A Guide to Forest Seed Handling (2024)

Introduction

Storage may be defined as the preservation of viable seeds from the time ofcollection until they are required for sowing (Holmes and Buszewicz 1958). Whenseed for afforestation can be sown immediately after collection, no storage isneeded. The best sowing date for a given species being raised in a nurserydepends on (a) The anticipated date of planting, itself dependent on seasonalclimate, (b) The time needed in the nursery for planting stock of that speciesto reach the right size for out-planting. Only rarely does best sowing datecoincide with the best date for seed collection. More often it is necessary tostore the seed for varying periods which may be

1. Up to one year when both seed production and afforestation are regularannual events, but it is necessary to await the best season forsowing.

2. 1 – 5 years or more when a species bears an abundant seed crop atintervals of several years and enough seed must be collected in a goodyear to cover annual afforestation needs in intermediate years of poorseed production.

3. Long-term storage for purposes of conserving genetic resources. Theperiod of storage will vary according to the seed longevity of thespecies and the storage conditions, but will be measured in decades inspecies which are easy to store.

The storage facilities to be provided must be related to the amount of seeds andthe period over which they are to be stored. It is a waste of money to createexpensive facilities capable of maintaining viability for 10 years if the seedswill never stay longer than nine months between collection and sowing. It isequally wasteful to spend money on seed collection, extraction and cleaning ifstorage conditions are so inadequate that the seeds are 90 % dead before theyreach the nursery.

For general accounts of seed storage of forest tree seeds, there are a number ofuseful 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 seedsand there is good reason to accept that the general principles established foragricultural crops apply also to forest trees. An excellent recent account ofthe subject as applied to agricultural seeds is contained in “Principles andpractices of seed storage” (Justice & Bass 1979) and there is also much usefulinformation in the slightly older publications of Roberts (1972) and Harrington(1970, 1972, 1973). Long-term storage for gene resource conservation is wellcovered by Cromarty et al. (1982).

Natural Longevity of Tree Seeds

The period for which seed can remain viable without germinating is greatlyaffected by its quality at the time of collection, its treatment between collectionand storage and the conditions in which it is stored. Nevertheless, seedlongevity varies enormously from species to species even if they are givenidentical treatment and storage conditions. Ewart (1908) divided seeds intothree biological classes according to the time for which they are capable ofretaining 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 innatural longevity of seeds of different species, his classification is too rigidto fit the variations between individuals, provenances and seed years in asingle species, or the possible variations in storage conditions. It is notpossible to define a standard set of “good” storage conditions equally suitableto all species, because species vary in their requirements for optimum conditions.Yet storage life of a given species will vary greatly according to theconditions in which it is stored.

Today two major classes of seed are recognised (Roberts 1973):

1. Orthodox. Seeds which can be dried down to a low MC of around 5% (wetbasis) and successfully stored at low or sub-freezing temperatures for longperiods.

2. Recalcitrant. Seeds which cannot survive drying below a relatively highmoisture content (often in the range 20–50% wet basis) and which cannot besuccessfully stored for long periods.

Within these two classes some further subdivision may be made, for examplebetween orthodox seeds with or without hard coats and between recalcitrant seedswhich can or cannot withstand low temperatures of below around 10°C. Within eachof the main classes there are still considerable differences between species inthe period for which viability is maintained under a given set of conditions.There may also be a distinction between truly recalcitrant species and speciesthat are just difficult; the latter may turn out to behave in an orthodox mannerif, 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 seedviability over a period of decades are hard-seeded. They include a number oftropical leguminous species. Examples of species of which at least some seedsmaintained viability after lengthy periods of storage in herbaria, cited byHarrington (1970) from the work of Ewart (1908) and Becquerel (1934), are:

Ambient conditions in herbaria storage can be considered good (fairly low relativehumidity and temperature) but well short of the combination of low initialMC, sealed storage and sub-freezing temperature now considered ideal for longtermstorage of orthodox species.

Recent research has provided more precise information on conditions of storage,initial germination and final germination of some species, but over shorterperiods. Examples are:

Sources: 1) Ffolliot and Thames 1983.
2) Doran et al. 1983.

As explained later in this chapter, modern thinking has defined low MC, lowtemperature and low oxygen pressure as the three most important constituents ofthe storage conditions which man should provide to maximize seed longevity inorthodox species. In the impermeable seedcoat nature has provided two of theseconstituents, a low MC and exclusion of oxygen. Full-sized but green leguminousseeds, sown immediately without drying, may germinate at once, indicating thatthe seedcoat has not yet developed an impermeable layer; no doubt the developmentof impermeability is synchronized in nature with the reduction of seedmoisture by natural drying to the optimum content for longevity. Hard seeds arethus a potent factor in extending seed life in all conditions of storage butconfer their most important benefits when storage facilities are limited andduring the potentially dangerous period between collection and entry intolong-term storage.

Not all leguminous seeds are equally long-lived, for example Koompassia malaccensisseeds have thinner seedcoats and deteriorate more rapidly in storage thanspecies such as Parkia javanica, and they need no pretreatment to overcomeseedcoat dormancy (Sasaki 1980 a). In Sudan seeds of Dalbergia sissoo storedless well at room temperature than those of local Acacia, Albizzia and Tamarindusspecies (Wunder 1966), while in Australia seeds of Acacia harpophylladeteriorate 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 alleucalypts can be kept viable for some years if stored with a low moisture contentin sealed containers at 3° – 5° C. The majority of species can be storedfor 10 years at room temperature with relatively little loss of viability(Turnbull 1975 f). Both E. deglupta and E. microtheca seeds deteriorate morerapidly if stored at room temperature, but storage life is improved if they arekept in air-tight containers at 3° – 5° C and recent evidence suggests that storageat -18°C is even better. In Thailand seeds of P. kesiya and P. merkusii retainedgood viability for four years if stored at below 8 % moisture content in sealedcontainers at 0° – 5° C (Bryndum 1975), while at least five years' good viabilityis possible with P. caribaea and P. oocarpa under similar conditions(Robbins 1983, a,b). Considerably longer periods have been recorded for somespecies of pine e.g. 30 years for Pinus resinosa in USA when stored in sealedcontainers at 1.1° – 2.2° C (Heit 1967b, Wang 1974). Tectona grandis is anorthodox tropical broadleaved species (Barner 1975b) but, since it produces goodseed crops in most years, there has been little stimulus to investigate optimumconditions 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 viabilityfor 6 years (79 % germination compared with the initial 88 %), while storage atbelow freezing temperature maintained a germination of about 60 % for up to 12years (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 islonger lived than A. macrophylla. Initial trials with the tropicalA. macrophylla indicated that good results could be obtained by drying freshseeds 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 inconsistentand less successful. With tropical species, it is likely that handling betweencollection and despatch and the largely uncontrollable conditions of air transitare more critical than in the easier temperate or subtropical species.

Orthodox species which rapidly lose viability unless they are given the optimumtreatment include species in the mainly temperate genera Populus, Salix andUlmus. Many of these lose viability within a few weeks under natural conditionsor if stored in ambient conditions of temperature and humidity, but can bestored for months or years if maintained at low temperature and low moisturecontent. Examples are of Ulmus americana stored successfully for 15 years at 3 %moisture content and -4° C (Barton 1961), and Populus sieboldii stored for 6years at -15° C over a desiccating agent in a sealed container (Sato 1949). InPopulus balsamifera and Salix glauca, reduction in germination after two yearsof sealed storage at -10° C was less than 6.5 % of initial germination (Zasadaand Densmore 1980); after three years there was very little change in Populusbut up to 40 % reduction in Salix.

