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X. Sorghum Genotypes as Experimental Subjects
J . R. QUINBY
rieties of the genotypes Malmarrna3Ma, and malMa2Ma3Ma4.A linkage
between height and maturity was broken and the resulting eight genotypes
are all in the same genetic background and are recessive at three height
loci. When the eight genotypes are grown in long nights, they flower at
about the same time (Miller et al., 1968a); but in the short nights of temperate zones in the summer, they flower at different times as shown in
Table I. Subsequently, a second mutation at the third maturity locus was
found in a 3-dwarf MILO variety (Quinby and Karper, 1961) and named
RYER MILO. The new variety was identified for maturity as being
Malma2ma,RMa, and was given the designation of 4 4 M . 38M was then
obtained as a segregation product out of a cross between 44M and SM60.
The list of MILO maturity genotypes now includes the ten strains shown
in Table I.
LOCUS FOURFOR MATURITY
The only recessive known at locus 4 was identified in HEGARI. Because
all the MILO maturity genotypes are dominant at locus 4, 4-recessive maturity genotypes in pure MILO could not be obtained. But several useful
maturity genotypes using recesives from BLACKHULL KAFIR, MILO, and
have been produced. These genotypes are KMH-1
(Ma,ma,ma,ma, ), KMH-2 ( ma,mu,ma3ma,), MH-3 ( mulma2nza:,Rma4).
C. PAIRSOF VARIETIES
AT ONE MATURITY
Fourteen pairs of genotypes that differ at only one maturity locus exist
.among the MILO maturity genotypes shown in Table I. In addition, KALO
and EARLY KALO differ at locus 2; and HEGARI and EARLY HEGARI at lOCUS
3. The origins of KALO and EARLY KALO and of HEGARI and EARLY HEGARI
have been presented previously (Quinby, 1967). Maturity loci 1 and 2
have been assumed here to control the synthesis of auxin and loci 3 and
4, the synthesis of gibberellin. These pairs of genotypes might be used to
verify or refute these assumptions.
In an effort to make tropical germplasm readily available to plant breeders of sorghum in temperate zones, more than 100 tropical varieties have
now been converted to temperate zone adaptation. Most tropical varieties
are dominant at all four maturity loci and the conversion to temperate
GENETIC CONTROL OF FLOWERING AND GROWTH I N SORGHUM
adaptation can be accomplished by substituting a recessive maturity allele
for a dominant one. The process is to cross a short, temperate variety to
the desirable tropical variety, to select short and early plants from the
segregating population, and to backcross to the original tropical variety.
The backcrossing is continued for four or five backcrosses or until the temperate, short-statured strain looks like the original tropical variety, except
for being short, when grown in the winter in Puerto Rico or Jamaica.
The Texas Agricultural Experiment Station and the U.S. Department
of Agriculture have distributed 62 converted lines and have several hundred more in some stage of conversion. The Pioneer Hi-Bred Company
also has numerous converted varieties. When selections are mzide from the
segregating population of the last backcrosses, it is possible to select lines
that are either dominant or recessive at the first maturity locus. The two
lines will look alike in the tropics in the winter but will differ in time of
flowering in temperate zones by 20 or 30 days. If any physiologist is interested in this kind of material, it would be possible, with a little advance
notice, to obtain a number of temperate and tropical pairs of varieties.
There are now converted varieties from low elevations in Nigeria and
from extremely high elevations in Ethiopia, where lowland varieties will
grow but not shed pollen. There are also varieties that are grown in the
summer in India and others that are grown in the winter. These varieties
should be ideal subject for certain studies of temperature effects.
Three height genotypes exist in MILO, all of which are of the maturity
genotype malMa2MaJMa4,The first is recessive at dw,; the second, at dwl
and dw,; and the third at dwl, dwL, and dw,. In addition, a number of
pairs of isogenic strains exist. The first member of each pair is a 3-dwarf
of the genotype dw,Dw,dw,dw,. The second member of each pair arose
as a tall mutation at locus 3 and has the genotype dw1Dw2Dw3dw4.
Early HEGARI and HEGARI are both DwldwLDw3dw4for height but the
former is unstable for height while the later is stable and produces no tall
mutations. Early HEGARI is Ma,Ma,ma3ma, for maturity while HEGARI is
MalMa2Majrna,.There is a possibility that recessive maj is, in some way,
associated with the unstable condition at height locus dw, in EARLY
HEGARI. Even though this might be due to an influence within a linkage
group, the linkage can be broken because KARPER (1953) distributed
HI-HEGARI, a forage hybrid of HEGARI maturity, that was selected from a
cross between HEGARI and a tall-mutant strain from EARLY HEGARI. Because of the instability, presumably at dwr, 2-dwarf and 1-dwarf height
genotypes exist in EARLY HEGARI.
