Seed storage temperature
What is meant by seed storage?
In a genebank, seed storage is the preservation of seeds under controlled environmental conditions which will prolong the viability of the seeds for long periods. Two types of seed stores are used for collections of genetic resources, those holding base collections and those holding active collections. The temperature, relative humidities, seed moisture contents, containers and distribution arrangements vary between these stores.
Why are seeds stored?
Seeds must be stored in a way which maintains their viability for long periods. Seeds left at ambient temperatures and relative humidities will lose their viability quickly whilst seeds stored in conditions of low moisture content and temperature will retain their viability for longer periods. Accessions held in a genebank are valuable and represent plants which are no longer available or which are endangered in their natural environment. These seeds must be conserved in the genebank for use in plant breeding in the future.
When should seeds be placed into store?
As soon as the seed has matured on the plant the slow process of deterioration begins. Therefore the sooner that seeds are placed into store the better. In practical terms this means that seeds collected in the field should be quickly returned to genebanks, processed and placed into store as soon as the cleaning, drying and packaging processes are complete.
How many seeds of each accession should be stored?
The IBPGR Advisory Committee on Seed Storage has recommended that for materials showing little morphological variation (genetically homogeneous) 3000 seeds are acceptable, but 4000 seeds are preferred, to represent each accession. For materials showing a large amount of morphological variation (heterogeneous) an accession should consist of at least 4000 seeds, but 12000 seeds are preferred. These sample sizes are recommended as number of seed but many genebanks would prefer to work with weight. An inter-conversion of numbers and weights is possible from the thousand seed weight of any accession. A list of approximate thousand seed weights of some common species is given in Appendix 2 of Cromarty, Ellis and Roberts (1982).
How should seeds be stored?
The conditions which prolong viability during storage have been well defined for seeds which are tolerant of desiccation. Storage conditions have been recommended by the IBPGR Advisory Committee on Seed Storage. For base collections, seeds of between 3-7 % moisture content should be stored in sealed containers. Sub-zero temperatures are acceptable, but -18 °C or less is preferred. For active collections sealed storage of seeds dried to 7% moisture content or less is recommended at temperatures of less than 15 °C. Unsealed storage is not encouraged. In particular, it is not recommended in tropical areas.
Table of thousand seed weights of species in your genebank
Fill in this table to use for future reference:
Seed storage temperature What is meant by seed storage? In a genebank, seed storage is the preservation of seeds under controlled environmental conditions which will prolong the viability of
Chapter 7 SEED STORAGE
Storage may be defined as the preservation of viable seeds from the time of collection until they are required for sowing (Holmes and Buszewicz 1958). When seed for afforestation can be sown immediately after collection, no storage is needed. The best sowing date for a given species being raised in a nursery depends on (a) The anticipated date of planting, itself dependent on seasonal climate, (b) The time needed in the nursery for planting stock of that species to reach the right size for out-planting. Only rarely does best sowing date coincide with the best date for seed collection. More often it is necessary to store the seed for varying periods which may be
Up to one year when both seed production and afforestation are regular annual events, but it is necessary to await the best season for sowing.
1 – 5 years or more when a species bears an abundant seed crop at intervals of several years and enough seed must be collected in a good year to cover annual afforestation needs in intermediate years of poor seed production.
Long-term storage for purposes of conserving genetic resources. The period of storage will vary according to the seed longevity of the species and the storage conditions, but will be measured in decades in species which are easy to store.
The storage facilities to be provided must be related to the amount of seeds and the period over which they are to be stored. It is a waste of money to create expensive facilities capable of maintaining viability for 10 years if the seeds will never stay longer than nine months between collection and sowing. It is equally wasteful to spend money on seed collection, extraction and cleaning if storage conditions are so inadequate that the seeds are 90 % dead before they reach the nursery.
For general accounts of seed storage of forest tree seeds, there are a number of useful references (Holmes and Buszewicz 1958, Magini 1962, Stein et al. 1974, Wang 1974, Barner 1975b). They deal mainly or exclusively with temperate species. More intensive studies have been made of the storage of agricultural seeds and there is good reason to accept that the general principles established for agricultural crops apply also to forest trees. An excellent recent account of the subject as applied to agricultural seeds is contained in “Principles and practices of seed storage” (Justice & Bass 1979) and there is also much useful information in the slightly older publications of Roberts (1972) and Harrington (1970, 1972, 1973). Long-term storage for gene resource conservation is well covered by Cromarty et al. (1982).
Natural Longevity of Tree Seeds
The period for which seed can remain viable without germinating is greatly affected by its quality at the time of collection, its treatment between collection and storage and the conditions in which it is stored. Nevertheless, seed longevity varies enormously from species to species even if they are given identical treatment and storage conditions. Ewart (1908) divided seeds into three biological classes according to the time for which they are capable of retaining viability under “good” storage conditions:
Microbiotic: seed life span not exceeding 3 years
Mesobiotic: seed life span from 3 to 15 years
Macrobiotic: seed life span from 15 to over 100 years.
