SEEDS, STORAGE OF

SEEDS, STORAGE OF. Seeds are by far the single most essential cultivated source of food, with wheat, rice, corn, soybean, barley, millet, and sorghum providing the vast majority of food for humans and their domesticated animals. While root and tuber crops, such as potatoes, yams, sweet potatoes, and cassava, are important sources of food in certain regions, they come in a distant second behind seeds. Besides serving as food, seeds also give rise to the next generation of crops. In general, annual crops are grown from seed (for example, grains, legumes, and vegetables), while perennial crops rely on vegetative propagation. Seeds are therefore stored not only to ensure a stable source of food, but also to provide the propagules from which additional crops can be grown.

Importance of Seeds

Modern agriculture is based on the establishment and growth of uniform stands of crops. Plant density should be uniform throughout a field, as should the plant age and stage of development. Uniform propagules and growing conditions are necessary to assure crop uniformity. Propagules are either produced sexually (as in seeds), or asexually (through cutting, graphing, budding, and division, etc.). Even vegetatively produced crops (such as tree fruits) use seed to produce rootstocks and in breeding programs. Seeds are composed of an embryo, stored food, and a protective covering. They fulfill a number of functions besides being the product of sexual reproduction. Seeds assist in the replication of individuals in a population, aid in dispersal of the species into new areas, and provide protection for the delicate embryo. The food sequestered in the seed to nourish the germinating embryo and growing seedling is the source of our food, while the embryo is the source of a new plant.

Seeds Stored for Food

By far the greatest bulk of stored seeds are used as food. Huge silos store wheat, corn rice, soybeans, and other grain, and legume crops prior to their being processed into flour, bread, corn meal, cornstarch, and the myriad other products we consume daily. Seed-derived vegetable protein can be mixed with colors and flavors and then extruded under moist heat and high pressure to produce various shaped and sized products that can be used in place of meat. Oils can be extracted from seeds, and sweeteners can be produced from the starch in seeds through enzyme digestion. Storage conditions for these seeds are designed to minimize loss of food value resulting from seed respiration and metabolism, and contamination or loss due to the growth of mold and insects, while at the same time retaining desired processing qualities. Low degrees of moisture and temperature are the two most important factors in storage of seeds for food. Both of these factors reduce seed respiration and the growth of pests.

Seeds Stored for Propagation

If seeds are not stored in a manner that maintains their vigor and viability, there will be no food for the next season. Seed vigor and viability is primarily based on generic factors, so differences occur among species, and in varieties within a species. The conditions under which seeds were produced can also affect seed longevity. Temperature, rainfall, humidity, nutrition, and diseases during seed production all influence seed viability, as does seed maturity when harvested. Mature seeds remain viable longer than immature seeds. Seed vigor usually decreases with time in storage; exceptions are seeds in which the embryo must develop or mature before they are ready to germinate (as with elm).

There are three general categories of seeds based on their desiccation tolerance and the time they remain viable in storage. Most annual and biennial crops, and horticultural crops produce orthodox, or desiccant-tolerant, seeds that can easily be stored for many years in a dry, cool storage environment. These crops include grains, legumes, vegetables, floral crops, and temperate fruit trees. By contrast, recalcitrant, or desiccant-intolerant, seeds are difficult to store and usually remain viable for only a few weeks or months before they are killed by desiccation. Desiccant-intolerant seeds do not enter dormancy after maturing. Their continued respiration and physiological activity leads to rapid deterioration. They must be planted while still fresh, or stored moist at low temperatures. Even if properly stored, they can only be kept for short periods of time before they succumb to fungal or bacterial rots, or exhaust their food reserves. They include tropical perennials (such as avocado, mango, and coconut) and some temperate deciduous trees (such as chestnut, buckeye, maple, and oak). Between these two extremes are the intermediate seeds that can be stored for a few years if maintained under proper conditions of temperature and humidity. They include tropical and subtropical perennials (for example, coffee, citrus, macadamia, and papaya), and some tree nuts (such as hazelnut, hickory, pecan, and walnut).

In order to maintain seed quality in storage, the relative humidity of the air (in percentage relative humidity, or RH) and the temperature of the seeds (in degrees Fahrenheit) should total, as a general rule, less than 100. This means that the relative humidity around seeds stored at room temperature (about 72°F) should be less than 28 percent, while it should be less than 63 percent for seeds stored in a household refrigerator (about 37°F). Storage life of orthodox seeds is doubled for every 10°F drop in temperature, or every 1 percent drop in seed moisture content. Seeds are hydroscopic, meaning they will absorb or lose water from the atmosphere until they come into equilibrium. At the same relative humidity, seeds containing mainly carbohydrates will contain more water than do oil-containing seeds. At a relative humidity of between 20 and 70 percent, the seed-moisture content of a carbohydrate containing wheat seeds is around 30 percent more than that of oil-containing soybean seeds at the same relative humidity. So the relative humidity around stored seeds must be adjusted to produce the same level of moisture in seeds of different composition.

