food and Agriculture of the Food and Agriculture Organization of the United Nations (FAO). The recommended preferred conditions for long-term seed storage, according to the Genebank Standards published in 1994 by FAO and the International Plant Genetic Resources Institute (IPGRI), are 3-7% seed moisture content, depending on the species, at - 18°C or cooler.
In the late 1980s, systematic research was initiated by the International Board for Plant Genetic Resources (IBPGR, now IPGRI) to investigate the effects of very low moisture content on seed longevity. Much of this research was based on the general assumption that the relative benefit in terms of longevity became greater for each successive reduction in seed moisture content, although this may vary among species. One of the most compelling reasons for undertaking research in this area was the prospect of developing 'low-input' alternatives for medium- to long-term cold storage of seed germplasm through its storage at room temperature (i.e. ultra-dry seed storage). Such an approach, it was believed, would allow for a secure and cost-efficient alternative for seed storage in cases where a reliable supply of electricity is not available for refrigeration of storage rooms. In line with its objectives to develop and broaden the applicability of conservation techniques, IPGRI actively supported research activities in this area, including significant work conducted in research institutes and genebanks in China.
However, in the early 1990s it was demonstrated that drying seed beyond a critical moisture content provided no additional benefit to longevity and may even accelerate seed aging rates. Further research on the determination of optimum seed moisture content led to a debate regarding the various parameters involved. In particular, questions were raised with regard to the critical relative humidity, storage temperature and their interaction, as well as the consequences of equilibrating seeds to levels of relative humidity lower than the critical level. The latter was prompted by the finding that the optimum seed moisture content level increased as the storage temperature was lowered, suggesting that there may be a danger of actually over-drying seed.
To make appropriate recommendations to the plant genetic resources community, in particular regarding the applicability of the ultra-dry seed technology, it was deemed necessary to resolve the basic questions of the optimal seed moisture level in relation to storage temperature, including the possibility of over-drying the seeds. It was in recognition of this need that IPGRI initiated the global ultra-dry seed storage experiment in 1995, in collaboration with the National Seed Storage Laboratory (NSSL) of the US Department of Agriculture, Agriculture Research Service, at Fort Collins, USA; the International Crops Research Institute for the Semi-Arid Tropics (ICRISAT) at Hyderabad, India, and the Institute of Crop Germplasm Resources of the Chinese Academy of Agricultural Sciences, Beijing, China (CAAS). During a meeting held in Rome in November 1994, representatives from NSSL, USA, the University of Reading, UK, and IPGRI designed and agreed upon the experimental protocol for the global project. The University of Reading could not participate in the subsequent implementation of the project but was kept regularly informed of progress.
During the Second International Conference on Seed Science and Technology, Guangzhou, China, 12-16 May 1997, a Satellite Symposium on ultra-dry seed storage and longevity was organized by IPGRI to assess the current state of the art in this research area.
Following a suggestion from Professor Mike Black, Editor of Seed Science Research, to publish the papers presented during the Symposium in this journal, a detailed outline for a supplement was developed by the same representatives of NSSL, University of Reading and IPGRI during a meeting held in Reading in January, 1998. They agreed that this supplement should include: (i) the papers presented during the Symposium reporting on the results of ultra-dry seed storage research performed by Chinese scientists; (ii) a paper presenting the first results of the global ultra-dry seed storage experiment; (iii) two position papers, presenting the University of Reading and NSSL perspectives in the ultra-dry seed storage debate; and (iv) a synthesis paper reviewing the cogent points of the debate on the various parameters involved in the determination of optimum seed moisture content, with special emphasis on the understanding thus far and the implications for application of the ultra-dry seed storage technology. The draft synthesis paper was shared with the representatives of the University of Reading who provided comments.
We would like to thank Professor Mike Black for his interest in this matter and for his support during the preparation of this supplementary issue. The assistance of David Nicholson and Jenny Howatson from CAB INTERNATIONAL in making the necessary arrangements for this publication is highly appreciated. Finally, the significant editorial contributions made by Christina Walters, without which this supplement could not have been produced, are gratefully acknowledged.
IPGRI intends to distribute this supplementary issue widely, to inform the user community of the progress achieved in this area, especially with regard to the most effective seed storage conditions related to seed moisture content and storage temperature. It is our hope that these research results will contribute to more efficient and effective seed storage and that they will stimulate further research on ultra-dry seed storage.
Jan Engels
Director, Genetic Resources Science and Technology
Group
Florent Engelmann
In vitro Conservation Officer
IPGRI, Rome
