MOST of the cultivated species of crop plants have been modified by breeding procedures to increase their usefulness as food, feed, and industrial products.

Let us begin our survey with wheat, the cereal of major importance among those grown primarily for food.

Wheat was brought into the North Atlantic coastal area by the several groups of colonists. All the groups brought varieties characteristic of their points of emigration. Diverse types thus were available. The ones best adapted to the new country persisted.

The first introductions were supplemented by additional introductions made by the successive waves of immigrants. Some came to have major importance: Red Lammas in Virginia; Mediterranean, in the southern Corn Belt; Purple Straw, in the Southeast; White Australian, on the west coast; Turkey, in Kansas, Iowa, and Nebraska; and the durums, in Minnesota, North Dakota, and South Dakota.

The improvement of wheat has followed the sequence of introduction, selection, and hybridization.

Most of the older varieties were gradually replaced by newer combinations, which in turn were replaced by still newer developments. The result has been the development of higher yielding types better adapted to the environment of the area of their culture.

The major improvement has been in resistance to various production hazards. These include greater resistance to hessian fly, green bug, and other insects; a greater winter hardiness; stiffer straw; and a higher resistance to smuts and rusts.

Covered smut has been particularly bothersome in the Pacific Northwest, where it is a soilborne parasite. Sources of resistance are available now to all of the physiologic races of importance.

Achieving resistance to stem rust has been more of a problem. As resistant types have been developed and become established commercially, new forms of rust to which commercial varieties of wheat are susceptible have arisen by mutation, hybridization, or other means.

New sources of resistance have had to be located, and such resistance has had to be incorporated into commercially acceptable types.

The use of newly discovered sources of resistance and the newer breeding refinements afforded by chromosomal substitution techniques offer promise of still further improvements. Much remains to be done in clarifying host-parasite relationships and establishing the genetic and chemical basis for physiological specialization.

Dwarf wheats, developed in the Pacific Northwest, appear to have a much higher yield potential than varieties in commercial use in 1960. Plant breeders have started to transfer the dwarf characteristics to wheats adapted to other sections, but the work had not progressed sufficiently in 1961 to assess the value of this development.

POTATOES may have been introduced in early colonial times. They were grown in New Hampshire as early as 1719. Breeding work during the next 100 years was limited. New varieties began to be developed from seed about 1840. Most of the important varieties were developed from true seed.

Cultivated potatoes are tetraploid—that is, they possess four basic chromosome sets. Most varieties initially had a high degree of male sterility. Male fertile varieties and selections have been developed. Polyploidy—three or more repetitions of the basic chromosome set—is favorable or at least is not disadvantageous under conditions of asexual propagation. The presence of extra chromosome sets, however, complicates breeding and genetic studies.

Considerable progress has been made nevertheless in developing varieties that possess satisfactory resistance to such diseases as scab, early blight, and fusarium wilt, certain types of virus, and late blight.

There remains the task of combining all these qualities and other desirable attributes, such as adaptation, yielding ability, and cooking qualities, into commercially acceptable types.

A few diploid seedlings—plants with two sets of chromosomes—are found in some seedling progenies. Exploratory work has begun to determine whether breeding operations can be conducted at the diploid level. If this can be done, all breeding operations should be simplified. The main deterrent appears to be a high degree of male sterility and the lack of a satisfactory technique for resynthesizing tetraploids.

SUGAR has become one of the cheapest of our common foods—thanks to a combination of breeding improvements, improved cultural practices, and improved extraction and refining techniques. Sugar is produced from both sugarcane and sugarbeets in the United States.

Cane has been grown for hundreds of years. Even the primitive tribes of today maintain a wide variety of recognizably different types, which undoubtedly arose largely by natural hybridization and selection.

When the culture of sugarcane expanded to commercial proportions, the types utilized were naturally occurring wild types or selections made by primitive man.

Following the usual pattern, diseases became of increasing importance as culture became more extensive. At first, to combat disease, one variety was substituted for another. The first sugarcane breeding stations were established in Java and Barbados in 1886. Previously there had been some question as to whether sugarcane could be propagated by seed.