In the tropics Aucoumea klaineana is a good example of an orthodox species whichis short-lived under ambient conditions. Germination of fresh seed is often over90 %, but after 30 days' storage in room conditions there is a significant dropin germination and this falls to zero after 100 days. Storage at 0 – 5° C and7 – 8 % MC in sealed containers with a chemical desiccator, Actigel, maintainsover 50 % germination for at least 30 months (Deval 1976). There is some indicationthat a further reduction of MC will conserve viability even better. Thusone 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 germinationof 70 % in the laboratory and 79 % in sand after 30 months; the same seedlot stored in sealed containers without Actigel had an MC of 9.9 – 10.4 % and agermination of 54 – 63 % in the laboratory and 62 – 67 % in sand. Other speciesof this type include Entandrophragma angolense which has a seed life of 6 weeksin room conditions but up to 6 years in cold storage (Olatoye 1968) and Cedrelaodorata which loses all germination capacity in 10 months at room temperaturebut suffers no loss in 14 months if stored at 5° C in a sealed jar (Lamprecht1956).

Some species may need special treatment to prolong viability for more than a fewmonths. fa*gus 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. Itis then suitable for sowing because such storage conditions constitute a suitablepretreatment to break dormancy. If longer storage is intended, MC shouldbe 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 andLeak 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-Masimbertand Muller 1975, Suszka and Kluczynska 1980). This technique has been successfullyapplied 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 (Mullerand Bonnet-Masimbert 1982).

Where storage conditions leave much to be desired, the longevity of orthodoxseeds without hard coats can be expected to be much inferior to the hard-coatedspecies. The nearer that conditions of storage approach the ideal for a givennon-hardseeded species, the less the difference between its longevity and thatof a hardseeded species. The best combination of MC and temperature will vary tosome extent between species, for example the above-quoted 8–10 % MC for fa*gussylvatica is considerably higher than the 5–6 % considered ideal for many forestand agricultural seeds.

Recalcitrant seeds

Recalcitrant seeds include a number of large seeds that cannot withstand appreciabledrying without injury; it is of interest that the overwhelming majorityof recalcitrant species listed by King and Roberts (1979) are woody.Temperate species such as Quercus and Castanea are commonly stored moist onlyfor short periods over winter. Reduction of storage temperature to near freezingwill prolong longevity. Bonner (1973 a) found that it was possible to storeacorns of Quercus falcata for 30 months and still obtain over 90 % germinationat the end of the period, provided that temperature was maintained at 3 ° C andMC 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 above40 % (Holmes and Buszewicz 1956, Suszka and Tylkowski 1980). Recent research inPoland has demonstrated good results from storing this species at >40 % MC inair-dry peat or air-dry sawdust in milk cans at -1° C. It is important to allowfree entry of oxygen and this is ensured by inserting several strips of cardboardat intervals between the lid and the edge of the can. In these conditionsgermination after 3 winters was in the range of 38 – 75 % and after 5 winterswas still about 12 % (Suszka and Tylkowski 1980). Temperatures below -5° Ckilled all the acorns, while a temperature of +1° C encouraged excessive pregermination(60 – 75 % after 3 winters, with radicles up to 25 cm long, comparedwith 12 % and radicles <0.5 cm long at -1°C). There may be possibilities ofstoring seeds after emergence of radicles (see p. 152). Recent research inPoland (Suszka and Tylkowski 1982) has indicated that best results are obtainedwith the recalcitrant Acer saccharinum by maintaining MC at the same percentage(50–52%) as when the seeds were freshly collected. For A. pseudoplatanus in theUK a minimum MC of 35 % is recommended (Gordon and Rowe 1982), while in Polandan MC of 24–32% and a temperature of -3°C have proved suitable to store samarasover three winters (Suszka 1978a).

Most short-lived recalcitrant tropical species are constituents of the moisttropical forests, where conditions conducive to immediate germination (highhumidity and high temperature) are prevalent throughout the year. Typical generaare Hevea, Swietenia, Terminalia and Triplochiton, as well as a number of Dipterocarpgenera such as Dryabalanops, Dipterocarpus and Shorea and some speciesof Araucaria. Dryabalanops is injured if dried below 35 % moisture content (MC)but still survives only about three weeks at over 35 % MC (King and Roberts1979). Triplochiton seed is naturally short-lived but can be stored for up to 22months at a temperature of around 6° C and a moisture content of between 12 and25 % (Bowen and Jones 1975). Azadirachta indica seeds also have a short periodof viability, although the species occurs in dry, not moist, tropical forestsand it is not clear whether it is a genuine recalcitrant or simply a short-livedorthodox species.

Orthodox and recalcitrant species sometimes occur within the same genus. In Acerand Ulmus, genera in which both orthodox and recalcitrant seed behaviour occur,the distinction in North American species is clearly between spring- and fallseeders.A. rubrum and A. saccharinum flower and seed in the spring. Their seedsare not dormant, and their storage behaviour is clearly recalcitrant. Other Acerspecies have fall-maturing seeds, which are dormant and orthodox in nature atmaturity. The same occurs in Ulmus. Seeds of U. crassifolia and U. serotinamature in the fall, and are orthodox in storage behaviour. Spring-seeding speciesof Ulmus are “weakly” recalcitrant (Bonner 1984b). In Araucaria,A. cunninghamii and other spp. in the Eutacta taxonomic group behave as orthodox.In Queensland seeds of A. cunninghamii of 5 provenances were air dried andstored at varying temperatures in sealed and unsealed containers. At the highertemperatures, +1.7°C and -3.9°C, germination started to drop after 17 months'storage and after 8 years was down to about half the initial germination rate insealed containers and about one third in unsealed containers. At the lowertemperatures of -9.4°C and -15°C, germination after 8 years' storage was littlechanged from the initial (41–44% compared with initial 49%) (Shea and Armstrong1978), and there was virtually no difference between sealed and unsealed containers.The rate of viability loss at the higher storage temperatures variedfrom provenance to provenance, but all stored better at the lower temperatures.Moisture content was not recorded but under local conditions air-dry seed isnormally in the range of 16–23% (Kleinschmidt 1980, cited in Tompsett 1982).Later trials with Papua New Guinea A. cunninghamii have shown that seeds can bedried from 21% to 7% MC without any effect on initial germination rate; effectson storage life are still under investigation (Tompsett 1982). A. hunsteinii inthe Intermedia group and A. angustifolia, A. araucana and A. bidwillii in theColymbea group are apparently recalcitrant. Arentz (1980) found that high viabilityof A. hunsteinii could be maintained for at least 6 months by storage at3.5°C and high MC; 37% was significantly better than 32%. Research reported byTompsett (1982) confirmed that MC should be maintained above 32%. Placing theseed in one polythene bag of 25 microns thickness inside a second bag is effectivein maintaining viability. The double thickness of polythene maintains ahigh MC but allows for some exchange of oxygen which is necessary to preserveviability of A. hunsteinii. A. angustifolia also needs a high MC; seeds died ifdried to less than 25–30% (Tompsett, in press).

For some temperate recalcitrant species, as indicated above, a relatively lowtemperature (just above or just below 0°C) has been found beneficial in extendingthe life of the seeds; low temperature to some extent compensates forthe high MC which must be maintained to prevent the early loss of viability. Insome tropical species, seeds are quickly killed if temperature is reduced toolow, just as they are quickly killed if MC is reduced too low. Among woodyspecies cited in King and Roberts (1979) are Theobroma cacao (killed below+10°C), Mangifera indica (damaged below +3° to +6°C) and, among the dipterocarps,Hopea helferi, Hopea odorata and Shorea ovalis (damaged below, respectively,+5°C, +10°C and +15°C). This susceptibility to chilling damage at temperaturesabove 0°C compounds the difficulty of storing these recalcitrantspecies, which seldom maintain viability for more than a few weeks or at mostmonths. This compares with a normal seeding periodicity of several years in mostdipterocarps, so there is no possibility as yet of conserving seeds in a viablecondition from one good seed year to the next.

Unlike orthodox species, in which viability is best preserved by maintaining aminimal respiration rate, it appears that active respiration is necessary tosurvival of seeds of most recalcitrants. Thus damage to recalcitrant seeds hasbeen reported not only from inadequate MC and too low a temperature but alsofrom lack of oxygen e.g. in Araucaria hunsteinii (Tompsett 1983), Heveabrasiliensis and Quercus spp. (cited in King and Roberts 1979).

