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V. Genetics and Physiology of Cell Elongation
GENETIC CONTROL OF FLOWERING AND GROWTH IN SORGHUM
plants probably results from inhibition to cell elongation due to an excess
of auxin and too little gibberellin. During the period from floral initiation
until emergence from the boot, the developing panicle is protected from
sunlight and the phytochrome in the panicle should be in the Pfjo0form.
If this is true, the developing panicle, in the absence of Pi,,, should synthesize auxin during the day as well as during the night, the amount depending on the height genotype. However, developing panicles of all genotypes should synthesize little or no gibberellin because of inhibition by
P,,,. The upper, expanding leaves, however, should synthesize both auxin
The height genes are assumcd to control levels of auxin and gibberellin,
and it is reasonable to assume that inhibition by PTs0and P,,, is the
mechanism involved. The hormone levels that are assumed to result from
dominant and recessive alleles at the four height loci are shown in Table
11. Alleles at height gene loci 1 and 2 are assumed to control levels of
auxin and alleles at loci 3 and 4 to control levels of gibberellin. Four dominant genes for height result in a low level of auxin and a high level of
gibberellin. For this reason, even though both maturity and height genes
control hormone levels, different functions must be assigned to the height
genes at loci 3 and 4 as compared to the maturity genes at loci 3 and
4. Apparently, dominants Mu, and Ma, result in a low level of gibberellin
and dominants Dw3 and Dw, in a high level of gibberellin.
Schertz et al. (1971) concluded that the dw, height locus in sorghum
may regulate the expressed level of peroxidase activity. There is no agreement about the function of peroxidase in plants at present but peroxidase
is generally thought to be involved in interactions with gibberellin or auxin.
How peroxidase may be involved in the control of plant growth is not
Several brachytic dwarfing genes exist in Zea mays L. and applications
of gibberellin in microgram amounts to plants of five of them result in
plants of tall height (Phinney, 1961). The probable explanation of this
response to gibberellin application in corn is that short plants contain too
much auxin for normal cell elongation unless some gibberellin is added.
This is the same response recognized in the early floral initiation in
sorghum that results from the presence of recessive
and greater synthesis of gibberellin. Presumably, any sorghum genotype that is high in
auxin content or is low in gibberellin content would grow taller if gibberellin were applied at the proper time and at appropriate concentrations.
C. HEIGHTGENESAND LEAFSIZE
Schertz (1973) has compared leaves of a doubled haploid of 4-dwarf
SA403 with leaves of a 3-dwarf obtained from a tall mutation of the same
J. R. QUINBY
Assumed Hormone Level and Height Genotype
of Certain Sorghum Cultivars”
Homozygous height genotype
and hormone level
Zero-dwarf or tall
TALLWHITE SOONER MILO
a a G g
a a g G
a A G G
A a G G
a a g g
a A G g
A a G g
a A g G
A a g G
A A G G
COMBINEK A F I R - ~ O
“A” indicates high, and “a” low, auxin level. “G” indicates high,
and “g” low, gibberellin level.
strain. His results, and those of others cited in his paper, indicate that
the taller plants had longer leaf blades and longer leaf sheaths than the
shorter plants. The gene involved in all these studies was dw,.
GENETIC CONTROL OF FLOWERING AND GROWTH I N SORGHUM
Quinby (1961) compared leaves of 1- and 2-dwarf MILO plants and
leaves of 2- and 3-dwarf MILO plants. In these cases, the comparisons were
between the effects of recessive dw, and dominant Dwl, and between recessive dw, and dominant D w p . Leaf sizes of the different height genotypes
were not significantly different. In view of the fact that the dw, is involved
in the synthesis of gibberellin and dw, and dw, with the synthesis of auxin,
it might be expected that recessive dw, might have a different effect on
leaf size than recessive dw, and dw,. Recessive dw, results in less gibberellin than dominant Dw,, but recessives dw, and dw, result in more auxin
than dominants Dwl and Dw2.
D. HEIGHTAND PEDUNCLE
Plants of 4-dwarf and 3-dwarf height appear to have peduncles as long
as taller genotypes. The published information on the subject of peduncle
length is inconsistent as reported by Schertz (1973), but it is apparent
that the shorter genotypes sometimes have longer peduncles than taller
genotypes and that there is not a consistent relationship between tall plants
and long peduncles.
The long peduncle lengths of plants with short internodes could result
from the fact that peduncle elongation occurs after the panicle is no longer
synthesizing large amounts of auxin, but could, also, result from the production of gibberellin in the still-enlarging panicle during daylight after
the panicle had emerged from the boot. Furthermore, any auxin that might
be produced in the upper leaves would not inhibit cell elongation in the
peduncle because auxin is reported to move in plants in polar fashion
(Leopold, 1964). Conversely, gibberellin produced in the leaves could
promote cell elongation in the peduncle because the movement of gibberellin in the plant may be either up or down.
