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17 May2009

Overstory #218 - Seed storage

Written by Anne Mbora.

altOptimal storage conditions can ensure that seeds retain viability as long as possible. (pictured: Leucaena leucocephala seeds)

Introduction

Seed can maintain viability for long periods if it is properly stored. When seed is stored in unfavorable conditions, it quickly dies. This article describes ways of keeping seed alive through proper seed storage.

Why should I store seeds?

Tree seed is stored for various reasons. All reasons for seed storage encompass some element of future use of the seed.

  • The main aim of seed storage is to keep the seed alive and in good condition.
  • Good seed years must be taken advantage of especially for species that only flower periodically. Ocotea usambarensis and Dipterocarpaceae for example, only flower once in several years. Similarly, many tree species do not produce the same amount of seed each year. Therefore, you may want to collect more seed in the year when production is high for use in years when production is low.
  • Conserving genetic information: the threat of extinction to provenances and species necessitates taking measures to conserve seed genetic information. Long-term seed storage, for example, conservation of seed by the Millennium Seed Project of The Royal Botanic Gardens, Kew, is one of the important measures of genetic conservation.
  • Preparation for the planting season: if the harvesting and planting seasons do not coincide, you may need to store seed to bridge the time between collection and sowing.
  • Future demand: the demand for a certain species may be very low when you collect the seed, but may be higher than the production at a later time. Under these conditions, you need to keep some seed in stock so that you can buffer the differences between demand and supply.
  • Avoiding deterioration: even if the time between collection and planting is very short, pests such as rodents, birds or beetles or fungi can attack the seed. You should store the seed in a place that is free from pests and pathogens.
  • Security: another reason for storing seed even if the time between collection and planting is short is to keep them safe. Demand for seed can be very high, such that it raises the probability of seed being stolen.
  • Food storage: You may wish to store seed so that you can use it at a later date as food. In some countries, tree seeds and fruits are eaten. Examples of such species include Tamarindus indica, Adansonia digitata, Ziziphus mauritiana and Acacia spp.

How should I prepare seed for storage?

Carry out seed drying just before storage and as quickly as possible after extraction. This helps to:

  • Avoid seed deterioration in storage and thus extend viability.
  • Reduce rotting of seeds in storage.
  • Improve resistance of seed to pests and diseases during storage.

Tree species are divided into three different classes according to the maximum period that their seed can be stored, taking into consideration their temperature and moisture content requirements. In nature, species show a range in their maximum storage period. The three classes guide you on the best storage period for each species. Storage time depends on storage conditions; under bad conditions, all seeds will die quickly, while under optimal conditions, lifespan is extended.

Recalcitrant seeds should be dried under shade to retain a high level of moisture content (20-40%), in order to keep them alive. The temperature at which the seed is stored should be around 12-15°C. Recalcitrant seeds cannot be kept for long under conventional conditions in seed storage containers. Examples of recalcitrant seed are Mangifera indica, Persea americana, Syzygium cuminii, Trichilia emetica and Vitellaria paradoxa.

Intermediate seeds can only withstand a limited level of drying - to a moisture content of 15-19%. They are sensitive to low temperature, thus should be stored in high temperatures. Intermediate seeds cannot be stored for a very long time before losing viability. You should use shade drying or air drying methods that are suitable for seeds extracted from pulpy fruits. Air dry for 1- 3 days before storing or plant the seeds immediately for better germination. Examples of intermediate seed are Azadirachta indica, Maesopsis eminii, Dovyalis caffra and Artocarpus heterophyllus.

Orthodox seed is commonly dried under direct sunlight for 2-3 days to a moisture content of 5-8%. The period of drying will depend on the relative humidity of the air, the species and the air temperature. Orthodox seeds can also be dried above the fireplace (smoke drying), or where farmers dry their firewood during the rainy season. You should be careful not to overheat and kill the seed. Orthodox seed can be stored for a fairly long period of time before losing viability. As a rule of thumb, for orthodox seeds, the mean viability period (seed life-span) doubles each time the moisture content is lowered 1% below the maximum 14-15% moisture content level that is acceptable for storage.

Seed can be treated against pests with locally available preservatives, for example, leaves of neem or the ash of any other tree species, before storage. Each container should have two labels - one inside and one attached outside with at least the species name, date of collection, number of parent trees, seed source, weight of seed and collector's name.

How long can I keep tree seed?

The length of time that seed remains alive varies according to the species or the place where the seed was collected. The length of time also varies between seeds of the same species that were collected from the same place. This time also depends on morphological, physical, physiological and health conditions of the seed. The length of time that seed stays alive depends very much on the storage conditions - when storage conditions are bad, all seed dies quickly.

In general, seed loses viability as time progresses until ultimately, all seeds die. The viability of seed is expressed as the percentage of the number of seeds that germinate from the whole lot after a given period of seed storage. The period for which seed can be stored varies from species to species - some can be stored for longer than this and others for much less. Seed that is stored under conditions that are less favourable to its survival will lose viability more quickly. This is an indication of the importance of storing seed properly.

As mentioned above, the period that seed can stay alive depends a lot on the species to which it belongs. Some species cannot be stored for long periods whatever the storage conditions. Other species can be stored for long periods so long as the storage conditions are good.

How should I store orthodox seeds?

