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VI. Influence of Photoperiod, Temperature, and Leaf Area on Flowering

VI. Influence of Photoperiod, Temperature, and Leaf Area on Flowering

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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



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.


The 60M, 80M, 90M, and lOOM maturity genotypes initiate floral buds

later than four earlier flowering genotypes of MILO in the short nights of



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



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.




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.





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.

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VI. Influence of Photoperiod, Temperature, and Leaf Area on Flowering

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