E. HEIGHTAND MATURITY
The published information on the relationship of plant height and
panicle length is inconsistent (Schertz, 1973). Panicle weight (head weight
less grain weight) of SM60 ( malma,ma3Ma,) was found to be greater than
that of SM80 (ma,ma,Ma,Ma,) (Quinby, 1972aj, and the inference is that
a higher level of gibberellin in SM60 caused greater growth of the panicle
in SM60. The heads of SMlOO (rna,Ma,Ma,Ma,) and SM80
(malma~Mu,Ma,)are noticeably more compact than heads of SM60
(malma,ma3Ma,) and SM90 (ma,Ma,ma,Ma,). The inference is that
lower levels of gibberellin in SMlOO and SM80 cause less elongation of
the rachis and rachilla in these two genotypes.
J. R. QUINBY
F. TYPESOF INTERNODE
Three types of internode distribution, associated with early, medium,
and late maturity, were recognized in sorghum in India (Ayyangar et al.,
1938), and were called ever-increasing, unimodal, and bimodal. These
three types of internode distribution occur in sorghum from June plantings
at Chillicothe, Texas (Quinby and Karper, 1945), and Ryer MILO
(Malma2ma2RMa4) is ever-increasing, 60M (Mama2ma,Ma,) is unimodal, and lOOM MILO (Ma,Ma,Ma,Ma,) is bimodal. In the ever-increasing type, the internodes are longer from the ground upward. In the unimodal type, internode eight, near the base of the culm, is shorter than
the internodes above and below it. In the bimodal type, an internode near
the base and another near the top of the culm are shorter than internodes
above and below them. A graph presented by Ayyanger et al. (1938)
shows that ever-increasing plants have long peduncles.
The physiological reason for the short internodes in unimodal and bimodal plants is not obvious. However, early flowering plants that must
have high gibberellin content in comparison to auxin content to allow early
floral initiation are ever-increasing in internode distribution. The inference
is that ever-increasing internode distribution is associated with high gibberellin content and that the short internodes that appear in unimodal and
bimodal plants are, in some way, associated with temporary high auxin
content. The short internode near the base elongates about the time a floral
bud is initiated and the short internode near the top, at the time the panicle
is large and still growing. A little later, the panicle begins to emerge from
the boot, or upper leaf sheath, and the elongation of the upper internodes
and peduncle is probably being influenced by the gibberellin being
produced by the panicle that has emerged into the sunlight.
Influence of Photoperiod, Temperature, and l e a f Area on Flowering
Miller et al. (1968a) grew 22 tropical and temperate varieties from the
world collection in monthly plantings throughout the year in Puerto Rico.
Part of their data was presented in different form by Quinby ( 1 9 7 2 ~ ) .
These data are the basis of the discussion that follows.
PI276769 is a late-maturing variety from Ethiopia that is planted there
in the spring and is harvested in January. This variety did not flower in
Puerto Rico until November when planted in any month from January
to August. Apparently, floral initiation took place in October when the
nights were about 12.2 hours long. PI291 227, a rabi (winter-growing)
GENETIC CONTROL OF FLOWERING AND GROWTH I N SORGHUM
variety from India, flowered in October in Puerto Rico when planted in
any month from March to August, Apparently, floral initiation took place
when the nights were about 11.8 hours long. TEXASBLACKHULL KAFIR
is a temperate variety that was grown extensively in the southern Great
Plains fifty years ago. This variety was not delayed greatly in flowering
in any planting, even when the nights were shorter than 11 hours.
Presumably, plants of PI276769 are so sensitive to inhibition by P7,,
that they produce little or no auxin during the day, and nights as long
as 12.2 hours are needed for plants to synthesize sufficient auxin to allow
floral initiation. PI291227 apparently has a critical dark period about 0.4
hour shorter than PI276769. Presumably, PI291 227 is slightly less sensitive to inhibition by Piy,,, and plants of this variety produced enough
auxin during the day, to supplement that produced during the night, to
substitute for the amount of auxin that could be produced in 24 minutes
of divkness during the period of floral induction. Presumably, plants of the
temperate variety TEXAS BLACKHULL KAFIR are so insensitive to inhibition
by P;,,, that they produce enough auxin during the day, to supplement
that produced during the night, to allow floral initiation regardless of the
length of the night.
Miller et al. (1968a) divided the varieties they grew into five classes
depending on the day lengths required to delay floral initiation. The data
show that tropical varieties of different maturities have different critical
dark periods and that tropical varieties need longer nights to allow floral
initiation than temperate varieties. Temperate varieties, many of which will
flower in continuous light, have no critical dark periods but differ in the
length of night that will delay floral initiation. All this information leads
to the conclusion that the photoperiodic effect is apparent only if the nights
are too short to allow the synthesis of sufficient auxin to allow early floral
Some information on the influence of temperature on the physiology
of flowering has accumulated, but how temperature affects time of floral
initiation is still not obvious. Lower temperatures have been observed to
hasten flowering in some varieties but to delay it in others (Quinby, 1967).
Hesketh et al. (1969) and Downes (1972) found that different temperatures caused a change in leaf numbers in a phytotron. Caddel and Weibel
(1971) found that the effect of night temperature on photoperiodic response depended upon the day temperature as well as on the variety and
that day temperatures were more important in determining length of
panicle development than in the time needed to reach floral initiation.
Quinby et al. (1973) found that alleles at all four maturity loci caused