麻豆传媒

Norman, Oklahoma

A WIND whips across the bare fields. With the season鈥檚 crop harvested,
surface soil is swept up in spirals. It rises into the air, obscuring the Sun
and turning the winter sky a paler shade of blue. Around the globe, from
Australia to the Central Asian republics of Turkestan and Uzbekistan and
California鈥檚 Central Valley, dust storms are a seasonal certainty. Little by
little, the world鈥檚 fertile soil is vanishing.

Each year, farmers harvest their wheat, barley, maize and rice leaving the
land exposed to the ravages of wind and rain. These forces erode agricultural
soil at a rate of around 1 per cent a year, says ecologist Stuart Pimm of the
University of Tennessee in Knoxville. The high-tech solution is to increase the
productivity of remaining land by pumping the ground full of chemical
fertilisers and using machines to plough up compacted soil. Such methods, with
fungicides and pesticides thrown in, have so far managed to feed most of the
world鈥檚 population, but not without enormous environmental and financial costs.
No more than a quick fix that puts back nothing permanent into the soil, these
cannot continue indefinitely.

But what if cereal crops only needed to be replanted once every few years
instead of annually? Perennial crops would hold the soil in place with their
extensive root systems. Roots also keep soil from compacting to the point where
water and new roots can no longer penetrate. And the correct mixture of plants
might even start the long process of soil regeneration. Plant geneticist Wes
Jackson dared to think this more than 20 years ago. Today, his team at the Land
Institute in Salina, Kansas, and a small band of researchers around the world
are pursuing the idea in earnest.

If they succeed, a new agricultural revolution is on the cards. But the scale
of the challenge is enormous. Today鈥檚 domesticated cereals are all members of
the grass family Poaceae and perennial species will need to come from
their wild relatives. But no wild species of cereal grain has been domesticated
since farming became established more than 6000 years ago.

Over the years, farmers have created cereals that behave in predictable ways,
so they can streamline planting and harvesting and prepare accurate budgets.
Their crops ripen uniformly, give large shatter-resistant seeds, and stay on the
stem until harvest time. The seed鈥檚 papery coat, or chaff, comes away easily
when the crop is threshed, and the seeds can be kept for up to two years and
still germinate uniformly when planted.

It has taken millennia to remove the ancient traits of individuality and lack
of predictability that once increased the wild plant鈥檚 chance of dodging
predators, pests and competitors. Selection by farmers has also forced cereal
plants to divert the energy previously expended on a strong root and vegetative
system into the production of plump seeds.

Not only must plant breeders such as Jackson find a short cut that mimics
these thousands of years of breeding, they must also create plants that can
compensate for the loss of one great advantage of annual cereals鈥攖he
ability of farmers to keep them one step ahead of pests by rotating them from
field to field.

But the rewards are potentially huge. Perennials offer advantages beyond
their ability to protect soil from erosion. They tend to retain more nutrients
than annual crops, and so need fewer fertilisers. Also aeons of self-reliance
have made them naturally more resistant to drought, disease and pests than
cultivated plants.

Researchers are building on these strengths to create alternatives to today鈥檚
crop plants. Jackson and his colleagues began by collecting the wild relatives
of common crops from native grass prairie. The prairie plants, the original
creators of America鈥檚 fertile soils, are dominated by perennials. If farmers
could one day plant a mixture of crops that mimic native vegetation, the
researchers believe they might be able to reverse the deterioration that modern
farming has caused.

Jackson鈥檚 team has identified four functional groups of prairie
plants鈥攇rain-producing grasses that grow and mature in warm seasons,
grasses adapted to cool seasons, nitrogen-fixing legumes and oil-producing
composites. Their work is now focused on a few species chosen for their
potential to produce high yields.

The nursery is stocked with eastern gamagrass (warm season), mammoth wild rye
(cool season), Illinois bundleflower (legume) and Maximilian sunflower
(composite). All are from the midwestern US, says plant biologist David Van
Tassel. Using traditional crossing methods, he selects plants which produce the
most and largest seeds and inter-breeds them. Over the next few years, Van
Tassel hopes to breed plants that closely resemble domestic crops but retain
their perennial habit.

The next step is to use these plants and others to create a crop mix that
mimics the wild prairie, establishing the web of fungi and nutrients that are
essential for healthy soil and attracting other species that encourage
perennials to give high grain yields. Work by Land Institute botanist Jon Piper
has shown that the prairie mix can limit the growth of weeds, provided there is
sufficient species diversity among the perennials.

Eventually, the researchers aim to provide farmers with a perennial crop
mixture which, after several years of growth, will become refined in each
location to give a mix that only needs replanting every five years or so. The
plants might be grown in strips, allowing the different species to be harvested
and replanted at different times. Or a homogenised mix could be cut as a whole
and the different grains separated mechanically after threshing. It may take a
few years before prairie mix is producing maximum yields and this will
undoubtedly cut into profits for a time, says Pimm. But he is confident that
perennials will be a viable alternative to annual crops. 鈥淔armers are a
conservative lot,鈥� he says, 鈥渂ut they understand the bottom line.鈥�

Jackson鈥檚 team is not alone in the search for perennial crops. About 340
kilometres to the southwest of Salina is the US Department of Agriculture鈥檚
Southern Plains Range Research Station, in Woodward, Oklahoma. Here, researchers
are breeding a better forage crop鈥攐ne that may soon replace annual sorghum
varieties. The USDA target is a natural mutant of eastern gamagrass, a relative
of maize, which plant breeder Chester Dewald has studied for more than two
decades.