J. R. QUINBY
Summary and Discussion of Genetic Control of Growth in Sorghum
If the control of growth is as simple as suggested in this chapter, there
is reason to wonder why the genetic control has been unrecognized for
so long. One may wonder, also, why the identity of the flowering stimulus
has remained so elusive.
The inhibitory effect of high as well as low levels of auxin might explain
the failure to hasten floral initiation with many species of plants. using applications of auxin to short-day plants growing in long days. These failures
have caused physiologists to conclude that endogenous auxins do not play
a central role in the process that leads to floral initiation. The fact that
applications of gibberellin to many long-day plants growing in short days
hastens floral initiation could indicate that an excess of auxin exists in such
plants. In such a case, additional auxin would not be expected to hasten
floral initiation. Short-day plants such as Xanthium, will initiate floral buds
following exposure to one long night and must need only a little auxin to
allow floral initiation. Such plants under short-night treatment should not
contain too much auxin; nevertheless, they do not respond to applications
of auxin. Perhaps the auxin level that promotes floral initiation in such
plants is so low that applications of auxin result in auxin levels high enough
to inhibit rather than promote floral initiation. The inhibition of both high
and low levels of auxin could explain why the floral stimulus was not
recognized long ago.
For 50 years plant breeders have been taught that inbred lines are burdened with numerous cryptic, recessive, deleterious genes. Assuming that
such deleterious genes prevent normal growth, how could the control of
growth be simple genetically? The notion about the complexity of the genetic control of plant growth spawned the development of the disciplines
of population and quantitative genetics. The interest in population genetics
diverted attention away from the obvious fact that a few genes such as
the maturity genes of sorghum have profound effects on duration of growth
and plant size.
Sorghum has many advantages as an experimental species. In the first
place, it is self-pollinated and inbred lines are vigorous. Only a few varieties were introduced to the United States about a century ago, and the
two most important ones were tropical varieties. The varieties were too
late and too tall to satisfy farmers who promptly selected shorter and
earlier maturing types that suited them better. As a result, a number of
dwarf and early varieties in similar genetic backgrounds originated.
In the 1930’s it seemed desirable to make a shorter SOONER MILO and
the tall EARLY MILO was crossed to DWARF YELLOW MILO. When F, rows
of this cross were grown, it became apparent that a linkage between tall
GENETIC CONTROL OF FLOWERING AND GROWTH IN SORGHUM
height and early maturity existed and that the inheritance of duration of
growth was relatively simple. Ultimately, the inheritance of maturity in
MILO was determined and three genes were recognized (Quinby and
Karper, 1945) .
The different maturity genotypes appeared to be the same size if grown
in 14-hours nights under which treatment they flowered at about the same
time. It was assumed, therefore, that the maturity genes, in some way,
controlled synthesis of a floral hormone that had no influence except on
time of floral initiation. At long last, an effort was made to see whether
the maturity genes of MILO might influence growth rates when the maturity
genotypes were grown in short nights and the genotypes had different times
of floral initiation and durations of growth. Contrary to expectation, maturity genes were found to influence growth rates (Quinby, 1972a). This
disclosure led me to the conclusion that the floral stimulus was probably
a combination of common hormones, and prompted the thinking that led
to the hypotheses presented in the previous pages and summarized in the
The genetic control of flowering in sorghum appears to be genetically
simple because only four gene loci have been recognized. The continuous
variation in flowering is thought to result from allelic series at the four
loci and because of complementary action between gene loci.
The floral stimulus appears to consist of auxin and gibberellin, and an
interaction between the two hormones produces the stimulus that changes
a vegetative bud into a fruiting bud. Auxin is produced largely during darkness, and gibberellin during daylight. Temperate varieties, because of less
inhibition by phytochrome, produce more auxin than tropical varieties during daylight. Early-flowering temperate varieties produce more auxin and
more gibberellin during daylight than late ones.
The PT3"form of phytochrome appears to inhibit synthesis of auxin
during daylight, and the PGGO
form to inhibit the synthesis of gibberellin
during darkness. Both forms of phytochrome are present in plants during
the day. Alleles at loci 1 and 2 are assumed to cause differences in sensitivity to inhibition by P,,",and alleles at loci 3 and 4 to cause differences
in sensitivity to inhibition by P,,,.
Dominant alleles at the maturity loci cause sensitivity and recessive alleles less sensitivity to ,inhibition by phytochrome. As a result, recessives
at loci 1 and 2 allow the synthesis of some auxin during daylight to supplement that produced in darkness. Recessives at loci 3 and 4 allow the synthesis of more gibberellin during daylight. The maturity genes appear,
through differences in sensitivity to inhibition, to regulate levels of auxin
and gibberellin that, in turn, control time of floral initiation and growth.
High, as well as low, levels of auxin inhibit floral initiation. Low levels