Breeding Willow for Short Rotation Coppice Energy Cropping - Industrial Crops: Breeding for BioEnergy and Bioproducts - page 75

Quality Traits
When breeding willow for biomass production for bioenergy, there are two broad
categories with distinct quality specifications based upon the energy conversion
process. It has been difficult to identify useful variation in gross energy value of the
biomass generated by different breeding strategies, but thermochemical energy
conversion requires that inorganic composition of the biomass is minimized. In
particular the elements N, K, Na, Cl, and Si can cause problems as they influence
volatile behavior, ash melting point (thereby making the ash difficult to use), and
the corrosive potential of the gases within the boiler and heat exchangers. The
proportion of the harvested biomass that is bark can strongly influence the concen-
tration of these elements [ 15 ] and is related to the ideotype chosen for yield (above).
Biological energy conversion processes focus on traits that maximize the accessible
proportion of the gross energy (although the residues may be subjected to thermo-
chemical conversion). The carbohydrates within the biomass must be accessible to
the biological system employed, which often requires lower lignin contents and
greater cellulose [ 16 , 17 ].
Breeding Strategies and Integration of New Biotechnologies
Utilization of natural genetic variation by traditional breeding methods is very
relevant to willow as the naturally occurring genetic variation is vast and largely
unexploited. In addition, the F 1 generation is exploited by vegetative reproduction,
so fixing alleles by inbreeding is avoided. The phenotype of the progeny is
sufficient for rapid selection to be made. An additional advantage of willow is
that the majority will flower very early in their life, when grown from seed rarely in
year 1 but routinely in year 2, and when grown from a cutting more likely in year
1. This allows the breeding program to move quickly, either backcrossing or
making recurrent selections for yield as soon as good phenotype (and genotype)
data becomes available. This is uncommon amongst tree species, even the closely
related Populus requiring 7 plus years before flowering.
However, such traditional methods rapidly hit their limitations. Some pheno-
types are difficult to ascertain, whether it be due to financial costs or time con-
straints. Traits such as disease resistance may be most powerfully deployed by gene
stacking, whereby multiple resistance sources are combined in one cultivar for
maximum durability against an evolving pathogen population. This is particularly
valuable in a perennial crop planted for a 20-year life and physically very difficult
to treat with agrochemicals. Following recombination the phenotype recorded by
the breeder may be resistant to the pathogen due to only one of the sources.
Confirmation of the incorporation of multiple sources requires a molecular genetics
approach.