Whether plants were selected for domestication because they produced seeds that stored well, or whether plants were bred to produce seeds that stored well as they were domesticated, the seeds of most cultivated plants are orthodox and can be stored for a few years at near ambient conditions of moisture and temperature. The vigor and viability of most seeds of cultivated annuals can be maintained for two to five years under favorable storage conditions of 30 to 60 percent RH (8 to 14 percent seed-moisture content) and 40 to 60°F. Difficult-to-store seeds can be kept for extended periods at very low temperatures. This technique of cryopreservation can maintainrecalcitrant seed for decades at liquid nitrogen temperatures (-320°F; -196°C). However, the vigor of seeds stored at even these low temperatures declines over time. The continued maintenance of specific seed lines requires that they periodically be removed from storage and used to produce a new crop of seeds. Seed storage facilities, therefore, need not only modern storage equipment, but also the land, personnel, and expertise to periodically grow the stored seeds under conditions that maintain their genetic purity.

High vigor is characterized by the seed's ability to germinate and for the embryo to grow under stressful conditions that can include compacted soil, pathogens, salinity, and cold or hot temperatures. Seeds with higher vigor possess greater storage potential and can be stored for longer periods before the seed becomes nonviable. Both vigor and viability decline gradually during the early stages of seed storage; they parallel one another initially, with vigor ultimately declining in advance of viability. Later in storage, there is a sharp decline in both, until the seed only produces a weak seedling that quickly succumbs to environmental or pathogenic stresses.

Seeds that are planted in a field should have been sorted to produce a group of seeds that will germinate and grow at the same rate to produce uniform plants. Cracked, deformed, and small seeds should be eliminated in this winnowing process. In addition to selecting robust seeds, various preplanting treatments can increase seed viability and ensure uniform seedling establishment. Seeds' viability can be increased by partially germinating them under optimal conditions before planting. Holding seeds in an aerated aqueous solution for a week or so and then immediately planting them or drying and planting them later can also increase viability. The osmotic strength of the solution allows the seeds to take up water but not to germinate. Another method is to enclose each seed in a porous clay coating that contains fertilizers or fungicides to assist the seedling during early stages of growth.

Preparation of Seeds for Storage

Seeds should be harvested after maturing on the plant, separated from the fruit or protective plant parts in which they developed, and cleaned. Desiccant-tolerant seeds can be slowly air dried over a few days at low relative humidity and then sealed in moisture-tight containers before being placed in a refrigerator. If the seeds are sufficiently dry, they can be stored in the freezer section. Desiccant-intolerant seeds must be stored moist and need to be planted as soon as possible.

Stored Seeds Preserve Genetic Resources

Before the advent of scientific plant breeding, farmers would keep seed from the previous harvest to plant for the next season. From this process, numerous land races of the major crops developed that were specifically adapted to the conditions of their environment, such as climate, cultural practices, diseases, pests, soils, weeds. The availability of commercially produced seed with improved quality characteristics, pest resistance, and yields have supplanted many of these local varieties, and many have disappeared. Older crop and heirloom varieties of many horticultural and ornamental plants have also been lost because no one kept their seeds. Recognizing that such seeds may contain traits and genes (for example, pest resistance) whose introduction could improve modern varieties, private and governmental groups have developed seed banks and germplasm repositories to collect and preserve seeds and other propagules for long periods of time. These facilities include the National Seed Storage Laboratory in Fort Collins, Colorado; the Seed Savers Exchange in Decorah, Iowa; and the National Genetic Resources Program (NGRP), authorized in 1990 by the U.S. Congress. The Plant Sciences Institute (PSI), part of the National Germplasm Resources Laboratory (NGRL), is located at the Beltsville Agricultural Research Center, Agricultural Research Service, U.S. Department of Agriculture.

See also Barley ; Biodiversity ; Crop Improvement ; Maize ; Rice ; Soy ; Storage of Food ; Vegetables ; Wheat .

BIBLIOGRAPHY

Agarwal, V. K., and J. B. Sinclair. Principles of Seed Pathology. 2d ed. Boca Raton, Fla.: Lewis, 1997.

Ashworth, Suzanne. Seed to Seed: Seed Saving Techniques for the Vegetable Gardener. 2d ed., rev. Decorah, Iowa: Seed Savers, 2002.

Basra, Asmarjit S., ed. Seed Quality: Basic Mechanisms and Agricultural Implications. New York: Food Products Press, 1995.

Copeland, Lawrence O., and Miller B. McDonald. Principles of Seed Science and Technology. 4^th ed. Boston: Kluwer, 2001.

Doijode, S. D. Seed Storage of Horticultural Crops. New York: Haworth, 2001.

Kelly, A. Fenwick. Seed Production of Agricultural Crops. New York: Wiley, 1988.

Whealy, Kent, ed. Garden Seed Inventory: An Inventory of Seed Catalogs Listing All Non-hybrid Vegetable Seeds Available in the United States and Canada. 5th ed. Decorah, Iowa: Seed Savers Exchange,1999.

Mikal E. Saltveit