Although Ewart’s classes were useful in drawing attention to the differences in natural longevity of seeds of different species, his classification is too rigid to fit the variations between individuals, provenances and seed years in a single species, or the possible variations in storage conditions. It is not possible to define a standard set of “good” storage conditions equally suitable to all species, because species vary in their requirements for optimum conditions. Yet storage life of a given species will vary greatly according to the conditions in which it is stored.
Today two major classes of seed are recognised (Roberts 1973):
Orthodox. Seeds which can be dried down to a low MC of around 5% (wet basis) and successfully stored at low or sub-freezing temperatures for long periods.
Recalcitrant. Seeds which cannot survive drying below a relatively high moisture content (often in the range 20–50% wet basis) and which cannot be successfully stored for long periods.
Within these two classes some further subdivision may be made, for example between orthodox seeds with or without hard coats and between recalcitrant seeds which can or cannot withstand low temperatures of below around 10°C. Within each of the main classes there are still considerable differences between species in the period for which viability is maintained under a given set of conditions. There may also be a distinction between truly recalcitrant species and species that are just difficult; the latter may turn out to behave in an orthodox manner if, for example, particular attention is paid to the methods of drying them.
Hard-coated orthodox seeds
Most, if not all, of the species which have been recorded as maintaining seed viability over a period of decades are hard-seeded. They include a number of tropical leguminous species. Examples of species of which at least some seeds maintained viability after lengthy periods of storage in herbaria, cited by Harrington (1970) from the work of Ewart (1908) and Becquerel (1934), are:
|158 years||Cassia multijuga|
|149 years||Albizzia julibrissin|
|115 years||Cassia bicapsularis|
|99 years||Leucaena leucocephala|
Ambient conditions in herbaria storage can be considered good (fairly low relative humidity and temperature) but well short of the combination of low initial MC, sealed storage and sub-freezing temperature now considered ideal for longterm storage of orthodox species.
Recent research has provided more precise information on conditions of storage, initial germination and final germination of some species, but over shorter periods. Examples are:
|Species||Conditions of Storage||Pre-storage germination |
|Post-storage germination |
|Prosopis juliflora 1)||Dry atmosphere of herbarium in S.W.||?||60||50|
|Acacia aneura 2)||Closed containers at room temperature (20–25°C)||56||60||13|
|A. hemsleyi 2)||” ” “||96||96||13|
|A. holosericea 2)||” ” “||95||84||14|
|A. leptopetala 2)||” ” “||73||72||18|
|A. victoriae 2)||” ” “||80||60||18|
As explained later in this chapter, modern thinking has defined low MC, low temperature and low oxygen pressure as the three most important constituents of the storage conditions which man should provide to maximize seed longevity in orthodox species. In the impermeable seedcoat nature has provided two of these constituents, a low MC and exclusion of oxygen. Full-sized but green leguminous seeds, sown immediately without drying, may germinate at once, indicating that the seedcoat has not yet developed an impermeable layer; no doubt the development of impermeability is synchronized in nature with the reduction of seed moisture by natural drying to the optimum content for longevity. Hard seeds are thus a potent factor in extending seed life in all conditions of storage but confer their most important benefits when storage facilities are limited and during the potentially dangerous period between collection and entry into long-term storage.
Not all leguminous seeds are equally long-lived, for example Koompassia malaccensis seeds have thinner seedcoats and deteriorate more rapidly in storage than species such as Parkia javanica, and they need no pretreatment to overcome seedcoat dormancy (Sasaki 1980 a). In Sudan seeds of Dalbergia sissoo stored less well at room temperature than those of local Acacia, Albizzia and Tamarindus species (Wunder 1966), while in Australia seeds of Acacia harpophylla deteriorate rapidly unless stored in sealed containers at 2–4 °C (Turnbull 1983).
Orthodox seeds without hard seedcoats
Many species in important genera of forest trees fall into this group, e.g. Pinus, Picea, Eucalyptus. Experience in Australia is that mature seeds of all eucalypts can be kept viable for some years if stored with a low moisture content in sealed containers at 3° – 5° C. The majority of species can be stored for 10 years at room temperature with relatively little loss of viability (Turnbull 1975 f). Both E. deglupta and E. microtheca seeds deteriorate more rapidly if stored at room temperature, but storage life is improved if they are kept in air-tight containers at 3° – 5° C and recent evidence suggests that storage at -18°C is even better. In Thailand seeds of P. kesiya and P. merkusii retained good viability for four years if stored at below 8 % moisture content in sealed containers at 0° – 5° C (Bryndum 1975), while at least five years’ good viability is possible with P. caribaea and P. oocarpa under similar conditions (Robbins 1983, a,b). Considerably longer periods have been recorded for some species of pine e.g. 30 years for Pinus resinosa in USA when stored in sealed containers at 1.1° – 2.2° C (Heit 1967b, Wang 1974). Tectona grandis is an orthodox tropical broadleaved species (Barner 1975b) but, since it produces good seed crops in most years, there has been little stimulus to investigate optimum conditions for long-term storage (Schubert 1974).
