R. H. EllisI have been asked to write concerning the background to the research in the Seed Science Laboratory at The University of Reading on seed longevity at low and very low moisture contents. Funding for that research ended more than 6 years ago, unfortunately. I therefore have little more to say beyond that already published. This review links the publications from that research programme in a brief, simple, largely chronological framework.
Department of Agriculture, University of Reading, Earley Gate, PO Box 236, Reading RG6 6AT, UK
The improved seed viability equation published at the beginning of the 1980s (Ellis and Roberts, 1980) resulted in two important changes in our general understanding of the quantitative relations between seed longevity and storage environment: the beneficial effect on longevity of low temperatures (e.g. less than ambient) was less than previously thought; while in contrast, the beneficial effect on longevity of reduction in seed moisture content at lower moisture contents was rather greater than previously thought. The practical question raised from that research was, what is the moisture content below which further desiccation no longer improves longevity?
In Autumn 1981 we therefore began what turned out to be a 4-year investigation with sesame (Sesamum indicum L.) which demonstrated that in hermetic storage the seed viability equation still applied at moisture contents as low as 2% (wet basis), the lowest moisture content investigated (Ellis et al., 1986). In order to obtain results reasonably rapidly, this investigation was carried out at 50°C. Nevertheless, even after 4 years' hermetic storage at 50°C with 2% moisture content, a substantial proportion of those sesame seeds remained viable. This considerable longevity at such a 'hostile' temperature was quite remarkable, and justified further research.
Subsequent investigations in a large number of species at the even higher storage temperature of 65°C (this temperature was selected primarily because research funders would not countenance a medium-term, let alone long-term, research programme) was successful in demonstrating that there was a low-moisture-content limit (which we termed the critical moisture content) to the application of the seed viability equation, and that this moisture content varied substantially among species (Ellis et al., 1988, 1989, 1990a,b, 1992). These very different seed moisture contents were all very similar, however, in terms of equilibrium relative humidity: about 10-12% relative humidity at 20°C, the temperature at which seeds were dried before subsequent hermetic storage (Ellis et al., 1989, 1990a,b, 1992).
Those results implied (for brevity I do not repeat all the caveats published at the time) that in order to maximize the longevity of orthodox seeds in genebanks where refrigeration to -18°C cannot be provided, seeds should first be dried at 20°C to moisture contents in equilibrium with about 11% RH and then be stored hermetically at ambient, or where possible, cooler temperatures (Ellis et al., 1989). This approach was subsequently termed 'ultra-dry' seed storage (IBPGR, 1992).
However, the value of the critical moisture content might vary with storage temperature (Ellis et al., 1989), in which case the storage of seeds at moisture contents lower than those recommended by the International Board for Plant Genetic Resources at its inception (IBPGR, 1976) would not necessarily result in better survival at ambient temperatures. Accordingly, at the end of 1989 an international collaborative programme of research was begun (Ellis et al., 1996). Parallel investigations in three laboratories (Reading, Wellesbourne, Wageningen) compared the results of 5 years' hermetic storage of ultra-dry seeds (i.e. those at moisture contents in equilibrium with about 10% RH at 20°C, between 2.0 and 3.7% moisture content for the five species studied) with storage at about 5-6% moisture content at both ambient (20°C) and sub-zero temperatures (-20°C).
That research and development programme showed that the lower seed storage moisture content (ultra-dry) did improve seed survival at 20°C substantially (and significantly), but that (not surprisingly during only 5 years) no loss in viability occurred at - 20°C and so it was not possible to distinguish between the two different moisture-content treatments in terms of seed survival at - 20°C (Ellis et al., 1996). That research also demonstrated most clearly the considerable benefits to seed longevity of reduction in storage temperature to -20°C, at both moisture contents investigated.
I take the opportunity to emphasize here that neither I nor my colleagues sought to replace the use of seed stores operating at - 20°C; the objective of our funded research on low-moisture-content seed storage was merely to determine what could be done by genebank curators and seedsmen in circumstances where adequate refrigeration could not be provided. It follows also that, while not irrelevant, the research objective was not to determine the most suitable moisture content for the storage of seeds at low temperatures (ca-20°C).
Nevertheless, with collaborators in a genebank in Madrid, Spain, we also published (Ellis et al., 1993) long-term results from one genebank's experience of 24 and 25 years' storage at cool and sub-zero temperatures, which showed that ultra-dry storage (or even slightly lower seed-moisture contents) was not damaging and indeed was very satisfactory. Figure 2 in that work is worth particularly close inspection with regard to any evaluation of putative dangers of ultra-dry seed storage.
