Environmental Engineering Reference
In-Depth Information
Haussmann et al. 2002; Bowers et al. 2003; Mace et al. 2009). Recently QTLs have been identified
in sorghum for sugar-related traits using traditional linkage mapping (Natoli et al. 2002; Murray
et al. 2008) as well as association mapping approaches (Murray et al. 2009). These QTLs may be
useful in marker-assisted breeding to improve the productivity of sweet sorghum. The sorghum
genome has also recently been sequenced (Paterson et al. 2009), which provides a tremendous
opportunity to further analyze genes in this species and to compare similar genes in other grass
species. The sorghum genome is relatively small in size (750 Mbp) and can act as a connecting
link between small cereal genomes such as rice (480 Mbp) and very large complex genomes like
sugarcane, which is related to sorghum. Comparative genomics using sorghum as a model C
4
grass
will also help us to understand the evolutionary aspect of genes involved in biofuel traits and aid in
designing strategies for enhancing biomass amenable to biofuel production.
1.2.3 B
iodiESEl
from
c
onvEntional
o
ilSEEd
c
ropS
Biodiesel is produced from fats and oils (triglycerides) by transesterification with alcohol. Usually
methanol is used to produce fatty acid methyl esters (Fukuda et al. 2001) and glycerol as a byprod-
uct. Almost any source of naturally occurring triglycerides can be used, but the composition of the
various fatty acids in the fuel affects its properties, including viscosity and crystallization tempera-
ture. These properties are especially important at lower temperatures. Poor performance in cold
weather is currently a major limitation to acceptance of biodiesel. Thus, in addition to increased oil
content, modification of fatty acid components of oilseed crops is a primary goal to produce high-
quality biodiesel. Davis et al. (2008) observed that saturated long-chain fatty acid methyl esters
were primarily responsible for crystallization in peanut biodiesel at higher temperatures than those
observed for soybean or canola biodiesels. They suggested that decreasing the concentration of
long-chain fatty acids, thus increasing the proportion of short-chain fatty acids by processing or by
plant breeding, will be necessary to improve the properties of peanut biodiesel at low temperatures.
Similarly, Krahl et al. (2007) advocated the use of plant breeding to improve the fuel qualities of
rapeseed methyl ester (RME) biodiesel by selecting for short-chain (C
12
-C
16
) fatty acids, although
Friedt and Luhs (1998) suggested that such progress may be limited by traditional plant breeding
methods and advocated transgenic approaches for oil modification in rapeseed.
In addition to fatty acid chain length, the degree of unsaturation in fatty acids will also affect
biodiesel properties. Peanut and soybean cultivars with high oleic (18:1) to linoleic (18:2) acid ratios
have been developed through conventional breeding and marker-assisted approaches (Takagi and
Rahman 1996; Chu et al. 2009). In peanut, a 1-bp insertion mutation was found to cause a frame-
shift in the
ahFAD2B
gene, leading to a loss of function of the enzyme that catalyzes the produc-
tion of linoleic acid from oleic acid. A polymerase chain reaction (PCR)-based cleaved amplified
polymorphic sequences (CAPS) marker was developed to screen for the presence of the mutant
allele (Chu et al. 2009). Because oleic acid is monounsaturated, it has good properties for biodiesel,
including lower crystallization temperature than saturated fatty acids and better oxidative stability
than linoleic acid (Knothe 2009). It is possible that these cultivars could be used to produce superior
biodiesel, although Davis et al. (2009) did not observe any significant differences in fuel viscosity
between the high- and low-oleic peanut cultivars in their study. However, Tat et al. (2007) observed
a reduction in the emissions of oxides of nitrogen (NO
x
) from burning high-oleic soybean biodiesel
compared with conventional soy biodiesel.
1.2.4 J
atropha
—a p
oSSiBlE
d
EdicatEd
S
ourcE
of
B
iodiESEl
Jatropha (
Jatropha curcas
L.) is gaining importance as an emerging biofuel crop. Jatropha is a
relatively long-lived tropical shrub or tree that produces oil-rich, usually inedible, nuts. The oil can
be used to produce biodiesel as with edible oils. Jatropha is reported to grow on marginal lands not
suitable for many other crops, so it should compete less for prime farmland. The major disadvantage
