NS/S utilizes a two-pronged approach to conserving crop genetic resources from the southwestern US and northwestern Mexico. Ex situ approaches involve conserving samples of crop seeds under frozen storage conditions, where they may remain viable (able to germinate) for long periods of time. We also utilize in situapproaches that support and encourage the ongoing relationship between people and plants through which both natural and human selection pressures continue to result in the development of new crop varieties – the same relationship between people and plants that produced the diversity present today.
The NS/S Seed Bank is at the core of our conservation efforts. It serves as a repository for seeds, guarded in a safe environment for the proverbial "rainy day". In this case, the rainy day is when a crop can no longer be found growing in a farmer's field. Domesticated crops depend on an intimate relationship with humans - they don't exist in the wild. Over thousands of years, traditional agriculturists have selected and saved seed from plants that expressed a diversity of traits of interest to them or their communities - the ability to mature before the first frost, a sweeter taste, faster cooking time, or resistance to specific insects or diseases. Local, regional and global food security depends on this diversity. A seed bank's primary function is to conserve this genetic diversity for the future. The seed bank houses approximately 1,900 different accessions of traditional crops utilized as food, fiber and dye by the Apache, Chemehuevi, Cocopah, Gila River Pima, Guarijio, Havasupai, Hopi, Maricopa, Mayo, Mojave, Mountain Pima, Navajo, Paiute, Puebloan, Tarahumara, Tohono O'odham, Yaqui, and other cultures.
Over one-half of the accessions are comprised of the three sisters -- corn, bean, and squash. Nearly 100 additional species of crops and crop wild relatives are being preserved in the NS/S Seed Bank, including unique and often rare crop varieties: red-seeded amaranth used to dye piki bread; black-seeded sunflowers used as a dye stuff; drought-tolerant beans grown in the Pinacate region in northwestern Mexico (perhaps one of the hottest regions in North America); Sonoran panic grass (Panicum sonorum) - once thought extinct; sunflowers containing genes for resistance to a commercially devastating sunflower rust; other sunflowers that are restricted to serpentine soils; lemon basil; chia - an important source of protein, oil, and fiber; red-seeded watermelons. All these and more contribute to the rich genetic legacy maintained by the many people and cultures that have inhabited and survived among the coastal deltas, lowland plains, bajadas, and high mountain plateaus contained within the southwestern U.S. and northwestern Mexico.
Each accession represents a specific crop "variety" grown by a particular farmer and is individually preserved and maintained. These landrace, folk or farmers varieties result from both natural and farmer selection pressures over time. Sealed samples of individual accessions are placed in frozen storage. When these samples begin to show decreased viability, they are regenerated. Regeneration involves removing the sample from the freezer, growing it at the Conservation Farm and replacing the previous frozen sample with newly produced fresh seed. For each accession, duplicate samples are taken in order to have a back-up in case any individual regeneration attempt fails.
Two of the key challenges to successful regeneration involve maintaining the genetic purity and integrity of each accession. The genetic purity of an accession is maintained by preventing cross-pollination between different accessions of the same crop, such as between different corn varieties, and accidental mixing of accessions during collection, storage, harvest or post-harvest processing. We take many precautions to ensure each sample is correctly labeled, seeds are not picked up off the floor, and equipment is cleaned between each use. The “genetic make-up”, i.e., the specific combination of genes and their frequency, constitutes the genetic integrity of a sample and is considerably more difficult to ensure, particularly over long periods of time. Periodic regeneration and appropriate sampling protocols, such as saving seeds from many different parents, are the primary strategies we utilize to maximize the likelihood of maintaining the genetic integrity of each accession. When the seeds from only a few individuals form the basis of the long-term sample, a genetic bottleneck may result. A bottleneck occurs when a subset of individuals come to represent a larger population. An integral part of our regeneration efforts includes taking care to select a random sample for storage and not to inadvertently impose our own biases relative to seed size, shape, or color.
Even under frozen storage in a seed bank, seeds lose viability over time. Regenerating aging freezer samples by growing them in the field and producing new healthy, viable seed occurs at the NS/S Conservation Center in Tucson, AZ, with help from partner farmers around the region.