Geneticist finds switchgrass could bridge bioenergy and conservation

An important part of the answer to today's energy woes could be blowing in grasslands, according to Agricultural Research Service (ARS) plant geneticist Michael Casler. He has spent the past 10 years breeding switchgrass (Panicum virgatum), a plant native to the U.S. that was an integral part of the tall grass prairies that once dominated America's Midwest. In a new study, part of the ARS' Rangeland, Pasture, and Forages program, he presents findings which show that the switchgrass varieties conservationists are keen on using for reestablishing vanished prairies, are genetically similar to the varieties with a high bioenergy potential.

As a breeder, Casler is mostly concerned with the plant's bioenergy-friendly attributes, including its ability to accumulate large amounts of biomass and tolerate environmental stress. Some quick facts about switchgrass as a biomass crop first:

• a recent ARS study on energy cropping systems shows switchgrass and hybrid poplar would produce nearly a three-fold greater reduction in greenhouse gas emissions compared to corn or soybean rotations (earlier post)
• as a perennial, switchgrass has deep roots with which the crop acts to increase soil carbon levels; boosting carbon storage in the soil mitigates the greenhouse effect and improves soil quality
• the energy balance of switchgrass based biofuels is up to between 5 and 10 times stronger than biofuels made from food crops like corn and soybean
• scientists have determined the sequences of about 12,000 switchgrass gene fragments so far; at least 12 of them are associated with genes that regulate the production and deposition of lignin, the cementing agent that holds plant cell walls together; this identification allows for the breeding of low-lignin varieties that can be converted more easily into liquid fuels

These basics show switchgrass is a highly promising energy crop. With this in mind, Casler began looking at switchgrass for more potential benefits: could the crop function as a restorer of once-pristine prairies? (For earlier examples of coupling bioenergy to conservation, see here and here). Historically, a sprawling seas of grasses once stretched from Montana and the Dakotas down to Texas, with pockets of prairie as far east as New York. Now, with much of this land fragmented or altered, only a patchwork of remnant prairies remains.

Numerous conservation efforts are examining how best to revive these vestigial prairies. But a question of genealogy always arises: which switchgrass varieties should be planted that will be in keeping with a site's genetic legacy? Some conservationists insist on using only long-established, local varieties of switchgrass. Others argue that modern-day cultivars can appropriately be used. Along with ARS scientist Kenneth Vogel, Casler set out to bring clarity to this debate and, hopefully, ease the task of grassland restoration:
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After two summers spent trekking native Midwestern prairies, plucking samples and sending them back to his laboratory, Casler discovered that today's agronomically important switchgrass cultivars are nearly identical genetically to their grassy ancestors.

Casler’s switchgrass subjects had their differences, but hardly any were attributable to broad geographic disparities. Actually, aside from subtle differences owed to variations in soil, climate, and slope, the broad switchgrass pool sitting before Casler was pretty homogeneous.

Plants from each individual population were as variable as those from geographically distant populations, and the remnant populations were very similar to the cultivars, he says. Part of this can be explained by the fact that people have been breeding switchgrass for only about 50 years, compared to the thousands of years of domesticating modern wheat or corn.

In fact, the most advanced cultivars he analyzed are only three to four generations removed from wild switchgrass, says Casler. He adds that these breeding-induced changes are small, since breeders only exploited a small amount of genetic variation already existing in native switchgrasses.

The good news about these findings is that so-called 'improved' switchgrass cultivars are, genetically speaking, very similar to populations of plants being used for native restoration. The difference between native and cultivated switchgrass is probably due to changes in the frequency of just a few genes that have little overall impact on switchgrass gene pools.

These findings make switchgrass, which was already enjoying modest agricultural fanfare, especially attractive.

Our findings show that switchgrass that’s grown for biofuel can also be grown for conservation and other uses without the fear of possible genetic contamination. We need to pay attention to the origin of switchgrass seed populations, but we’ve learned that seeds can be transferred widely within the hardiness zone in which they originated. - Michael Casler

Switchgrass as a source of renewable energy still requires more research before its full potential is realized. Casler says that the plant’s biofuel future probably lies in specially designed seed mixtures with supporting role-type plants—including beneficial legumes that fix their own nitrogen.

If Casler’s right, fields of soft, willowy switchgrass growing alongside native legumes like pure prairie clover and Illinois bundleflower could someday provide us with a source of green energy as well as a window into our country’s verdant past.

The study's findings are good news for prairie restorers, who can confidently tap a wider pool of switchgrass cultivars and local varieties for conservation projects. And switchgrass growers can take satisfaction knowing their fields still are, in many ways, symbolic of the country's rich grassy past.

ARS is the U.S. Department of Agriculture’s chief scientific research agency.

Image courtesy: Stephen Ausmus.

References:
ARS: Switchgrass: Bridging Bioenergy and Conservation - September 13, 2007.

ARS: Scientists Turn Genetic Keys to Unlock Bioenergy in Switchgrass - April 20, 2007.

ARS: Energy Farming With Switchgrass Saves Carbon - July 19, 2007

Biopact: European project finds nitrogen damages biodiversity - biomass stripping coupled to bioenergy could offer conservation strategy - October 13, 2007

Biopact: Tallgrass Prairie Center to implement Tilman's mixed grass findings - September 02, 2007

Biopact: Study: greenhouse gas balance of different energy cropping systems - June 18, 2007