Others have been concerned with seed vigour (as an indicator of seed deterioration) following storage at low moisture contents (Vertucci and Roos, 1990). We responded immediately to the suggestion that ultra-dry storage could be damaging to seed vigour by publishing evidence that in three pasture legumes, after 18 months' hermetic storage at 40°C, the rapidity of germination upon rehydration (a measure of vigour) was greatest after storage at moisture contents in equilibrium with 11% RH at 20°C than at higher moisture contents (Ellis et al., 1991). Subsequent and comprehensive results for changes in lettuce (Lactuca sativa L.) and sunflower (Helianthus annuus L.) seed germination and vigour (mean germination time, root length and seedling dry weight) during hermetic storage at 35°C similarly showed that seed vigour after storage was greatest at seed moisture contents in equilibrium with about 8-10% RH at 20°C than at higher or lower moisture contents (Ellis et al., 1995).
Finally, readers may also wish to consult Steiner and Ruckenbauer (1995). Those authors concluded from analyses of 'the Vienna Sample of 1877' that 'ultra-dry, long-term seed storage under ambient conditions can successfully be achieved with the intention of cutting down risks and costs in germplasm conservation'.
References
Ellis, R.H. and Roberts, E.H. (1980) Improved equations for the prediction of seed longevity. Annals of Botany 45, 13-30.
Ellis, R.H., Hong, T.D. and Roberts, E.H. (1986) Logarithmic relationship between moisture content and longevity in sesame seeds. Annals of Botany 57, 499-503.
Ellis, R.H., Hong, T.D. and Roberts, E.H. (1988) A low-moisture-content limit to logarithmic relations between seed moisture content and longevity. Annals of Botany 61, 405-408.
Ellis, R.H., Hong, T.D. and Roberts, E.H. (1989) A comparison of the low-moisture-content limit to the logarithmic relation between seed moisture and longevity in twelve species. Annals of Botany 63, 601-611.
Ellis, R.H., Hong, T.D. and Roberts, E.H. (1990a) Moisture content and the longevity of seeds of Phaseolus vulgaris. Annals of Botany 66, 341-348.
Ellis, R.H., Hong, T.D., Roberts, E.H. and Tao, K.-L. (1990b) Low moisture content limits to relations between seed longevity and moisture. Annals of Botany 65, 493-504.
Ellis, R.H., Hong, T.D. and Roberts, E.H. (1991) Seed moisture content, storage, viability and vigour. Seed Science Research 1, 275-277.
Ellis, R.H., Hong, T.D. and Roberts, E.H. (1992) The low-moisture-content limit to the negative logarithmic relation between seed longevity and moisture content in three subspecies of rice. Annals of Botany 69, 53-58.
Ellis, R.H., Hong, T.D., Martin, M.C., Pérez-Garcia, F. and Goméz-Campo, C. (1993) The long term storage of seeds of seventeen crucifers at very low moisture contents. Plant Varieties and Seeds 6, 75-81.
Ellis, R.H., Hong, T.D. and Roberts, E.H. (1995) Survival and vigour of lettuce (Lactuca sativa L.) and sunflower (Helianthus annuus L.) seeds stored at low and very-low moisture contents. Annals of Botany 76, 521-534.
Ellis, R.H., Hong, T.D., Astley, D., Pinnegar, A.E. and Kraak, H.L. (1996) Survival of dry and ultra-dry seeds of carrot, groundnut, lettuce, oilseed rape, and onion during five years' hermetic storage at two temperatures. Seed Science and Technology 24, 347-358.
IBPGR (1976) Report of IBPGR Working Group on Engineering, Design and Cost Aspects of Long-term Seed Storage Facilities. International Board for Plant Genetic Resources, Rome.
IBPGR (1992). Annual Report 1991. International Board for Plant Genetic Resources, Rome.
Steiner, A.M. and Ruckenbauer, P. (1995) Germination of 110-year-old cereal and weed seeds, the Vienna Sample of 1877. Verification of effective ultra-dry storage at ambient temperature. Seed Science Research 5, 195-199.
Vertucci, C.W. and Roos, E.E. (1990) Theoretical basis of protocols for seed storage. Plant Physiology 94, 1019-1023.
© CAB INTERNATIONAL, 1998
