Seed Preservation

Conservation of crop genetic resources is important to the long-term health of the world's food production systems. Genetic diversity provides the raw materials for selecting and improving plant traits such as resistance to pests, diseases, and environmental stresses. Genetic engineering has greatly increased our ability to manipulate genes for the benefit of agriculture, including transferring genes between unlike species. However, until individual genes that code for a specific trait or set of traits can be designed and developed in the lab, researchers and plant breeders must rely on existing genes. Thus, saving and preserving seeds, and their genes, is critical to future food security and stability.

Providing access to a reservoir of plant genetic resources has been the goal of ex situ (outside of the place of origin or natural occurrence) conservation at both the national and international levels. Ex situ strategies include the storage of plant genetic resources in seed banks, clonal repositories, and living collections. An extensive system for the ex situ conservation of plant genetic resources has been developed in the United States under the auspices of the Agricultural Research Service (ARS), the main research agency of the USDA (United States Department of Agriculture). The National Plant Germplasm System (NPGS), established to preserve and promote the use of plant genetic diversity, is a collaborative effort between state, federal, and private entities to acquire and manage plant genetic resources, including wild and weedy crop relatives, landraces , obsolete cultivars, and elite lines or populations of agricultural, horticultural, industrial, and medicinal crops. Though the NPGS focuses on building a strong, competitive U.S. agricultural industry, all germplasm held in the NPGS collections is made available to researchers around the world upon request.

Germplasm from all over the world is preserved in the NPGS system. Because many of the commercial crops produced in the United States are from nonnative sources, American agricultural productivity has depended on plant introductions from other countries, particularly from the tropics and subtropics. There are over four hundred thousand accessions from more than ten thousand species in the U.S. germplasm reserves. Responsibility for maintaining and distributing this large collection is divided between different NPGS sites, such as the eight National Germplasm Repositories, four Regional Plant Introduction Stations, the National Seed Storage Laboratory (NSSL), and other NPGS sites. Each site is charged with maintaining different species.

The NSSL preserves the base collection of the NPGS and conducts research to develop new technologies for preserving seed and other types of plant germplasm. Seeds are stored either in conventional storage at -18°C or in cryogenic storage (liquid nitrogen) at -196°C. The National Germplasm Repositories are responsible for acquiring, preserving, increasing, evaluating, documenting, and distributing plant genetic resources of specific genera. United States germplasm collections of maize (corn), pumpkins, sunflowers, melons, cucumbers, and carrots are maintained at the North Central Regional Plant Introduction Station located in Ames, Iowa. The Western Regional Plant Introduction Station in Pullman, Washington, maintains lettuce, beet, bean, chickpea, forage and turf grass, and pea germplasm. Collections of pears, strawberries, blueberries, raspberries, and others are maintained at the National Clonal Germplasm Repository in Corvallis, Oregon, as living plants or, in the case of wild species, as seeds. Fruit and nut tree germplasm is maintained at the National Clonal Germplasm Repository in Davis, California.

Long-term storage of seed samples carries with it some inherent problems. The primary objective of seed banks is to maintain the genetic diversity and integrity of germplasm. Maintaining seeds in frozen storage requires adequate temperature and humidity controls. Even under ideal conditions, however, seeds eventually begin to lose viability—the ability to germinate. Thus, periodically, new seed needs to be produced in order to replace aging seed samples. This process is referred to as regeneration. The loss of unique genotypes within a collection—whether from natural causes, small sample sizes, or random drift—nonetheless would constitute a change in the presence or frequency of genes within a germplasm collection.

During regeneration, plants are exposed to the risks inherent with agriculture—insects, diseases, drought, hail, temperature extremes, and wind, depending on whether they are grown under field or greenhouse conditions. When susceptible genotypes succumb to insect or disease pressures, the genetic variability and integrity of the collection may be compromised unless the genes lost are present in surviving genotypes. Thus, the need to regenerate a collection must be weighed against the need to minimize risks associated with regeneration. Additionally, genetic contamination through cross pollination or accidental mixing during post-harvest processes such as cleaning can also result in a loss of integrity.

Many small, independent organizations are also involved in seed saving. Together, they have created what is known as the heirloom seed movement. These organizations, groups, and individuals have helped bring about global awareness of genetic erosion—a reduction in the number of varieties, and, hence, genetic diversity—in commercially available vegetable and crop seed. Of the approximately five thousand heirloom varieties of vegetables available in the 1984 seed catalogs, 88 percent were no longer available by 1998. On average, there is a 6 percent loss in available varieties every year. Comparison of a USDA inventory of varieties available at the beginning of the twentieth century with a list of holdings in the NSSL at the end of the twentieth century revealed that only 3 percent of this germplasm survived in American germplasm reserves.

More than twenty-five small seed companies in the United States focus their efforts on slowing or preventing the loss of open-pollinated, heirloom seeds. Founded in 1975, Seed Savers Exchange collects, maintains, and distributes precious heirloom seeds through a network of eight thousand members. The vast collection of rare, heirloom seeds includes over eighteen thousand varieties of tomatoes, beans, peppers, squash, peas, lettuce, corn, melons, garlic, and watermelons from countries around the world.

see also Biodiversity; Cultivar; Native Crops; Seeds; Vavilov, N. I.

Suzanne C. Nelson

Bibliography

USDA-ARS/National Seed Storage Laboratory Preservation Page. 1997. [Online] Available at http://checkers.nssl.colostate.edu/preserve/prestour.htm.

USDA-ARS/North Central Regional Plant Introduction Station Page. 1999. [On-line] Available at http://www.ars-grin.gov/ars/MidWest/Ames/.

Whealy, Kent. Garden Seed Inventory, 5th ed. Seed Savers Publications, 1985.

NATIVE SEEDS/SEARCH

Native Seeds/SEARCH (NS/S) conserves the traditional seeds, crops, and farming methods that have sustained Native peoples in the southwestern United States and northwestern Mexico. Since 1983, the NS/S collection of crops and wild crop relatives has grown to nearly two thousand varieties of corn, beans, squash, amaranth, chilis, cotton, and other crops grown by Apache, Hopi, Maricopa, Mojave, Mountain Pima, Navajo, Paiute, Puebloan, Yoeme, and other farmers.