Environmental Engineering Reference
In-Depth Information
Similar brown-midrib (
bmr
) mutations have been reported in sorghum and are also associ-
ated with reduced lignin content. Oliver et al. (2005) tested isolines of sorghum carrying the
bmr6
or
bmr12
mutation. Interestingly, in one specific genetic background (a hybrid of A
Wheatland × RTx430), the
bmr12
mutation did not affect grain yield compared with the wild-type
hybrid. Thus, in sorghum it appears possible to maintain grain yield and reduce lignin in the
stover within the same hybrid. It has also been observed that in sorghum,
bmr
mutations did not
increase susceptibility to
Alternaria
, and in some lines the
bmr
genes were actually associated
with increased resistance to disease caused by
Fusarium
(Funnell and Pedersen 2006). A pos-
sible explanation for these observations is that the phenylpropanoid lignin precursor molecules are
redirected into biochemical resistance pathways.
Several studies have been conducted to identify QTL associated with digestibility and other
forage properties of leaves and stems in maize (Cardinal et al. 2003; Wei et al. 2009; Lorenzana et al.
2010). Perhaps because of varying environmental effects, differences in phenotypic evaluation, and
differing genetic backgrounds, only a few common QTL are apparent when these studies are com-
pared. Because of the tremendous genomic resources available for maize (Guillaumie et al. 2007;
Penning et al. 2009), including a fully sequenced genome (Schnable et al. 2009), some of these QTL
can be putatively assigned to a causative underlying gene on the basis of their location in the genome
(Wei et al. 2009). Still, the biosynthesis of plant cell walls is a very complex process, and very few
major QTLs have been identified. For example, Lorenzana et al. (2010) reported 152 QTLs for vari-
ous stover composition traits in maize, each with relatively small effects.
In addition to utilizing crop residues, dedicated photoperiod-sensitive maize and sorghum
varieties are being developed for the cellulosic ethanol industry. Under the long day lengths of
temperate summers, these plants will not flower and will continue to grow vegetatively, producing
large quantities of biomass.
1.3.2 p
ErEnnial
g
raSSES
aS
d
EdicatEd
B
iofuEl
c
ropS
Plant-based products (like agricultural and forestry residues) and paper and fiber wastes provide
various feedstocks for the emerging cellulosic biofuels industry, but these materials alone cannot
meet the increasing demand. Perennial grasses grown specifically for biomass production have
the potential to meet a large portion of this demand for energy feedstock. Some of the species
currently being studied for biomass production in warmer climates include sugarcane and energy-
canes (
Saccharum
hybrids), napiergrass (
Pennisetum purpureum
Schum.), bermudagrass [
Cynodon
dactylon
(L) Pers.], and giant reed (
Arundo donax
L.). In cooler temperate locations, native prairie
grasses including switchgrass (
P. virgatum
L.), big bluestem (
Andropogon gerardii
Vitman), prairie
cordgrass (
Spartina pectinata
), and little bluestem (
Schizachyrium scoparium
) are being studied for
their potential as biomass crops (Gonzalez-Hernandez et al. 2009). Several species and hybrids in
the genus
Miscanthus
are also promising candidates for bioenergy production in temperate regions.
This is by no means an exhaustive list of plant species with biomass production potential, although
these are among the most studied of the perennial grasses. Most of these crops are undomesticated
or only a few generations removed from their wild progenitors, and so there should be room for
improvement to develop superior cultivars specifically suited to production of large quantities of
high-quality biomass. In the development of these cultivars, a combination of traditional breeding,
molecular approaches, and transgenic plant technologies should be considered.
In the early 1990s, the U.S. Department of Energy identified switchgrass as a highly promis-
ing source of biomass for energy and fuels (Sanderson et al. 1996). Since then, a considerable
amount of research has been conducted on the utilization and improvement of switchgrass for bio-
fuel purposes. Switchgrass had been used for years as a forage crop, and several named cultivars
have been widely planted. Most sources identify two major ecotypes: lowland and upland. The
lowland ecotypes tend to be thicker stemmed, taller, and higher yielding plants adapted to wetter
sites, whereas the upland ecotypes, although thinner stemmed and lower yielding, are adapted to
