Environmental Engineering Reference
In-Depth Information
products, the diversion of food and feed to fuel has become controversial. Biofuel production from
food crops such as sugarcane, sunflower, soybean, sugarbeet, rapeseed, and maize has been blamed
for triggering a food crisis in recent years. Whether or not increased biofuel production has really
displaced a significant quantity of food is still highly debated. Nonetheless, this does raise a ques-
tion: which should be given a priority when it comes to making a choice between energy and food
resources? The answer to this paradox lies in utilization of crop residues and the many potential
dedicated nonfood biofuel crops, including perennial grasses, such as switchgrass (
Panicum virga-
tum
L.) and
Miscanthus
spp.; fast-growing trees including poplar (
Populus
spp.) and willow (
Salix
spp.); fiber crops such as kenaf (
Hibiscus cannabinus
L.); and oil-rich nonedible crops such as
Jatropha curcas
L. and
Millettia pinnata
(L.) Panigrahi. Production of biofuels from food crop
residues or from dedicated nonfood lignocellulosic crops utilizes the whole plant, thus capturing
more energy per unit of land area.
Liquid biofuel production from plant cell-wall material is almost a half-century-old practice
(Himmel and Bayer 2009), but its potential is only beginning to be realized. One of the greatest
obstacles to producing liquid fuels from these materials is that conversion of cellulosic matter
into fermentable sugars is much more difficult than conversion of starch. This “cellulosic” etha-
nol is known as a second-generation biofuel. This chapter addresses the progress made in the
development of crop cultivars for first- and second-generation biofuels, with a major emphasis
on perennial grasses for second-generation biofuels, and considers further improvements that
could be made in the future. This chapter will focus on traditional plant breeding approaches
and molecular tools available to the plant breeder. Transgenic approaches, which will certainly
be important in the future, will be considered in detail in Chapter 3 of Section 1 of this hand-
book. Finally, this chapter will conclude with a brief look toward the future possibilities of other
alternative biofuel sources.
1.2
the Past and Present—ethanol and BIodIesel:
the FIrst-GeneratIon BIoFuels
1.2.1 E
thanol
from
S
ugarcanE
Brazil has a long history of producing ethanol fuel from fermentation of sugar from sugarcane.
However, large-scale production did not begin until the late 1970s with the government-mandated
ProAlcohol program, which made it compulsory to blend ethanol with gasoline. Even then, success
of this program was mixed, and the popularity of ethanol among consumers in Brazil tended to vary
with the price of oil. Consumer acceptance of fuel ethanol increased dramatically with the introduc-
tion of flex-fuel vehicles, those able to operate on any blend of gasoline and ethanol (Matsuoka et al.
2009). In addition to liquid fuel, electricity is produced by burning the leftover sugarcane bagasse,
which increases the energy efficiency of the whole process. To date, the Brazilian sugarcane-based
ethanol industry is the most successful example of biofuel production in the world. Many sources
now consider Brazil to be “fuel independent.” Although technological advances in mechaniza-
tion and processing were critical for this industry to thrive, genetic improvement of the feedstock
undoubtedly also played a significant role.
Modern sugarcane is a complex hybrid derived primarily from
Saccharum officinarum
and
Saccharum
spontaneum
.
S. officinarum
is believed to contribute the sweet stalk trait, whereas
S. spontaneum
contributes genes for stress tolerance and disease resistance. After the initial crosses
were made,
S. officinarum
was used as the recurrent female parent in multiple backcrosses, result-
ing in several hybrids, which became the foundation stock for modern sugarcane cultivar develop-
ment (Jannoo et al. 1999; Lakshmanan et al. 2005). Because of the large size, high ploidy level, and
complexity of the sugarcane genome, molecular tools available to sugarcane breeders are some-
what lagging behind those for crops with simpler genomes such as rice.
S. officinarum
is octa-
ploid (2
n
= 80), and cultivated sugarcane is even more complex (2
n
= 100-130). Still, because of