Whereas some temperate recalcitrant species have been stored successfully forseveral years, seed longevity in tropical recalcitrants can be measured in daysor weeks. The amount of research on tropical species is still small, especiallyon forest species, and it is possible that seed longevity could be prolongedbeyond a few weeks if the best combination of seed maturity, speed, conditionsand degree of drying, and most suitable storage temperature could be determinedfor each species. King and Roberts (1979) suggest a research strategy.

Factors Affecting Longevity in Storage

Seed condition

Even in ideal storage conditions seed will soon lose viability if it is defectivefrom the start. Factors to be considered are:

Seed maturity. Fully ripened seeds retain viability longer than seeds collectedwhen immature (Stein et al. 1974, Harrington 1970). Certain biochemical compounds,essential for preserving viability, may not be formed until the finalstages of seed ripening. These include dormancy-inducing compounds in certainspecies, and dormancy is sometimes associated with seed longevity. In a fewspecies e.g. Gingko biloba, Fraxinus excelsior, seed embryos are underdevelopedwhen the seed is shed. Maturation of these embryos is necessary before sowingbut need not be done before storage. In Fraxinus excelsior drying of freshlycollected samaras to 9–10% MC, followed by storage in sealed containers at -3° C,gives satisfactory results provided that successive moist warm and moist coldtreatments are applied after storage (Suszka 1978a). For details of treatmentssee p. 184.

Parental and annual effects. In seed harvest, quantity and quality often gotogether. The percentage of sound seeds in a high-yielding mother tree is usuallyhigher than in one with a poor crop. Similarly, a given mother tree will havea higher percentage of sound seeds in a good crop year than in a poor one.Collection from high-cropping mother trees in a seed year is likely to yieldseeds with the best longevity in storage. On the other hand, high-cropping“wolf” trees should be avoided because of their potentially undesirable woodproperties, even though they may produce long-storing seeds.

Freedom from mechanical damage. Seeds damaged mechanically in extraction, cleaning,dewinging etc. rapidly lose viability. The danger is greatest for specieswhich have thin or soft seedcoats. Excessive heat during extraction or dryingalso damages seed. Care should be taken to use the minimum times, lowest temperaturesand minimum machine speeds necessary during the preparation of seedfor storage (Stein et al. 1974). In some species, damage during dewinging may bereduced by partly restoring the moisture content between extraction from conesand dewinging, since moist seeds suffer less mechanical damage than dry ones(Nilsson 1963, Barner 1975b).

Freedom from physiological deterioration. Poor handling in the forest, duringtransit or during processing causes physiological deterioration of seeds even ifmechanical and fungal damage are absent. Adequate ventilation of orthodox seedsis necessary to avoid rapid respiration and overheating, while recalcitrantseeds must be protected against excessive drying.

Freedom from fungi and insects. For species stored at low temperature and lowmoisture content, the storage conditions themselves should prevent the developmentof fungi and insects. It is necessary, however, to avoid collection ofcrops showing a high incidence of fungal or insect attack and to carry out alloperations of collection, transport, processing etc. as quickly as possible toensure seed is not already damaged before it goes into storage. Attack by fungiand insects is most rapid on the forest floor, so collection from the groundshould be carried out as soon after fruit fall as possible. Fungicidal treatmentcannot be generally recommended since it can be harmful to seeds (Magini 1962);many fungicides are only effective when dissolved in water and are inappropriatefor dry storage. Insects are usually killed if seeds are dried at temperaturesabove 40° – 42° C. For seeds which cannot be dried, other measures may beneeded. For example seeds of Quercus are fumigated with serafume or other chemicalsor heated in warm water for control of weevils (Belcher 1966, Olson 1957),while methyl bromide or carbon bisulphide are also commonly used to kill insects(Boland et al. 1980).

Initial viability. Seed lots with high initial viability and germinative capacityhave a higher longevity in storage than those with low initial viability.Germination tests, preceded if necessary by appropriate pretreatment to overcomedormancy, should be carried out on a sample of each seed lot before storage, inorder to determine how long the seed is likely to retain viability in storage.Longevity of the viable seeds is correlated with the percentage which germinatein the initial test. As an example, samples of two seed lots of the same species,from which 80 % germination of fresh seeds is normally expected, mightgive results of 90 % and 50 % initial germination. Not only would storage of thesecond seed lot involve wasting space in storing dead seed, but even the 50 % ofinitially viable seeds are likely to lose their viability more quickly in storagethan would the 90 % viable seeds in the first seed lot. Deterioration ininitial viability may not be serious if the seeds are to be sown within a fewweeks or months, but only good quality seed should be stored for long periods(Holmes and Buszewicz 1958, Magini 1962). In long-term storage of agriculturalseeds for genetic conservation, it is recommended that no seeds should be acceptedfor storage which have an initial viability of less than 85 % of thatconsidered typical for the species or variety in question (IBPGR 1976). It maybe noted that initial viability and germinative capacity are frequently theresultant of the factors described in previous paragraphs (seed maturity, mechanicaldamage, fungal or insect attack).

Storage conditions and ageing of seeds

In common with all other living things, seeds are subject to ageing and, eventually,to death. In the case of orthodox seeds, the process of ageing and deteriorationis so greatly affected by the conditions of storage that the “age”of seeds, expressed solely in terms of the period elapsed since ripening andharvesting, is an inadequate measure of the degree to which they have “aged” inthe sense of losing viability and progressing towards the irreversible deteriorationof death. The term “physiological age” is commonly used to describe thedegree of deterioration of seeds measured by their reduced capacity for germination.Nomographs for the effects of temperature and MC on physiological ageingof seeds have been constructed for several agricultural crops (Ellis and Roberts1981). As an example, the nomograph for barley indicates that the same degree ofdeterioration (from initial 95 % to final 50 % germination) would occur in about16 days in seeds stored at 25° C and 21 % MC, but in about 100 years in seedsstored at 8° C and 8 % MC. Both seed lots would have the identical physiologicalage, though stored for very different periods of time. Similar effects can beexpected in orthodox seeds of forest trees.

A number of physiological changes in cell tissues may be associated with physiologicalageing in seeds. They include (1) Loss of food reserves caused by respiration,e.g. decrease in proteins and non-reducing sugars, accompanied by increasein reducing sugars and free fatty acids (2) Accumulation of toxic orgrowth-inhibiting by-products of respiration (3) Loss of activity of enzymesystems (4) Loss of ability of dried proteid molecules to recombine to formactive protoplasmic molecules on subsequent rehydration (5) Deterioration ofsemi-permeable cell membranes (6) Lipid peroxidation, leading to production offree radicals which react with, and damage, other components in the cell (7)Alterations to DNA in cell nucleus, causing genetic mutations as well as physiologicaldamage (Roberts 1972, Harrington 1973, Villiers 1973). It is stilluncertain to what extent these various effects are the causes or only the symptomsof deterioration, but it has been suggested (Villiers 1973) that the productionof free radicals is the first effect of ageing and that damage to theseveral systems in the cells is the subsequent result of the release of freeradicals.

Whatever the exact mechanism of seed deterioration, there is a consensus that,in orthodox seeds, loss of seed viability is largely governed by the rate ofrespiration. Any measures which reduce the rate of respiration without otherwisedamaging the seed are likely to be effective in extending longevity duringstorage. These are the control of oxygen, the control of moisture content andthe control of temperature. In recalcitrant seeds the safe minimum levels ofoxygen, moisture content and temperature, and hence of respiration, are allconsiderably higher than those for orthodox seeds but, provided levels aremaintained above the safe minima for each species, it appears that longevity canbe extended by keeping them as close to the minima as possible in order to avoidan excessively high respiration rate.

