Tải bản đầy đủ - 0trang
G . H. COONS, F. V. OWEN, A N D DEWEY STEWART
the high-yielding capacity and globose root shape of the garden beet.
He has succeeded in doing this, but sucrose percentages in the segregates
from the initial hybridizations are significantly below those of the sugar
beet. By backcrossing and selecting to retain the globose root shapea character that would facilitate mechanical harvesting-the sucrose
percentage may be enhanced. The rapid increase in machine harvesting, which is now a practice on approximately 90 per cent of the total
acreage of sugar beets in this country, has emphasized the economic
importance of a root shape that will make easier the pulling of the
root from the ground.
2. Wild Species of Beta
a. Beta maritima L. The wild relatives of the sugar beet represent
a great and essentially untouched gene bank. Mention has been made
oi Munerati’s (1932) obtaining factors for leaf spot resistance by crossing sugar beets with the biotypes of Beta maritima that occur along
the mouth of the Po River in Italy. There is no question that factors for
Cercospora leaf spot resistance were obtained, but the breeding job involved was both long and tedious.
Certain collections of B. maritima made in 1925, usually from a
single plant or a plant colony, when tested by artificial inoculations in
the greenhouse and in field tests in Colorado, showed high resistance to
Cercospora leaf spot. Certain of these collections, when tested by Coons
et al. (1932) in New Mexico, were highly resistant to curly top. In a
few cases, a collection showed resistance to both diseases. Hybridizations with sugar beets were made and the segregating generations were
selected for both leaf spot resistance and curly top resistance. The outlook of obtaining resistant strains in this way was promising but not
more so than from the selections made from the sugar beet itself. Since
breeding work with the sugar beet did not present the problems of
ridding the progenies of multicrowns and rootiness, the emphasis on
wild hybrids gradually dwindled.
I n 1951, Stewart revived the investigations with B. maritima, first
carefulIy screening a wide selection of types to find individuals with
high leaf spot resistance. Already, plants whose natural leaf spot resistance is as great as that of the most resistant inbred types have been
found. If the genes in the B. maritima plants prove complementary, the
way is open for increase of leaf spot resistance beyond what has as yet
been found in sugar beets.
b. Other Species of Beta. The other wild species of sugar beet may
afford a germ plasm resource that conceivably may decisively affect
sugar beet breeding. Within the genus Beta, the various authors recog-
SUGAR BEET IMPROVEMENT
nize from 12 to 14 species. Coons (1954) has listed Beta vulgaris L.,
maritima L., B. pat& Ait., B. macrocarpa Guss., and B. atriplicifolia
Rouy as belonging to the Vulgares group. All of these intercross readily
and they are undoubtedly closely related. A second group, designated as
Patellares, comprises three species, B . patellaris Moq., B. procumbens
Chr. Sm., and B. webbiana Moq. These have vinelike growth habit, are
perennial, have monogerm, nutlike seed balls, and do not develop a
fleshy storage root. A third group is the Corollinae, consisting of B.
trigyna Wald. et Kitt., B. lomatogona Fisch. et Mey., B. macrorhiza
Stev., and B. foliosa Hausskn. These plants occur chiefly in Asia Minor,
but B. trigyna has been found in Hungary and in the Crimean Peninsula. They are all perennials and are characterized by flowers in which
the perianth is more or less corolloid. They have enlarged, woody storage roots which permit these species to withstand extreme drought. A
fourth group, Nanae, contains only the dwarf, alpine species B. nana
Boiss. et Held. Certain of these species are of interest because they exhibit characters sought at present for sugar beet improvement. Beta
lomatogona and B. nana and all the species of the Patellares group are
monogerm types. B. macrorhiza is characterized by having a very large
seed ball. B. foliosa apparently has capacity for withstanding both cold
The 2 n chromosome number of the sugar beet is 18, and this number holds for nearly all other species of Beta. Notable exceptions exist in
B. trigyna and B. patellaris, which have 36 as the 2 n chromosome number. It is reported that B. lomatogona exists as both an 18- and a 36chromosome form. The hexaploid form of B. t r i g p a is known also from
Chief attention is now focused on B. patellaris, B. procumbens, and
B. webbiana because these species are apparently immune to Cercospora
beticola and were once thought by Coons (1953a) to be immune to curly
top, since inoculations in greenhouse and field exposures in New
Mexico failed to bring about obvious signs of curly top. However,
Murphy and Giddings (1954) found that B. patellaris became infected
with curly top during extreme exposures in Idaho in 1953. The three
varieties are reported as being unfavorable food plants for the sugar
beet nematode; at least cysts are not found on the roots, according to
The desirability of hybridizing sugar beets with these species has
long been recognized, but the hybrids which are rather easily obtained
have not been viable, usually dying when the young plants are only a
week or two old. Stewart (1950) was able to bring a sugar beet x B.
procumbens to flower and the F, plant set seed when pollinated with
G . H. COONS, F. V. OWEN, AND DEWEY STEWART
pollen from the sugar beet. Unfortunately, the plants of the backcross
were as unthrifty as the F, generation.
