STORAGE OF HARDWOOD SEEDS

by

F.T. Bonner¹

Tree improvement programmes and the artificial regeneration efforts that accompany them require good seed supplies. Good seed must be collected and cared for properly to prevent loss of quality. Seed storage for short-term preservation of germ plasm is an integral part of most tree improvement programmes. This paper presents recommendations for the care and storage of hardwood tree seeds. For more detailed reviews of seed storage, readers should refer to Barton (1961) and Roberts (1972). Holmes and Buszewicz (1958), Sohönborn (1964), Stein et al. (1974) and Wang (1974) are excellent sources that deal with the storage of tree seeds.

PRE-STORAGE CARE

Seed quality can suffer from improper handling of seeds between collection and storage. Much of this potential damage is related to moisture content of fruits and seeds. Most species fall into one of three classes for pre-storage care: (1) seeds that must be dried for extraction and storage; (2) seeds that must be kept moist for storage; and (3) seeds that must be kept moist for extraction, then dried for storage. Table 1 lists many of the world's important hardwood genera according to the pre-storage care their seeds require.

Seeds that must be dried for extraction and storage are generally the easiest to care for. Multiple fruits, such as Casuarina, Eucalyptus, Liquidambar, or Platanus, may be dried in the sun or in kilns with artificial heat, then tumbled or macerated to extract the seeds. Once dried, the pods of the legumes in this group, such as Acacia, Gleditsia, or Robinia, can be flailed or macerated for seed extraction. When seed moisture reaches 6 to 10 percent ², these seeds are ready for storage.

The second class contains seeds which are very difficult to store, such as Aesculus, Quercus, and Dipterocarpus. Aesculus and Quercus seeds should be cleaned by water flotation, then kept moist at all times. If moisture drops below 25 percent in some of these species (Q. nigra, for example), they lose viability. Seeds of Hopea and other dipterocarp genera rapidly lose viability when moisture contents fall below 19 to 33 percent (Tang and Tamari 1973). Even if sowing is scheduled a few days after collection, these seeds should be kept cool and moist.

Table 1. Some important hardwood genera classed according to the pre-storage handling requirements of their seeds.

The third class of seeds contains mainly drupes and other fleshy fruits. It is usually desirable to extract the seeds from their fleshy tissues for storage. Extraction is much easier in macerators if this tissue has not been allowed to dry. Once cleaned, the seeds should be dried to below 10 percent moisture for storage. Care during storage is the same as for the first class of seeds. Stein et al. (1974) reviewed information on depulping fleshy fruits.

MEASURING SEED MOISTURE CONTENT

Seed moisture content should be monitored during processing to determine when it is low enough to permit storage, and it should also be monitored during storage to determine if proper conditions are being maintained. The official moisture-testing methods of the International Seed Testing Association (ISTA) can be used. For many hardwood species even drying at 103°C for 17 hours is prescribed, but toluene distillation is required for Fagus (ISTA 1976). Seeds with high oil contents must be tested with the toluene method, as even drying can cause significant loss of seed lipids through volatilization. Most of the hardwoods listed in Table 1 are not included in the ISTA Rules, because the demand for official test certificates has not been sufficient.

Because these ISTA methods are useful mainly in testing laboratories and since rapid determinations are often needed during seed processing, electric moisture meters which give instant, nondestructive measurements have become popular. Although they are probably less accurate than the official ISTA methods, the meters usually are accurate enough for decisions affecting storage. Electric meters must be initially calibrated against other moisture determinations and at present there are few calibration charts available for hardwood seeds. Examples of calibration for four North American hardwoods are given in Table 2. Calibration charts are easy to prepare for other species.

There are about a dozen types of commercially available meters, all built for small seeds. These meters cannot be used on Juglans, Quercus, or other large seeds. Winged seeds, such as Acer and Fraxinus, are also difficult to measure. Their wings prohibit smooth flow of seeds into the measurement chamber, causing irregular air spaces and, therefore, variable readings.