Three species have been used for sugar production in various parts of the world—Saccharum officinarum, S. spontaneum, and S. barberi. Improvement has been done by crossing desirable varieties within S. officinarum, or by crossing S. spontaneum or S. barberi with S. officinarum, followed by nobilization and selection. Nobilization consists in backcrossing to the "noble" canes, S. officinarum.

Various virus diseases, red rot, and root rot have been the major diseases. Considerable success has been achieved in developing resistant types. Sugarcane is a high-order polyploid (probably octoploid).

In plants that reproduce exclusively by seeds, the individuals that have a greater or fewer number of chromosomes than typical tend to be eliminated by natural selection. This relationship does not hold if the individuals are propagated vegetatively. This is the situation in sugarcane, and chromosome numbers may be 80 to 160.

A measure of the progress that has been made is that the yield of dry sugar per acre has increased more than 1,000 percent since 1844.

Approximately one-fourth of the sugar consumed in the United States comes from sugarbeets. This crop is of recent culture and may be considered as the direct contribution of plant breeding.

A German chemist demonstrated in 1749 that the sugar from beets and the sugar from cane are identical. The first factory for the commercial extraction of sugar from beets was built about 1800. Extensive breeding work with sugarbeets was conducted in Germany and France, from which we imported seed until about 1930.

The European varieties were susceptible to curly top virus and cercospora leaf spot. The continuance of the sugarbeet industry required that varieties resistant to these two diseases be developed. The development of U.S. Number 1 (resistant to curly top) and the demonstration of the feasibility of the overwintering method of seed production provided the start of a domestic sugarbeet seed industry based on locally developed varieties.

Marked progress has been made since 1930 in the development of varieties adapted to the important producing areas and combining high yields of sugar with resistance to the worst diseases.

Major developments in production practices have lowered labor requirements. Machinery has been developed for the mechanization of harvesting operations and for the "shearing" of seed to reduce the seedball to a single-seeded condition. This reduced the labor requirements for thinning. The discovery and utilization of the mono-germ type followed.

Both cytoplasmic and genic sterility exist in beets. These types of sterility differ in their mode of inheritance. Their utility in plant breeding lies in the fact that both types are male sterile. The male-sterile plants can be used as female parents in the production of hybrid seed and the commercial utilization of hybrid vigor without the cost and volume limitations that would be involved in the production of hybrid seed by hand pollination. Considerable progress has been made in developing and evaluating the necessary stocks.

EARLY research to improve tomatoes was done primarily by private breeders and seed companies. Considerable work has been done since 1910 by public agencies to develop varieties resistant to disease. Marglobe was one of the most important of the first wilt-resistant types. Progress has been made in developing special types for canning and for growing in greenhouses. F1 varietal hybrids have become available for home and market gardens.

Because the hybrid seed is produced by hand pollination, the cost is too great for extensive commercial utilization. A number of genetic male-sterile types are available. They may be used eventually to simplify production of hybrid seed and to permit the commercial utilization of hybrid vigor.

Most of the varieties of peas grown in the United States were introduced from England. They were not well adapted to our climatic conditions, and we had to develop adapted types that would fill different requirements for canning and for the home and market gardens.

Progress has been made in developing varieties having the growth habit and features of pods and seed the two outlets require and in combining these with resistance to wilt, a serious disease of peas.

One of the first major improvements in green beans was the reduction in pod fiber—the so-called stringless varieties. Diseases later became of major importance, and attention was directed toward the development of resistance to anthracnose, bacterial blight, rust, and mosaic.

Regional adaptation and the differing requirements for market gardens and canning have complicated the breeding problem, but progress has been made.

Of the several types of lettuce, the "crisp head" type is the one usually offered for sale. Once it was grown mostly in the Western States. Varieties resistant to mildew, brown blight, and tip burn have been bred.

Until about 1940, the head lettuce grown in the East was primarily of the "butter head" type, but consumer demand led to a shift to the "crisp head" type. Seed of this type was produced only in California, and the varieties developed for the West were poorly adapted to eastern conditions. Breeding programs were started in the East in the late 1920's, and a number of acceptable "crisp head" varieties have been developed and released.

