Environmental Engineering Reference
In-Depth Information
5-year period from 2003 to 2008, and the U.K. program has continued to focus on producing
and selecting elite genotypes, using molecular techniques to characterize the National Willow
Collection and to develop markers for rapid selection of desirable traits (Hanley et al. 2002, 2007;
Trybush et al. 2008).
As improved willow germplasm from Canada and Sweden was tested in the New York State in
the 1990s, it became apparent that rust was having a significant impact on
S. eriocephala
and potato
leaf hopper was debilitating varieties with
S. viminalis
or
S. schwerinii
in their genetic background.
Thus, a willow breeding program was initiated by L. Abrahamson, R. Kopp, and L. Smart in New
York to develop varieties specifically selected for conditions in North America with improved yield
over the varieties selected in the Toronto program. Initial crosses were completed in 1998 at the
SUNY College of Environmental Science and Forestry using techniques for controlled pollinations
developed by R. Kopp (Kopp et al. 2001, 2002a, 2002b). A large and diverse breeding collection
of willows was assembled with major collection efforts in 1995, 2000, and 2001 across portions of
the Northeast and Midwest United States (Smart et al. 2005). Most individuals were collected from
natural stands of
S. purpurea
and
S. eriocephala
in New York, Pennsylvania, Ohio, and Wisconsin.
Molecular marker analyses have been used to demonstrate high genetic diversity and a relatively
high degree of heterozygosity among the natural populations of
S
.
purpurea
and
S. eriocephala
in New York (Lin et al. 2009). By 2007, the willow collection maintained at the Tully Genetics
Field Station contained over 700 accessions representing more than 20 species and species hybrids.
In addition to the varieties collected, contributions of bred and native collections from U.S. and
overseas collaborators and acquisitions of commercial and horticultural varieties from nurseries
were also obtained. Since 1998, more than 600 crosses were completed in this program, seven
varieties were patented, and elite varieties are being broadly tested in trials across North America
(Smart and Cameron 2008).
A major thrust of all of the breeding programs has been to identify heritable traits that are
indicative of long-term biomass productivity in field-grown plants and that can be measured in
juvenile plants. Cross-sectional stem area, number of stems per plant, and susceptibility to rust have
been found to be heritable in
S. eriocephala
(Cameron et al. 2008). Using a multivariate approach,
stem area, length of growing time, and insect damage have been identified as traits that can be
scored in field-grown juvenile plants and used to evaluate future relative performance (Tharakan
et al. 2001). These studies provide willow breeders with the tools they need to quickly and efficiently
screen thousands of seedlings for high biomass production.
A second major goal in breeding programs was to identify a parent or a combination of parents
that will produce superior offspring. Successfully predicting the performance of an individual parent
and/or the performance of a combination of specific parents to generate improved progeny has been
shown in a study of 34 full-sib F
1
S. eriocephala
families (Cameron et al. 2008). Furthermore,
parents with low similarity indices, as indexed by amplified fragment length polymorphism (AFLP)
fingerprinting, can produce offspring that display large amounts of phenotypic variability, thereby
increasing the probability of producing willow clones exhibiting desirable extreme phenotypes
(Kopp et al. 2002b). Crossing success rate in a breeding program is often dependent upon the
fertility of the parents. Many species of
Salix
are polyploid (Suda and Argus 1968; Thibault 1998),
and progeny from interspecific crosses sometime display intermediate levels of ploidy (Thibault
1998; MacAlpine et al. 2008). Flow cytometry has been used to estimate ploidy levels of parents
used in crosses and identify triploids and pentaploids that are sterile (MacAlpine et al. 2008). The
U.K. program has used this technique to successfully increase their crossing success rate to greater
than 58% (MacAlpine et al. 2008).
28.8 Future commercIalIzatIon oF WIlloW BIoenerGy croPs
As a novel crop that requires specialized equipment for planting and harvesting, has relatively high
establishment costs, requires a long-term commitment to recover that outlay, and is aimed at the