According to evidence summarized by Bowen and Whitmore (1980), most Agathis spp. are orthodox. For example, an appropriate treatment of A. australis in one study (dried to 6 % MC, then stored in sealed containers at 5° C) preserved viability for 6 years (79 % germination compared with the initial 88 %), while storage at below freezing temperature maintained a germination of about 60 % for up to 12 years (Preest 1979). The same seed stored at higher MC or temperatures (15 – 20 % MC. or 15° – 20° C) had lost all germination power within 14 months. A australis seed is inherently longer lived than A. robusta which in turn is longer lived than A. macrophylla. Initial trials with the tropical A. macrophylla indicated that good results could be obtained by drying fresh seeds from about 65 % to 20 % before despatch by air (period in transit 14 days) and by further drying in the recipient country for 5 days at 16° C and 14 % RH. final MC was 6 % and germination 75 %. However, later trials were inconsistent and less successful. With tropical species, it is likely that handling between collection and despatch and the largely uncontrollable conditions of air transit are more critical than in the easier temperate or subtropical species.
Orthodox species which rapidly lose viability unless they are given the optimum treatment include species in the mainly temperate genera Populus, Salix and Ulmus. Many of these lose viability within a few weeks under natural conditions or if stored in ambient conditions of temperature and humidity, but can be stored for months or years if maintained at low temperature and low moisture content. Examples are of Ulmus americana stored successfully for 15 years at 3 % moisture content and -4° C (Barton 1961), and Populus sieboldii stored for 6 years at -15° C over a desiccating agent in a sealed container (Sato 1949). In Populus balsamifera and Salix glauca, reduction in germination after two years of sealed storage at -10° C was less than 6.5 % of initial germination (Zasada and Densmore 1980); after three years there was very little change in Populus but up to 40 % reduction in Salix.
In the tropics Aucoumea klaineana is a good example of an orthodox species which is short-lived under ambient conditions. Germination of fresh seed is often over 90 %, but after 30 days’ storage in room conditions there is a significant drop in germination and this falls to zero after 100 days. Storage at 0 – 5° C and 7 – 8 % MC in sealed containers with a chemical desiccator, Actigel, maintains over 50 % germination for at least 30 months (Deval 1976). There is some indication that a further reduction of MC will conserve viability even better. Thus one seed lot of initial germination 76 % in the laboratory and 79 % in sand, when stored in sealed containers with Actigel, had an MC of 4.6 % and a germination of 70 % in the laboratory and 79 % in sand after 30 months; the same seed lot stored in sealed containers without Actigel had an MC of 9.9 – 10.4 % and a germination of 54 – 63 % in the laboratory and 62 – 67 % in sand. Other species of this type include Entandrophragma angolense which has a seed life of 6 weeks in room conditions but up to 6 years in cold storage (Olatoye 1968) and Cedrela odorata which loses all germination capacity in 10 months at room temperature but suffers no loss in 14 months if stored at 5° C in a sealed jar (Lamprecht 1956).
Some species may need special treatment to prolong viability for more than a few months. Fagus sylvatica can be conserved overwinter by maintaining MC at 20 – 30 % and storing part-filled in sealed polythene bags at 0 – 5°C for 100 days. It is then suitable for sowing because such storage conditions constitute a suitable pretreatment to break dormancy. If longer storage is intended, MC should be reduced to 8–10 % by drying in a current of air at room temperature (15–20°C). The nuts are then placed in sealed containers and stored at -5° to -10° C and will keep for several years (Nyholm 1960, Suszka 1974, Rudolf and Leak 1974). Later research in France and Poland confirmed the above MC of 8–10 % and the advantages of sealed containers for long term storage (Bonnet-Masimbert and Muller 1975, Suszka and Kluczynska 1980). This technique has been successfully applied on a large scale (17 tons of beechnuts from 51 different sources) in France. Germination has been maintained over periods of 4 to 6 years (Muller and Bonnet-Masimbert 1982).
Where storage conditions leave much to be desired, the longevity of orthodox seeds without hard coats can be expected to be much inferior to the hard-coated species. The nearer that conditions of storage approach the ideal for a given non-hardseeded species, the less the difference between its longevity and that of a hardseeded species. The best combination of MC and temperature will vary to some extent between species, for example the above-quoted 8–10 % MC for Fagus sylvatica is considerably higher than the 5–6 % considered ideal for many forest and agricultural seeds.
Recalcitrant seeds include a number of large seeds that cannot withstand appreciable drying without injury; it is of interest that the overwhelming majority of recalcitrant species listed by King and Roberts (1979) are woody. Temperate species such as Quercus and Castanea are commonly stored moist only for short periods over winter. Reduction of storage temperature to near freezing will prolong longevity. Bonner (1973 a) found that it was possible to store acorns of Quercus falcata for 30 months and still obtain over 90 % germination at the end of the period, provided that temperature was maintained at 3 ° C and MC between 33 % (initial) and 37 % (final). A lower MC or a higher temperature (8° C) both reduced germination. For Quercus robur MC should be maintained above 40 % (Holmes and Buszewicz 1956, Suszka and Tylkowski 1980). Recent research in Poland has demonstrated good results from storing this species at >40 % MC in air-dry peat or air-dry sawdust in milk cans at -1° C. It is important to allow free entry of oxygen and this is ensured by inserting several strips of cardboard at intervals between the lid and the edge of the can. In these conditions germination after 3 winters was in the range of 38 – 75 % and after 5 winters was still about 12 % (Suszka and Tylkowski 1980). Temperatures below -5° C killed all the acorns, while a temperature of +1° C encouraged excessive pregermination (60 – 75 % after 3 winters, with radicles up to 25 cm long, compared with 12 % and radicles 3 capacity at 5°C) for short or medium term storage, but also has access to the long-term storage facilities (at -20°C) of the Regional Genetic Resources Unit (described in appendix 3), which are also at Turrialba (Chang 1980).
