Environmental Engineering Reference
In-Depth Information
drier conditions (Bouton 2008). 'Alamo' and 'Kanlow' are two commonly planted lowland culti-
vars, whereas the upland cultivars include 'Cave-in-Rock,' 'Blackwell,' and 'Trailblazer,' among
many others (Sanderson et al. 1996). Ploidy in switchgrass is quite variable, ranging from diploid
to dodecaploid, with most of the lowland types being tetraploid and most of the uplands being
octaploid (Bouton 2008). One of the first studies utilizing molecular markers in switchgrass was
a diversity assessment of a representative sample of the germplasm. Because no species-specific
markers had been developed at the time, sorghum chloroplast DNA probes were used to identify
RFLPs. Of 80 probe/enzyme combinations, only one polymorphism was found, located within the
ribulose-1,5-bisphosphate carboxylase large subunit gene (
pLD 5
). This indicates that there is rela-
tively little genetic diversity within the switchgrass chloroplast genome, but the surprising result
was that this one marker clearly differentiated the two ecotypes (Hultquist et al. 1996). Around the
same time, Gunter et al. (1996) analyzed genetic diversity in switchgrass using RAPD markers.
Although many more polymorphisms were observed in this study, cluster analysis of the marker
data separated the accessions into upland and lowland groups, lending additional support to the
genetic distinction between the two ecotypes. Recently, EST sequences were used to develop SSR
markers for switchgrass (Tobias et al. 2006). Markers such as these will be useful tools for creating
a genetic map for this species.
Switchgrass is propagated by seed, although it is highly self-incompatible. Thus, switchgrass
cultivars are actually populations or synthetic varieties maintained by generations of random mat-
ing. The possibility of producing F
1
hybrid seed in switchgrass has been proposed. Martinez-Reyna
and Vogel (2008) created reciprocal F
1
hybrids between the cultivars 'Kanlow' and 'Summer' by
bagging a panicle of each parent together in the same bag, taking advantage of self-incompatibility.
Twelve such crosses were made, and the hybrid seed bulked to capture the variation within each
parent. These are contrasting ecotypes, but both are tetraploid, thus allowing crossing to take
place. Although the cultivars themselves are heterogeneous, significant high-parent heterosis was
observed for biomass yield and plant height in the F
1
generation when the plants were planted in
swards (Vogel and Mitchell 2008), but only moderate midparent heterosis for second season yield
was observed in individually spaced plants (Martinez-Reyna and Vogel 2008). When the hybrids
were advanced to the F
2
and F
3
generations (actually syn
2
and syn
3
generations), the observed
heterosis disappeared (Vogel and Mitchell 2008). These studies identified the tetraploid upland
and tetraploid lowland ecotypes as heterotic groups for switchgrass breeding. However, the hetero-
geneity within switchgrass accessions makes the establishment of seed production fields difficult
if the goal is to produce true F
1
seed. To produce true F
1
hybrids, clonal propagation of parent
lines would be required to establish seed production fields. Traditional vegetative propagation in
switchgrass is slow and cumbersome, although tissue culture techniques for rapid cloning have
been developed (Bouton 2008). An alternative is to produce semihybrids (Brummer 1999). In this
scheme, two parent populations from contrasting heterotic groups are planted together in isolation
and allowed to randomly mate. The resulting seed will be approximately 50% hybrid and 50%
nonhybrid.
Vegetatively propagated perennials offer the advantage of perpetuated hybrid vigor and genetic
uniformity, although initial planting is more labor-intensive than for seeded crops. Energycanes,
which are propagated from cane cuttings, were developed by sugarcane breeders with the USDA in
Florida and Louisiana. Although the main product of sugarcane is sucrose for food or fuel, energy-
canes were developed specifically for the purpose of producing energy and biofuels. Whereas sug-
arcanes are usually highly advanced backcrosses, most energycanes are F
1
or BC
1
hybrids between
cultivated sugarcane and closely related species, often
S. spontaneum
. Legendre and Burner (1995)
showed that these early-generation hybrids tended to be superior to sugarcane cultivars in biomass
yield and ratooning ability. They also have greater cold tolerance and can be overwintered in mild
temperate locations where sugarcane will not survive. In addition to sugarcanes grown for sucrose
production, Tew and Cobill (2008) define two types of energycanes. Type I energycanes were devel-
oped as dual-purpose plants, from which the sugars can be extracted for direct fermentation to