Storage atmosphere

The most obvious method of reducing the rate of aerobic respiration is to excludeoxygen from the atmosphere surrounding the seeds. This can be done byreplacing oxygen by other gases such as CO₂ or nitrogen, or by using a partialor complete vacuum. In an example with lettuce cited by Roberts (1972), seedswere stored in sealed containers at 6 % MC and 18° C. After 3 years, seed storedin an atmosphere of pure oxygen had 8 % viability, those in air 57 %, those innitrogen or argon or CO₂ 78 % and those in a vacuum 77 %. The value of excludingoxygen during storage of dry orthodox seeds has also been demonstrated inPinus radiata (Shrestha, Shepherd and Turnbull 1984). Best results were obtainedwith a storage atmosphere of nitrogen, followed by CO₂, while vacuum and airboth gave poorer results. At the highest temperature used, 35°C, at which deteriorationin viability was most rapid, the loss in final germination after 50weeks' storage in sealed containers at 8% MC was 8% in nitrogen, 14% in CO₂, 21%in vacuum and 29% in air. The same ranking was obtained by comparing the speedof germination and the vigour of germinated seedlings (measured as dry weight 49days after sowing). Although increases in seed longevity of this magnitude havebeen achieved experimentally, some of the methods are expensive to apply and theeffects on seed life are less dramatic than the effects of differences in temperatureand humidity (Goldbach 1979). Exclusion of oxygen will prevent aerobic,but not anaerobic, respiration, whereas reduced MC and temperature will decreasethe level of both. While systematic predictions have been made of seed longevityunder a range of temperature and MC for several agricultural crops (Ellis andRoberts 1981), similar quantitative predictions of the effect of oxygen levelson longevity are lacking.

One simple method which is recommended is to fill sealed containers as nearlyfull as possible. If there is only a small amount of air inside the container ascompared with the volume occupied by the seeds, oxygen will be consumed and CO₂produced. The resulting high CO₂/O₂ ratio is probably favourable for seed longevityin orthodox seeds (Goldbach 1979).

Whereas the complete exclusion of oxygen from the storage atmosphere appearsbeneficial to most dry orthodox seeds, there is evidence that some oxygen isnecessary for recalcitrant seeds. Seeds of Araucaria hunsteinii, which had aninitial germination of 56%, had all died within a month if stored in pure nitrogen,in two months if stored in 1% oxygen, and in three months if stored in5% oxygen, while germination was still 18% after four months' storage in 10%oxygen (Tompsett 1983, 1984). Storage in a polythene bag of 25 micron thickness,which was ventilated (21% oxygen) periodically when opened for extraction ofsamples, gave results very similar to those in 10% oxygen. King and Roberts(1979) record a general consensus that adequate ventilation (i.e. adequateoxygen), is necessary for the successful storage of recalcitrant seeds atrelatively high MC, as well as for storage of imbibed seeds of orthodox species.

Seed moisture content

The relationships of seed moisture content on a wet weight or fresh weight basisto seed MC on a dry weight basis, and of the equilibrium moisture content ofseeds to the relative humidity of the surrounding atmosphere, are important inseed processing and are explained on pp. 121–125. They are equally important inseed storage. In the first case manipulation of RH can effectively change MC ofseeds to the optimum for storage, in the second case MC can be maintained at ornear that optimum by maintaining a suitable RH in the atmosphere around andbetween the seeds.

Effect of MC. In orthodox seeds, moisture content is probably the most importantsingle factor in determining seed longevity (Holmes and Buszewicz 1958). Reductionin MC causes a reduction in respiration and thus slows down ageing of theseed and prolongs viability. Harrington (1959), cited by Barner (1975b), hasrelated MC to various processes within and around the seed as follows:

Prevention of fungal activity is more easily achieved by controlling MC than bycontrolling temperature. If MC and RH are high enough, fungal activity is possiblebetween -8° C and +80° C (Roberts 1972) and it is easier to keep MC below12–14 % (or RH to the equilibrium of around 65 %) than to maintain sub-zerotemperatures.

Within the range of 4 to 14 % MC, Harrington (1963, 1970) has suggested a ruleof thumb applicable to many agricultural species - the life of the seed isdoubled for every 1 % decrease in MC, Schönborn (1965) found a relationship of asimilar order when measuring the respiration rate, expressed in terms of CO₂production, of Picea abies. At 20° C and 20 % MC the seed gave off 80 ml. CO₂per hour per kg of seed; at 20° C and 5 % MC, the rate of CO₂ production wasreduced to 0.11 ml/hr/kg, a reduction of nearly a thousand times for a differenceof 15 percentage points in MC.

An MC of 4 – 8 % is considered safe for most orthodox species; 5 % ± 1 % isrecommended for long-term storage for genetic conservation (IBPGR 1976). Oilyseeds will usually tolerate drying to a somewhat lower moisture content (calculatedon the basis of total fresh weight) than will non-oily seeds (Harrington1970). Drying below 4 % can lead to damage or more rapid loss of viability insome species, although certain species can be dried to a considerably lower MC.Betula papyrifera was successfully stored at 0.6 % MC without injury (Joseph1929 cited in Holmes and Buszewicz 1958), while Schönborn (1965) succeeded indrying small samples of Picea abies, Pinus sylvestris, Pseudotsuga menziesii andLarix decidua down to 0 % MC without any observable drop in germination after 6months, compared with germination at the normally applied 6 – 8 % MC. Drying inthis case was done, not by exposing the seeds to high temperatures but by leadinga current of dry air through the seeds at 20° C. The same treatment killedPinus strobus and Abies alba, and earlier attempts by Barton (1961) to storeseeds of several species of Pinus and Picea at 0 % MC also failed. Below about 2% MC desiccation injury becomes a strong possibility in many species. Drying tovery low MC is also more costly than drying to the usual 4 – 8 % and is likelyto be used only in exceptional cases. Methods of drying are described on pp.95–107.

Some orthodox forest trees store best at appreciably higher MC. As mentioned onp. 134, 8–10% is recommended for fa*gus sylvatica. For seeds of Abies spp., an MCof 12–13 % is recommended for storage of one to three years, but for longerperiods this should be reduced to 7–9 % (Barner 1975b). Species which benefitfrom storage at higher than average MC also need particular care in the timingand speed of drying.

Fluctuation in the moisture content of seed in storage due to open storagewithout humidity control or to frequent opening and resealing of sealed containersresults in deterioration in the germinability of the seeds (Wang 1974,Stein et al. 1974). In fact a steady MC slightly above the optimum is usuallyless harmful than one which fluctuates between the optimum and a higher moisturecontent.

Some cases have been reported in which the usual trend of decreasing seed longevityassociated with increasing MC is reversed at or near the moisture content offully imbibed seeds. If the species in question needs exposure to light in orderto germinate, it is possible to store fully imbibed but ungerminated seeds forsome time in the dark. Fraxinus americana was stored at 22° C at varying MCswith the following results (Villiers 1973):

It has been postulated that imbibed seeds can repair damage to cell membranes,enzymes and DNA in the cell nucleus caused by free radicals in a way which isnot possible for seeds at lower MC. Prolonged imbibed storage may, however, bedifficult in practice, because of the need to maintain constant high moisturefor imbibition and adequate oxygen without allowing the seeds to germinate orencouraging the multiplication of fungi and bacteria (Roberts 1981).

Moisture content is also important in recalcitrant seeds, but in this case thecritical MC is the minimum to which it is allowable to dry the seeds rather thanthe maximum content for prolonged storage. For many of the large temperatehardwood seeds, moisture contents in the range of 25 – 79 % are appropriate(Wang 1974). Storage should be carried out at close to the minimum safe MC,since the higher the MC the higher the respiration rate and the more rapid theloss of viability. Higher respiration rates release higher amounts of energy andthere is a risk of overheating and death of the seed, unless great care istaken to provide adequate aeration. High MC also increases fungal activity andthe spread of rot. Wang (1974) quotes results from two seed lots of Acersaccharinum; germinability of one lot stored at 58 % MC and 1 – 2° C droppedfrom 94 % to 12 % after 6 months' storage, while that of the other, stored at 45% MC at the same temperature, was still 78 % after 16 months. Loss of viabilityin this species may be sudden. Tylkowski found that seeds stored in sealedbottles at 50–52 % MC and -1° to -3°C had over 90 % germination after 18 monthsbut this dropped to nearly zero after 24 months (Suszka and Tylkowski 1982).

