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VIII. Morphological Effects of Hybrid Vigor in Sorghum

VIII. Morphological Effects of Hybrid Vigor in Sorghum

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Leaf blades of hybrids are larger than comparable leaf blades of parents,

from the first embryonic leaf upward until maximum leaf size is reached

in hybrids (Quinby, 1970). Above that point, leaf blades of hybrids are

sometimes smaller than those of parents and, because some hybrids have

fewer leaves than parents, the largest leaf blade on a hybrid may be smaller

than the largest leaf blade of a parent. The smaller upper leaves of hybrids

has the following logical explanation.

Leaves of grasses are larger from the ground upward until a maximum

is reached; above which the leaf blades become progressively smaller. Borrill (1959) attributed the smaller upper leaves of grasses to the onset of

floral initiation in the shoot apex and to less cell extension in the upper

leaves. Data presented by Quinby (1970) can be interpreted to mean that

the inhibiting influence of the developing terminal inflorescence on size

of the meristem from which the upper leaves originate is greater in hybrids

than in patients. The largest leaf of the hybrids, in his study, was the fourth

or fifth from the top and, of the parents, the third from the top. The upper

leaves of hybrids may not only be smaller but the inhibiting influence appears earlier in hybrids.

Argikar and Chavan (1957), Quinby (1963), and Liang (1967) measured either the third or fourth leaf blade from the top of the plant and

found that these leaf blades of hybrids were larger than those of parents.

The data mentioned in the previous paragraph show that comparisons between either third or fourth leaves from the top of parents and hybrids

are not exactly valid, but it is apparent that leaves of hybrids are larger

than comparable leaves of parents except for the two or three upper leaves

whose growth is inhibited by the developing inflorescence.

Because of the smaller upper leaves of hybrids, some hybrids have

smaller leaf blade areas during the period of grain development than parents (Quinby, 1970). Stickler and Pauli (1961a) reported that all leaves

of a sorghum plant that were alive during the period of seed development

contributed to grain yield. Nevertheless, in a second paper, Stickler and

Pauli (1961b) presented data that showed that leaf sheaths contributed

to grain yield if the leaf blades had been removed but contributed nothing

if the leaf blades were intact. In view of the fact that sorghum has photosynthetic areas, other than the leaves, such as the developing seeds, glumes,

peduncle, etc., and because sorghum plants make little use of leaf sheaths

in photosynthesis, it appears that the photosynthetic areas of either parent

or hybrid sorghum plants are adequate. Apparently, greater leaf blade area

of hybrids is not a major cause of greater grain yield.





Quinby and Karper (1954) reported that some sorghum varieties are

taller or shorter than expected in view of their height genotypes and attributed the tallness or shortness to a modifying complex. It is realized

now that allelic series at the four known height gene loci could also account

for the unexpected tallness or shortness. Most presented grown hybrids

are 3-dwarfs and are recessive at the same three loci, but there is considerable difference in height among them just as there is among parents.

Grain hybrids are usually taller than the average of their parents

(Quinby e t a ! . , 1958; Arnon and Blum, 1962; Quinby, 1963; Kambal and

Webster, 1966; Liang, 1967; Kirby and Atkins, 1968; Patanothai and Atkins, 1971) as are forage hybrids (Chavda and Drolsom, 1970); and tall

height has been reported to be a manifestation of hybrid vigor. Plant height

is made up of the length of the internodes that make up the stem, the

length of the peduncle, and the length of the head. Kambal and Webster

(1966) reported the length of each component, and all were longer in

hybrids. The length of the stem depends on the amount of cell elongation

as well as on number of internodes, and hybrid vigor appears to increase

cell elongation.


The amount of tillering in sorghum, as in other species, is determined

by the effectiveness of apical dominance. The inhibiting influence of apical

dominance on axillary bud development is known to be hormonal. The

amount of tillering varies in both parents and hybrids and much or little

tillering is not unique to either. Karper and Quinby (1937), Quinby and

Karper (1946), and Quinby (1963) concluded that greater tillering is a

manifestation of hybrid vigor in sorghum. However, Kambal and Webster

(1966) and Beil and Atkins (1967) found little difference in amount of

tillering between parents and hybrids, and Chiang and Smith (1967) found

that, on the average, hybrids had fewer tillers than parents. The data do

not show conclusively that greater tillering is a manifestation of hybrid





Argikar and Chavan (1957) found no consistent difference and Niehaus

and Pickett (1966), and Liang (1967), and Quinby and Liang (1969)

found little difference between parents and hybrids in leaf number. Apparently, hybrid vigor does not increase leaf number.