Improvements in vegetables thus involve other attributes than greater yield and resistance to disease. Another goal has been to increase the range of adaptation of a crop. In some instances, as in lettuce, that can be solved by the development of several varieties of the desired type, each adapted to some specific set of soil and climatic conditions.

Preferences of consumers for fresh, frozen, or canned vegetables impose requirements that the plant breeder also must consider.

CORN, first among the feed grains, is native to the Americas. It was an important crop of the Indians before the first voyage of Columbus. By 1700, both flint and "gourd seed" dents were being grown, but we know little about the varieties that were developed and grown between 1700 and 1850. Mass selection was the major breeding procedure for developing new varieties. Some varietal hybridization was practiced, perhaps to produce new populations from which new varieties could be developed.

The experimental basis for hybrid corn was laid by the early work of G. H. Shull at the Station for Experimental Evolution in Cold Spring Harbor, N.Y.

His studies indicated that there was a marked loss in vigor upon inbreeding but that when inbred lines were crossed some of the resulting hybrids were superior to the parental varieties. He visualized the commercial use of this hybrid vigor, but he did not explore the possibility further.

Following Shull's early work, a number of experiment stations conducted studies on the inbreeding of corn. The consensus appeared to be that the inbred lines obtained were too weak and low yielding to make the commercial use of hybrids feasible.

Then came the development of the "double cross" by D. F. Jones, of the Connecticut Agricultural Experiment Station. This development placed the burden of commercial seed production upon vigorous, high-yielding, single-cross parents, rather than the weak inbred parents previously visualized.

Thereafter inbreeding and hybridization studies were undertaken on a greatly expanded scale by the Department of Agriculture, the State experiment stations, and private seed companies. Important contributions in the development of both theory and practice were made by each group before hybrid seed was first offered for sale in the late 1920's.

It was estimated that 0.3 percent of the corn acreage was planted to hybrid seed by 1933. The utilization of hybrid seed has increased rapidly since that date and exceeded 95 percent of the acreage in 1960.

Adapted hybrids are now available for all sections in the United States where corn is of importance.

The use of hybrids has affected production directly and indirectly. The increased uniformity and resistance to lodging of hybrids have fostered more harvesting by machines. Hybrids have made the use of larger quantities of fertilizers economically feasible. The total production of corn has increased steadily on fewer acres at lower costs.

The use of cytoplasmic sterility—failure of pollen production presumably conditioned by the cytoplasm and transmitted only through the female parent—to facilitate the production of hybrid seed has increased since 1956. Perhaps more than half of the seed produced for the Corn Belt in 1960 involved the utilization of this procedure. The use of cytoplasmic sterility should reduce the hazards of seed production but will not reduce the cost.

CORN also has industrial uses.

The Department of Agriculture in 1941 established Utilization Research and Development Divisions in Wyndmoor, Pa., Peoria, Ill., New Orleans, La., and Albany, Calif. A major assignment was to develop new industrial outlets of agricultural products and crop residues. They have been responsible for many developments.

There is, however, another approach to increased industrial use of agricultural crops—the genetic modification of a particular crop to fill some special industrial need.

The development of waxy corn is an example of the possibilities of genetic modification.

Waxy corn, introduced from China in 1907, has a form of starch different from that of normal corn. The starch from waxy corn has pasting and viscosity properties like those of tapioca. Before the Second World War, about 175 thousand tons of tapioca starch were imported from the Dutch East Indies annually. This source of supply was lost, and the culture of waxy corn was started to fill this important need.

Double-cross hybrids having the waxy-starch characteristics were developed, and production began in 1943. Production has increased steadily since then. More than 1 million bushels of this type have been milled annually since 1950.

Work has been started to develop high-amylose corn. Types are available in which the starch has 70 to 80 percent of the straight-chain component, amylose.

Somewhat higher percentages are desired by industry. If such types can be developed, amylose starch would find an important outlet in the production of films and fibers.

Amylose starch has properties somewhat like those of cellulose, and its utilization in paper is a possibility. If it works out, the annual consumption of this type of corn could be several million bushels annually.

Sorghums are used for forage, sirup, and grain. Grain sorghums have become a major feed grain.

