Tải bản đầy đủ - 0 (trang)
V. Improvements in Methods of Breeding Wheat

V. Improvements in Methods of Breeding Wheat

Tải bản đầy đủ - 0trang



ment of special techniques for determining varietal resistance to weather

hazards and pests of various kinds. Methods for measuring quality have

been greatly improved and simplified. More complete knowledge of the

genetic constitution of the wheat plant has greatly facilitated the choice

of parental varieties, the selection of progenics, and breeding procedures.

1. Earl9 Methods of Breeding

Continuous mass selection of the largest and best-appearing heads of

the heaviest and plumpest grain separated by wind or sieves was generally recommended and used at the turn of the century, as it had been

since the time of the Romans and even before. Publication of The

Origirz of Species by Darwin in 1859 provided a theory that greatly stimulated the belief that important improvements could be achieved by continuous selection.

lie Couteur and Shirreff initiated the practice of selecting single

heads and plants early in the nineteenth century. Shirreff emphasized

the importance of the initial selection and believed that little or nothing

was gained by later selection. Hallett (1861) proposed a “pedigree

method” which consisted essentially of selecting the largest grain from

the longest and largest head from the best plant each year and continuing this year after year. Four years of selection doubled the length of

the head, trebled the number of grains per head, and increased the tillering fivefold. He believed that yields per acre were also greatly increased.

Some breeders followed the method developed by Shirreff, but Hallett’s

philosophy predominated until the close of the century and did not disappear until the end of the first decade o r later of the present century.

Willet M. Hays of the Minnesota Station in about 1895 applied to

wheat breeding the concept of the progeny test to determine the relative

superiority of selected strains. He developed the centgener method of

planting for spacing the plants and thereby facilitated the selection of

the best individual plants each year. The method was widely adopted

throughout the United States.

This, in brief, was the prevailing theory and practice until the publication of Johannsen’s pure-line theory in 1901. This publication plus

accumulated general experience, including carefully controlled experimental trials at a number of experiment stations, turned the attention

of wheat breeders to the selection of pure lines and the determination

of their relative value. Most of the improved varieties of the first three

decades of the twentieth century were the product of this method. They

include such varieties as KANRED, NEBRASKA NO. 60, CHEYENNE, IOBRED,




and others.

Hybridization, it is true, was practiced by a number of breeders before 1900, notably Farrer in Australia; Saunders in Canada; and Blount,

Pringle, Jones, and Spillman in the United States, but it received relatively little attention. The principal purpose of hybridization was to

induce variation o r to “break the type” and thereby afford greater opportunity for selection. Consequently, parents were chosen more or less

at random. A few breeders had clearly in mind the possibility of combining in a single variety the desirable characteristics of two or more.

Saunders recognized the need f o r early maturity when he crossed Hard

Red CALCUTTA with RED FIFE to produce MARQUIS, and Spillman set out

to combine resistance to shattering and winter hardiness in the Washington hybrids. Not until several years after the rediscovery of Mendel’s

laws, however, was there general recognition of the value of hybridization and the need for a careful choice of parents based on their known


A development traceable directly to a, better understanding of genetics is the backcrossing technique, first proposed by Harlan and Pope

(1922) and used especially by Briggs (1938) and associates in California. It has also been used extensively in the breeding of rust-resistant

durums but sparingly elsewhere. Percival (1921) and Clark (1936) reviewed the methods generally used during the last century and in the

early part of the 20th century.



2. Objectives in Breeding

Another development of first importance is the clearly defined objectives of most modern wheat-breeding programs. Early efforts were directed mostly to increasing yields but without any clear concept of what

determined yield. It was assumed that varieties differed in “yielding

capacity,” as, without doubt, they do and also that “yielding capacity”

is synonomous with actual yield, which usually is not the case. Plant

breeding often was regarded as an ast; this implied that breeders acquired a special skill that enabled them to choose heads or plants that

would produce superior yields. Undoubtedly, there is some truth in

most of these assumptions, but there is also a great deal of error. This

concept again was not accepted universally. Carleton (1900, p. 54), for

example, stated that “yield after all is not a distinct quality in itself but

is the combined result of a number of qualities acting independently and

not thought of a t all.”