A sorghum plant continues to produce leaves in the meristem until a

floral bud is initiated, and the production of an additional leaf delays

flowering by about 3 days (Sieglinger, 1936). Quinby (1967), without

showing leaf numbers, presented data that show that hybrids flowered, on

the average, 3 or 4 days earlier than the average of parents. Liang (1967),

and Quinby and Liang (1969) presented data that show that, on the averaage, parents and hybrids had the same leaf number but that hybrids

flowered earlier. than parents. The shorter period from planting to flowering

in hybrids was shown by Quinby and Liang (1969) to be made up of

a 1.2-day shorter period from planting to floral initiation and a 2.6-day

shorter period of panicle development.

Niehaus and Pickett (1966) and Chiang and Smith (1967) have reported late flowering rather than early flowering to be a heterotic effect

in sorghum but their diallel studies included parents that produced late

hybrids due to complementary action of maturity genes.

Early flowering of hybrids is usually not caused by lower leaf number;

but lower leaf number would cause earliness if it occurred. Early flowering

of hybrids is caused by more rapid development of the meristem prior

to floral initiation and by more rapid development of the panicle. A difference in rate of growth in favor of hybrids is probably involved in early

flowering because the meristems of hybrids become larger than those of

parents in a shorter time; and earliness could result without hybrids having

fewer leaves than parents.




Measurements of root growth of a sorghum hybrid and its two parents

were made by McClure and Harvey (1962) using radiophosphorus. A

difference in root growth in favor of the hybrid did not exist until after

heading. At the time of heading, the roots of parents occupied more soil

volume than those of the hybrid. A week later at time of flowering and

at maturity, the hybrid root system was more extensive than that of parents

because only the hybrid continued to expand its root system after


The lesser root growth of hybrids during early stages of growth when

above ground parts were growing rapidly might be expected because auxin

concentrations that promote stem growth are known to suppress root

growth (Thimann, 1937). The reason for the cessation of root growth after

heading in parents and the continuation of root growth after heading in

hybrids is not obvious and is not apparent in the literature.









Seeds produced by sorghum hybrids are frequently heavier than the

average seed weight of parents (Niehaus and Pickett, 1966; Kambal and

Webster, 1966; Chiang and Smith, 1967) but Beil and Atkins (1967) and

Kirby and Atkins (1968) found seeds of parents of hybrids not to differ

significantly in seed weight. Liang (1967) found that seeds of some hybrids

were not significantly heavier than those of the heavier parents but seed

of hybrids were significantly above mid-parental values. Whenever mean

values of parents and hybrids were presented (Argikar and Chavan, 1957;

Quinby, 1963; Liang, 1967), the data show that seeds of hybrids were

usually between those of parents in weight but that seeds of hybrids were

sometimes lighter and sometimes heavier than those of parents. It is apparent that larger seed size is not a consistent difference between parents

and hybrids.

Quinby et al. (1958) found that hybrids produced seeds whose average

test weight per bushel was 0.6 pound above the average of the heavier

parents. Kambal and Webster (1966) found that, on the average, hybrids

produced grain that was slightly higher in test weight per bushel than parents. However, 10 hybrids were below the lowest parent in test weight,

and 53 hybrids were below the mid-parental value in test weight. Apparently, hybrids do not consistently have seeds of greater test weight than

parents. The influence of hybrid vigor on test weight per bushel is in doubt.



The length of the grain filling period of parents and hybrids was determined by Quinby (1972b), and he cited some earlier reports. He found

that final kernel weights of male parents were above those of hybrids but

that differences in age of kernels when parents and hybrids reached maximum kernel weight were not significantly different. Hybrid vigor appeared

not to lengthen the period of starch accumulation in the endosperm.



Arnon and Blum (1962) determined the protein content of the seeds

of the variety MARTIN and several hybrids and, although their results were

not consistent, reported that seeds of hybrids usually had a lower protein

content than the seeds of MARTIN. Kambal and Webster (1966) and Liang

(1967) found that seeds of hybrids were lower in protein content than

the average of parents. Collins and Pickett (1972), in a diallel study involving four female, eight male, and the 48 hybrids made from them, found



that only four hybrids contained a higher percentage of protein than parents and no hybrid was superior to either parent in percentage of lysine.