Sorghums have been introduced from various areas. Those of African origin have proved to be most useful. Before we had hybrids, the production of grain sorghums was concentrated in the Plains States. The varieties once grown were tall and hard to harvest, but the double dwarf, or combined-height, types came into use after 1920 and nearly replaced the taller kinds.

A genic male sterile was discovered in 1936. Plans were developed to utilize it to produce hybrid seed. It proved to be unsatisfactory, and others were found and investigated. Before the genic sterility was in extensive commercial use, a cytoplasmic sterile strain was discovered that proved to be more adaptable to commercial production of hybrids.

The first hybrids utilizing this sterile source were available for commercial planting in 1956. The utilization of hybrid sorghum has progressed faster even than hybrid corn. About 70 percent of the grain sorghum acreage was planted to hybrid seed in 1960.

Alfalfa is a widely grown and valuable hay crop. Colonists brought alfalfa (lucerne) from Europe. Much later it was introduced into California from Chile. Alfalfa was grown all over the United States by 1900.

Modification of the crop at first was almost entirely by natural selection. Northern common types had fall dormancy and the ability to persist through cold winters. The important varieties that were developed through introduction and selection during the early period were Grimm, Baltic, Cossack, and Ladak.

Wendelin Grimm emigrated to Carver County, Minn., in 1857, bringing seed of alfalfa from his native Germany. The original importation was not winter hardy, but seed from surviving plants were saved year after year, and so Grimm, a winter-hardy variety, was developed. It became one of the most widely distributed varieties in the northern sections until bacterial wilt attacked it.

After scientists determined the nature of bacterial wilt, alfalfas from Russian Turkestan, northern India, western China, and northwestern Iran were found to have some resistance. A few resistant plants also were found among some adapted varieties.

State and Federal alfalfa-breeding programs were enlarged and coordinated to concentrate on wilt resistance and other breeding problems.

From this work have come such improved varieties as Ranger, Buffalo, Atlantic, and Vernal. They represent marked improvements in disease resistance and adaptation to the areas for which they are recommended. These new varieties could be called synthetics, because they were produced by recombination of individually selected and tested clones, or inbred lines.

Many research workers have set as their ultimate goal the production of hybrid alfalfa.

Pioneer work in the breeding of grasses in the United States was that of W. M. Hays with timothy at the Minnesota Agricultural Experiment Station. Improvement work began later at several stations. Selection was the primary breeding technique, but precautions were not taken to prevent cross-pollination. A number of improved varieties were distributed, but most of them were adapted to only a limited area.

Rust has been one of the major diseases. It has been relatively easy to isolate and select resistant types of timothy. Because timothy usually is grown for hay with alfalfa or clover, types are needed that will be ready for cutting when the legume is ready. Varieties like Shelby and Marietta nearly fill the requirement.

Types especially adapted for pasture use have been sought. Progress has been made in isolating prostrate types, but seed supplies have been inadequate because of low yields of seed.

The work with timothy has been done almost entirely by evaluating and propagating clones, but the work with other species of grass has attempted to utilize hybrid vigor more completely.

In types such as bromegrass and orchardgrass and reed canarygrass, clonal selection has been followed by the recombination of a relatively small number of selected clones into a synthetic. The level of hybrid vigor in such material would be greater than the vigor of the parent clones.

Pensacola bahiagrass contains about 20 percent of plants that exhibit self-incompatibility. Such plants when self-pollinated set less than 2 percent of seed. When grouped in pairs, seed sets as high as 90 percent may be obtained.

This method of hybridization has been used to produce Tifhi-1. This F1 hybrid has produced 69 pounds more beef per acre per year than an unimproved variety that was used for comparison. The same breeding procedure could be used with several other perennial pasture plants.

Buffalograss is dioecious—that is, the staminate and pistillate flowers are borne on separate plants. This phenomenon has been used to produce the F1 hybrid Mesa. Seed fields are prepared by vegetatively interplanting selected clones of the two dioecious parents in the seed fields.