Loss of viability in storage, in addition to reducing the number of plants which can be produced by a given seed lot, may result in a shift in the genetic constitution of the seed being stored. This could be particularly important in forest trees which are predominantly outbreeding, variable populations. First, loss of viability may occur more rapidly in some genotypes than others; if losses are high, say 50 % of the total, the genotypes with short-lived seeds may be eliminated altogether. Yet they may have valuable traits for adaptation, growth or disease resistance as growing trees, and in any case they contribute to genetic variation in the species which it is the purpose of genetic conservation to preserve. Secondly, it is an accepted fact in agriculture that chromosome damage or change occurs and accumulates in the seed in storage, and that the risk of such heritable gene mutations depends not so much on the age of the seed as on changes in its viability (Roberts 1972, Barner 1975b). A seed lot which has suffered a serious loss of viability is likely to have experienced some gene mutations among the survivors, but there is very little direct evidence that heritable mutations are induced under good storage conditions which lead to only small losses in viability.
The high standard of storage conditions recommended by the IBPGR and referred to above, if combined with regular testing of seed and regeneration as soon as germination falls to 85 % of the initial germination rate (Ellis et al. 1980) should minimize the risk of genetic change in storage. It is possible that still lower temperatures would increase longevity even more. Research on storage in liquid nitrogen has been pursued for some years and considerable progress has been made, but testing for several more years will be needed before the method can be recommended for general adoption in gene banks (IBPGR 1981).
Moist storage without control of MC or temperature
Suitable for storage of recalcitrant seeds for a few months over winter. Seeds may be stored in heaps on the ground, in shallow pits in well-drained soils or in layers in well ventilated sheds, often covered or mixed with leaves, moist sand, peat or other porous materials (Holmes and Buszewicz 1958, Magini 1962). Seeds stored outdoors are kept moist by rain or snow, but those under shelter may need to be moistened periodically (Stein et al. 1974). The aim is to maintain moist and cool conditions, combined with good aeration to avoid overheating which may result from the relatively high rates of respiration associated with moist storage. This may be accomplished by regular turning of the heaps of seed (Aldhous 1972) or by inserting bundles of straw or twigs into them (Magini 1962).
This method is suitable for short-term storage of large-seeded hardwood species in the temperate zone e.g. Quercus, Castanea, Aesculus. It is unlikely to be suitable for tropical recalcitrant species because ambient temperature is too high.
Outdoor stratification, a method of overcoming internal dormancy, is described on pp. 178–180. It is properly to be considered as a seed pretreatment, but it serves the incidental function of storing the seed for a few weeks or months and the method used is closely akin to those described in this section.
Moist cold storage, with control of temperature
This method implies controlled low temperatures just above freezing or, less commonly, just below freezing (Magini 1962). Moisture can be controlled within approximate limits by adding moist media e.g. sand, peat or a mixture of both to the seed, in the proportions of one part media to 1 part seed by volume, and remoistening periodically, or more accurately (but more rarely) by controlling the relative humidity in the cold store. The latter type of control is often too expensive (Magini 1962, Holmes and Buszewicz 1958). Respiration rate is reduced and storage life prolonged by the low temperature, but seed should not be stored in sealed gas-proof containers which would limit oxygen supply. Closed polyethylene bags of 4 – 10 mil (100–250 microns) thickness will allow exchange of oxygen and CO2 with air outside, while severely restricting exchange of moisture (Stein et al. 1974).
The method is suitable for the same temperate recalcitrant genera as listed in the previous section, and with temperatures of 0 – 5° C should extend viability up to 1 ½ to 2 years. Sub-freezing temperatures have given improved results in a few cases but frequently injure seeds with high MC and should be used only after research has demonstrated their applicability to the species in question.
Less is known about the application of this method to tropical species, but it merits much more investigation that it has so far received, for the dipterocarps and genera such as Araucaria, Agathis and Triplochiton. As mentioned earlier there is evidence that some species are killed at low but above-freezing temperatures and Gordon (1981) has proposed a division of recalcitrant seeds into those which can withstand temperatures below 10° C without loss of viability and those which cannot. Tamari (1976), summing up several years of research on dipterocarps in Malaysia, concluded that the best treatment for several species was (1) Dry at a temperature not above 35° C, to reduce MC to 35 % (2) Seal with a fungicide inside polythene bags (3) Store at 15° C, or for 3 weeks at 15° C followed by further storage at 10° C. This treatment has been successful in extending longevity from a week or two up to two months in Hopea helferi, but this is still a long way from providing safe storage between seed years, reported to vary from 3 – 6 years apart in many dipterocarps (Tang 1971). Storage at 3.5°C and MC over 32 % over periods of at least 6 months has been successful on the recalcitrant Araucaria hunsteinii (Arentz 1980).