Apparently, hybrids usually have seeds that contain slightly less protein,

on a percentage basis, than parents and it is obvious that hybrid vigor

does not increase percent protein in sorghum seed.







Hybrids produce more grain and stover than parents. Hybrid plants become taller and hybrid panicles larger than those of parents in less time.

Early flowering of hybrids is caused by a shorter period of growth prior

to floral initiation and a shorter period of panicle development rather than

by a difference in leaf number between parents and hybrids. Root growth

prior to heading is greater in parents than in hybrids but the above ground

growth of hybrids is greater. The fact that hybrid vigor shows more in

grain than in stover yield is attributed to the fact that hybrids grow faster

than parents and because growth is exponential. Processes that do not involve rate of cell division, like the deposition of starch or the accumulation

of protein in the endosperm, show little effect due to hybrid vigor. Hybrid

vigor appeared not to lengthen the period of starch accumulation in the


All the information on plant morphology of parents and hybrids seems

to indicate that any character that is affected by rate of growth will be

different in parents and hybrids. It seems logical to assume that hormone

levels in parents and hybrids must be different.


Genetic Control of Hybrid Vigor in Sorghum

Gregor Mendel (1 865) observed hybrid vigor in an F, hybrid of tall

and dwarf peas and called the phenomenon luxuriance. Fifty years ago,

geneticists working with Zea mays L. could not reconcile the stimulation

of hybrid vigor with genetic principles and, for that reason, assumed that

heterozygosity could not account for hybrid vigor. More important to this

discussion, they also assumed that the reduced growth of inbreds was due

to small deficiencies in metabolism and that recessive alleles at numerous

gene loci were involved.

The possibility that plant hormones, rather than metabolic deficiencies,

might be responsible for differences in plant growth has largely been ignored by plant breeders; and this is true in spite of the fact that varieties

of self-pollinated crops such as sorghum are not adversely affected by inbreeding. This lack of interest in the hormonal control of plant growth



has been due to the fact that no mechanism to control hormone levels

had been recognized. If such a mechanism has now been recognized, there

is reason to revise some of the theories that are the basis of practices in

plant breeding. The theory that varieties of self-pollinated crops such as

sorghum are burdened with numerous, small metabolic deficiencies is now


Hageman et al. (1967) has postulated that hybrids are superior to parents in having better balanced metabolic systems. They suggested that the

fundamental metabolic systems involved in growth and yield needed to

be recognized and, particularly, the optimum levels of activity of each enzyme. The idea that the genetic control over growth is hormonal is probably not in conflict with their concept and they have recognized that “a

single enzyme, hormone, vitamin, or growth factor could be solely responsible for the enhanced growth of a hybrid.” Nevertheless, they have stated

that “the complexities of metabolism preclude a single factor from being

the universal underlying cause of hybrid vigor.”





Data in Table I11 (Quinby and Karper, 1946) show that plants heterozygous at locus 1 when locus 2 is homozygous recessive were later to

flower than either homozygous genotype. But when locus 2 was homozygous dominant, the genotype heterozygous at locus 1 was earlier to

flower than the later homozygous genotype. The heterozygous genotype

Malmalma2ma, was much later to flower than either homozygous genotype

and produced a much greater yield of heads. However, the difference in

duration of growth between genotypes was great and the influence of duration of growth and of hybrid vigor cannot be separated. However, the

heterozygous genotype Ma,maIMa,Ma, was only 3 days earlier to flower

than the homozygous genotype MalMalMa,Ma, but produced a yield of

heads 60% greater due largely to more heads per plant. Heterozygous

genotypes for maturity were different from homozygous genotypes in both

maturity and yield of heads and heterozygosity was important largely because of interaction among genes at different loci rather than between alleles within a heterozygous locus.

Differences, due to gene interaction, between pairs of hybrids that differ

only in being homozygous or heterozygous at one locus are shown in Table

IV. These data were presented previously (Quinby and Karper, 1948),

but, at that time, the maturity genotypes of most of the parents were not

known. Now that the maturity genotypes are known, it is possible to draw

conclusions regarding the genetic cause of hybrid vigor that could not be

drawn in 1948.

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VIII. Morphological Effects of Hybrid Vigor in Sorghum

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