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XIII. Physiology of Flowering in Sorghum

XIII. Physiology of Flowering in Sorghum

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classification ( 1963) the Milo maturity genotypes would be quantitative

short-day plants because they will flower in continuous light but are

hastened in flowering by short days.

Salisbury (1963) credits Spector ( 1956) with classifying sudangrass,

a grass-sorghum, as a quantitative short-day plant at high temperature

and a day-neutral plant at low temperature. Also, Spector (1956) has

classified one unnamed sorghum variety as a quantitative long-day plant

with no temperature effect. There is reason to question the classification

of any sorghum variety as a Iong-day plant.

Lane (1963), as discussed in Section IV, A, made use of sorghum to

determine what part phytochrome played in inducing floral initiation.

His conclusions were that the flowering times of maturity genotypes of

Milo did not seem to result from differences in total phytochrome, as

determined phytometrically on seedlings, or in the rate that the active

form converts in darkness to the inactive form. It is now unclear whether

measurements such as those reported by Lane truly reflect pigment

conversion. However, Lane (personal communication) found that low

temperature delayed both dark conversion of seedling phytochrome and

the flowering of mature plants. It follows that the effect of temperature

could be easily explained or correlated with the simplified phytochrome

model. Using a saturating irradiance of far-red light (to convert Pfr to

Pr) at the end of the photoperiods, or after an intervening period of

darkness, Lane found that the later a variety is in flowering, the more

obligatory is a dark period following the photoconversion of phytochrome to its inactive form. Thus, genetic control over flowering in Milo

appears not to involve phytochrome in any way but to control some step

in the synthesis of the floral stimulus in darkness which comes after

phytochrome has “dark converted to its inactive form. It appears, therefore, that the necessary conversion Pfr to Pr effectively shortens the dark

period that a plant can use and that the length of the period of conversion is influenced by temperature.

Sorghum is not a favorable laboratory species because 9 induction

periods were needed to induce floral initiation (Lane, personal communication). Also, no one has grown normal sorghum plants in a growth

chamber. However, single gene differences that exist in Milo genotypes

should arouse the interest of physiologists.


Discussion and Summary

Sorghum is a tropical species that can be grown in temperate zones

because mutations that allow early floral initiation have occurred. The

time of floral initiation is controlled by four gene loci. Tropical varieties



are dominant at locus 1 and usually at the other three loci also. Temperate zone varieties are recessive at locus 1; or, if dominant at locus 1,

are recessive at either locus 2 or 4.

The expression of the four genes is influenced by environment, particularly by photoperiod and temperature. Sorghum is a short-day species

and many varieties, even temperate zone varieties, are hastened in

maturity in the tropics or under short-day treatment in temperate zones.

The influence of temperature is not as easy to demonstrate as the influence of photoperiod, but varieties grown at different locations in Texas

with similar latitudes but different elevations are either hastened or

delayed in maturity by the lower night temperatures at the higher

elevations. The faster or slower floral initiation depends on the maturity

genotype of the variety being grown. In the field, the interaction between

temperature and photoperiod always exists and it is not easy to recognize the separate effects of temperature and photoperiod. However, some

varieties with high critical photoperiods were observed to be much later

when grown in the summer in Mississippi when temperatures are high

than when grown in Florida in the winter when night temperatures are

much lower.

The number of leaves on a sorghum plant depends on the time of

floral initiation. If floral initiation is delayed, more leaves are initiated.

For this reason, early-flowering plants always have few leaves and lateflowering plants, many leaves. Leaves are initiated in the meristems of

growing sorghum plants in 3 to 6 days, the difference in rate of initiation

being a varietal characteristic. Sorghum seeds usually have 5, 6, or 7

embryonic leaves.

Dominant and recessive alleles at the four gene loci have important

interactions that influence time of floral initiation. When locus 1is dominant, dominance at the other three loci causes lateness. When locus 1 is

recessive, dominance at locus 2 or both loci 2 and 3 causes earliness.

Heterozygosity at locus 1 has an unexpected interaction. Late floral

initiation occurs when locus 1 is heterozygous and locus 2 is recessive.

If locus 2 is dominant, heterozygosity at locus 1 does not delay floral

initiation. Heterozygosity at loci 2, 3, and 4 has no apparent effect on

time of floral initiation.

Four independently inherited maturity gene loci have been recognized

in sorghum; the search for additional loci has been fruitless. Varieties

can be identified for dominance or recessiveness at the four gene loci,

and the identity of 28 varieties is known. Some of the varieties originated

in the United States, and their parentages are known. For this reason, it

is possible to recognize the results of some of the plant breeding work

that has been done. Shortening stature and changing maturity and adap-



tation are worthy accomplishments, but it is obvious that plant breeders

were recombining maturity and height genes and little else.

Allelic series exist at all the maturity gene loci, and the series consist

of both dominant and recessive alleles. Four gene loci and different

alleles at those loci produce a range in flowering at Plainview, Texas,

from 44 to 90 days. The allelic series at each locus must be long, and it is

thought that four gene loci and allelic series at those loci could account

for the continuous variation exhibited by the sorghum species.

It was obvious when the first three maturity gene loci were recognized more than 20 years ago that photoperiod influenced the expression

of the genes. For this reason, it was reasonable to think of the maturity

genes as “photoperiod” genes. It was equally obvious that temperature

was influencing time of floral initiation in some way, and several fruitless

years were spent in looking for “temperature” genes. No “temperature”

genes could be found, and it finally became apparent that temperature

operated through the known maturity genes in some way. Because genotypes that are identical as far as dominants and recessives at the four

gene loci are concerned differ in time of floral initiation and in response

to photoperiod, it seems reasonable to assume that different alleles differ

in sensitivity to temperature. If this is true, it is also logical to assume

that the maturity gene loci control response to photoperiod and that

alleles that differ in response to temperature influence response to photoperiod. Thus, time of floral initiation is controlled by interaction of

dominants and recessives at the four gene loci and by what alleles exist

at those loci. Also, there is at least a little insight into how photoperiod

and temperature interact to influence time of floral initiation.