Less research has been done on tropical recalcitrant species, but there is someevidence e.g. in Triplochiton, that viability may be significantly prolonged ifthe minimum MC for the species can be determined and if particular care is takenover the period of drying down to that MC (Bowen and Jones 1975). Trials ofShorea platyclados in Malaysia indicated that a gradual reduction of MC to 20 –27 %, followed by sealing in charcoal, sawdust or vermiculite at 15° – 22° Callowed storage for at least one month, compared with the week or so of naturalviability (Tang 1971). On the basis of experiments carried out on Shoreaparvifolia and Dipterocarpus humeratus, Maury-Lechon et al. (1981) recommendedreduction of MC to between one quarter and one half of the initial MC in freshlycollected fruits. Although tropical recalcitrant seeds cannot yet be stored formore than short periods, there is a growing body of useful research on theproblem. King and Roberts (1979) contains a good summary of achievements andpossible approaches.

Storage temperature

Temperature, like moisture content, is negatively correlated with seed longevity;the lower the temperature the lower the rate of respiration and thus thelonger the life-span of the seed in storage. Harrington (1963, 1970) suggestedanother rule of thumb for agricultural seeds - between 50° C and 0° C, every 5 °C lowering of storage temperature doubles the life of the seed. For orthodoxseeds, which can be dried to a low moisture content, still greater longevity canbe assured by storage at sub-freezing temperatures. For long-term storage forgenetic conservation of agricultural seeds, a temperature of -18° C has beenrecommended as the “preferred” standard for most species and -10° C as an“acceptable” standard for those species known to have an intrinsic high viability(IBPGR 1976). Much lower temperatures have been used with success on anexperimental basis, e.g. in liquid helium at -269° C, but the high cost ofmaintaining such low temperatures for a long period would outweigh any (as yetunproven) advantages in increased longevity.

Choice of storage temperature varies considerably according to species and theperiod for which the seed is to be stored. The lower the temperature that has tobe maintained in a cold store, the higher the cost, and provision of subfreezingtemperatures may be unnecessary if seed is to be stored for only a yearor two for afforestation projects. Holmes and Buszewicz (1958) noted that, in anumber of experiments on conifers, the superiority of sub-freezing temperaturesbecame evident only after prolonged storage over periods of about 5 years ormore. Sub-freezing temperatures (-18°C) appear to prolong viability in tropicalorthodox species such as Eucalyptus deglupta and Flindersia brayleyana (Turnbull1983).

Some seeds keep well at room temperature, e.g. many leguminous and rosaceousgenera, Eucalyptus, Tilia and many other hard-seeded or stone fruits. Mostspecies, however, only keep well for longer periods at lower temperatures. In3 – 5 years' storage of most conifers and Alnus and Betula, the crucialtemperature seems to lie at a maximum of +4° C. The temperature should thereforebe kept at 1 to 4° C. For longer periods of storage, say 5 – 15 years, thetemperature should be -4 to -10° C. For Abies, however, a temperature of -4° Cis used for short storage periods and -10° to -20° C for longer periods (Barner1975b).

Temperature and moisture factors are so interrelated that it is very difficultto separate them. Seeds at a relatively high moisture level can be stored forconsiderably longer periods at near freezing temperatures than at higher temperatures,while higher storage temperatures (30° C) are less harmful when themoisture content of the seed is low. In short, it can be said that the criticalmoisture content lies at a higher level when storage temperatures are low thanwhen they are intermediate or high, i.e. to some extent, a low temperature cancompensate for a high moisture content, and vice versa. (Holmes and Buszewicz1958). It is, however, necessary to avoid any risk of freezing damage caused byice formation in seeds of high MC. Roberts (1981) has suggested that 20% MC maybe the critical upper limit for storage at 0°C, 15 % for -20°C and 13% for-196°C. If seeds are dried to 4–8% MC as commonly recommended for orthodox species,there should be no danger of freezing damage, even at temperatures wellbelow zero.

As mentioned in Chapter 6, the equilibrium moisture content of many seeds at agiven RH varies with temperature. Barton and Crocker (1948) have shown that, ata range of RH from 35 % to 76 %, the amount of water contained by seeds increasedprogressively as temperature decreased from 30° C to 10° C. Speciesincluded Pinus as well as several agricultural crops. At low (35%) RH moistureabsorption by dry seeds was approximately the same at 5°C as at 10°C, but athigher (55% and 76%) RH the seeds absorbed less moisture at 5°C than at 10°C,thus reversing the trend above 10°C. Change of equilibrium moisture content withchange of temperature can be of importance in open storage. In sealed containersthe effect is minimal because the final EMC is dominated by the initial MC ofthe seeds and not the moisture of the enclosed air.

As with moisture content, repeated fluctuations in temperature lead to loss ofviability. As far as possible, temperature should be maintained at a uniformlevel.

The effect of temperature on seed longevity of temperate recalcitrant species issimilar to that of orthodox species - within certain limits the lower the temperaturethe longer the period of viability. Some tropical species are killed bytemperatures above freezing e.g. some dipterocarps at <14° C (Gordon 1981),cacao at <10 °C and mango at < 3–6 ° C (King and Roberts 1979). Seeds of Hopeahelferi stored at 15° C with high moisture content in unsealed polythene bagsretained 98 % germination after 37 days and 80 % after 60 days (Tang and Tamari1973). Germination was much reduced if temperature was dropped to 10° C orraised to 25°–28° C. Shorea ovalis is another species which does not withstandlow temperatures; it stores best at 21° C. Shorea talura, in contrast, storeswell at 4° C and 40 % MC (wet weight basis); after six months germination wasreduced from an initial 95 % to 69 % (Sasaki 1980 b). Other species of dipterocarphave a much shorter seed life.

For most temperate recalcitrant species the lower the temperature down to 0° Cthe longer the safe storage period. Below-freezing temperatures, on the otherhand, often kill recalcitrant seeds which need to be stored at a high moisturecontent (Harrington 1970, Wang 1974). Some success has been achieved in storingQuercus seeds in the USA by maintaining them at 35 – 45 % moisture content and-1° to +3° C temperature (Bonner 1978). Temperature can be critical because lessthan -1° C will usually kill the seeds, while temperatures above 2° or 3° Ccause excessive germination. In Europe more northerly species of Quercus can bestored at slightly lower temperature -1° to -3° C and 38 – 45 % MC (Suszka &Tylkowski 1980).

Light

Light, particularly ultra-violet light, is reported to be harmful to seed(Harrington 1970), but very few studies have been made. Use of opaque metalcontainers would be preferable to glass jars or bottles for species which areaffected by light. But light appears to be much less important than eithermoisture content or temperature.

Choice of Storage Method

A number of different storage methods are available, as described below. Themain factors affecting choice are the seed characteristics of the species inquestion, the period for which it is to be stored and the cost. If more than onemethod is suitable to maintain viability for the required period, the simplestand cheapest will normally be chosen.

Storage at ambient temperature and humidity

Seeds can be stored in piles, single layers, sacks or open containers, undershelter against rain, well ventilated and protected against rodents (Holmes andBuszewicz 1958, Magini 1962, Stein et al. 1974). Best results are obtained incool, dry climates. In these conditions several species of Pinus, EucalyptusPseudotsuga and Tectona will store satisfactorily for at least six months, whileleguminous trees with impermeable seedcoats and naturally low MC, e.g. AcaciaProsopis, Robinia will retain viability for years (Magini 1962, Stein et al.1974).

