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Inside USDA's National Seed Storage Lab

Sunday, December 1, 1996
filed under: Fertility

The “Fort Knox” of the seed world is not surrounded by imposing fences. Its windows have no bars, and there are no armed security guards at its front door. Yet for the billions of us who must eat to survive, the contents of the National Seed Storage Laboratory (NSSL) are priceless — more valuable than gold. From the outside, NSSL’s three-story brick building on the campus of Colorado State University in Fort Collins is not unlike facilities found at any number of universities around the country. Inside, however, exists a one-of-a-kind collection of nearly 250,000 samples of seed gathered from around the world (not counting duplicate samples). These samples (called “accessions”) represent some 3,000 different plant species, with each sample containing anywhere from 1,500 to 3,500 seeds.

The job of those who work in this building is to make time stand still for those seeds as much as possible, ensuring that the seeds’ full genetic diversity remains available for use by future generations.

Why is that so important?

Dr. Steve Eberhart, NSSL director, points to a map containing photos of many of the world’s key agricultural crops and their corresponding centers of origin. Very few of those crops, he notes, are native to North America. “Almost every crop we have — sunflower being one of the exceptions — was introduced here from someplace else,” Eberhart explains. Given the ever-dynamic nature of the plant world, new disease strains are always evolving; new insect challenges develop; and all sorts of other foes pop up to confront our essential crops. In a world of few certainties, we can be certain that each new decade will bring new and unforeseen problems for production agriculture.

By going back to the center of a plant species’ origin, scientists often are able to find outright resistance or other survival mechanisms which allowed the “forefathers” of modern-day crop seeds to persevere within their native environment. Those wild plants usually are very unacceptable for a farmer’s field: Often nonuniform in appearance, they typically have poor yield capacity and are generally inferior for commercial applications. However, they may possess one critical trait — resistance to a particular disease, for instance — that makes their seeds invaluable for modern-day agriculture.

Sunflower provides an excellent example. One of the few crops indigenous to North America, it encompasses some 50 species and subspecies — all of them represented in the collection of the National Seed Storage Laboratory. The combined genetic diversity of all those sunflower species has barely been tapped — in large part because of the inherent difficulties of inter-specific crossing. Sunflower germplasm utilized thus far, however, has allowed for hybridization of the crop, brought resistance to several major diseases (rust, downy mildew, Verticillium wilt), and greatly enhanced the economic viability of sunflower. Yet the evolvement of new disease strains and the ongoing need for more “tools” (drought tolerance, insect resistance, better stalk strength) underscore the need for continually expanded access to sunflower’s huge pool of wild germplasm.

Receiving, evaluating seed quality and preserving that pool — and the pools of thousands of other plant species — is the core mission of the National Seed Storage Laboratory.

The NSSL is part of the National Plant Germplasm System (NPGS), which is operated by the USDA Agricultural Research Service. The Fort Collins facility’s ever-expanding holdings constitute the “base” collection of seeds, with a duplicate “active” collection for each species being located at one of four regional plant introduction stations and crop-specific collections under the NPGS framework. (See the article on page 26.)

Public and private scientists seeking seed samples for their research receive them (at no charge) from the appropriate regional station, not from the NSSL at Fort Collins. The only time seeds leave the base collection is when a regional station runs short of a particular accession. A limited supply of seeds is then sent to the regional station for “growouts” (i.e., population increases to replenish the diminished supply).

After more than a decade of lobbying for funds and planning its construction, the National Seed Storage Laboratory opened in 1958. It began to consolidate collections of seeds which had been gathered by various individuals and institutions over the years — and often stored under haphazard conditions, resulting in seeds sometimes losing their viability and thus becoming useless.

By the mid-1980s, the NSSL building was filled beyond capacity, with more than 200,000 accessions stored within its walls. That number had grown to about 235,000 by the end of the decade. Several years of campaigning Congress to fund a larger facility finally culminated in 1992 with the opening of an addition which increased space four-fold and now provides modernized vaults that can store and protect up to 1.5 million seed samples.

Though the NSSL does include some ornamental and endangered species of seeds, the bulk of its collection consists of agricultural seeds. Most economically significant food, fiber and industrial crops have crop germplasm committees which assist the National Plant Germplasm System as to priorities for their particular crop, i.e., what needs to collected and evaluated and where in the world collectors should go to find those needed accessions. (Dr. Gerald Seiler, USDA-ARS, Fargo, N.D., chairs the Sunflower Crop Germplasm Committee and also has served as curator of the sunflower wild species collection.)

What happens once a seed sample arrives at the National Seed Storage Laboratory? In the past, the first step was to bring down its moisture content to what was considered the “optimum” level for long-term storage. That level typically was around six percent, regardless of the plant species.

