Abstract
Seed genebanks around the world are seeking economical ways to store seeds as a means of conserving plant biodiversity. Zheng et al.1 suggest that the use of ‘ultra-dry’ technology2,3, in which the seeds are dried to a water content of less than 5%, can extend the longevity of some seed species sufficiently to reduce or eliminate the need for refrigeration. This would benefit in particular some developing countries, such as China, for which they say the cost of cold storage is prohibitive.
Main
It is widely accepted that drying seeds can increase their longevity3,4,5 and there is general agreement that, moisture contents being equal, the same seed lot will survive longer if stored at lower temperatures3,4,5. The salient issue is whether the longevity achievable with various preservation protocols is sufficient for the conservation of germ plasm.
Base collections need seeds to survive for decades3. Zheng et al.1 report good survival of ultra-dry Eucommia seeds after two years of storage at ambient temperatures. However, ultra-dry technology has several technical problems, including: (1) a general decline in germination within five years; (2) a narrow range of allowable moisture contents for seeds stored at high temperatures; and (3) a tremendous variability in longevity among different seed cultivars5.
Relatively successful storage has been reported for ultra-dry seeds of a single rape-seed cultivar in which 100% and 42% of seeds germinated after 11 and 18 years, respectively, but this is not representative of the performance of Brassica species under ultra-dry storage conditions5. Our germination records (www.ars-grin.gov/nssl; data available on request) show that refrigerated storage is required to achieve the seed longevities required for ex situ germplasm conservation.
Ultra-dry technology might seem to be economical, but this does not take into account the high cost of establishing the optimum water content for individual seed accessions or the labour-intensive expense of frequent viability monitoring and regenerations. Added to this is the risk of losing valuable germ plasm when seeds die during storage or accessions are regenerated from small populations. When these factors are considered, refrigeration, by comparison, is a bargain.
References
Zheng, G.-H., Jing, X.-M. & Tao, K.-L. Nature 393, 223–224 (1998).
Cost-effective Long-term Seed Stores (International Board for Plant Genetic Resources, Rome, 1986).
Report of the Expert Consultation on Genebank Standards (Food and Agricultural Organization of the United Nations/International Board for Plant Genetic Resources, Rome, 1992).
Justice, O. L. & Bass, L. N. in Principles and Practices of Seed Storage (Agriculture Handbook no. 506, US Government Printing Office, Washington DC, 1978).
Seed Sci. Res. (suppl.) 8, (1998).
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Walters, C., Roos, E., Touchell, D. et al. Refrigeration can save seeds economically. Nature 395, 758 (1998). https://doi.org/10.1038/27363
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DOI: https://doi.org/10.1038/27363
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