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XII. Implications to Plant Breeding
GENETIC CONTROL OF FLOWERING AND GROWTH I N SORGHUM
appears to be due to allelic series at the four maturity loci. The simple
genetic control over growth may explain the inability of sorghum breeders
in the United States to increase yield of varieties. Sorghum breeders
shortened duration of growth and stature and made sorghum more suitable
to be harvested mechanically, but did not increase yielding capacity until
cytoplasmic male-sterility was found and hybrid vigor could be used. The
evidence presented here emphasizes that a breeder cannot change maturity
genotype without changing hormone levels that, in turn, influence growth
rate and adaptation. The corollary of this is that a breeder will not change
yield without changing maturity genotype.
The problem of finding maximum hybrid vigor in the sorghum species
for any particular environment apparently depends on choosing strains to
cross at the inception of a breeding program that, between them, contain
the most favorable complimentary maturity alleles for that environment.
The hybrid vigor that occurs in sorghum hybrids at present must result
from differences in complementary action between recessive alleles or between dominant alleles. For instance, REDLAN and CAPROCK are the female
and male parents of a vigorous hybrid. They have similar but not identical
parentages (Quinby, 1967) and both are ma,Ma,Ma,Ma, in maturity
genotype. Because of their parentages, the recessive at the first locus in
REDLAN and CAPROCK are likely to be a little different because the two
KAFIR parents used in Oklahoma and in Texas to produce REDLAN and
CAPROCK differ by about 3 days in time of flowering. The dominants at
loci 2 and 3 in both REDLAN and CAPROCK must have come from the KAFIR
parents and might be identical but could be slightly different. The dominants at the fourth locus in REDLAN and CAPROCK probably came from
and MILO, respectively, and are probably different. But all the differences must be in complementary action between recessive alleles or between dominant alleles.
Hybrids made with female parents of the genotypes ma,mazma3ma4or
Malma,manma, crossed to male parents of the genotype ma,Ma,Ma,Ma,
have not yet been evaluated and it remains to be seen whether heterozygosity due to dominants and recessives is more effective in producing
hybrid vigor than heterozygosity due to two recessive or two dominant
alleles. But it appears that an opportunity to produce parents to make
higher yielding hybrids still exists because hybrids heterozygous at some
of the four maturity local have not yet been evaluated. In addition, numerous dominant alleles not previously available in temperate zones are now
available in the temperate varieties recently converted from tropical
Duration of growth is an important part of adaptation and one plant
breeding problem in sorghum is to identify parents that produce vigorous
J. R. QUINBY
hybrids in each of a number of different maturities. Because of the association of high yield with long duration of growth reported by Quinby and
Karper (1945) and Dalton (1967), farmers are inclined to grow hybrids
that are as late to flower as temperature conditions permit. Under irrigation
in temperate zones, the length of the favorable growing season sets the
limit on suitable durations of growth; but, on dryland, the amount of soil
moisture may set the limit. Double cropping makes short duration of
growth necessary in many areas. There is evidence in the data presented
by Quinby (1972a) to indicate that a hormone level that allows early floral
initiation and flowering inhibits vegetative growth. This association may
account for the difficulty of finding extreme hybrid vigor in hybrids of extremely early maturity.
It is not yet apparent to what extent local adaptation exists. RS610 is
adapted from the Gulf Coast of Texas to South Dakota, a span of 20 degrees in latitude, and is grown in Israel, South Africa, the Argentine, and
Australia. CSHl, one of the first hybrids put into production in India, is
adapted in the kharif season between latitudes 9 and 32 degrees (Rachie,
1970). A REDBINE X HEGARI-derivative hybrid that is Ma,ma, at the first
maturity locus is a late, dual-purpose hybrid in the United States but a
mid-season grain hybrid at lower latitude in Rhodesia and Venezuela. It
appears that vigorous hybrids have wide adaptation to differences in latitude or elevation. Nevertheless, different hybrids must be grown in North
Dakota and Texas and, in India, different varieties, if not hybrids, are
grown in the kharif and rabi seasons. Differences in resistance or susceptibility to diseases or insects is also important in determining adaptation.
The first two hybrids put into production in India were named CSHl
and CSH2. Both have CK60A as the female parent. IS84 is the male parent of CSHl and IS3691 the male parent of CSH2. IS84 is a yellow endosperm FETERITA derivative and is recessive at the first maturity locus.