Much information as to yield factors is still lacking, but few probably

will doubt that wheat breeders are on solid ground in breeding for re-



sistance to stem rust in spring wheat for the northern Great Plains, in

breeding for appropriate degrees of winter hardiness in winter wheats,

and in breeding for resistance to all diseases and insect pests and weather

hazards so f a r as is feasible and for areas where they are known to be

important. The important difference is not that the modern wheat

breeder ignores yield, but that he uses what information he has more

effectively in producing higher-yielding varieties. His efforts are more

effective because it is much easier, less expensive, and less time-consuming to test selections for reaction to individual factors that affect yield

than i t is to determine relative yields. If a modern wheat breeder uses

the term “yielding capacity’’ at all, he means yielding capacity under

a specified set of environmental conditions.

A closely related achievement also of importance is the relatively

more precise knowledge of the kind of wheat needed for each wheatgrowing area. The wheat breeder has learned, mostly the hard way,

that nowhere in the United States are varieties wanted that mature as

late as those of northern Europe or as early as those grown in India:

that varieties even earlier than TURKEY are needed for the southern

Great Plains; that in much of the northern Great Plains varieties of

spring wheat susceptible to rust have little chance of successful competition with similar varieties that are resistant to rust; that short stiffstrawed varieties are needed for the Pacific Northwest. There is now

a clearer concept of the degree of winter hardiness needed for each principal wheat-growing area and also general recognition of the fact that

winter hardiness in one area does not necessarily mean winter hardiness

elsewhere. If it is necessary, as seems probable, to accept a compromise

between extremes of winter hardiness and of early maturity in hard

winter wheat, current knowledge as to the needs with respect to each

should make i t easier to attain a suitable compromise.

3. Resistance to Disease, Insect, and Weather Haaards

Certainly among the most important advances in techniques are those

for testing the resistance o r tolerance of varieties and selections to specific disease, insect, and weather hazards. Wheat breeders no longer

wait for natural epidemics but produce what is needed where and when

it is wanted. This method, first proposed by Bolley (1905) soon after

1900 for breeding varieties of flax resistant to wilt, has been all but universally adopted for other crops and diseases wherever feasible. Seed

is inoculated with specific races of the bunt organism and seeded late to

insure suitable temperatures for infection or, in the case of dwarf bunt,

the seed is planted in infected soil. Plantings for leaf and stem rust

resistance are provided with border o r so-called spreader rows of a sus-



ceptible variety which, in turn, are artificially inoculated and watered

with overhead sprinklers to provide suitable atmospheric humidity for

the germination of the rust spores. Cherewick (1946) has recently described the methods used i n Canada for establishing rust epidemics in

experimental plots. Cartwright and LaHue (1944) have developed a

technique for testing varieties and selections for resistance to Hessian

fly by means of which 20,000 or more may be tested in a single season.

Platt and Farstad (1946) have described a method for insuring epidemics of sawfly that has proved most useful in breeding for resistance

to this insect in Canada and the United States. It consists essentially

of seeding short rows of the material to be tested on summer fallow and

adjacent to infected stubble of the preceding crop. The rows are seeded

at right angles to the stubble in order to insure like infestation of all

rows, since infestation depends materially on the distance the flies must

migrate. An important feature, emphasized by Platt and Farstad in

the interests of economy of labor, is to estimate infestation rather than

make actual counts. The latter is very laborious and time-consuming,

and estimates were found by them to be sufficiently accurate for most

purposes. Similar methods incidentally are widely used and with satisfactory results i n determining relative resistance to rust, bunt, Hessian

fly, winterkilling, lodging, shattering, and other diseases and hazards.

The development of greenhouse techniques not only for determining

resistance to diseases and insects but also for growing two or more generations per yeas has been most important. Resistance of selected lines

can be determined before the time for seeding in the field the following

spring. The F 2 or segregating generation of a cross is reached a year

earlier by growing the F1 generation in the greenhouse. Closely related

is the practice of seeding a crop in the field in Arizopa, southern California, or Mexico, and shipping the product back to the northern United

States for planting the following spring. Relatively rapid increases in

the quantity of seed of promising varieties have been made in this way.

The relative winter hardiness of new varieties or selections for Central and Southern States is frequently determined by seeding them in

Northern States where partial but not complete killing may be expected.

Artificial freezing tests have been used although not extensively, largely

because equipment is expensive and not readily available. The use of

winter-hardiness nurseries and others for similar purposes has been

greatly facilitated by the co-ordinated co-operative programs previously

described. Relative shattering and lodging are now easily, simply, and

inexpensively determined by permitting border rows to stand for several

weeks after the normal harvesting date and estimating the loss due to

either or both.