Gahi-1 pearl millet is a hybrid composite made from the random inter-pollination of four selected inbred lines. The seed produced represents a combination of F1 hybrid seed and selfed or sibbed seed—the result of interpollination among plants of the same parental line. The nonhybrid seed is selectively eliminated, through competition, in heavy seedings, and an essentially pure stand of hybrid plants remains.

Yields obtained by this procedure have exceeded those of the parental check by more than 50 percent.

Cytoplasmic male sterility has been discovered in pearl millet. This will provide an alternative procedure for the production of hybrid seed.

As work with the various forage species progresses, growing emphasis may be placed on procedures that permit wider utilization of hybrid vigor.

THE MAIN SPECIES of cotton grown in the United States are sea-island and Egyptian (Gossypium barbadense) and upland (G. hirsutum).

Sea-island is outstanding in length and quality of fiber, but its production has been limited to South Carolina, Georgia, Florida, and the islands near South Carolina. Wilt became a problem in parts of this area about 1895.

Erwin F. Smith, of the Department of Agriculture, determined the cause of the disease. Resistant varieties were developed. Before resistant types of the upland could be developed, the production of cotton had to be abandoned in many areas in the Southeast.

The boll weevil, first found in Texas in 1892, moved eastward and did so much damage that the late-maturing, long-staple types could not be grown profitably. Early efforts of plant breeders were devoted to the development of productive early types, which tended to escape serious damage. Fiber qualities, particularly length, were sacrificed for earliness.

After productive early varieties were available, greater attention was directed toward increasing the length of the fibers.

Varieties of Egyptian cotton were introduced into the irrigated sections of Arizona and southern California in 1903, but as a group they were poorly adapted. Selection, however, was continued, and improvement in earliness and productivity was gradual.

In 1908 two types differing from normal were found and increased. One of them was later named Yuma. A selection from Yuma was later named "Pima," which continues to be important in the Salt River Valley of Arizona.

Cotton often is cross-pollinated. The extent varies with varieties and the number of pollinating insects. This chance hybridization has provided the variability that permitted the isolation of the many different varieties.

Controlled hybridization also has been used to produce new, variable populations, in which further selection could be practiced.

The breeding methods most commonly used have been mass selection or the selection of individual plants. When pollination is not controlled, neither leads to a high degree of uniformity. Thus variability was a factor that permitted the rapid change in varieties when wilt and the boll weevil became serious.

The commercial utilization of first-generation hybrids of cotton has been suggested by many workers. Four methods have been proposed: Inter-planting of the desired parents, with cross-pollination to be effected by bees; controlled hand pollinations; the use of genetic or cytoplasmic sterility; and induced male sterility induced by chemicals (gametocides).

For various reasons, only the use of gametocides has been tried on a commercial scale. Further developmental work is required before this method of producing hybrid seed can be considered entirely satisfactory.

Soybeans have been grown in China for several thousand years, but only 50 thousand acres were grown in the United States in 1907. Then the crop was used largely for forage.

The possibility of utilizing soybeans as an oilseed crop was visualized about 1920. Mills were built, and by 1929 there had developed a considerable demand for high-oil, yellow-seeded varieties.

Some of the leading varieties in 1929 were Illini, Dunfield, Mukden, and Scioto, which were direct introductions from the Orient or selections from such introductions.

It became apparent that various combinations of the desired attributes could not be obtained readily by a direct selection. Between 1930 and 1940, hybridization (followed by selection) or hybridization and backcrossing (followed by selection) became the standard breeding procedures.

Most of the varieties released since 1940 were developed following controlled hydridization.

Breeding of soybeans has been effective in isolating types superior in yielding ability, resistance to lodging and shattering, adaptation to suit various requirements as to maturity, and resistance to disease.

Yielding ability seems to be conditioned by relatively few genes, because transgressive segregation—types that exceed the normal range of the two parents—for yield has been observed in the progeny of many crosses. Resistance to lodging and shattering are required for satisfactory harvesting by combines.

Few of the original introductions were adapted to the Northern States. Varieties having a suitable daylength response are now available. Evidence of the importance of this development is that in Minnesota soybeans were grown on 97 thousand acres in 1938 and on 3.1 million acres in 1958.