Dry storage with control of MC but not of temperature

Orthodox seeds will retain viability longer, when dried to a low moisture content(4 – 8 %), as described on pp. 126–128, and then stored in a sealedcontainer or in a room in which humidity is controlled, than when stored inequilibrium with ambient air humidity. Storage life is further prolonged ifcool, but not controlled, temperature conditions can be provided, e.g. at highlatitude or altitude and in a cellar or other room screened from direct sunlight.

Seed is sometimes stored in open containers in a room maintained at an RH of15 – 20 % by dehumidifying machinery. In forestry it is more common to rely onpredrying of the seed to the correct MC, followed by storage in full, sealedcontainers. Provided that the containers are not opened too frequently and thatthe sealing is effective, the method will maintain a low MC for many years. Itis cheaper than using a humidity-controlled room, especially during periods whenonly a little seed is in store, and it is not subject to hazards of mechanicalbreakdown.

This method is suitable for a range of species including many Pinus andEucalyptus species, for which it should maintain viability for one or moreyears.

Dry storage with control of both MC and temperature

This storage treatment is the standard practice for many orthodox species whichhave periodicity of seeding but which are planted annually in large-scale afforestationprojects. For many species a combination of 4 – 8 % MC and 0 to +5° Ctemperature will maintain viability for 5 years or more. Some cool-temperategenera benefit by storage at sub-freezing temperatures, e.g. -4°C or lower forAbies (Barner 1982), -10°C for fa*gus (Suszka 1966, 1974), -5°C for fa*gus (Mullerand Bonnet-Masimbert 1982), -18°C for Pinus strobus, Populus deltoides andothers (Wang 1980). Pinus merkusii is an example of a tropical pine which respondswell to storage at low temperature and MC. 80 % germination was obtainedafter 3 years' storage at 2° C and 6 – 10 % MC of seeds of the Zambales(Philippines) provenance, while seeds stored in equilibrium with room temperatureand humidity showed a significant loss of germination after 3 – 4 months(Gordon et al. 1972). P. caribaea and P. oocarpa behave similarly in this respect.In addition to true seeds, this method is also suitable for certain typesof fruits. For example in the Jari project of Brazil depulped, cleaned and driedstones of Gmelina arborea are successfully stored in sealed containers at 5° Cand 6 – 10 % MC (Woessner and McNabb 1979). The fresh stones have a germinationof 90 % and after two years of storage germination is still 80 %.

Dry storage for long-term gene conservation

The preferred storage treatment for long-term conservation of gene resources oforthodox agricultural seeds is -18° C temperature and 5 % +-1 % MC (IBPGR 1976).This is likely to be equally appropriate for orthodox seeds of forest treesrequiring storage for genetic conservation. The quantity of seeds requiring thisstandard of storage is small in comparison with the quantities used each yearfor operational afforestation, and the cost per kg of seed is higher. For manycountries it would therefore be desirable that forest and agricultural cropgenetic resources should share a common long-term storage facility. A goodexample is that of the Banco Latino Americano de Semillas Forestales at CATIE,Turrialba which has its own seed store (55 m³ capacity at 5°C) for short ormedium 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), whichare also at Turrialba (Chang 1980).

Loss of viability in storage, in addition to reducing the number of plants whichcan be produced by a given seed lot, may result in a shift in the genetic constitutionof the seed being stored. This could be particularly important inforest trees which are predominantly outbreeding, variable populations. First,loss of viability may occur more rapidly in some genotypes than others; iflosses are high, say 50 % of the total, the genotypes with short-lived seeds maybe eliminated altogether. Yet they may have valuable traits for adaptation,growth or disease resistance as growing trees, and in any case they contributeto genetic variation in the species which it is the purpose of genetic conservationto preserve. Secondly, it is an accepted fact in agriculture that chromosomedamage or change occurs and accumulates in the seed in storage, and thatthe risk of such heritable gene mutations depends not so much on the age of theseed as on changes in its viability (Roberts 1972, Barner 1975b). A seed lotwhich has suffered a serious loss of viability is likely to have experiencedsome gene mutations among the survivors, but there is very little direct evidencethat heritable mutations are induced under good storage conditions whichlead to only small losses in viability.

The high standard of storage conditions recommended by the IBPGR and referred toabove, if combined with regular testing of seed and regeneration as soon asgermination 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 stilllower temperatures would increase longevity even more. Research on storage inliquid nitrogen has been pursued for some years and considerable progress hasbeen made, but testing for several more years will be needed before the methodcan 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. Seedsmay be stored in heaps on the ground, in shallow pits in well-drained soils orin layers in well ventilated sheds, often covered or mixed with leaves, moistsand, peat or other porous materials (Holmes and Buszewicz 1958, Magini 1962).Seeds stored outdoors are kept moist by rain or snow, but those under sheltermay need to be moistened periodically (Stein et al. 1974). The aim is to maintainmoist and cool conditions, combined with good aeration to avoid overheatingwhich may result from the relatively high rates of respiration associated withmoist 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 (Magini1962).

This method is suitable for short-term storage of large-seeded hardwood speciesin the temperate zone e.g. Quercus, Castanea, Aesculus. It is unlikely to besuitable for tropical recalcitrant species because ambient temperature is toohigh.

Outdoor stratification, a method of overcoming internal dormancy, is describedon pp. 178–180. It is properly to be considered as a seed pretreatment, but itserves the incidental function of storing the seed for a few weeks or months andthe 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, lesscommonly, just below freezing (Magini 1962). Moisture can be controlled withinapproximate limits by adding moist media e.g. sand, peat or a mixture of both tothe seed, in the proportions of one part media to 1 part seed by volume, andremoistening periodically, or more accurately (but more rarely) by controllingthe relative humidity in the cold store. The latter type of control is often tooexpensive (Magini 1962, Holmes and Buszewicz 1958). Respiration rate is reducedand storage life prolonged by the low temperature, but seed should not be storedin sealed gas-proof containers which would limit oxygen supply. Closed polyethylenebags of 4 – 10 mil (100–250 microns) thickness will allow exchange ofoxygen 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 inthe previous section, and with temperatures of 0 – 5° C should extend viabilityup to 1 ½ to 2 years. Sub-freezing temperatures have given improved results ina few cases but frequently injure seeds with high MC and should be used onlyafter research has demonstrated their applicability to the species in question.

Less is known about the application of this method to tropical species, but itmerits much more investigation that it has so far received, for the dipterocarpsand genera such as Araucaria, Agathis and Triplochiton. As mentioned earlierthere is evidence that some species are killed at low but above-freezing temperaturesand Gordon (1981) has proposed a division of recalcitrant seeds intothose which can withstand temperatures below 10° C without loss of viability andthose which cannot. Tamari (1976), summing up several years of research ondipterocarps in Malaysia, concluded that the best treatment for several specieswas (1) Dry at a temperature not above 35° C, to reduce MC to 35 % (2) Seal witha fungicide inside polythene bags (3) Store at 15° C, or for 3 weeks at 15° Cfollowed by further storage at 10° C. This treatment has been successful inextending longevity from a week or two up to two months in Hopea helferi, butthis is still a long way from providing safe storage between seed years, reportedto vary from 3 – 6 years apart in many dipterocarps (Tang 1971). Storageat 3.5°C and MC over 32 % over periods of at least 6 months has been successfulon the recalcitrant Araucaria hunsteinii (Arentz 1980).

In some recalcitrant species newly germinated seeds may retain viability betterunder moist cool storage conditions than ungerminated seeds. Gordon (1981)reported that some pregerminated seedlots of Quercus spp. showed no significantchange 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 proportionof the seeds which were viable but ungerminated when placed in the same bagsdied during the same period.

Other methods

Other storage methods have been used in the past, but are not yet of wide application.They include (Magini 1962, Stein et al. 1974):

1. Storage of recalcitrant seeds in running (not stagnant) water.

2. Storage under partial vacuum.

3. Storage in gases other than air e.g. nitrogen or CO₂.

4. Coating individual large seeds with paraffin or latex to prevent moistureexchange. This method may also be used to maintain moisture content duringshipment.