Unfortunately, no simple or even dependable methods for testing

relative drought resistance have been developed other than relative yields

in a dry area for a long period of years.

4. Testilzg for Comparative Yields

Field plots of various sizes seeded with an ordinary grain drill were

used for determining relative yields of varieties long before the end of

the nineteenth century. Usually there were single plots only of each

variety, although occasionally duplicate, triplicate, or even quintuplicate seedings were made. One-tenth acre was a common size plot. It

was recognized early that soil variation of ten seriously vitiated experimental results ; consequently, much attention was devoted in the early

years of the century to the selection of uniform land for experimental

fields, uniform preparation of the land, improvement in uniformity by

drainage, etc., and in extreme cases to the relocation of experimental

fields in order to have reasonably uniform land. The use of replicated

plots in the United States has been almost universal since about 1915,

largely as a result of the application of statistical methods to field experiments, I n recent years the combine has been used to harvest experimental field plots with a material saving in labor.

Probably the most important single advance in methods of comparing

yields was effected by the substitution of rod-row plots for centgeners.

Rod-row plots were first extensively used by Norton (1907). The universal adoption of this and similar methods has resulted in a saving of

labor, and hence in an opportunity to compare more selections, which it

is difficult for those who have not used a centgener machine to appreciate. There can be little doubt also that modern rod-row tests are more

accurate than those made by the centgener method, although there appear to be no data to prove it. An important contribution to the accuracy of rod-row tests was the demonstration by Kiesselbach (1918) a t

the Nebraska Station of border effect, and as a consequence the general

adoption of multiple-row plots.

Kezer (1906) correctly believed that he had made a n important contribution when by substituting a movable for a fixed frame, he and five

men were able to plant the equivalent of 275 centgeners per day as compared with 80 with the machine previously used. But contrast this with

100 rod-rows or 150 head rows per hour per man by modern plant-breeding crews. Many yield nurseries are now seeded with drills powered by

small tractors, harvested and bound into bundles mechanically, and

threshed with improved threshing machines having probably twice the

capacity of those available in 1900. Magee (1951), for example, has



estimated that three men with a tractor-mounted drill used by him “can

seed more than 5 men using hand seeders.”

One thousand centgeners was a big nursery around 1900. It is not

unusual for a modern wheat-breeding and yield nursery to comprise

10,000 or even 20,000 rows. The improvement, it should be noted, is

due not only to better techniques of planting, harvesting, threshing, etc.,

but also to changes in specific objectives, as noted above, which greatly

reduce the time devoted to each item. For example, many selections are

now discarded without harvesting with perhaps no more than a record

of their defects. With the centgener method and the philosophy accompanying its use, detailed notes were recorded often for each individual

plant in each centgener.

5. Techniques for Measuring Quality

Striking improvements in methods of measuring or comparing quality have been made since Saunders used the chewing test to determine

the quality of MARQUIS. At least four distinct categories must be recognized: milling quality; quality for bread; quality for cake, cookies, crackers, and similar products collectively called pastries ; and in the case of

durum wheat, quality for macaroni.

a. Milling Quality. A good milling variety in the eyes of the miller

is one that mills easily and produces a high yield of flour, especially of

patent flour. Improvements in techniques for determining or measuring

milling quality have been few. They have consisted mostly of standardization and refinements in the operation of experimental mills, and there

have been some improvements in the mills themselves, especially adjustments to enable satisfactory tests to be made on small samples. Recent

studies indicate that it may be possible to relate poor milling properties

to definite physical or chemical characteristics of the grain and thus

facilitate the identification of poor milling varieties.

b. Quality for Bread. Perhaps the most important advances i n techniques relating to measuring quality are those f o r quality of bread. The

history of the subject is surprisingly confusing in view of the rather

clear picture that has emerged in recent years. The literature is voluminous, and only the barest outline can be presented here. Larmour (1940)

has given an informative account of much of the research in the field u p

to about 1940 as it relates to a comparison of hard red spring and winter


Quality of undamaged or normal wheat and flour for bread has long

been known to depend on quantity and quality of protein. Protein content is easily determined, but until recent years there was no adequate

test for protein quality. The latter was at best inaccurately estimated



from the baked loaf and then only when interpreted in terms of the

quantity of protein in the flour; that is, if differences in the quality of

bread could not be explained by differences in protein content, they were

automatically attributed to protein quality. Interpretation was usually

difficult and inaccurate, and especially so if the protein contents of the

varieties being compared were different.