OPTIMUM STORAGE CONDITIONS

The two objectives of storage are to maintain seed stocks as cheaply as possible until they are needed, be that several months or ten years, and to preserve as much seed quality as possible. Seeds stored several years will lose some quality, but if proper steps are taken these losses can be minimized. The seed manager must consider what storage length is required, what losses in seed quality are acceptable, what facilities are available, and what other economic factors apply. The best storage facility that money can buy may be more than is needed.

Tree seeds can be divided into two groups according to storage procedures: (1) species that can be kept at low moisture content, and (2) species that must be maintained at a high moisture content. Seeds of the first group are easy to store over long periods, if seed moisture content is maintained between 5 and 10 percent. At temperatures of 0° to 5°C, seeds in this condition should store well for at least 5 years. For storage of more than 5 years, subfreezing temperatures may be better. For storage over only one winter, temperatures above 5°C can be tolerated. The hard seeds of the Leguminosae family and a few other species are much more tolerant of high temperature, and refrigerated storage is not as important for them.

Species that can be kept at low moisture content (Group I) are, in general from genera whose seeds should be dried for storage (classes 1 and 3 in Table 1). Documented examples of this group include: Acer sacoharum (Yawney and Carl 1974); Betula alleghaniensis (Clausen 1975); Carya ovata (Bonner 1976); Eucalyptus spp. (Krugman 1974); Fagus sylvatica (Machanicek and Vrabec 1973); Fraxinus excelsior (Barton 1945); Nyssa aquatica (Bonner and Kennedy 1973); Populus tremula (Schreiner 1974); Prunus serotina (Grisez 1976); Tectona grandis (Kimariyo 1973); and Ulmus pumila (Heit 1967).

Some of these species must be handled carefully. Fagus, for example, should be dried slowly in two stages (Rudolf and Leak 1974), and Populus seed must be dried immediately after collection (Schreiner 1974).

Other species from either group can probably be stored successfully with the low moisture-low temperature method, but accurate data are sparse. The tropical hardwoods of these groups, such as Triplechiton scleroxylon and Gmelina arborea, are known to have short-lived seeds. Storage at 0° to 5°C and 5 to 10 percent seed moisture may not ensure viability of these seeds for 5 years or more, but 2 or 3 years may be possible. More research is needed on these problems.

Table 2.  Calibration data for measuring seed moisture content of four North American hardwood species with a Burrows electric meter (also known as Dole or Radson meter). ¹

1 Mention of trade names is for information only and does not imply endorsement by the U.S. Forest Service.

2 Data from Bonner (in press).

Seeds that must be kept at high moisture content (Group II) are very difficult to store without great losses of viability. Moderate success has been achieved with the seeds of Quercus by maintaining them at 35 to 45 percent moisture and -1° to +3°C (Bonner 1973; Suszka 1976). When moisture contents must be kept in this range, temperature must be carefully controlled. Temperatures colder than -1°C will usually kill the seeds, but temperatures above 2° or 3°C cause excessive sprouting of stored acorns. Also, air-tight storage containers cannot be used because some gas exchange must take place between the acorns and the atmosphere. At high moisture contents, respiration proceeds at a rapid pace, and complete blockage of gas exchange kills the acorns. Polythylene bags with a wall thickness of about 4 mils ³ allow sufficient gas exchange, but block water vapour loss.

Thinner bags allow too much water vapour to escape, which dries the acorns, while thicker bags inhibit gas exchange.

Storage requirements of other genera in Group II have not been studied as much as those of Quercus species. There are indications, however, that procedures similar to those for Quercus would be effective for other temperate zone species of this group, such as Aesculus, spring-maturing Acer, Castanea, and Corylus. Juglans nuts can apparently be stored for up to 4 years in cool, moist conditions (Brinkman 1974). Tropical zone hardwoods with large, moist seeds are very difficult to store, and the same methods also apply here. Research on storage of tropical hardwood seed is badly needed.

MOISTURE CONTROL

Of the two important variables in seed storage, moisture control is more critical than temperature control. Even at low temperatures, potential for damage to Group I seeds exists at several levels of moisture (Harrington 1973).