In some recalcitrant species newly germinated seeds may retain viability better under moist cool storage conditions than ungerminated seeds. Gordon (1981) reported that some pregerminated seedlots of Quercus spp. showed no significant change in the number of living seedlings after one year’s storage in 500 gauge (125 microns) polythene bags lightly sealed at 3° C, whereas a large proportion of the seeds which were viable but ungerminated when placed in the same bags died during the same period.
Other storage methods have been used in the past, but are not yet of wide application. They include (Magini 1962, Stein et al. 1974):
Storage of recalcitrant seeds in running (not stagnant) water.
Storage under partial vacuum.
Storage in gases other than air e.g. nitrogen or CO2.
Coating individual large seeds with paraffin or latex to prevent moisture exchange. This method may also be used to maintain moisture content during shipment.
Some form of container is necessary for most seed storage, to facilitate access to, and handling of, individual seed lots while keeping them separate, to make the best possible use of storage space, to provide protection against animal and insect pests and, for some seeds, to prevent passage of moisture and gases between the enclosed and the outside atmosphere. Many types of container have been used for tree seeds; they may be conveniently divided into (1) Materials freely permeable to moisture and gases (2) Materials completely impermeable, when sealed, to moisture and gases (3) Materials resistant, but not completely impermeable, to moisture.
Materials freely permeable to moisture and gases
These include hessian or burlap sacks, cotton bags and containers of paper, cardboard and fibreboard. Hessian and cotton have the advantage that seed triers can be inserted through the cloth mesh to withdraw samples for testing without the need to open the mouth of the container. The resilience of the cloth will close the hole and avoid subsequent loss of seed, which is not possible with containers based on paper or paperboard (Harrington 1973). Hessian and cotton are also robust materials which can be used more than once.
None of these materials is entirely proof against insect and rodent pests, and all are freely permeable to water vapour and gases. For orthodox seeds in uncontrolled conditions they are therefore suitable only for rather short storage periods; these can be extended in the case of hardcoated seeds or where ambient conditions are cool and dry. If seeds are stored in large containers after drying to the correct MC, the outer seeds themselves provide some barrier to the passage of moisture. Viability of the inner seeds may thus be preserved for a period even though there is some deterioration from increased MC in the outer layers. If a seed store has facilities for controlling both temperature and relative humidity, then permeable containers can be safely used for orthodox seeds for several years, provided that pests can be excluded.
For moist storage of recalcitrant seeds, open or freely permeable containers such as hessian sacks should be used in order to allow free exchange of air and so avoid the overheating which can occur if moist, rapidly respiring seeds are enclosed without adequate ventilation. Periodic spraying of the sacks may be necessary to maintain the high MC which is appropriate for this type of seeds.
Materials completely impermeable, when sealed, to moisture and gases
After drying of orthodox seeds to the correct MC, the MC may be maintained in storage by dehumidifying the whole storage space. Another very efficient way, commonly used in storing forest seeds, is to place the seed in sealed moistureproof containers. This avoids the need for expensive dehumidification equipment. For long-term storage the most effective method is a combination of moistureproof containers with controlled low temperatures provided by refrigeration. An added advantage of most materials in this type is that they also exclude oxygen and so reduce still further the rate of respiration. Impermeable sealed containers are not suitable for storing recalcitrant seeds nor are they suitable for orthodox seeds at high MC, which deteriorate more rapidly in sealed than in open storage. Some seeds absorb moisture quickly, so it is important that they be sealed inside the container as soon as possible after drying is complete, preferably within the drying room itself.
Moistureproof containers include tin or aluminium cans and drums, glass jars of the Mason or Kilner types, plastic vials and laminated aluminium foil packages. Rigid and unbreakable metal cans provide maximum protection against mechanical damage to the seeds and are equally suitable for storage and subsequent shipment. Containers are only as moistureproof as their sealing. For rigid containers screw-top or clamp-down gasketed lids should be used, if periodic opening for seed extraction and subsequent resealing are anticipated; aluminium foil should be heat-sealed. The effectiveness of sealing is particularly important in long-term storage. Three types of container are considered suitable for hermetically sealed long-term storage of agricultural seeds: glass jars or vials; metal cans; and laminated foil packets. They should be equally appropriate for orthodox forest seeds. But the report by IBPGR (1976) recommended sealed metal cans as the most reliable and convenient. It noted that the seals on screw-cap jars are not always perfect and that further experience of the lasting qualities of laminated foil packets is needed before they can be recommended for general use in storage which will often last for several decades.