There is a positive correlation between duration of growth and plant

size. SM90 and 9QM genotypes of Milo differ only at locus 1, SM90

being recessive ma, and 90M being dominant Ma,. When the two

genotypes were grown at Chillicothe, Texas, in 1944, dominance at locus

1 increased duration to flowering by 53 days, leaf number by 92%,height

by 77%,and more than doubled total dry weight of the plant.

Heterosis in plants has not been satisfactorily explained genetically.

The information in the sorghum literature, although scanty, leads to the

conclusion that hormones as well as nutrition are involved in heterosis.

Parents of grain hybrids are almost invariably recessive at locus 1

and, because there should be no dominance, flowering should occur at

some time between that of the parents. However, hybrids usually flower

earlier than either parent, and this earliness is considered to be a manifestation of heterosis. A rule of thumb to use in calculating the flowering

time of a sorghum hybrid is to subtract 3 days from the average flowering

time of the two parents.



What physiological reactions lead to floral initiation in plants is

unknown at present, but the maturity genes appear to be a part of some

regulatory process. In the only work done on sorghum to study the

flowering process, genetic control over flowering in Milo appeared not

to involve phytochrome in any way but to control the synthesis of the

floral stimulus in darkness after phytochrome had been converted to its

inactive form. Sorghum is not a favorable laboratory species because 9

induction periods are needed to induce floral initiation. However, single

gene differences that exist in Milo genotypes should arouse the interest

of physiologists at work on the flowering process.


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Pinus radiata

M. Raupach

Division of Soils. Commonweolth Scientific and Induslriol Research Organization.

Glen Osmond. South Australia

I. Introduction .

. . . . . . .

I1. The Importance of the Species .

. . .

I11. Characteristics of Growth and Climatic Tolerance

A. Description .

. . . . . .

B. Seasonal Growth .

. . . . .

C . Climatic Limitations for Forest Use .


. . .

1V. Soil Factors Restricting Growth .

A . Limited Volume of Exploitable Soil .


B. Unsatisfactory Water Regime .

. .

C Poor Nutrient Status .

. . . .

D. Biological Factors

. . . . .

V. Assessment of Limiting Factors .

. . .

A. Foliar Analysis .

. . . . .

B. Pot Experiments .

. . . . .

C . Field Experiments

. . . . .

D. Soil Analysis .

. . . . . .

E . Nutrient Balance .

. . . . .

VI . Effective Addition of Fertilizers .

. . .

VII . Field Practices .

. . . . . .

A . General .

. . . . . . .

B . The Ash-Bed Effect .

. . . .

. . . .

C . Thinning and Pruning .

VIII Conclusion .

. . . . . . .

References .

. . . . . . .








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The fact that native forest species are inadequate or unsuitable for

timber production in many countries has led to the extensive establishment of exotic softwood forest plantations Trials with a number of species have usually resulted in the choice of two or three of them for

plantation establishment on a wide scale. One of the most accepted

species is Pinus rudiata ( D. Don) . In areas with suitable climate and

with soils that are not too infertile. this tree has proved to be a rapid





producer of timber. And it is because of this high rate of growth that it

has now become established almost as a monoculture over wide tracts

of land in a number of countries, mainly in the southern Hemisphere.

It is timely to examine the requirements of the species as far as

present knowledge allows because a number of very real problems have

arisen now that the most suitable soils have been planted up and

attention is turned to the improvement of less suitable ones. A decline

observed in the productivity of aging stands and second and third

plantation crops is causing concern, especially as it often occurs where

previous production was high. Soil conditions, fertilizer requirements,

and methods for the detection of incipient nutritional deficiencies will be

examined in this paper. It should be emphasized that, unlike agricultural

crops, a monospecific managed forest plantation sets up a rather

specialized system of biological activity and nutritional recycling and

that the balance of this system can sometimes be easily upset.

Recent reviews by Tamm (1964), RaIston (1964), GesseI (1962),

Stoeckeler and Arneman (1960) and earlier studies referred to in these

accounts have considered forest species generally. But as Ingestad

(1960) has pointed out with reference to P. sylvestris, variations between

species within conifers alone are so large that generalizations cannot be

applied with certainty. As a result, many details have to be worked out

separately for each species. As an example of this, P . pinaster often grows

on sites too poor for P. radiata.

Reference should be made to two recent bibliographies on P. radiata

(Pert, 1963; Scott, 1960) which have assembled much of the older

literature. Of these only Scott is annotated, and the scope of his treatment

is rather different from that of this review.


The Importance of the Species

Pinus radiata (D. Don) (syn. P . insignis Douglas) is commonly

known as Monterey pine and belongs to a group of hard pines having

generally three but sometimes two needles in each fascicle and retaining

their cones in a ripe condition for a number of years. Seed is shed freely

from these cones during this time, and regeneration of the forest is tided

over periods of adverse conditions by this means (Scott, 19600).

The species is native to a total area of 30,000 acres of a narrow

coastal strip in southern California, to two islands in the Santa Barbara

group, and to the Mexican island of Guadalupe, which is some 500

miles farther south (McDonald, 1957). In the past its distribution was

far more extensive than today, and it existed under greater extremes of

temperature. Although the tree is of very limited economic importance in

its native habitat, its use as an exotic elsewhere has prompted studies on

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XIII. Physiology of Flowering in Sorghum

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