Orthodox seeds should be stored in clean, dry airtight containers. The containers should be properly filled with seed, and then closed off after the seed is put in. Gourds can also be used as seed storage containers.

If properly dried, orthodox seeds can be stored at room temperature. Most seeds can be stored for longer periods in refrigerated conditions (O°C to 5°C or even down to -20°C). Some hard-coated species can be stored at room temperature almost as long as when refrigerated.

Orthodox seeds of most species will remain viable for 1-2 years when stored under fair to good conditions at the farmer and NGO level. Viability can be extended by reducing seed moisture contents and by lowering the storage temperature.

As a rule of thumb, the mean viability period doubles each time the moisture content is lowered by 1% below the 14-15% moisture content which is the maximum content for successful storage. In other words if the viability was 2 years at 14% then if you drop the moisture content to 13% it will be 4 years. Similarly, again as a rule of thumb, the mean viability period doubles each time the mean storage temperature is lowered by 5°C. This ranges from the highest temperatures encountered during the drying of orthodox seeds (50°C) down to O°C.

In general, orthodox seeds are stored at 5-8% moisture content and at temperatures of 0-5°C.

How should I store intermediate seeds?

These seeds can withstand relatively low moisture contents 12%), but they cannot withstand low temperatures 5°C). After the seeds' moisture content is lowered, store them in a clean, dry, airtight container for 4-6 weeks at ambient temperatures. Do not store them in refrigerated surroundings.

How should I store recalcitrant seeds?

Recalcitrant seeds need to be stored at high moisture levels. They can only be stored for 1-2 weeks before losing viability. These seeds cannot withstand surroundings with low temperatures or low relative humidity. For short-term storage, put recalcitrant seeds in permeable containers to allow ventilation.

To maintain their moisture content, recalcitrant seeds should be mixed with a medium (sawdust, peat or vermiculite) that is slightly moistened with distilled or de-ionized water. Seeds and the medium should be mixed at a ratio of 1:2, and then stored at ambient temperatures and kept moist.

How do I keep relative humidity low during storage?

Orthodox seeds must always be kept in a well-ventilated cool, dark and dry place. The seeds' initial moisture content should be low enough to avoid respiration.

If small portions of seed are likely to be removed frequently from the cold storeroom, seeds should be stored in small amounts. Seed can be stored in small sealed plastic bags within a larger container. The volume of air within the container should be low compared to the volume of seeds in the container. If the amount of air is large, the seeds will absorb a lot of moisture from the fresh air each time the seed container is opened. Place substances that absorb moisture such as roasted rice husks, newspaper, charcoal, or silica gel in the container as the volume of seed decreases.

Note that seeds with high oil content have a lower moisture content than seeds with a low oil content and high protein or starch contents.

References

Albrecht J. (ed). 1993. Tree seed handbook if Kenya. Nairobi: Kenya Forestry Research Institute/Deutsche Gesellschaft fur Technische Zusammenarbeit.

Indonesia Forest Seed Project. 2001. Demo room poster on storage. Bandung: Indonesia Forest Seed Project.

Mortlock W 1998. Guideline 1: Native seed storage for revegetation. Yarralumla: FloraBank.

Mulawarman, Roshetko M.J., Singgih M.S. and Djoko I. 2003. Tree seed management - seed sources, seed collection and seed handling: a field manual for field workers and formers. Bogor: Winrock International and World Agroforestry Centre. 54 pp.

Robbins A.M.J. and Shrestha N.B. 1986. NR Study-note F120d. Basic rules for storing tree seed. Tree seed handling: a manual for field staff in Nepal. Field Document 11. Bandung: National Tree Seed Project.

Schmidt L. 2002. Guide to handling if tropical and subtropical forest seed. Humlebaek: DANIDA Forest Seed Centre.

Thomsen K. and Stubsgaard F. 1998. Easy guide to controlling seed moisture during seed procurement. Humlebaek: DANIDA Forest Seed Centre.

Original Source

This article was excerpted with the kind permission of the publisher from:

Mbora, A. 2006. "Seed storage." In: Kindt R., Lillesø J.P.B., Mbora A., Muriuki J., Wambugu C., Frost, W., Beniest J., Aithal A., Awimbo J., Rao S., Holding-Anyonge C. Tree Seeds for Farmers: a Toolkit and Reference Source. Nairobi: World Agroforestry Centre. Agroforestry Tree Seeds for Farmers Tree Seed Toolkit

About the Publisher

The World Agroforestry Centre is a non-profit research organisation whose vision is a rural transformation in the developing world resulting in a massive increase in the use of trees in rural landscapes by smallholder households for improved food security, nutrition, income, health, shelter, energy and environmental sustainability. We are one of the 15 centres of the Consultative Group on International Agricultural Research (CGIAR). Headquartered in Nairobi, Kenya, we operate five regional offices located in Brazil, Cameroon, Indonesia, Kenya, and Malawi, and conduct research in eighteen other countries around the developing world. We receive our funding from over 50 different governments, private foundations, international organisations and regional development banks.

The World Agroforestry Centre (ICRAF)
United Nations Avenue
PO Box 30677, GPO 00100
Nairobi, Kenya
Tel: +254 20 7224000, via USA +1 650 833 6645
Fax: +254 20 7224001, via USA +1 650 833 6645
E-mail: icraf@cgiar.org
Internet: www.worldagroforesty.org

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