Materials resistant, but not completely impermeable, to moisture
They include polyethylene and other plastic films and aluminium foil. These materials are resistant to the passage of moisture but, over a long period of time, there will be a slow passage of water vapour tending to equilibrate the RH inside with that outside the container. Some of the figures quoted by Justice and Bass (1979) for transmittal of water vapour appear surprisingly high, e.g. 0.13 g per 100 square inches (645 cm 2 ) per 24 hours for low density polyethylene film 10 mil (250 microns) thick and about ten times this figure for low density film 1 mil (25 microns) thick. However, the standard conditions for testing these materials are 0% RH on one side and 90–100% on the other. The RH gradient during storage is never so severe as this and hence the rate of passage of water vapour is much less rapid in practice. In one test using 6 mil (150 microns) high density polyethylene the rate of passage over two years from an outside RH of 95–100% at 20°/30°C was four times that from an outside RH of 50% at 10°C (Justice and Bass 1979). The thicker the film, the greater the resistance to passage of water vapour and, for a given thickness, high density polyethylene is more resistant than low density.
Although polyethylene is not suitable for long-term storage of orthodox seeds for genetic conservation, it is very suitable for short-or medium-term storage and has given excellent results for up to 5 years’ storage of Pinus caribaea and P. oocarpa seeds in Honduras, with no significant change in MC. For Honduran conditions a thickness of at least 4–5 mil (100–125 microns) is recommended; thinner polythene can permit a significant passage of water vapour in time and is also subject to mechanical damage in handling (Robbins 1983 a, b). Harrington (1973) considered 3 mil (75 microns) high density or 5 mil (125 microns) regular suitable for temperate conditions and 7 mil (175 microns) high density or 10 mil (250 microns) regular as adequate for even severe tropical conditions. Proper sealing of bags is essential and can be done by a combination of heat and pressure. In the past hot irons were used, but sealing can now be done more efficiently and conveniently by commercial heat sealers, of which a number of different models is now on the market.
Different materials, each alone slowly permeable to water vapour, may become completely impermeable when laminated together. Various combinations of laminated polyethylene, aluminium foil and kraft paper proved completely impermeable to water vapour over a two year period, even when there was a high differential between the inside and outside RH (Justice and Bass 1979).
Use of desiccants in containers
If orthodox seeds are dried to the correct MC and stored in sealed impermeable containers, the MC should remain constant for years. If, however, the seeds are stored in moisture resistant but not completely impermeable material such as polythene bags, or if it is necessary to open and reclose the containers periodically to extract seeds, there will be a slow build-up of moisture in time. A convenient way to prevent this is to enclose some desiccant such as silica gel in the containers. The capacity of silica gel to adsorb moisture depends on the relative humidity of the ambient air, as shown in the following table (after Harrington 1972):
Moisture content of silica gel in equilibrium with various relative humidities
|% RH||% H2O adsorbed||% RH||% H2O adsorbed|
A convenient method is to use silica gel treated with cobalt chloride, which changes colour from blue to pink about 45% RH; the corresponding equilibrium MC for many orthodox species would be 7–9% (see graphs Figs. 6.23 and 6.24 ). Dried silica gel is enclosed with the seeds and, whenever the granules turn pink, the silica gel is removed and reactivated by drying in an oven at 175° C and cooling in a sealed container before reuse. A weight of silica gel equal to one tenth the weight of seeds is recommended (Harrington 1972). Care should be taken not to include too much silica gel which could lead to overdrying of the seeds. Even with silica gel of one tenth the weight of seeds, the MC of seeds enclosed at 6% would be lowered to below 5% during the initial phase of storage. More frequent reactivation of silica gel would preserve an equilibrium of RH and seed MC at lower levels than the 45% and 7–9% mentioned above but would forego the convenience of the colour indicator.
Another use for desiccants is where the MC of seeds is known to be higher than the optimum for sealed storage, for example because only air-drying is possible. As mentioned on p. 127, the enclosure with the seed of approximately an equal weight of silica gel in sealed containers should reduce the MC of the seed to a suitable level and maintain it. As an example
|1 kg (oven-dry weight) of seed at initial 19% MC (dry weight basis) contains||190 g H2O|
|1 kg (oven-dry weight) of seed at 6% MC (dry weight basis) contains||60 g H2O|
|Therefore moisture to be removed||=||130 g H2O|
|RH in equilibrium with 6% MC||=||25 % RH|
|At RH 25%, 1 kg silica gel adsorbs||125 g H2O|
Therefore a weight of silica gel equal to the weight of seed will reduce the initial 19% MC to just over 6% MC for storage.
Choice and use of container
The following factors, which should be considered when choosing the best storage container for a given use, are based on those listed by Stein et al. (1974): When seed requires further drying in storage, do not use a tight-closing container because enclosing excess moisture is harmful to the seed (Barton 1961). Use a tight-closing container if gain in seed moisture content can be damaging and relative humidity in the storage facility is high.
Containers and seed can quickly gather unwanted condensation when brought out of cool or subfreezing storage. Warming to room temperature is recommended before opening a container brought out of such storage.
4 to 10 mil (100 – 250 microns) polyethylene bags will greatly restrict exchange of moisture, but still allow exchange of oxygen and carbon dioxide with air outside. Such exchange may be beneficial or harmful, depending on species.
