Environmental Engineering Reference
In-Depth Information
Three cycles of phenotypic selection for high IVDMD resulted in a linear increase in IVDMD
of +1.7%/cycle and an associated linear decrease of -3.3%/cycle in lignin concentration (Casler
et al. 2002). Because IVDMD is not a plant trait per se, but defined only by an anaerobic interaction
between plant tissue and enzymes secreted by rumen microbes (Tilley and Terry 1963), it is
likely that selection acted largely upon lignin concentration and composition. Low-lignin, high-
digestibility genotypes tended to also have lower ratios of p -coumaric/ferulic acid than high-lignin,
low-digestibility genotypes (Sarath et al. 2008). Reductions in lignin concentration appear to have
resulted from reductions in cortical fibers and secondary wall thickenings in switchgrass stems
(Sarath et al. 2005).
Selection for increased IVDMD has also resulted in a significant decrease in winter survival
(Casler et al. 2002). Some families in the high IVDMD populations continue to have high winter
survival rates indicating that this apparent genetic correlation may be broken with selection,
allowing simultaneous improvements in IVDMD and winter survival (Vogel et al. 2002b). Genetic
correlations of forage yield and IVDMD indicate that it should be possible to improve both traits
simultaneously (Talbert et al. 1983). The breeding research on improving IVDMD and forage yield
demonstrate the need for multiyear evaluation of breeding nurseries in the environments in which
the plant materials will be used to ensure that selected plants are exposed to stresses present in
normal production environments (Casler et al. 2002; Vogel et al. 2002b).
22.7.4 S witchgraSS h yBridS
All current cultivars of switchgrass are either open-pollinated seed increases of prairie remnants
or synthetic populations of superior plants selected for agronomic performance. In both cases,
random intermating occurs among large numbers of plants and there is little opportunity to utilize
structural or genomic information that would lead to improved performance, such as hybrid vigor.
The partial genetic isolation between upland and lowland ecotypes of switchgrass is reminiscent of
complementary heterotic gene pools in maize ( Zea mays L. ssp. mays ). Maize breeders created these
heterotic gene pools by choosing parental lines simply based on their observed heterotic patterns
(Tracy and Chandler 2006). Many generations of selection reinforced and solidified these patterns.
Initial crosses of upland × lowland switchgrass have demonstrated an average of 19% midparent
heterosis for biomass yield of spaced plants, whereas upland × upland and lowland × lowland
hybrids of similar genetic origin showed no heterosis for biomass yield (Martinez-Reyna and Vogel
2008). When evaluated as sward plots, upland × lowland hybrids averaged 30-38% high-parent
heterosis (Vogel and Mitchell 2008). There appears to be some natural genetic complementation
between upland and lowland ecotypes that results in significant heterosis of these hybrids. These
results suggest the need for breeding and selection methods that incorporate efficient methods of
evaluating and selecting parents for combining ability. If these results follow the maize model,
selection for combining ability could strengthen what already appears to be significant hybrid vigor
between upland and lowland ecotypes.
Commercial production of F 1 hybrid switchgrass cultivars will require a mechanism to
propagate parental clones in the thousands or tens of thousands. Efficient and repeatable methods
for regenerating switchgrass plants from in vitro cultured cells and tissues have been developed
(Denchev and Conger 1994; Alexandrova et  al. 1996a, b) including a method for regenerating
switchgrass plants from cells in suspension culture (Dutta and Conger 1999). Thousands of plants
can be transplanted into alternating rows of two parental clones using existing transplanting
technologies for horticultural crops. In many cases, this equipment is unused for much of the
year and switchgrass hybrid seed production fields can be transplanted at times of the year when
vegetable-transplanting equipment is not in use. Use of two heterotic parental clones and physical
isolation from other switchgrass will ensure that nearly 100% of the seed harvested from the entire
seed production field will be hybrid seed. Because switchgrass is a long-lived perennial and seedling
transplants can be planted on spacings that eliminate or minimize interplant competition and the
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