Once seed moisture reaches the proper level for storage, it must be maintained. Cold storage facilities with controlled humidity can be built, but they are very expensive. At the usual storage temperatures of 3° to 5°C, relative humidity will be in the 90's inside the cooler, and dried seeds will take up moisture from the atmosphere unless protected. The most common system for storing Group I tree seeds is to dry them to the desired moisture content and then refrigerate the dried seeds in moisture-proof containers such as fibre or metal drums with polyethylene or foil liners. If the humidity of the entire cooler is controlled, then Group I seeds can be kept in containers that are not moisture-proof, such as cotton or burlap bags. Under these conditions, the high moisture seeds of Group II would have to be stored in moisture-proof containers, since seeds stored in containers which are not moisture-proof will gain or lose moisture in equilibrating with the ambient humidity.

Moisture contents at equilibrium differ among species, and variation in seed lot characteristics can cause differences as high as 2 percent within a species (Harrington 1973). Among hardwoods, species of Group I have similar equilibration values, but the larger Quercus acorns of Group II vary greatly (Table 3).

Values for Quercus acorns cannot be determined precisely, thus the values in Table 3 are only approximations. During storage, acorn weights decrease continually due to respiration, which also causes moisture percentage to increase slowly during storage. In long-term acorn storage, final moisture percentages are almost always higher than the original percentages.

Equilibrium moisture contents of species listed in Table 3 at the same relative humidity, but higher temperature, would be slightly higher. Because starch will absorb more moisture than lipids (Harrington 1973), starchy seeds, such as those of Aesculus and Quercus, will usually have higher equilibrium moisture contents than seeds with high lipid contents.

TEMPERATURE CONTROL

For short-term storage of the seeds that are dried to 5 to 10 percent moisture, temperature can be quite flexible. The old agricultural rule-of-thumb can be used: if the sum of temperature in oF plus relative humidity in percent is less than 100, then conditions are satisfactory for storage from extraction and cleaning until sowing (4–5 months). If long-term storage is needed to hedge against seed-crop failures, then good cold-storage facilities are necessary. To keep seeds stored at low moisture content for 1 to about 5 years, and to keep seeds stored at high moisture levels for any length of time, refrigeration that will hold temperatures a few degrees above freezing is essential. Seeds stored at 5 to 10 percent moisture can be kept much longer than 5 years by using temperatures of about -18°C (Stein et al 1974). However, subfreezing temperatures are more expensive to maintain than above-freezing temperatures, so the need for saving seed stocks more than 5 years should be considered carefully.

Table 3.  Equilibrium moisture contents at 40% and 95% relative humidity for some hardwood seeds (determined over saturated salt solutions at 4° – 5°C).

Species of Group II which are extremely difficult to store, such as the sub-genus Leucobalanus of Quercus (white oaks) and the tropical dipterocarps, can presently be held only a few months at most. These seeds should be kept moist at temperatures slightly above freezing. Sprouting will be retarded but not prevented entirely, and the chances of harmful fermentation decreased.

CONCLUSIONS

Current seed technology can provide satisfactory storage for most low-moisture seeds (Group I) for 5 years or more, but seeds of high moisture content (Group II) can rarely be stored more than 2 or 3 years. Some can be stored only a few months. Within each group there is a wide range of sensitivity to storage conditions, which sometimes calls for special handling, but the basic approach of temperature and moisture control still applies. Many important hardwoods, especially tropical species, have not retained viability well in past storage trials, and more research is needed. Special attention should be paid to seed moisture content and the methods required to maintain moisture at the desired level.

LITERATURE CITED

Barton, L.V. 1945
Viability of seeds of Fraxinus after storage. Contrib. Boyce Thompson Inst. 13: 427–432.

Barton, L.V. 1961
Seed preservation and longevity. Interscience Publishers, Inc., New York. 216 p.

Bonner, F.T. 1972
Measurement of moisture content in seeds of some North American hardwoods. Proc. Int. Seed Test. Assoc. 37: 975–983.

Bonner, F.T. 1973
Storing red oak acorns. USDA For. Serv. Tree Planters' Notes 24(3): 12–13.