A container that is easy to open and close is desirable when quantities of seed are likely to be added or removed repeatedly. In order to minimize temperature and relative humidity fluctuations, open only when necessary. Alternatively, store seed in small containers, so that the entire contents can be stored or emptied at one time.
For orthodox seeds, fill containers completely to ensure minimum exchange of moisture between the seed and the entrapped air and, more importantly, to limit the amount of oxygen enclosed.
When exchange of moisture through the container walls must be eliminated or restricted, the container must be made impermeable or of moisture resistant material. The longer the storage period and the higher the differential between external RH and RH within the container, the more impermeable the material must be.
When seeds are fragile and easily damaged, a rigid-walled container should be used. Moisture-proof plastic bags are often used as liners for rigid containers.
Choose a container shape and stacking arrangement which facilitates uniform temperature and aeration throughout the storage facility.
Some containers may be made of substances that are harmful to tree and shrub seeds (Barton 1954). Unproven containers should be tested for toxicity.
7.1 Airtight containers used for storing seed, Division of Forest Research, CSIRO Canberra. (FAO/Division of Forest Research, CSIRO Canberra)
|7.2 Interior view of cold storage room at Humlebaek, Denmark. (DANIDA Forest Seed Centre)|
|7.3 Examples of different types of container used for storage or shipment in Denmark. (DANIDA Forest Seed Centre)|
Static electricity can build up slowly on some materials such as PVC. This makes them difficult to clean between exhaustion of one seed batch and insertion of the next.
It should be stressed that no one container or packaging material can be the best for all sizes, conditions and objectives that occur in seed packaging. The relative merits of the various containers will have to be weighed against their disadvantages and costs before deciding the final choice.
Design and Engineering of Seed Storage Facilities
The weight of seeds to be kept in store can be estimated in the manner indicated in Chapter 3 and will depend on the annual planting area, the maximum number of years’ seed supply to be stored at any one time because of seeding periodicity, and the number of seeds per kg, for each species. Weight of seed in kg can be converted to net volume in litres (or in g to cm 3 ) by a factor related to average specific gravity. An average factor of 2.0 is appropriate for many forest species and corresponds to an apparent specific gravity of 0.5 (true specific gravity would be slightly higher because of the air-spaces between the seeds).
For conversion from net volume to gross storage space, allowing for shelving, ventilation, air spaces within and between containers, access and fittings within the cold room, a factor of about X8 is commonly used (Magini 1962); this is with fixed shelving. Use of mobile shelving may double the quantity of seed which may be stored in a given space (IBPGR 1976); in this case a factor of about X4 is appropriate. Thus 500 kg of seed of S.G. 0.5 would need a gross storage space of 500 × 2 × 8 = 8000 litres or 8 m 3 if fixed shelving were used and 4 m 3 with mobile shelving. Where relatively few seed lots and large quantities of each lot are being stored, it is possible to use standard sizes of container, each filled to the brim, and for shelving space to be adapted to fit container size exactly. Under these conditions considerable savings in storage space can be effected. Thus in the Danish seed store at Humlebaek, which uses fixed shelving, a factor of only 3.12 has been calculated (Barner 1982 a).
Design and equipment
The design and machinery for refrigerated storage is a matter for refrigeration engineers. Some guidance as to the features which should be included in any quotation for installation may be obtained from the excerpts from the IBPGR (1976) report which appears in Appendix 2 and the example of the facilities installed by the Regional Genetic Resources Project at Turrialba (Goldbach 1979) which appears in Appendix 3. It should be noted that both these documents refer to long-term storage of agricultural seeds for purposes of genetic conservation.
It is essential that designs and equipment be adapted to local conditions and local resources. The best installation in the world is of little use if it cannot be maintained, so it is essential to investigate the local provision for servicing and spares before committing oneself to any particular item. The reliability of mains electricity services and the need for a voltage controller and standby generator are of primary importance. Ready availability of a spare compressor may also be necessary.
The correct siting of a seed store may reduce the need for much expensive equipment. For example a tropical country with variable climate and topography might solve many problems by moving its store from a hot humid coastal site to the dry rain-shadow side of a mountain at 2000 m. In such a case a well-ventilated room might provide perfectly suitable conditions for several years’ storage for relatively “easy” species such as pines and eucalypts and could be supplemented by one or more deep-freeze chests for small quantities of more “difficult” species requiring sub-freezing temperature. The value of deep-freeze chests was stressed by the IBPGR (1976) and its comments are reproduced in Appendix 4.
The benefits of exemplary seed collection, processing and storage methods may be largely lost if care is not taken over shipment from seed store to nursery. It is seed viability at the time of sowing, rather than at the time of despatch from the seed store, which determines the number of healthy plants produced from a particular seed lot. It is therefore essential to provide shipment methods which will ensure the minimum loss of viability in the interval between storage and sowing. The selection of appropriate packing material will depend on the characteristics of the species, the quantity to be shipped, the length of time in transit, the mode of transport and the temperature and moisture conditions to which the shipment will be exposed (Baldwin 1955).
