See related
article, Land Use and Climate Change
Some ORNL researchers
don't want some green plants to be carbon copies of their parents. They
hope to genetically manipulate these plants to fix more carbon from
the air by photosynthesis so they grow faster and hold more carbon in
their stems, leaves, and roots. The goal is to grow trees and grasses
that can produce more fuel and store more carbon in the soil.
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Biomass
in forests and fields takes up carbon dioxide. It can be converted
to low-carbon fuels, construction wood, and other products. Illustration
by Brett Hopwood.
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These researchers
work for DOE's Bioenergy Feedstock Development Program, which is managed
at ORNL. "Our program manages tree and grass crops in experimental systems
to maximize carbon production for energy and carbon sequestration,"
says Lynn Wright, program co-manager. "Our research focuses on developing,
cultivating, and harvesting fast-growing plants for energy instead of
using fossil fuels. We are trying to increase the production of carbon
in hybrid poplar trees and switchgrass aboveground to improve energy
production. We also want to use these plants to conserve or add carbon
belowground for sequestration."
In the early days
of the program, researchers tried to optimize the aboveground growth
of plants. They developed fast-growing varieties and hybrids that grew
rapidly and used inputs like water and fertilizer efficiently. More
recently, genetic manipulation has moved to the front burner. In manipulating
plants such as hybrid poplar trees, plant geneticists talk about allocating
the carbon between the aboveground stems and leaves and the belowground
roots of the plant. They also talk about partitioning (dividing) it
among three types of plant cell wall components—cellulose, hemicellulose,
and lignin.
What researchers
would like to do is create a plant that is very high in cellulose aboveground,
increasing its energy conversion potential. Cellulose is a polysaccharide
containing chains of 6-carbon sugars. Enzymes can split cellulose into
individual sugar molecules, which can then be converted by microorganisms
into ethanol. When used as a fuel, ethanol is cleaner than gasoline.
Because the carbon in plants is taken from the air during photosynthesis,
burning ethanol from cellulose contributes very little net carbon dioxide
to the atmosphere.
This "designer"
plant would also be high in lignin in the roots. Lignin consists of
carbon-containing phenolics that are amorphous in structure. It resists
digestion by enzymes, making it less susceptible to microbial degradation
and more effective at sequestering carbon in the soil.
"To customize
a plant species genetically to boost its carbon content, we must understand
the fundamental biological processes that control carbon allocation
and partitioning," says Jerry Tuskan, an ORNL plant geneticist. "We
hope to make progress in this area through our wood chemistry and genetic
studies using an experimental population of hybrid poplar trees."
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Hybrid
poplar trees may be genetically modified to store more carbon
in the soil.
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Tuskan, Stan Wullschleger,
Tim Tschaplinski, and Lee Gunter, all of ORNL's Environmental Sciences
Division (ESD), and Brian Davison of the Chemical Technology Division
are involved in this study, which is partly funded by the Laboratory
Directed Research and Development Program at ORNL. Collaborating in
this study are researchers from DOE's National Renewable Energy Laboratory
(NREL).
In this
population, explains Tuskan, each tree has the same grandparents but
its genetic characteristics are unique. "It's like shuffling a deck
of cards for each tree," he says. "Each tree will have a different arrangement
of diamonds, clubs, hearts, and spades. The progeny will have characteristics
different from those of their grandparents and each other."
At the end of August 2000,
the experimental planting was a year old. Each tree will be lifted,
roots and all, and ORNL and NREL researchers will measure the trees
to determine their energy content and the relative amounts of cellulose
and lignin aboveground and belowground. The trees that contain the most
carbon in their trunks or roots will be further analyzed biochemically
by the project’s plant physiologists to identify specific compounds
in the cell wall.
"Using genetic
techniques, we will identify genetic markers, or the DNA sequences flanking
the genes, that are responsible for desirable cell wall traits, such
as high carbon content in lignin in the roots," Tuskan says. "These
markers will allow us to identify regulatory regions that control carbon
allocation and other genes that control partitioning either aboveground
or belowground. We will provide our findings to the energy and forest
products industry to help them customize crops to get a desired product."
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Switchgrass,
a perennial grass that grows deep roots, can be converted to ethanol.
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Janet Cushman
of ESD, who co-manages the DOE bioenergy feedstock program, says that
switchgrass is also ideal for producing ethanol and sequestering carbon,
largely because its roots can go 7 m (20 ft) deep. The same genetic
manipulations proposed for tree crops can be applied to this perennial
grass of North America. If concerns about increasing concentrations
of carbon dioxide in the atmosphere lead to policies to promote carbon
storage and encourage use of renewable energy, there will be even more
incentives to switch from fossil fuels to biomass.
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ORNL's
Bioenergy Feedstock Development Program
ORNL's
Chemical Technology Division
ORNL's Environmental
Sciences Division