Storage Containers

Some form of container is necessary for most seed storage, to facilitate accessto, and handling of, individual seed lots while keeping them separate, to makethe best possible use of storage space, to provide protection against animal andinsect pests and, for some seeds, to prevent passage of moisture and gasesbetween the enclosed and the outside atmosphere. Many types of container havebeen used for tree seeds; they may be conveniently divided into (1) Materialsfreely permeable to moisture and gases (2) Materials completely impermeable,when sealed, to moisture and gases (3) Materials resistant, but not completelyimpermeable, 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 trierscan be inserted through the cloth mesh to withdraw samples for testing withoutthe need to open the mouth of the container. The resilience of the cloth willclose the hole and avoid subsequent loss of seed, which is not possible withcontainers based on paper or paperboard (Harrington 1973). Hessian and cottonare also robust materials which can be used more than once.

None of these materials is entirely proof against insect and rodent pests, andall are freely permeable to water vapour and gases. For orthodox seeds in uncontrolledconditions they are therefore suitable only for rather short storageperiods; these can be extended in the case of hardcoated seeds or where ambientconditions are cool and dry. If seeds are stored in large containers afterdrying to the correct MC, the outer seeds themselves provide some barrier to thepassage of moisture. Viability of the inner seeds may thus be preserved for aperiod even though there is some deterioration from increased MC in the outerlayers. If a seed store has facilities for controlling both temperature andrelative humidity, then permeable containers can be safely used for orthodoxseeds for several years, provided that pests can be excluded.

For moist storage of recalcitrant seeds, open or freely permeable containerssuch as hessian sacks should be used in order to allow free exchange of air andso avoid the overheating which can occur if moist, rapidly respiring seeds areenclosed without adequate ventilation. Periodic spraying of the sacks may benecessary 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 instorage by dehumidifying the whole storage space. Another very efficient way,commonly used in storing forest seeds, is to place the seed in sealed moistureproofcontainers. This avoids the need for expensive dehumidification equipment.For long-term storage the most effective method is a combination of moistureproofcontainers with controlled low temperatures provided by refrigeration. Anadded advantage of most materials in this type is that they also exclude oxygenand so reduce still further the rate of respiration. Impermeable sealed containersare not suitable for storing recalcitrant seeds nor are they suitablefor orthodox seeds at high MC, which deteriorate more rapidly in sealed than inopen storage. Some seeds absorb moisture quickly, so it is important that theybe 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 ofthe Mason or Kilner types, plastic vials and laminated aluminium foil packages.Rigid and unbreakable metal cans provide maximum protection against mechanicaldamage to the seeds and are equally suitable for storage and subsequent shipment.Containers are only as moistureproof as their sealing. For rigid containersscrew-top or clamp-down gasketed lids should be used, if periodicopening for seed extraction and subsequent resealing are anticipated; aluminiumfoil should be heat-sealed. The effectiveness of sealing is particularly importantin long-term storage. Three types of container are considered suitablefor hermetically sealed long-term storage of agricultural seeds: glass jars orvials; metal cans; and laminated foil packets. They should be equally appropriatefor orthodox forest seeds. But the report by IBPGR (1976) recommendedsealed metal cans as the most reliable and convenient. It noted that the sealson screw-cap jars are not always perfect and that further experience of thelasting qualities of laminated foil packets is needed before they can be recommendedfor 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. Thesematerials are resistant to the passage of moisture but, over a long period oftime, there will be a slow passage of water vapour tending to equilibrate the RHinside with that outside the container. Some of the figures quoted by Justiceand Bass (1979) for transmittal of water vapour appear surprisingly high, e.g.0.13 g per 100 square inches (645 cm²) per 24 hours for low density polyethylenefilm 10 mil (250 microns) thick and about ten times this figure for low densityfilm 1 mil (25 microns) thick. However, the standard conditions for testingthese materials are 0% RH on one side and 90–100% on the other. The RH gradientduring storage is never so severe as this and hence the rate of passage of watervapour 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 RHof 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 topassage of water vapour and, for a given thickness, high density polyethylene ismore resistant than low density.

Although polyethylene is not suitable for long-term storage of orthodox seedsfor genetic conservation, it is very suitable for short-or medium-term storageand has given excellent results for up to 5 years' storage of Pinus caribaea andP. oocarpa seeds in Honduras, with no significant change in MC. For Honduranconditions 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 andis 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) regularsuitable for temperate conditions and 7 mil (175 microns) high density or 10 mil(250 microns) regular as adequate for even severe tropical conditions. Propersealing 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 efficientlyand conveniently by commercial heat sealers, of which a number of differentmodels is now on the market.

Different materials, each alone slowly permeable to water vapour, may becomecompletely impermeable when laminated together. Various combinations of laminatedpolyethylene, aluminium foil and kraft paper proved completely impermeableto water vapour over a two year period, even when there was a high differentialbetween 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 impermeablecontainers, the MC should remain constant for years. If, however, the seeds arestored in moisture resistant but not completely impermeable material such aspolythene bags, or if it is necessary to open and reclose the containers periodicallyto extract seeds, there will be a slow build-up of moisture in time. Aconvenient way to prevent this is to enclose some desiccant such as silica gelin the containers. The capacity of silica gel to adsorb moisture depends on therelative humidity of the ambient air, as shown in the following table (afterHarrington 1972):

Moisture content of silica gel in equilibrium withvarious relative humidities

A convenient method is to use silica gel treated with cobalt chloride, whichchanges colour from blue to pink about 45% RH; the corresponding equilibrium MCfor many orthodox species would be 7–9% (see graphs Figs. 6.23 and 6.24 ). Driedsilica gel is enclosed with the seeds and, whenever the granules turn pink, thesilica gel is removed and reactivated by drying in an oven at 175° C and coolingin a sealed container before reuse. A weight of silica gel equal to one tenththe weight of seeds is recommended (Harrington 1972). Care should be taken notto include too much silica gel which could lead to overdrying of the seeds. Evenwith 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 frequentreactivation of silica gel would preserve an equilibrium of RH and seed MC atlower levels than the 45% and 7–9% mentioned above but would forego the convenienceof the colour indicator.

Another use for desiccants is where the MC of seeds is known to be higher thanthe 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 equalweight of silica gel in sealed containers should reduce the MC of the seed to asuitable level and maintain it. As an example

Therefore a weight of silica gel equal to the weight of seed will reduce theinitial 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 storagecontainer 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 containerbecause enclosing excess moisture is harmful to the seed (Barton 1961).Use a tight-closing container if gain in seed moisture content can be damagingand relative humidity in the storage facility is high.

Containers and seed can quickly gather unwanted condensation when brought out ofcool or subfreezing storage. Warming to room temperature is recommended beforeopening a container brought out of such storage.

4 to 10 mil (100 – 250 microns) polyethylene bags will greatly restrict exchangeof moisture, but still allow exchange of oxygen and carbon dioxide with airoutside. Such exchange may be beneficial or harmful, depending on species.

A container that is easy to open and close is desirable when quantities of seedare likely to be added or removed repeatedly. In order to minimize temperatureand relative humidity fluctuations, open only when necessary. Alternatively,store seed in small containers, so that the entire contents can be stored oremptied at one time.

For orthodox seeds, fill containers completely to ensure minimum exchange ofmoisture between the seed and the entrapped air and, more importantly, to limitthe amount of oxygen enclosed.

When exchange of moisture through the container walls must be eliminated orrestricted, the container must be made impermeable or of moisture resistantmaterial. The longer the storage period and the higher the differential betweenexternal RH and RH within the container, the more impermeable the material mustbe.

When seeds are fragile and easily damaged, a rigid-walled container should beused. Moisture-proof plastic bags are often used as liners for rigid containers.

Choose a container shape and stacking arrangement which facilitates uniformtemperature and aeration throughout the storage facility.