IS3691 is a yellow endosperm HEGARI derivative and is dominant at the
first maturity locus. IS84, IS3691, and CK60A all originated in the breeding program at the Texas Agricultural Experiment Station at Lubbock,
Texas. CK60A is dominant at the second maturity locus, and CSH2 might
be considered to be a tropical hybrid.
CSHl might be considered to be a temperate hybrid because it is homozygous recessive at the first locus and might be suitable to grow in the
temperate zone in the United States except that it is a 2-dwarf and too
tall. Actually, CSHl is probably adapted from 9 to 40 degrees north latitude. A tropical grain hybrid of 2-dwarf height like CSH2 is suitable to
be used as a late-maturing forage hybrid in the temperate zones.
Experience has shown that the most suitable hybrid for the area of the
Coastal Plains in Texas is not the most suitable hybrid on the High Plains
GENETIC CONTROL OF FLOWERING AND GROWTH IN SORGHUM
of Texas largely because maturities of hybrids are different in the two areas
because of a two-month difference in planting date. In addition, the two
areas differ in elevation by as much as 1500 meters. It is apparent that
there is some local adaptation; but, at the same time, some hybrids are
suitable in both the tropical and temperate zones and from sea level to
as high as 1500 meters.
The Texas Agricultural Experiment Station and the U.S. Department
of Agriculture have been converting tropical varieties to temperate zone
adaptation and shortening them in stature in an effort to make tropical
germplasm available for use in the temperate zones. All these converted
tropicals might well be evaluated as parents of tropical hybrids as soon
as a female parent of the genotype Ma,ma,ma,ma, is available. Such hybrids could be produced in the temperate zone and only a testing program
in the tropics would be needed to recognize the superior hybrids. It seems
logical to think that a converted variety from a high elevation in Ethiopia
might be the male parent of a suitable hybrid for high elevations in Ethiopia or a converted variety from a lower elevation in Nigeria, a parent
for a suitable hybrid for Nigeria. After being recognized, such hybrids
could then be produced in the tropics in the suitable season, or in one
of the temperate zones.
At the time the program of converting tropical varieties to temperate
zone adaptation was begun, it seemed logical to convert varieties from
most of the seventy groups that were thought, at the time, to constitute
the species. The idea was to make a diversity of germplasm available in
the temperate zones even though some of the varieties appeared not to
be agronomically suitable. Now that the conversion of the first group of
varieties has been accomplished, it seems that the objective might well be
changed. It is apparent that the alleles at the maturity gene loci are
important in determining yield, and it seems logical now to convert the
best of the tropical varieties without regard to the botanical group to which
they belong. Since there are so many good tropical varieties in existence,
it might be well to observe the F, plants of each cross in the winter season
in the tropics. It might then be well to continue to convert only those tropical varieties that produce vigorous F, plants.
Doggett (1970) has presented an excellent review of the theories that
are the basis of theories of population improvement and recurrent selection
and has suggested how suitable sorghum populations might be established.
Gardner (1973) has concluded that increases in yield from population improvement in sorghum as high as 6 % per year are theoretically possible.
J. R . QUINBY
If the genetic control of plant growth is hormonal and is as genetically
simple as it appears to be, recurrent selection programs to increase yield
alone appear to be unnecessary because allelic series, rather than modifiers,
account for the continuous variation. Also, the use of composites to allow
population improvement should be reevaluated because the assumed deleterious linkages do not exist if the modifiers do not exist.
So-called “variation” has come to be much sought after in many plant
breeding programs, and x-irradiation or some other method of producing
cryptic mutations have been resorted to in some instances. But, if the coiltrol of growth is hormonal and is genetically simple, the variation sought
after would be ineffective because mutations of structural genes probably
would result only in abnormalities.
Finding extreme hybrid vigor in hexaploid, as compared to diploid or
tetraploid, species could be difficult. In a diploid species with differences
at 2 loci, 4 homozygous and 5 heterozygous genotypes would be possible.
