Sugar Beet, Energy Beet, and Industrial Beet - Industrial Crops: Breeding for BioEnergy and Bioproducts - page 90

extensive during early season growth [ 54 ], and inferring gene functions and impor-
tant biochemical pathways (including signal perception and transduction) among
differentially expressed genes allowed better estimates of the magnitude of these
changes. For example, an unusual involvement of hydrogen peroxide in stimulating
germination appeared to activate stored lipid metabolism during germination, but
only under stress conditions [ 77 ], and in essence defined a component of seedling
vigor, which is an otherwise nebulous concept because it is difficult to measure
empirically.
For energy beets, breeding progress and approaches are still in their infancy, and
as with sugar beet variety development, private companies such as KWS (Einbeck,
Germany) and its US subsidiary Betaseed Inc. (Shakopee, MN) are taking the lead
in variety development. Interest in fodder beet as a source of germplasm for
creating energy beets was rekindled due to the high root yields of fodder beet
relative to sugar beet. However, most if not all of this yield boost is due to increased
water content and thus is not relevant for biomass energy production [ 78 ]. Thus,
well-adapted, high sucrose content sugar beets will likely be the best choice for
founder populations of energy beets. Breeding dedicated energy beets has some
advantages over breeding sugar beets. A major consideration for sugar beets is the
loss of sucrose to molasses, which is caused in part by anionic sucrose molecules in
solution charge balancing with cationic sodium and potassium ions as well as amino
acids. Breeding against these loss-to-molasses (e.g., melassinogenic) substances
can be problematic [ 79 ] but is a goal of most sugar beet breeding programs. The
level of impurities is not a known issue for energy beet, so there is more freedom in
the ability to breed for high dry matter varieties without selection against high
impurity levels, although mechanical properties of the beet root such as density,
elasticity, and slicing or crushing resistance will still be important [ 4 ]. Commercial
energy beet varieties are being marketed in Europe and are not suitable for sugar
processing due to excessive impurities but are instead being targeted for biogas
production. Dedicated energy and industrial beets could be bred with pigments
(either superficial epidermal pigments or wholly pigmented root tissue) to distin-
guish them from other crop types, as many fodder beet varieties have pigmented
root epidermis but colorless internal tissues.
High biomass, defined as the yield of dry matter, is desirable for an energy crop.
Relationships between water and dry matter, and dry matter and sucrose content,
have a genetic and physiological basis. The literature has addressed some of these,
but a clear picture is yet to emerge. An inverse correlation between sucrose content
and root yield may largely be explained by differences in water content
[ 80 ]. Between extreme high and low sucrose germplasm, consistent differences in
the proportion of dry matter were evident [ 81 ]. Water content decreases early
during development but appears to remain constant thereafter through the growing
season [ 48 , 82 , 83 ]. Sucrose is readily fermentable, so varieties with a high dry
matter content as well as a high proportion of sucrose to dry matter may be the best
germplasm resources for energy beet breeding. The easy fermentation of sucrose
would provide a direct boost to bioenergy yields and the low non-sucrose dry matter