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VI. Influence of Photoperiod, Temperature, and Leaf Area on Flowering
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
J. R. QUINBY
differences in response to temperature and that no two varieties responded
to differences in temperature in the same way. Also, varieties produced
more leaves at either high or low temperatures than at intermediate
If the mechanism of control of hormone levels that has been assumed
is correct, gibberellin would be synthesized largely during daylight and
auxin largely during darkness. Presumably, temperatures during the day
would affect synthesis of gibberellin more than synthesis of auxin. Conversely, night temperatures would affect synthesis of auxin more than synthesis of gibberellin. That some varieties are hastened in floral initiation
by cool night temperatures while others are delayed could result from the
fact that some varieties, because of genotype, need more (or less) auxin
and some, more gibberellin for early floral initiation. If this is true, the
temperature effect could be the influence of temperature on the rate of
chemical reaction and nothing else. However, nothing is understood about
the nature of sensitivity to inhibition by phytochrome, and temperature
might have an influence on sensitivity.
Hendricks and Borthwick ( 1963) have reported that the reversion
of PT3,, to P,,, in darkness is hastened by increase of temperature. As
mentioned in Section VI, A, a difference in critical dark period of 24
minutes can cause a difference of a month in time of flowering. For this
reason, an increase in temperature could influence time of floral initiation
by lengthening the period of time during darkness when phytochrome is in
the P660 rather than in the P730form.
The fact that sorghum plants continued to initiate leaves at temperatures
too high or too low to allow early floral initiation seems to indicate that
homone levels that allow cell division are not as precise as the levels that
allow floral initiation (Quinby et al., 1973).
The data of Miller et al. (1968a), as presented by Quinby (1972c),
show that both tropical and temperate varieties initiate floral buds at
slightly different lengths of night in the different monthly plantings. It is
presumed that these differences are caused by the differences in temperature that occurred from month to month. The influence of temperature
apparently varies enough from variety to variety to cause parents that will
flower together in one planting to flower several days apart in another
planting even though the nights differ in length by only a few minutes during the periods of induction.
C . LEAFAREA
The 60M, 80M, 90M, and lOOM maturity genotypes initiate floral buds
later than four earlier flowering genotypes of MILO in the short nights of
GENETIC CONTROL OF FLOWERING AND GROWTH I N SORGHUM
the summer in Texas and have more leaves at time of floral initiation
(Quinby, 1972). Genotype 60M initiated a floral bud two days later than
the four earlier genotypes and had nine leaves fully exposed, rather than
eight, at the time. Leaf nine of 60M was almost twice as large as leaves
eight of the earlier flowering genotypes. The total exposed leaf area of
60M at time of floral initiation was almost twice that of the earlier genotypes. Genotype 80M had a leaf area more than four times greater than
that of SM100, SM90, SM60, and SM80; and 90M and lOOM had leaf
areas almost 15 times as great as these four genotypes at time of floral
initiation. The different leaf areas of the various maturity genotypes could
indicate that the leaves of the later-maturing genotypes, that are sensitive
to inhibition by phytochrome, produced less of the floral stimulus per unit
area; and thus needed larger leaf areas to synthesize sufficient amounts
of the floral stimulus to induce floral initiation.
Quinby (1967) considered sorghum to have a disadvantage as an experimental subject because of the 8 or 10 long nights needed to induce
floral initiation. This conclusion was based on the finding of Keulemans
(1959), who reported that the juvenile stage in sorghum is 3 weeks and
that 10 to 14 long nights were needed to induce floral initiation. Lane
(1963), working with four MILO maturity genotypes, found that 12 consecutive 14-hour nights were needed to induce floral initiation if the longnight treatment began when the plants were 7 days old. Caddel and Weibel
(1972) found that five long nights were sufficient to induce floral initiation
after plants of three sorghum cultivars were 15 days of age. It is realized
now that the longer periods of induction assumed by Keulemans and Lane
included several days when the small plants did not have leaf areas large
enough to synthesize appreciable floral stimulus.
Presumably, genotypes that are early flowering and relatively insensitive
to inhibition by phytochrome need smaller leaf areas to synthesize sufficient floral stimulus to induce floral initiation than more sensitive genotypes. If this is true, a genotype relatively insensitive to inhibition by phytochrome would begin synthesis of hormones earliei and at a more rapid
rate than more sensitive genotypes. The result would be differences in time
of floral initiation like those shown in Table I.