Bonner, F.T. 1976
Storage and stratification recommendations for pecan and shagbark hickory. USDA For. Serv. Tree Planters' Notes 27(4): 3–5.

Bonner, F.T. and H.E. Kennedy, Jr. 1973
Storage of water tupelo seeds. USDA For. Serv. Tree Planters' Notes 24(4): 7–8.

Brinkman, K.A. 1974
Juglans L. Walnut. In Seeds of woody plants in the United States. USDA Agric. Handb. 450. p. 454–459.

Clausen, K.E. 1975
Long-term storage of yellow and paper birch seed. USDA For. Serv., North Central For. Exp. Stn., St. Paul, MN, Res. Note NC-183. 3 p.

Farmer, R.E. Jr. 1975
Long term storage of northern red and scarlet oak seed. Plant Propagator 21(1): 11–14.

Grisez, T.J. 1976
Black cherry seeds stored 8 years. USDA For. Serv. Tree Planters' Notes 27(3): 20–21, 24.

Harrington, J.F. 1973
Problems of seed storage. In Seed ecology: proceedings. p. 251–263. W. Heydecker, ed. Penn. St. Univ. Press, University Park. 578 p.

Heit, C.E. 1967
Propagation from seed. Part II: Storage of deciduous tree and shrub seeds. Am. Nurseryman 126(1): 12–13, 86–94.

Holmes, G.D. and Buszewicz, G. 1958
The storage of seed of temperate forest tree species. For. Abs. 19: 313–322, 455–476.

International Seed Testing Association. 1976
International rules for seed testing: rules 1976. Seed Sci. Technol. 4: 1–180.

Kimariyo, P.E. 1973
Handling hardwood seeds in Tanzania. In International symposium on seed processing: seed problems. Bergen, Norway. IUFRO. Vol. II. Paper 16. 7 p.

Krugman, S.L. 1974
Eucalyptus L'Herit. Eucalyptus. In Seeds of woody plants in the United States. USDA Agric. Handb. 450. p. 384–392.

Machanicek, J. and Vrabec O. 1973
(Long-term storage of Beech nuts at the Teplice state forest directorate.) Lesnictvi 19. p409–416. (In Czech., Eng. sum.).

Roberts, E.H., ed. 1972
Viability of seeds. Syracuse Univ. Press. 448 p.

Rudolph, P.O., and Leak, W.B. 1974
Fagus L. Beech. In Seeds of woody plants in the United States. USDA Agric. Handb. 450. p. 401–405.

Schönborn, A. von. 1964
Die Aufbewahrung des Saatgutes der Waldbäume. (Storage of forest tree seed.) B.V. Verlagsgesellschaft. Munich. 158 p.

Schreiner, E.J. 1974
Populus L. Poplar. In Seeds of woody plants in the United States. USDA Agric. Handb. 450. p. 645–655.

Stein, W.I., Slabaugh, P.E. and Plummer, A.P. 1974
Harvesting, processing, and storage of fruits and seeds. In Seeds of woody plants in the United States. USDA Agric. Handb. 450. p. 98–125.

Suszka, B. 1976
Studies on the long-term storage of acorns. USDA. Final report for PL-480 study E21-FS-44. Unpubl. 111 p.

Tang, H.T. and Tamari, C. 1973
Seed description and storage tests of some Dipterocarps. Malay. For. 36(2): 38–53.

Wang, B.S.P. 1974
Tree-seed storage. Dept. Environ. Can. For. Serv. Pub. 1335. 32 p.

Yawney, H.W. and C.M. Carl, Jr. 1974
Storage requirements for sugar maple seeds. USDA For. Serv., Northeast. For. Exp. Stn., Upper Darby, PA. Res. Pap. NE-298. 7 p.

¹  Principal Plant Physiologist and Project Leader, Forest Tree Seed Laboratory, maintained at Starkville, Mississippi, by the Southern Forest Experiment Station, Forest Service - USDA, in cooperation with Mississippi State University.

² All seed moisture contents in this paper are expressed as percentages of the wet weight, as outlined in international testing rules (International Seed Testing Association, 1976).

³ 1 mile = 1/1000 inch = 0.025 mm