In a tetraploid species with differences at 4 loci, 16 homozygous and 65
heterozygous genotypes would be possible. In a hexaploid species with
differences at 6 loci, 64 homozygous and 665 heterozygous genotypes
would be possible. The problem of finding the superior heterozygous genotype among 665 rather than 65 or 5 would be greater; and, of more importance, some of the 64 homozygous genotypes would be made up of combinations that would be vigorous, due to epistasis, without being
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GENETIC CONTROL OF FLOWERING A N D GROWTH I N SORGHUM
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ION ABSORPTION BY PLANT ROOTS'
T. K. Hodges
Department of Botany and Plant Pathology, Purdue University, Lafayette, Indiana
I. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . .
11. Overview of Nutrient Absorption by Roots
111. Energy-Dependent and Active Ion Transport
A. Terminology . . . . . .
B. Active Transport of S
IV. Kinetics and Selectivity of Ion Absorption . . .
V. Energetics of Ion Transport . . . . . . . . . . . . . . . .
VI. Proposed Model for Ion Absorption by Roots . . . . .
Sustained growth of higher plants requires light, carbon dioxide, water,
and mineral ions. One of the most fundamental problems of plant growth
is how inorganic ions enter root cells and then move through the root and
up to the shoot. It is well known that green leaves convert light energy
into chemical energy (NADPH and ATP) by photosynthesis and provide
the roots with an energy supply in the form of reduced carbon compounds.
Probably the most vital function of the root is the utilization of this energy
in procuring essential inorganic ions. Although roots have other important
functions, such as anchorage, providing a pathway for water and nutrient
transfer to the shoot, metabolism for their own growth, synthesis of growth
regulators, their unique ability to extract and concentrate inorganic ions
selectively ranks as one of their most important functions.
During the last-50 years much effort has been devoted to elucidating
nutrient absorption and transport in roots, and although progress has been
considerable, much still remains to be learned.
In this article I shall restrict the discussion to ion absorption by cells
(mainly root cells) and not consider long distance transport from cell to
'Paper No. 5090 of the Journal Series of the Purdue University Agricultural
Experiment Station, Lafayette, Indiana.
T. K. HODGES
cell or organ to organ. As it seems relevant to our understanding of ion
absorption by roots, I will also consider ion transport in other tissue, such
as storage roots, leaves, algae, and in organelles, such as mitochondria and
chloroplasts. My emphasis on ion absorption by cells, including the flux
of ions across both the plasma membrane and tonoplast, is based not only
on the primacy of these processes, but also on very exciting studies involving relatively new techniques and methods of analysis being used for investigating these phenomena.
Only selected works are considered here, and for additional coverage
the reader is referred to the recent books or reviews by Briggs et al.
(1961), Sutcliffe (1962), Jennings (1963), Hope (1971), Gauch (1972),
MacRobbie ( 1970, 1971 ), Epstein ( 1972a, 1973), Higinbotham ( 1973)
and to the Annual Reviews of Plant Physiology, where various aspects
of ion transport are considered annually.
Overview of Nutrient Absorption by Roots
Most studies concerning the mechanism( s ) of nutrient absorption have
been conducted with excised roots (Hoagland and Broyer, 1936; Epstein,
1973). Excised roots usually function for several hours, at least with regard to ion absorption, as if they had never been removed from the shoot
(Hoagland and Broyer, 1936; Jackson and Stief, 1965). Hoagland and
Broyer (1936) were the first to show the value of using excised roots that
were low in salt content for studying nutrient absorption. Such roots accumulate salts in a short time and one can easily measure the increase in ion
content either chemically or by using the radioisotope of a specific
When low-salt, excised roots are placed into a warm (room temperature) salt solution such as KCl, they absorb both the cation and anion
rapidly during the first 10-30 minutes, and this is followed by a gradually
decreasing rate of absorption, which continues for several hours. In such
experiments it is advisable to include Ca?+ in the experimental solution
since this ion is essential for maintaining the functional and structural integrity of plant membranes (Epstein, 1961; Marinos, 1962; Foote and
Hanson, 1964). The initial phase of absorption is readily reversible; i.e.,
an absorbed radioisotope washes out of the roots if they are transferred
to water or a salt solution that does not contain the radioisotope. The
phase of rapid absorption is little affected by temperature, anaerobiosis
or metabolic inhibitors, indicating that it is a physical rather than a metabolically linked absorption (Butler, 1953; Briggs and Robertson, 1957).
The portion or volume of the root into which ions enter rapidly and rever-