High and fluctuating temperatures and adverse humidity are the chief causes of viability losses during shipment (Stein et al. 1974). These factors are identical with those that cause deterioration in freshly collected fruits between the collecting site and the processing depot, as described on pp. 83–84. However, seeds between storage and sowing should start with advantage of having had optimum conditions of temperature and moisture content during the storage period. In fact, maintenance of storage conditions during transit would be ideal, but is often not possible (Stein et al. 1974).
Provided that the initial moisture content of the seeds is correct, it can be easily maintained during transit by the use of sealed containers. In some cases the seeds can be despatched in the same containers in which they were stored. In others it may be advisable to transfer them from a large container in storage to a smaller container for despatch. Individual nurseries may require only a small quantity of a given seed lot. In addition, small and light packages are often less subject to mechanical damage in transit than large, heavy ones. Magini (1962) recommends separate packages of 1 – 20 kg but not larger. A variety of moisture-proof or moisture-resistant material is available, as described earlier in this chapter under storage containers. Polyethylene of 4–8 mil (100–200 microns) has the advantage of restricting moisture passage while allowing exchange of oxygen and CO2.
Sealed containers are highly suitable for orthodox species, of which the seeds must be kept dry during transit. The addition of a desiccant such as silica gel may be a useful additional insurance if there is any risk that the seeds may absorb moisture while being transferred from storage container to shipment container. Seeds of recalcitrant species, on the other hand, are best left unsealed, since the effect of some loss of moisture is less harmful than that of the overheating which can occur as a result of rapid respiration in sealed bags at ambient temperatures. They should be well-mixed with pulverized sphagnum moss, ground peat, coconut fibre or sawdust, that has been moistened and squeezed dry. A mixture of equal weights of dry packing and water will give adequate moisture content to these materials (Baldwin 1955). In the case of international transit, however, an inert non-organic substance such as moist vermiculite is likely to be more acceptable to quarantine authorities.
Sealed moisture-proof containers should always be used for long journeys, e.g. form one country to another, of orthodox species of short longevity, provided that the initial MC is correct. But if orthodox seeds are being forwarded soon after collection and without having been dried to the appropriate MC for storage, it is preferable to ship in bags permeable to air rather than to seal with excessively high MC. A number of species with resistant seedcoats or pericarps, such as Tectona and many leguminous species, are able to withstand prolonged periods in ambient conditions; cotton or paper bags or hessian sacks are perfectly suitable for these species.
Large, moist seeds can be sealed individually with paraffin wax or latex. In the method described by Baldwin (1955), paraffin wax is heated to 71°–77° C and seeds or nuts dipped for a few seconds in a screen-type container, which should be shaken vigorously during the immersion. The waxed seeds should be packed in soft material so that the wax is not scraped off during transit. At the time of sowing the wax must be partly scraped off to permit the entry of water.
Protection against high or rapidly fluctuating temperatures is more difficult, but care should be taken to avoid placing the seeds close to local hotspots such as radiators and hot pipes. For very sensitive seeds, temperature effects can be mitigated by the use of insulating material in the packaging. Sub-zero temperatures do not usually affect dry seeds but may cause damage to recalcitrant seeds which must be kept moist. Premature germination is another risk which affects moist seeds. During storage, germination can be restricted by the use of low temperatures just above freezing, but the higher temperatures encountered during transit may induce germination in a substantial number of seeds. Seeds which are prone to germinate when held in moist packing may be treated with an inhibitor such as maleic hydrazide (Baldwin 1955).
No matter what type of seeds is being despatched, it is necessary to take precautions against mechanical damage to seeds and against losses due to damage to the containers in transit. Double wrapping is often advisable, for example a sealed polythene bag should be placed inside a stout canvas bag. Stout drum cartons with sealed polythene or aluminium-foil containers inside provide an especially effective combination for seeds which need to be kept dry. If the inner bag is labelled, this is also an insurance against accidental defacement of the outer label. Clear labelling is essential and the consignee should be advised of despatch by means of an appropriate seed consignment note or seed issue form (see Appendix 1).
Stein et al. (1974) have provided a useful check list of helpful practices in seed shipment, reproduced hereunder:
Double wrap the seed. Enclose the seed container in a sturdy, preferably rigid, outer container.
Small or moderate size containers generally withstand shipment better than large containers.
Fill containers completely to minimize air content and jostling of seeds during shipment.
All packages should bear a good identifying label on the innermost covering and another one within the container.
For long distances, shipment of sensitive seeds by air is desirable.
Seed packages should permit ready opening and reclosing if destined for export to a country requiring fumigation. In addition a copy of the phytosanitary certificate should be readily available to quarantine authorities e.g. by sealing it in an envelope which is firmly attached to the outside of the package.
Seed storage facilities at nursery sites or district forest stations are inferior to those at the central seed store. Shipments should therefore be timed so that seeds can be sown with the minimum delay after receipt.
Chapter 7 SEED STORAGE Introduction Storage may be defined as the preservation of viable seeds from the time of collection until they are required for sowing (Holmes and Buszewicz 1958).