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- Author or Editor: Eric 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.
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.]
Infrared spectroscopy was used to measure biochemical changes during bean (Phaseolus vulgaris L.) seed imbibition. Transmission spectroscopy of excised embryonic axes revealed changes in lipid phase (gel to liquid crystalline) and protein secondary structure within the first 15 min of hydration. Spectral changes in seed coats, cotyledons, and axes during the first 2 hr of imbibition (measured in vivo) were detected using photoacoustic sensing. Onset of seed respiration could be detected as early as 15 min after addition of water. CO2 production, demonstrated by the appearance of a double peak centered at 2350 cm-1, increased with time of imbibition. Infrared photoacoustic spectroscopy of intact seeds holds promise as a method for non-invasive viability assessment.
The low quality of some seed lots received by germplasm repositories such as the National Seed Storage Laboratory can thwart efforts to regenerate seed for storage. This germplasm is in danger of irretrievable loss. The aim of this work is to promote the germination, and hence regeneration, of such low quality seeds through sterile culture of the isolated embryos. Hybrid (B73×LH51) maize seeds were aged 5 y at 32°C and 0.037 g H2O g-1 dry wt. Vigor - but not viability -declined under these conditions. The effects of four factors on growth and germination were systematically examined. These were: seed pretreatments; antibiotics and fungicides; nutrients; and growth substances. Amongst the pretreatments, none surpassed partial hydration of seeds for 24 hr to 0.55 g H2O g-1 dry wt at 25°C prior to embryo dissection. Thiram (2.4 mg mL-1) and kanamycin (50 ug ml1) effectively controlled bacterial and fungal growth with no deleterious effects on growth during culture of the isolated embryos. Exogenous sucrose (optimum 5 % wt/vol) significantly stimulated radicle growth in both deteriorated and non-deteriorated embryos. No other organic or inorganic nutrient stimulated growth. Naphthalene acetic acid did not affect growth while kinetin reduced radicle growth and stimulated coleoptile growth. Gibberellic acid (GA3 at 10-5M) significantly stimulated radicle growth in deteriorated embryos, whereas it promoted coleoptile growth in both deteriorated and non-deteriorated embryos. These data suggest GA or a GA-stimulated process may limit the growth of aged embryos.
Practical experience has generally been the source of guidance for seed storage from one season to the next. Our ancestors soon realized that avoiding moist warm conditions and protecting seeds from predators was necessary if seeds were to survive till the next planting season. Simple experiments, using different combinations of temperature and seed moisture content and/or relative humidity, showed that much longer storage periods could be attained by lowering one or both of these factors. Drying seeds and storing them in air-tight containers, or even under vacuum, at subambient temperatures could produce longevities of years or even decades. Many myths were recorded in the popular literature about longevities of centuries or even millennia. Recent research on the biochemistry and biophysics of deterioration have led to new theories on longevity that have turned our thinking upside down. A discussion of both practicalities of storage and theoretical aspects will be presented. Simplified recommendations are proposed for determining the most cost-effective approach for seed storage under various environmental and economic conditions.
Short term soaking of seeds does not appear to be detrimental to seed viability and may provide a means of testing seed viability non-destructively. Seeds of corn (Zea mays L.) and rice (Oryza sativa L.), differing in viability, were soaked for 0.5, 1, 2 and 4 hr in distilled water at room temperature. Analyses of pH, protein/polypeptides (BCA assay and absorbance at 280 nm), and potassium (and other metals), were done on individual seed leachates. After each time period seeds were germinated for 7 d to determine viability. For both corn and rice, pH remained constant between 0.5 and 4 hr of soaking. Protein concentration gradually increased during the 4 hr soak in both corn and rice, but varied with seed lot. Potassium was the most common metal excreted and increased 3 to 4 fold between 0.5 and 4 hr of soaking. Although seed to seed variability in any given lot was high, in general, low viability seeds lost more cellular constituents than high viability seeds.
Seeds of the recalcitrant species Litch i chinénis and Euphoria longan were stored in humid conditions at 8-10C under three different atmospheres: air, 80% nitrous oxide (N20)/20 % oxygen, and 100% nitrous oxide. The combination of anesthetic and oxygen extended storage longevity of both species. Oxygen was required for maintenance of viability; seeds stored under 100% N20 lost germinability at the most rapid rate. Lychee seeds retained 92% of control germination after 12 weeks under 80% N20/20% 02, while those under air lost 56% viability. Longan seeds lost all viability after 7 weeks under air, yet retained 70% of their control germination under 80% N20/20% 02. The combination of anesthetic and oxygen atmospheres could provide a new approach to recalcitrant seed storage.