Dewald discovered the mutant in central Kansas. Normally a single stalk of
gamagrass has both male flowers producing pollen and female flowers that give
rise to seeds. In Dewald鈥檚 plants, however, almost all the flowers are female,
increasing the crop鈥檚 yield. In the wild, these 鈥渟ex reversal mutants鈥� are
pollinated by normal gamagrass nearby. By manually crossing the two in his
laboratory, Dewald is trying to produce a self-pollinating plant with maximum
seed yield. 鈥淲e never know exactly what we鈥檙e going to get, but every year I鈥檓
like a kid at Christmas,鈥� he says.

The trick is to keep the perennial qualities while breeding in features
attractive to farmers. In practice, this means retaining the deep root system
that holds soil in place and makes gamagrass more drought resistant than most
annual crops. Using wild relatives, Dewald is breeding in other desirables, such
as stronger stalks to support more grain and shatter-resistant seed heads. He is
also experimenting with gamagrass from Guatemala. Its huge leaves make the plant
a 鈥渂igger photosynthetic factory, so it should produce more grain鈥�, says
Dewald.

At the Rodale Institute in Kutztown, Pennsylvania, Peggy Wagoner is also
using traditional methods to produce a perennial variety of wheat. Wild
triga鈥攆ormerly called intermediate wheatgrass鈥攐riginates from the
eastern Mediterranean and southwestern Asia and was introduced to the US early
this century as a forage grass. 鈥淭riga is a species with a lot of potential for
developing into a perennial grain,鈥� says Wagoner. It has a strong stalk, is
easily threshed and the plants mature in synchrony. In addition, the grain
contains nearly 66 per cent more protein than domestic wheat.

At the moment, the yield from wild triga is only about a fifth of that from
annual wheat varieties, but Wagoner is working to change that. 鈥淎 lot of the
energy goes to the roots for over-wintering,鈥� she explains, so there is less
photosynthetic energy available for producing grain. She is trying to create
plump grain by interbreeding different types of wild triga from a variety of
countries. Wagoner has now produced second generation plants and will start
marketing the crop once the third is ready.

Wild triga is unlikely to replace traditional wheat, however, because it
lacks gluten, the sticky protein that gives dough its elasticity. It would have
to be mixed with normal wheat flour for bread making. Even so, it could be a
valuable alternative crop in areas such as the mid-Atlantic region of the
US, where hilly ground increases the amount of erosion caused by rain.

Classical crossing methods have proved effective in the search for perennial
crop plants, but they are slow. Some plant breeders aim to take a short cut by
using genetic engineering. The key is to find genes that are linked to
domestication and then insert these into wild plants.

Evolutionary geneticist John Doebley of the University of Minnesota in Saint
Paul is working on maize. By making genetic comparisons between domesticated
maize and its wild perennial relative, Doebley and his colleagues have found
several domestication genes. These include one for apical dominance, which
controls how much branching occurs off the main stalk. Agricultural plants
usually have a single strong stalk supporting the crop, rather than the many
smaller branching stalks found in wild varieties. In maize, the domesticated
variety carries no more than 25 large dense ears of corn, while the wild
varieties have several hundred tiny ears. Doebley expects that the gene which
suppresses branching might one day be inserted into the genome of perennial
plants so they can develop ears similar to those of domesticated maize.

Doebley has been lucky in finding a gene that has a dominant effect on a
trait. Rarely is the situation this simple. Genetic control of traits more often
involves multiple genes with unequal effects. This complexity is at the root of
the problems faced by molecular geneticist Andrew Paterson of Texas A&M
University in College Station in his work on sorghum.

Sorghum is closely related to a persistent perennial weed called
Johnsongrass, which is hated by farmers worldwide because its ability to survive
underground over winter means that once it invades a field it is difficult to
remove. Paterson believes that this obstinancy may hold the key not only to
making sorghum a perennial, but also to controlling the weed itself.

The secret of the success of Johnson-grass lies in its network of underground
stems known as rhizomes, which allow it to store nutrients deep in the soil and
escape temperature fluctuations at the surface. 鈥淪eventy per cent of Johnson
grass dry weight is rhizome,鈥� says Paterson. He is trying to clone the genes for
rhizomes, so that he can understand how these structures are formed and
function. So far, he has mapped the approximate location of nine rhizome genes
and located six further genes which play smaller roles.

Removing rhizome genes from the weed could limit its tenacity, while adding
the genes to cultivated sorghum and other annuals could give them perennial
abilities. Paterson cautions that it may take him a full decade to complete his
task, and even then he may find that controlling rhizome formation is not enough
to produce a high-yielding perennial crop.

In the meantime, farming is slowly approaching a crisis point. Topsoil is
still vanishing, and what remains is becoming sterile and compacted. In this
environment, advocates of perennial agriculture are locked in a race against
time.

* * *

Mother earth

SOIL is a living biosystem. One square metre of soil contains thousands of
tiny creatures and billions of microorganisms. The exact process by which they
create new soil remains a mystery.

Everyone knows the benefits of earthworms and other creatures that help
maintain soil structure by not allowing it to become too compacted. Now the
spotlight is falling on symbiotic systems called mycorrhizae, which are made up
of plant roots and the fungi that live in or on them. Although they steal sugars
from plants, mycorrhizae also expand the surface area of the roots, allowing
them to capture more nutrients from the soil.

In natural ecosystems, soil also contains string-like filaments called hyphae
which are the primitive roots of multicellular fungi. These multinucleated cells
pervade the soil, entwining and penetrating plant roots. Hyphae are thought to
help plants gather rare nutrients such as phosphorus.

By using fungicides and fertilisers, farmers are unwittingly destroying
microorganisms that could help crops to grow. 鈥淭he underground scene may really
be the most important thing,鈥� says David Van Tassel, from the Land Institute in
Salina, Kansas.