Sugar Beet, Energy Beet, and Industrial Beet - Industrial Crops: Breeding for BioEnergy and Bioproducts - page 87

resistance-breaking strains of the virus, and transgenic approaches are being tested
[ 43 ]. Also very important is resistance to the foliar Cercospora leaf spot fungus
(caused by Cercospora beticola ), which has the potential to defoliate the crop if
uncontrolled. Genetic resistance is controlled by at least two genes, to as many as
eight, and, along with sucrose content, is one of the few traits in beets with
demonstrated quantitative inheritance. Other diseases can be yield limiting in
certain areas, such as curly top (Geminiviridae, Curtovirus ) in the Western USA
and powdery mildew (caused by Erysiphe betae ) in the UK. Sugar beet cyst
nematode ( Heterodera schachtii ) is the major pest of beets worldwide and is
yield limiting. Major achievements for beet breeding relate to developing germ-
plasm resistant or tolerant to each of these diseases and pests [ 2 , 25 ]. However,
combining these into a common genetic background is still problematic, primarily
because their precise genetic control is generally not well defined and many of the
relevant genes have not yet been identified; thus marker-assisted technologies for
trait stacking are still in their infancy [ 44 ].
Target Traits and Current Breeding Goals
The sugar beet taproot consists of water (~75 %), soluble solids (~20 %; ~75 % as
sucrose), and insoluble solids (~5 %) [ 4 , 45 - 47 ]. Values range by genotype and
environment, and genotype x environment interactions for sucrose and marc
(non-sucrose dry matter) are rarely significant [ 48 , 49 ]. Sucrose biosynthesis occurs
as in other plants, although the mechanism of sucrose accumulation is uncertain
[ 50 - 52 ]. Sucrose and betalain pigments accumulate in root parenchyma vacuoles.
Sucrose, but not betalain, is concentrated within the innermost five of 12-15
concentric cortical rings around the point of maximum root girth. These features
begin developing within 3 weeks after germination and may be discriminated as
significantly different between varieties by 10 weeks of age and continue to
increase through the growing season [ 53 , 54 ]. Varieties result from repeatedly
selecting high sucrose segregants in heterogeneous breeding populations. Sucrose
percent is quantitatively controlled with high heritability. Schneider et al. [ 55 ]
detected five quantitative trait loci (QTLs) associated with sucrose content in
multilocation replicated field trials using a molecularly mapped population. Inter-
estingly, only two QTLs influenced root yield (mass), which is generally considered
a nonadditive trait.
Sucrose yield is determined by total harvest weight multiplied by their propor-
tion of sucrose, minus loss during storage and processing. Root sucrose content is
expressed as a percent of fresh weight primarily because of the ease of specific
gravity and refractometric measures and later polarimetry [ 41 ]. Nearly unanimous
consent exists for a strong negative correlation between sucrose content and root
yield [ 56 ]. Few factors in beets could simultaneously influence both yield (e.g.,
mass/area) and physiological (e.g., proportion of total mass) components, and of
these, water exerts the greatest effect on both yield and percent sucrose.