This difficulty stemmed principally from the general experience, as

shown by Thomas (1917), Coleman et al. (1927), and Shollenberger

[cited by Larmour, (1940) 1, that the relation between protein content

and loaf volume by the methods then used is nonlinear. Flours with a

medium protein content would generally produce better bread than

those with a low protein content but would also usually produce as good

bread as flours with very high protein content. Yet it was known that

high protein flours used in blends with low protein flours would improve

the latter in proportion to protein content.

This paradox was cleared up by a long series of investigations, including those by Larmour and MacLeod (1929), Geddes and Larmour

(1933), Larmour and Brockington (1933) , Ofelt and Larmour (1940),

Larmour (1940), and especially Finney and Barmore (1944,1945,1948).

Briefly, it was found that the baking test formulas and procedures then

used were such that the potential value of high protein flour was not

expressed, As a corollary, it was found that when proper formulas and

techniques are used, the relation between protein content and loaf volume

is perfectly or almost perfectly linear, at least within the range ordinarily found.

Another important discovery was that the slope of the regression line

is a varietal characteristic and such that for a large number of varieties

differing in protein quality, they fall into a €an-shaped pattern with

rela.tively little difference between them at low protein levels and wide

differences a t high protein levels. This means that for the first time

cereal chemists have an adequate and reasonably accurate measure of

protein quality.

Very recently it has been found that exposure of the growing wheat

to excessively high temperatures and relatively low humidities during the

latter part of the fruiting period may deleteriously affect protein quality

and distort the usual linear relation between protein content and loaf

volume. Other environmental factors may require consideration but

even so the bread-baking test is far superior to that of fifty years ago.

G. Pastry Quality. The fact, long suspected but not proved until recently that pastry quality depends more on the physical properties of

the flour and on the characteristics of the protein than on the protein

content, has been of great assistance in evaluating varieties for pastry



purposes. The most generally acceptable evaluation is by means of the

cooky-baking test. Cake-baking tests are generally less useful because

small differences are obscured, probably because flour constitutes less

than half of the ingredients in a cake formula. Cooky-baking tests apparently were first proposed by Alexander (1933), first used for evaluating varieties by Fifield et al. (1936-1950), and since greatly improved

by Finney and Yamazaki (1946) and Finney et al. (1950).

d. Bread-Baking Tests f o r Soft Wheat. A t the beginning of the century and for many years thereafter, soft wheats for the Eastern States

were evaluated by bread-baking tests. One reason was that soft wheats

were then extensively used for bread and family flours. Another reason,

probably not widely accepted, was the assumption that varieties of poor

quality for bread were suitable for pastries and vice versa. This, as is

now well known, is only partly true. Important here, especially in relation to future breeding, is the discovery that the suitability of certain

varieties of soft wheat for bread or for pastries depends on their protein

content, which is determined mostly by environment. If high in protein

content, they may be used for bread, and if low in protein content, they

may be used for cakes, cookies, crackers, and other pastry products. This

is true, however, only for those varieties that yield flours having the

necessary desirable physical properties.

e. Macaroni Quality. Macaroni tests for quality of durum varieties,

the only tests available during the early years of the century, are especially time-consuming and like the bread-baking tests require considerable grain. Relative quality of durum varieties is now regularly

determined by the disk test first proposed by Fifield e t al. (1937). I n

this method, the wheat is milled to produce semolina in the usual way.

Semolina dough is then pressed into disks one-fourth of an inch thick and

with about the diameter of a silver dollar. Translucency and color, on

which quality for macaroni depends, can be determined from them as

easily and as accurately as from macaroni. In terms of labor and time,

the disk test is seven or eight times as efficient as the macaroni test.

Macaroni is still sometimes made for final evaluations of promising new

varieties but otherwise is seldom needed.

f. Ancillary Quality Tests. Baking tests whether of bread or cookies

are time-consuming and relatively expensive. They also require more

flour than is usually available in early-generation selections from a cross.

