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VII. A Concept for the Future

VII. A Concept for the Future

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81



A BIOCHEMICAL APPROACH TO CORN BREEDING



do not support the concept of either ( a ) more enzyme, or ( b ) more

efficient systems. This suggested that the enzymes and enzyme systems

are more efficiently organized as a whole in the hybrid.

FENDERS



FENDERS



L,,J

10

LEFT

DOORS



I



10

RIGHT

DOORS



t



ID

LEFT

FENDERS



10

RIGHT

FENDERS



L

m

J

14

LEFT

DOORS



6

RIGHT



10

RIGHT

FENDERS



10

LEFT

FENDERS



L n J

10

LEFT

DOORS



10

RIGHT



LOUT L,,YS

t



v



Production / hr.

4 Cars

ASSUME EACH LINE HAS THE SAME AMOUNT OF STEEL. AND THAT EQUAL

AMOUNTSOFSTEELARE USEDFOR DOORS AND FENDERS.



FIG. 11. Diagrammatic analogy to illustrate the need for balance of systems

and integrated control of these systems. (Note: The word “fender” is defined as

that part of the body that covers the wheels.)



In this connection it may be useful to visualize how closely a plant

and its metabolic system resemble a modern, completely automated

industrial factory (Fig. 11). Comparable parts or systems could be

considered as:

Factory



Plant



Automated machines tf Enzymes or enzyme systems

Competition for doors and fenders tf Competition for energy and metabolites

Assembly lines tf Metabolic pathways

Computerized control c-f Environmental and hormonal control



In a factory, the size of the building may, but need not, indicate the

rate or total of production. The rate and total production depends on

the efficiency of the individual machines, and on the properly coordinated rates of operation of all machines in all assembly lines. Flow lines

1 and 2 (left to right) of Fig. 11 could, given time, reconvert components

and produce 5 cars, but not within the prescribed time.

As with the factory, the initial size of a plant or its meristem cannot

be the basic cause of hybrid vigor. Rather, this cause must lie in the

efficient and timely operation of the individual enzymes and in the

proper balance and integration of the enzymatically catalyzed metabolic

systems. To be sure, a larger plant, tissue, or cell may confer certain

advantages to the metabolic system through providing greater amounts

of needed “raw products.” Without greater efficiency of metabolism,



82



R. H. HAGEMAN, E. R. LENG, AND J. W. DUDLEY



however, such enhanced supply could be useless or actually detrimental

to metabolism as a whole.

From current knowledge of metabolism and the simple analogy of

the “factory” operation a possible biochemical explanation of heterosis

can be developed. Admittedly, present evidence is not sufficient to fully

clarify this complex situation. However, the consistent finding that

highly heterotic ( in growth and yield) hybrids are usually intermediate

between their parental inbreds in activity levels of important metabolic

enzymes is significant. It is clear that major metabolic processes as a

whole tend to be limited in rate by the least efficient (or slowest) reaction involved. Obviously, then, every highly homozygous inbred parent

must have some important reactions which are severely limited in rate;

i.e., in these lines, there must be serious gene-controlled shortcomings

in function of major enzyme systems. While it seems most logical to

assume that these shortcomings are deficiencies in function, it is also

possible that they represent higher-than-optimum levels of enzyme

activity, leading to repressive levels or detrimental competition for

certain metabolites.

In any event, it is clear that the highly heterozygous hybrid possesses

a more favorable genetic constitution for overall enzymatic efficiency

than does either of its parents. This suggests the following possible

situations:

1. Gene action in the control of major metabolic enzymes is usually

not complete dominance-i.e., the heterozygote is usually intermediate

between the homozygous parents in enzyme activity.

2. The “intermediate” level of enzyme activity found in hybrids is

adequate for most major metabolic processes, and may actually be more

effective for overall metabolism than a higher level.

3. One “dose” of a gene-a single favorable allele-is sufficient for

a satisfactory level of function of the enzyme it controls.

4. Allele “A” may specify one enzyme, allele “a” a different enzyme;

thus, the hybrid could possess both enzymes or enzyme forms,

The most likely explanation of the heterosis phenomenon rests on the

fact that the hybrid between two inbred parents is likely to have a

better-balanced metabolic system. The inbred lines, on the other hand,

having had their genetic complements rapidly and effectively fixed

by inbreeding, tend to have unbalanced systems, with some enzymes

controlled at high levels, some at medium levels, and some genetically

limited to low or ineffective levels of activity. The fact of linkage,

resulting from the presence of only 10 pairs of chromosomes adds a

complicating factor to the inbreeding situation since “favorable” and

“unfavorable” genes may be tightly linked,



A BIOCHEMICAL APPROACH TO CORN BREEDING



83



Utilization of these ideas in a planned breeding program is still

almost completely in the realm of theory. Yet, if the assumptions set

forth above are valid, they point the way toward the development of

highly efficient selection techniques.

The first and most important step is to work out the fundamental

metabolic systems involved in growth and yield. Of particular importance is the determination of the major enzymatic controls involved in

these processes, and, particularly, the optimum levels of activity of each

such enzyme in combinations with specified levels of the other enzymes.