More recently, however, NSSL research has shown that there is no single optimum moisture content for all species’ seed samples “An oilseed like sunflower may store well at six percent,” says NSSL curator Dr. Loren Wiesner. “But it can be different for a starch- and protein-containing seed like wheat. Through our research, we’ve found that optimum moisture contents vary by species — from five percent up to 12 or 14 percent, depending upon their composition.”

After extensive investigation, NSSL research leader Dr. Eric Roos and his staff concluded that the best way to prepare seeds for long-term storage was to place them in a “equilibration chamber” set at a relative humidity of 25 percent and a temperature of five degrees Centigrade (about 40 degrees Fahrenheit). Those conditions allow the seed to attain its own optimum moisture for long-term storage. In other words, “the seed knows best.”

Once equilibrium has been attained (usually two to three weeks), the sample is removed from the chamber and a test is conducted to verify the seeds’ moisture content. NSSL technicians clean up the sample if necessary (remove trash, inert materials, etc.) and then perform viability tests to determine each sample’s germina-tion percentage and check for dormancy.

After testing, samples going into conventional storage (as opposed to liquid nitrogen, discussed on page 24) are placed inside foil laminated bags, which are then heat-sealed to hold the sample at the equilibrilized moisture content. The bags are then placed into a tray inside the huge cold-storage vault, which is kept at a temperature of minus (-) 18 degrees Centi-grade (zero degrees Fahrenheit). Each foil bag has a location bar code and a serial number bar code. Those codes tell where the sample belongs in the vault (location) and what’s inside the bag (serial).

How long can those seeds remain inside that vault without losing their viability? No one knows the definitive answer, as the laboratory has existed for just a few decades. That’s where re-testing enters in. In the past, for those samples with 85 percent or higher germination, NSSL staff would re-test every 10 years; for those below 85 percent, every five years. However, studies have shown that under the -18 C. conditions, most species’ seeds store so well that re-testing is not needed more than once every 10 years — and then only on more-susceptible species such as onions, lettuce and peppers. If those “touchier” species do not show lowered germination levels, there’s no need to test the more-stable species.

Along with the new procedures for conventional storage, the other major change in seed storage technology over the past several years has been the introduction of cryopreservation. Located in a large room on the lower level of the National Seed Storage Laboratory are dozens of five-foot-diameter steel tanks containing liquid nitrogen at minus (-) 196 degrees Centigrade. Long tubes filled with seeds are held in trays perched within the vapor phase of the liquid nitrogen. (The vapor zone is at “just” -160 degrees C.). NSSL is the only facility in the world that currently stores seed routinely under liquid nitrogen.

Why store seeds in this manner? In a word, longevity. “We have no idea how long seeds can stay alive in liquid nitrogen,” says Eric Roos. “It’s certainly going to be several decades; maybe even centuries.” While they’re in this type of ultra-cold storage, the seeds show no metabolic activity. “Even molecular motion is slowed down,” Roos notes. Not all seeds are candidates for liquid nitrogen storage, for one of two reasons.

First, some species are damaged by the cooling/warming process. The hard seed coats of certain legumes, for example, will fracture or shatter. High-oil crops like sunflower also are negatively affected (though for reasons not well understood). The only possible danger to seed viability occurs while the seeds are being “cooled down” (placed in the nitrogen vapor zone) or “warmed up” (being removed). Every sample placed in liquid nitrogen goes through a 24-hour exposure test in which the seeds are placed in the tank, left for 24 hours and then taken out. If germination is reduced by 10 percent or more, this type of storage is not feasible for that sample.

Secondly, because of the physical limitations and cost of liquid nitrogen storage, it’s not logistically or economi-cally feasible for large seeds such as corn or soybeans to be placed in liquid nitrogen tanks. As a general rule of thumb, says Loren Wiesner, any seeds smaller than wheat or barley will be placed into liquid nitrogen storage (assuming they’re not adversely affected), while wheat, barley and any others of that size or larger go into conventional (-18 degrees C.) storage.

t costs about four cents a year to keep a seed sample under conventional storage at the National Seed Storage Laboratory and about 15 cents per year for a sample under liquid nitrogen storage.

That’s a tremendous bargain, Wiesner affirms, given the value — current and future — of the materials being stored. Production agriculture has been and always will be faced with all sorts of challenges affecting crop yield and quality; but if we don’t possess germplasm with disease resistance or other esssential traits, meeting those challenges will be impossible.

“We can’t buy it,” Wiesner emphasizes in reference to that germplasm. “It’s natural, and we must preserve it.” And since no one knows what forms of disease or other problems will appear in the coming years, it’s important to collect and maintain as much genetic diversity as possible.

The NSSL is not a “museum for seed,” concludes its curator. The millions of seeds inside its walls constitute a huge storehouse of potential — potential which, once germinated, will be relied upon to feed future generations for decades and centuries to come. — Don Lilleboe
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