Plants of a homozygous variety growing in a row may begin to flower
over a period as long as 10 or 12 days (Quinby, 1967; Miller et al.,
1968b). These extreme differences in time of flowering of plants of the
same maturity genotype are probably caused by differences in leaf area
among plants. Plants that emerge a day early or are favorably located are
larger and intiate floral buds earlier than others. Plants that are disadvantaged are smaller and may lay down two or three more leaves before floral
initiation than the larger plants in the row. The assumption is that the
J. R. QUINBY
disadvantaged plants need more exposed leaves to have sufficient leaf area
to synthesize enough of the floral stimulus to allow initiation.
The literature, that has been reviewed by Salisbury (1963), indicates
that plants must reach a certain age or a certain size before their leaves
will be sensitive to the environment that promotes the production of the
floral stimulus. But, if maturity genes of sorghum control hormone levels,
it would not be necessary to assume a juvenile or “ripeness to flower”
stage because, in young plants, small leaf area rather than insensitivity to
inductive environments would inhibit floral initiation.
Influence of Maturity Genotype on Plant Growth and Adaptation
If the floral stimulus consists of auxin and gibberellin, and the maturity
genes control levels of the two hormones, it would be inconceivable that
the maturity genes would not affect rate of growth and development during
the vegetative period prior to floral initiation. Evans (1969) has reviewed
the literature on the multiple effects of the photoperodic stimulus and has
listed flower development, sex expression, .growth rate, cambial activity, leaf
shape, dormancy, senescence, and tuberization as being some of the many
plant responses to day-length control.
In sorghum, data have been interpreted to mean that a hormone level
that causes early floral initiation inhibits growth of the meristem and leaves
(Quinby, 1972). Likewise, a hormone level that delays floral initiation
promotes growth of the meristem and leaves. This difference in response
to hormone level between organs might be expected. Thimann (1937)
found that different organs of a plant have different optimun concentrations of auxin that promote growth.
Nitsch (1963) surmised that climatic conditions cause changes in the
balance of endogenous growth factors, and suggested that neither photosynthesis nor mineral nutrition is crucial in the control of the course of
plant development. He recognized that a regulatory system played a key
role and that the reception of the climatic stimulus could involve
Alleles at maturity gene loci control auxin and gibberellin levels and
the synthesis of the two hormones is influenced by both photoperiod and
temperature is discussed in Section VI. For that reason, adaptation, or
lack of it, appears to depend on maturity genotype.
GENETIC CONTROL OF FLOWERING AND GROWTH IN SORGHUM
Morphological Effects of Hybrid Vigor in Sorghum
If yield of grain and stover is the measure of performance, hybrid vigor
necessarily must show in differences in morphology between parents and
hybrids. Differences in morphology between the two might indicate what
kinds of genes are involved in hybrid vigor. An interpretation of the sorghum literature that might indicate where hybrid vigor is and is not influencing plant growth and development follows.
NUMBEROF SEEDS PER HEAD,AND
Data presented by Quinby (1970) show that the weight of the panicle
(head less grain) is greater in hybrids than in parents even though the
panicles of hybrids develop in less time. Liang (1967) found that panicles
of hybrids were larger (length x width) than those of the larger parent.
Patanothai and Atkins ( 1971) have presented graphs that show that the
weight of the fruiting body of hybrids, before kernel development, was
greater than that of parents. It appears that panicles of hybrids grow to
be larger than those of parents, and in less time.
Increased grain yield is one universally recognized manifestation of hybrid
vigor (Stephens and Quinby, 1952; Argikar and Chavan, 1957; Quinby
et al., 1958; Arnon and Blum, 1962; Quinby, 1963; Niehaus and Pickett,
1966; Kambal and Webster, 1966; Chiang and Smith, 1967; Beil and Atkins, 1967; Liang, 1967; Kirby and Atkins, 1968; Nagur and Murthy,
1970; Patanothai and Atkins, 1971). A greater number of seeds per plant
has been recognized as a most important component that contributes to
greater grain yield of hybrids (Argikar and Chavan, 1957; Arnon and
Blum, 1962; Quinby, 1963; Kambal and Webster, 1966; Beil and Atkins,
1967; Kirby and Atkins, 1968; Ali-Khan and Weibel, 1969; Blum, 1970);
but Patanothai and Atkins (1971 ) have presented data that show that hybrids do not always have more seeds per plant than one parent.
Quinby (1963) found that RS610, a widely grown hybrid, produced
82% more grain than the average of its parents but produced only 31%
more stover. Graphs presented by Patanothai and Atkins (1971) show
much greater differences between hybrids and parents in head weight than
in stover weight. These differences in grain and stover production, due
to hybrid vigor, are probably explained by the fact that growth is exponential and that the limit of stover production is determined in the first onethird of the life cycle and the limit of grain production in the second onethird of the life cycle.