When this information is available, breeding material can be screened

for activity levels of the various enzymes. Simple inheritance studies can

then be conducted, optimum combinations of levels of activity can be

identified and assembled through proper genetic combination. The

resulting hybrid material can be expected to give superior performance.

The results from studies of nitrate reductase provide an example of the

possibilities of such a breeding approach. Enough is already known

about genetic diversity and hereditary patterns of NR activity to create

hybrids with specified NR activity levels. If other major metabolic

systems can be as accurately evaluated as NR activity, it should be much

easier to screen breeding material efficiently and to produce desirable

combinations than is now possible where yield testing in the field is the

major selection tool.

ACKNOWLEDGMENT



One of us (RHH) wishes to express his appreciation to Professor John Grafius

for his encouraging comments and actions and to Professor 0. T. Bonnett for his

interest and counsel. It is gratefully acknowledged that without the assistance and

contributions of Leonard Beevers, Donna Flesher, Almut (Gitter) Jones, J. W.

Kniprneyer, J. F. Zieserl, Jr., David M. Peterson, and L. E. Schrader this manuscript could not have been written. This work has been supported by National

Science Foundation Grants Nos. 1995, 4407, G 9862, GB 263, and GB 3750.

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PRESERVATION OF SEED STOCKS

Edwin James

National Seed Storage Lobaratory, United States Department of Agriculture,

Fort Collins, Colorado



I. Introduction .

. . . . . . . .

11. Theories Regarding Seed Deterioration .

.

.

A. Depletion of Food Reserves .

. . . .

B. Changes in Protein Structure .

. . . .

C. Inactivation of Enzymes and Respiration .

.

D. Development of Fat Acidity .

. . . .

E. Mutagenic Effects .

. . . . . .

111. Methods of Preserving Seeds

. . . . .

A. Effect of Climate .

. . . . . .

B. Control of Both Temperature and Humidity .

C. Control of Humidity Only .

. . . .

D. Storage in Moistureproof Containers

.

.

IV. The National Seed Storage Laboratory .

. .

A. Historical .

. . . . . . . .

B. Operation .

. . . . . . . .

C. Kinds of Seeds Stored .

. . . . .

D. Contributions to Scientists .

. . . .

References

. . . . . . . . .

I.



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Introduction



From the time of his nomadic existence man has had the problem of

maintaining harvested seeds for planting. Planters living in the temperate

zone have fared better, in this regard, than those of the humid tropical

areas of the world, Even now all peoples continue to depend on seed

stocks for survival. Primitive peoples found that storage requirements for

seeds to be planted were different from the requirements of seeds used

for food.

To have seed for the next crop, primitive man suspended his unthreshed crops from roofs to dry, later storing the dried seed in straw

bags, baskets, earthenware jars, or pits. Some of these methods are still

used in some of the undeveloped countries, Why seeds maintained

higher viability under certain conditions was unknown until the beginning of plant science in the eighteenth century.

Although some factors related to seed longevity became clear through

87



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EDWIN JAMES



systematic research, the problem of seed deterioration remains to be

solved. Longevity of seed of many species has been extensively reviewed

by Crocker (1938, 1948), James ( 1961, 1963), and Owen (1956), as has

the storage of seeds for food and feed by Anderson and Alcock (1954).

I shall therefore not touch on these factors, but limit my discussion to

preservation of germ plasm as seeds. First, however, an examination of

some of the theories relating to seed deterioration is in order.

II.



Theories Regarding Seed Deterioration



A. DEPLETION

OF FOOD

RESERVES

The earliest theory concerning the loss of seed viability was based on

the supposition that the food supply for the living embryo had been

exhausted. This theory was abandoned because it soon became apparent

that many dead seeds appeared perfectly sound and seemed to have had

an ample supply of food reserves. Furthermore, some seeds having an

abundance of reserve food deteriorate more rapidly than others with

limited amounts. Seeds of Zea mays L. more than 700 years old, found

in the Mesa Verde cliff dwellings, appear perfectly sound upon surface

examination, but no viable seeds have ever been found among them.

Oxley (1948) proposed that seed viability is lost when some undetermined, unstable organic compound in the seeds becomes exhausted. In

more recent years Harrington (1960) reasoned that even though there

are enough food reserves in seeds to ensure long life, the moisture content may be high enough to support respiration but too low to provide

for the translocation of food materials to the embryo; as a result, the

embryo dies. Although Harrington presented no data to support this

theory, it may have some validity. I have found that excised embryos of

snap beans and soybeans, stored at 70°F. and 70 percent relative humidity, had a shorter life than that of the intact seeds, The reason for the

shorter life of the embryos was not determined, however.

B. CHANGES

IN PROTEIN

Smucru~~

Ewart (1908) stated: “Longevity [of seeds] depends not on the food

materials or seed coats, but upon how long the inert proteid molecules

into which the living protoplasm disintegrates when drying, retain their

molecular grouping which permits their recombination to the active

protoplasmic molecules when the seed is moistened and supplied with

oxygen.” No experimental evidence was presented to support this theory.

With this reasoning, disintegration of proteid molecules should be excessive in seeds dried to very low moisture contents. Struve (1959), however, concluded that corn dried to 0 to 4 percent moisture, sealed with



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