The percentage and quality of oil are under genetic control but are subject to some environmental modification. Oil content has been increased through increased yields and a higher oil content of seed. Many of the newer varieties are superior in oil content to their parental strains. Much soybean oil is used in the manufacture of shortenings, margarine, and paints.

The protein in soybean varieties ranges from 35 to 50 percent. Oilmeal, the residue after the oil is extracted, is an important protein concentrate for feeding livestock. The protein concentrate also has many industrial uses, including the making of plastics and adhesives.

Increased emphasis has been devoted in the last few years to the breeding of new varieties with a higher percentage of protein.

In the early years of production in the United States, soybeans were relatively free of diseases and insect pests. Several diseases and the root knot nematode have become more bothersome in recent years. This development has been so new that the genetic basis for resistance is still undetermined in several instances.

The production of varieties that have adequate resistance to all of the major diseases remains a task for the future.

EFFORTS to improve ornamental plants have varied with time and the method of propagation.

At first probably no more was involved than the selection of the more attractive types that grew wild and their transfer to cultivated conditions.

Further progress depended on the type of propagation used. The rose, one of the most ancient of cultivated flowers, is an example. Several wild species are involved. Most of them are confined to the North Temperate Zone. During the 16th century, propagation was largely by cuttings, and only a few varieties were cultivated. Some 21 double varieties were cultivated in Europe by 1800.

Now the total number of varieties of rose probably would be close to 20 thousand. This big increase has come about by hybridization and the evaluation of the resultant seedlings.

The development of large numbers of new varieties in other forms also depends on the evaluation of seedling progenies. The development of new types under vegetative propagation is limited to the recognition and maintenance of chance bud sports.

The breeding of flowers and ornamentals has been done largely by amateurs, with some participation by private seed firms and public agencies. Some of the work undoubtedly has suffered from a lack of understanding of genetics, yet the most modern techniques sometimes have been used. I give several examples.

Stock, Mathiola incana, has both single- and double-flowered forms. Doubleness, the desired type, is conditioned by recessiveness for a single gene. The double-flowered forms are sterile and must be propagated from heterozygous singles, which yield only 25 percent of double-flowered plants.

In one such strain, a mutation occurred that kills all of the pollen grains carrying the mutant gene. When the pollen lethal is linked with the gene for single flowers, approximately 50 percent of the plants produce double-flowered plants. Still later a trisomic type, called "slender," was discovered. Trisomics are 2n+1 types, in which one chromosome of the complement is present in triplicate. The extra chromosome involved in "slender" is the same one that carries the gene for doubleness. Because of the unequal fertility of gametes carrying the extra chromosome, the selfed progeny of the slender type yields approximately 90 percent of double-flower plants. Furthermore, the single-flowered plants can be recognized in the seedling stage and discarded. Thus, although the desired double type cannot be propagated directly by seed, genetic techniques have made it possible to increase the frequency of doubleness from 25 to approximately 100 percent.

Colchicine is an effective agent for doubling the chromosomes in many plant forms. Such doubled chromosome forms, called tetraploids, have larger flowers and usually a sturdier form of growth.

Colchicine treatment has been used to produce tetraploid forms of many of the annual flowers, such as snapdragons, marigolds, and petunias. Tetraploid forms in vegetatively propagated types, such as forsythia and lily, have also been produced.

Crosses between tetraploids and diploids yield triploid progeny, which are sterile. Where seed propagation is required, triploid forms must be propagated anew each generation. Where vegetative propagation is possible, desirable triploids may be maintained indefinitely. The tiger lily and certain varieties of cannas and day lilies are triploids which are propagated vegetatively.

In plants propagated by seed, species hybridization has played an important role in improvement. Roses, dahlias, gladiolus, amaryllis are common flowers that involve extensive species hybridization in their ancestry.

The limit to the usefulness of species hybridization is determined by the percentage of seed set and the viability of such seed. Viable seedlings often can be obtained from otherwise sterile crosses by use of embryo culture. In this method, young embryos from developing seeds are dissected out and transferred to an appropriate nutrient media. Embryo culture has been used extensively in the breeding of iris.

Alfred Stefferud

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