Recently, Gaskill (1954) obtained viable hybrids by crossing chard
and B. procumbens. The plants have come to flower. Whether the backcross to beet is viable has not as yet been determined. He has been able
to obtain evidence that, with respect to infestation with the sugar beet
nematode, the hybrid partakes of the character of B. procumbens.
Previously, the failure to obtain viable hybrids from matings of the
sugar beet and species in the section Patellares has been due to the
necrosis of the roots of the F, seedlings, the top growth apparently being
normal. Coe (1954) has developed a grafting technique, whereby the
sugar beet serves as a foster root for the unthrifty F, seedling. This
grafting technique has made it possible to grow many F, plants to the
flowering stage, but the sterility of these species hybrids has impeded
progress in the development of sugar beet types carrying the genes for
disease resistance and for nematode immunity found in B. procumbens,
B. patelloris, and B. webbiana.
VII. THE FUTURE
Inbreeding of beets has not been a common practice in European
laboratories and, aside from the work of Tracy and the years of effort
by Deming, few breeders in the United States have made a sustained
effort with sugar beets at all comparable to what has been done with
other cross-pollinated plants, notably Zea mays. Clearly, many recessive factors are not revealed in a crop largely increased by mass selection methods. There is now increased interest in further exploration of
the genic resources of the sugar beet itself by the production of great
numbers of inbreds. A major project of the Breeders’ Forum is the production and test of such inbred material as a gene resource.
All of these efforts can only result in a greatly augmented array of
breeding material, the building stones to fashion new and better plants
in the future. We may therefore forecast very great steps in sugar
beet improvement. The obstacle to the easy production of abundant
hybrids, presented by plants whose flowers are perfect, is minimized by
the discoveries of cytoplasmic male-sterility and Mendelian male-sterility. The surface has just been scratched in utilizing these powerful
tools both in a practical way and in genetic research. There are
also great possibilities in utilizing self-fertility and self-sterility-approaches that have been opened by the researches of Owen (1942) and
Helen Savitsky (1952a) , but which as yet have scarcely been explored.
Abegg (1940a) gave a list of characters in sugar beet that had been investigated. To these he assigned symbols. The usefulness of simple Men-
SUGAR BEET IMPROVEMENT
delian factors such as monogermness is being demonstrated, and one can
only speculate on possibilities when genetics research throws light on
other characters in the sugar beet and demonstrates their inheritance.
Polyploidy has at present hardly been utilized with sugar beets as a
genetic tool, but it may be the key opening the way for certain wide
crosses between the species of Beta. The results from European work
that are beginning to come in concerning the productiveness of triploids,
certainly demand thoroughgoing exploration with American varieties.
As indicated, the research in the United States has simply shown that
polyploidy itself is not an automatic way whereby production may be
increased. The finding of highly productive polyploids and their use,
either as tetraploids or as triploids, have not thoroughly covered the
possibilities in this field of research.
The control of certain serious diseases by resistant varieties, although affording a fair measure of assurance against crop failures, is
certainly not a closed matter, because losses are still heavy. We may
forecast that the improved sugar beet in the next two decades will incite
wonder as to why present-day varieties once were prized. As said, control of virus yellows and sugar beet nematode is still to be achieved, but
evidence given by Coons (1952) and Rietberg (1954) indicates that disease resistance breeding may be effective against virus yellows. There
are indications also that breeding research may be useful against the
sugar beet nematode (Rietberg, 1954).
Once certain morphological and disease resistance goals are gained,
or even some half-way point is reached, there remain the great problems concerned in improving the sugar beet as a living machine. There
is a job of making it a better, more efficient sugar producer; reducing its
complement of harmful nitrogen; removing the melassigenic elements;
in short, as physiological research explains the metabolism of the sugar
beet, the breeder must be ready to build on these findings. The sugar
beet needs to be improved in storagability, both with respect to resistance to decay and oxidation rate. These fields of research as investigated by Gaskill (1952b), Stout and Smith (1950), and Nelson and
Oldemeyer (1952) already are showing great promise. I n another
approach to sugar beet improvement, the results given by Wood et al.
(1950) indicate that tolerance to cold exposures may be increased, the
seedlings which have survived subfreezing temperatures giving progenies with greater cold tolerance than the parental material.
The jobs yet to be done serve to make us humble in appraising the
improvements that have been registered. The advances that have come
since the White Silesian beet was picked out by Achard give encouragement in facing the problems of the future.