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- Author or Editor: E. E. Roos x
Much emphasis has been placed on the need to preserve plant genetic resources (13, 17, 18, 28, 31, 50). As a reuslt, the number of accessions added to gene banks around the world has risen dramatically over the past 10 to 12 years. The International Board for Plant Genetic Resources (IBPGR), during its first decade of work (1974–84), arranged for the collection of ≈ 121,000 samples of germplasm from more than 90 countries around the world (19). At the USDA National Seed Storage Laboratory, accessions in storage have increased from 91,000 in 1976 to more than 204,000 in 1986. These accessions represent ≈370 genera and 1960 species. The large influx of samples during recent years has placed greater responsibility on germplasm curators to ensure that samples are properly handled, including periodic germination testing and regrowing of samples when needed.
Seldom are seeds harvested and immediately planted without undergoing at least a brief storage period. Exceptions would be certain seeds designated as “recalcitrant” (not readily storable) (45) which must be planted immediately, or viability is soon lost. Examples include many tropical plants as well as many of our temperate trees. The life span of these recalcitrant seeds may be of the order of a few days to several months (22). Another case where freshly harvested seeds may not undergo storage would be breeding materials where the object is to produce as many generations a year as possible. In this case, seeds are often harvested in an immature state and planted immediately. However, normally most seeds are stored several weeks or months before being planted. Longer storage periods, 1 to 5 years, are necessary for seeds which may be expensive or difficult to produce, or for those cultivars which are not produced every year due to lower demand by growers. Finally, germplasm banks, such as the USDA National Seed Storage Laboratory, may wish to preserve seeds for decades or even centuries (26).
Coated and raw (uncoated) lettuce (Lactuca sativa L.) seed obtained from commercial sources were subjected to 6 storage conditions (ranging from 21° C, 90% relative humidity (RH) to 5°, 40% RH) for a period of 3 years. Four types of packaging material differing in moisture-barrier properties were used. Samples were removed periodically for moisture and germination tests. Under poor storage conditions, coated seed deteriorated more rapidly than the raw seed controls. Under favorable storage conditions, both coated and raw seed retained full viability for the 3 years.
Seeds of 14 vegetable and 2 flower species were placed in paper envelopes and submerged in liquid nitrogen (LN2, –196°C) for periods up to 180 days with no adverse effects on germination. Vigor tests with beans (Phaseolus vulgaris L.), lettuce (Lactuca sativa L.), and peas (Pisum sativum L.) also indicated no reduction in germination or vigor after exposure to LN 2. Liquid nitrogen offers promise as a medium for long-term storage of seed germplasm.
Achieving the optimum moisture content for long-term seed storage usually requires that seeds be dried after receipt at a genebank. Soybean (Glycine max L.) seeds were dried using four procedures: over concentrated H2S O4, over silica gel, at 15% relative humidity (RH), or in an oven at temperatures of 30, 35 and 40C. Following dying seeds were stored at 40C for 10 days and at 5C for one yr. Seeds were evaluated for germination and vigor (root length, dehydrogenase, and leachate conductivity). Initial moisture content (mc) was reduced from 8.3% to between 6.6% (24 hr at 30C) and 4.6% (H2S O4, 30 days). Germination and vigor of seeds was essentially unchanged immediately following the drying treatments. Storage for 10 days at 40C reduced germination by up to 12% while storage for one yr at 5C had a similar effect (14% maximum loss) for most treatments. The treatments having the lowest drop in germination after one yr of storage treatment were the silica gel and the 30C oven treatments, which dropped only 3% in germination. Drying at 15% RH, also resulted in a lower loss in germination. In all three tests, vigor of seeds after storage at 40C was higher than controls for the the silica gel and 15% RH treatments as well as for the 30C and 35C oven treatments. Storage at 5C gave similar results for all three vigor assessments.
An index “internal slope” derived from the cumulative frequency distribution of individual seed leachate conductivities is related to seed quality; the larger the index value the less variation among individual seeds in a sample (100 seeds) and the higher the seed quality. We have recently developed data acquisition/instrurment control/data smoothing/data analysis software which accesses frequency and cumulative frequency distributions of individual seed conductivities and the derived index on an almost continuous basis from the start of the first soaking.
At present, lack of convergence with regard to curve fitting may occur necessitating multiple sampling times. A “window in time” approach is described whereby index estimates during a two-hour interval within the index stability phase are averaged. Evidence of the method's ability to assess seed vigor will be presented.
The moisture content of snap bean (Phaseolus vulgaris L.) seeds used in commercial plantings ranged from 7.7 to 13.7% on a fresh-weight basis. Bean seeds having initial seed moisture contents above 12% had higher field emergence than lower moisture seeds particularly at soil temperatures below 10°C. The high-moisture seeds quickly lost moisture when planted in very dry soil. Laboratory germination was improved a lesser amount by raising initial seed moisture content.
All kinds of plant seeds evolve volatile compounds during storage. However, a reliable deterioration forecast method is still not established using volatile evolution, even though some preliminary work indicated a relationship between volatile evolution and seed deterioration (Fielding and Goldsworthy, 1982; Hailstones and Smith, 1989; Zhang et al., 1993). Here we review some of the previous work concerning seed volatiles and present some more recent research on the effects of seed moisture content on deterioration. We found that volatile evolution from seeds was controlled by seed moisture level. Generally, seeds tended to evolve more hexanal and pentanal under extremely dry conditions (below 25% equilibrium RH). The production of hexanal and pentanal decreased with increasing seed moisture level. On the other hand, methanol and ethanol increased with increasing seed moisture. All of the volatile compounds accumulated in the headspace of the seed storage container during storage. Therefore, it should be possible to use different volatiles to indicate the deterioration of seeds stored under different moisture levels. We suggest that hexanal may be used for seed assessing deterioration under dry storage conditions (below 25% equilibrium RH), while ethanol may be used for seeds stored under higher moisture conditions (above 25% equilibrium RH). [References: Fielding, J.L. and Goldsworthy, A. (1982) Seed Sci. Technol. 10: 277–282. Hailstones, M.D. and Smith, M.T. (1989) Seed Sci. Technol. 17: 649–658. Zhang et al. (1993) Seed Sci. Technol. 21:359–373.]