Some containers may be made of substances that are harmful to tree and shrubseeds (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 ForestResearch, 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 makesthem difficult to clean between exhaustion of one seed batch and insertion ofthe next.

It should be stressed that no one container or packaging material can be thebest for all sizes, conditions and objectives that occur in seed packaging. Therelative merits of the various containers will have to be weighed against theirdisadvantages and costs before deciding the final choice.

Design and Engineering of Seed Storage Facilities

Storage capacity

The weight of seeds to be kept in store can be estimated in the manner indicatedin Chapter 3 and will depend on the annual planting area, the maximum number ofyears' 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 beconverted to net volume in litres (or in g to cm³) by a factor related to averagespecific gravity. An average factor of 2.0 is appropriate for many forestspecies and corresponds to an apparent specific gravity of 0.5 (true specificgravity 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 fittingswithin the cold room, a factor of about X8 is commonly used (Magini 1962); thisis with fixed shelving. Use of mobile shelving may double the quantity of seedwhich may be stored in a given space (IBPGR 1976); in this case a factor ofabout X4 is appropriate. Thus 500 kg of seed of S.G. 0.5 would need a grossstorage space of 500 × 2 × 8 = 8000 litres or 8 m³ if fixed shelving were usedand 4 m³ with mobile shelving. Where relatively few seed lots and large quantitiesof each lot are being stored, it is possible to use standard sizes ofcontainer, each filled to the brim, and for shelving space to be adapted to fitcontainer size exactly. Under these conditions considerable savings in storagespace can be effected. Thus in the Danish seed store at Humlebaek, which usesfixed 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 refrigerationengineers. Some guidance as to the features which should be included in anyquotation for installation may be obtained from the excerpts from the IBPGR(1976) report which appears in Appendix 2 and the example of the facilities installedby the Regional Genetic Resources Project at Turrialba (Goldbach 1979)which appears in Appendix 3. It should be noted that both these documents referto long-term storage of agricultural seeds for purposes of genetic conservation.

It is essential that designs and equipment be adapted to local conditions andlocal resources. The best installation in the world is of little use if itcannot be maintained, so it is essential to investigate the local provision forservicing and spares before committing oneself to any particular item. Thereliability of mains electricity services and the need for a voltage controllerand standby generator are of primary importance. Ready availability of a sparecompressor 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 mightsolve many problems by moving its store from a hot humid coastal site to the dryrain-shadow side of a mountain at 2000 m. In such a case a well-ventilated roommight provide perfectly suitable conditions for several years' storage forrelatively “easy” species such as pines and eucalypts and could be supplementedby one or more deep-freeze chests for small quantities of more “difficult”species requiring sub-freezing temperature. The value of deep-freeze chests wasstressed by the IBPGR (1976) and its comments are reproduced in Appendix 4.

Seed Shipment

The benefits of exemplary seed collection, processing and storage methods may belargely lost if care is not taken over shipment from seed store to nursery. Itis seed viability at the time of sowing, rather than at the time of despatchfrom the seed store, which determines the number of healthy plants produced froma particular seed lot. It is therefore essential to provide shipment methodswhich will ensure the minimum loss of viability in the interval between storageand sowing. The selection of appropriate packing material will depend on thecharacteristics of the species, the quantity to be shipped, the length of timein transit, the mode of transport and the temperature and moisture conditions towhich the shipment will be exposed (Baldwin 1955).

High and fluctuating temperatures and adverse humidity are the chief causes ofviability losses during shipment (Stein et al. 1974). These factors are identicalwith those that cause deterioration in freshly collected fruits between thecollecting site and the processing depot, as described on pp. 83–84. However,seeds between storage and sowing should start with advantage of having hadoptimum conditions of temperature and moisture content during the storageperiod. In fact, maintenance of storage conditions during transit would beideal, but is often not possible (Stein et al. 1974).

Provided that the initial moisture content of the seeds is correct, it can beeasily maintained during transit by the use of sealed containers. In some casesthe seeds can be despatched in the same containers in which they were stored. Inothers it may be advisable to transfer them from a large container in storage toa smaller container for despatch. Individual nurseries may require only a smallquantity of a given seed lot. In addition, small and light packages are oftenless subject to mechanical damage in transit than large, heavy ones. Magini(1962) recommends separate packages of 1 – 20 kg but not larger. A variety ofmoisture-proof or moisture-resistant material is available, as described earlierin this chapter under storage containers. Polyethylene of 4–8 mil (100–200microns) has the advantage of restricting moisture passage while allowing exchangeof oxygen and CO₂.

Sealed containers are highly suitable for orthodox species, of which the seedsmust be kept dry during transit. The addition of a desiccant such as silica gelmay be a useful additional insurance if there is any risk that the seeds mayabsorb moisture while being transferred from storage container to shipmentcontainer. Seeds of recalcitrant species, on the other hand, are best leftunsealed, since the effect of some loss of moisture is less harmful than that ofthe overheating which can occur as a result of rapid respiration in sealed bagsat ambient temperatures. They should be well-mixed with pulverized sphagnummoss, ground peat, coconut fibre or sawdust, that has been moistened andsqueezed dry. A mixture of equal weights of dry packing and water will giveadequate moisture content to these materials (Baldwin 1955). In the case ofinternational transit, however, an inert non-organic substance such as moistvermiculite 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, providedthat the initial MC is correct. But if orthodox seeds are being forwarded soonafter 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 withexcessively high MC. A number of species with resistant seedcoats or pericarps,such as Tectona and many leguminous species, are able to withstand prolongedperiods in ambient conditions; cotton or paper bags or hessian sacks are perfectlysuitable for these species.

Large, moist seeds can be sealed individually with paraffin wax or latex. In themethod described by Baldwin (1955), paraffin wax is heated to 71°–77° C andseeds or nuts dipped for a few seconds in a screen-type container, which shouldbe shaken vigorously during the immersion. The waxed seeds should be packed insoft material so that the wax is not scraped off during transit. At the time ofsowing 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 suchas radiators and hot pipes. For very sensitive seeds, temperature effects can bemitigated by the use of insulating material in the packaging. Sub-zero temperaturesdo not usually affect dry seeds but may cause damage to recalcitrant seedswhich must be kept moist. Premature germination is another risk which affectsmoist seeds. During storage, germination can be restricted by the use of lowtemperatures just above freezing, but the higher temperatures encountered duringtransit may induce germination in a substantial number of seeds. Seeds which areprone to germinate when held in moist packing may be treated with an inhibitorsuch as maleic hydrazide (Baldwin 1955).

No matter what type of seeds is being despatched, it is necessary to take precautionsagainst mechanical damage to seeds and against losses due to damage tothe containers in transit. Double wrapping is often advisable, for example asealed polythene bag should be placed inside a stout canvas bag. Stout drumcartons with sealed polythene or aluminium-foil containers inside provide anespecially effective combination for seeds which need to be kept dry. If theinner bag is labelled, this is also an insurance against accidental defacementof the outer label. Clear labelling is essential and the consignee should beadvised of despatch by means of an appropriate seed consignment note or seedissue form (see Appendix 1).

Stein et al. (1974) have provided a useful check list of helpful practices inseed shipment, reproduced hereunder:

1. Double wrap the seed. Enclose the seed container in a sturdy, preferablyrigid, outer container.

2. Small or moderate size containers generally withstand shipment betterthan large containers.

3. Fill containers completely to minimize air content and jostling ofseeds during shipment.

4. All packages should bear a good identifying label on the innermostcovering and another one within the container.

5. For long distances, shipment of sensitive seeds by air is desirable.

6. Seed packages should permit ready opening and reclosing if destinedfor export to a country requiring fumigation. In addition a copy ofthe phytosanitary certificate should be readily available to quarantineauthorities e.g. by sealing it in an envelope which is firmlyattached to the outside of the package.

Seed storage facilities at nursery sites or district forest stations are inferiorto those at the central seed store. Shipments should therefore be timedso that seeds can be sown with the minimum delay after receipt.