Consequently, there has been a n urgent need for simple chemical or

physical tests that require less time and less flour. This need has in part

been supplied by various tests including the (1) viscosity test, (2) doughball, Pelshenke, or fermentation time test, (3) pearling test, (4) mixogram curves, (5) water-absorption test, (6) the sedimentation test, and



others. None of these can be completely substituted for baking tests,

but each provides a certain kind of information that is most useful in

evaluating quality and especially so in early generations, since each test

requires only a small quantity of grain. Finney and Yamazaki (1953)

have recently developed an alkaline viscosity test, and Pamazaki (1953)

has developed a n alkaline water-retention capacity test for soft wheat

flours that correlates very highly with the cooky test. Micro-milling and

micro-baking test procedures, developed by Finney and Pamazaki

(1946) and Finney et al. (1950), are now available that require only a

small quantity of grain and hence are useful for testing quality in early




Little was known about the nature of wheat diseases or how to control them before the beginning of the twentieth century. Loose smut

and stinking smut or bunt were widespread, and the latter especially

threatened to become a limiting factor in wheat production in many

areas. Control methods known a t that time were inconvenient or ineffective or both and were not generally used until severe losses had occurred. Both stem and leaf rust were recognized as important diseases

in many sections. Other diseases were present, but most of them had

not been identified and fully described as to symptoms, causal organisms,

and manner of spread.

1. #tern Rust

a. History and Distribution. Some important facts regarding stem

rust had been established before the twentieth century, such as the general characteristics of the causal organism, the role of the barberry as

an alternate host, and the fact that the variety of stem rust that attacks

oats, for example, does not attack wheat.

Laws to enforce the eradication of the barberry were passed in Connecticut, Rhode Island, and Massachusetts between 1726 and 1766, long

before the relationship between barberry bushes and stem rust infection

was clearly established. The existence of physiologic races that attack

some varieties of wheat but not others was not known, nor had the role

of wind-blown spores in producing stem rust epidemics been discovered.

Carleton (1900) and others had recognized differences in susceptibility

of varieties to stem rust, but resistant varieties, as we know them today,

were not available.

b. Development of Resistant Varieties. The discovery of the outstanding resistance to stem rust in IUMILLO durum wheat and in YAROSLAV

emmer marked the beginning of the first successful attempts to breed



resistant varieties. So far as the writers are aware, the details of this

discovery have never been published. The first significant observations

regarding the rust resistance of these varieties were made a t the South

Dakota Agricultural Experiment Station a t Brooking8 in 1902, when

Mr. John 8. Cole, then in charge of the cereal-breeding plots, reported

in part as follows : *

“Rust damage to common spring wheat was so great that no comparisons could be made that were of any value. . . Durum wheats from

Spain, Italy, and Bulgaria . . show but little promise. One, however, NO.

1736, a wheat of peculiar type from Italy, appears to be almost perfectly resistant to black rust and promises to yield well.’’ NO. 1736 is

IUMXLLO, which later provided most of the genes for the resistance of

THATCHER to stem rust. Cole also stated: “Results with emmers were

very striking, especially in the matter of rust resistance. Four varieties

were found to be highly resistant to rust and yielded about 40 bushels

per acre, whereas the yield of other varieties grown under the same conditions, but which rusted severely, went as low as 4 bushels.” Among

these four resistant varieties was YAROSLAV emmer, which McFadden

later crossed with MARQUIS in producing HOPE and H-44 and from which

most of the genes for resistance in common spring wheats other than

THATCHER have been derived.

Cole’s observations were verified during the famous stem rust epidemic of 1904 and again in 1905. By 1904, the varieties mentioned by

Cole were being grown at other stations where observations regarding

their rust resistance served to call attention to the possibilities of using

them i n breeding programs. Fo r further details, the reader is referred

to Ausemus’ (1943) excellent review of breeding for resistance to stem

rust and other diseases in wheat and other small grains.

c. Discovery of Physiologic Races. Another contribution of basic

importance regarding the rust fungus was the discovery in 1917 of the

existence of physiologic races that attack different varieties of wheat.

This outstanding contribution was made by Stakman and his associates

at the Minnesota Experiment Station (Stakman and Piemeisel, 1917 ;

Levine and Stakman, 1918).

d. Barberry Eradication. The disastrous stem rust epidemics in 1904

and 1916 were chiefly responsible for the region-wide barberry eradication laws passed by North Dakota in 1917 ; by South Dakota, Minnesota,

Iowa, Nebraska, Colorado and Michigan in 1918 ; and later by Montana,



* Report of the cooperator’s work on cereals at the South Dakota Agricultural

Experiment Station during the season of 1902. Nov. 7, 1902. (Unpublished.) Filed

with the Division of Cereal Crops and Diseases, Bureau of Plant Industry, Soils, and

Agricultural Engineering.




Wyoming, Wisconsin, Illinois, Indiana, Ohio, Pennsylvania, Virginia,

West Virginia, Missouri, and Washington. By 1950 about 340,000,000

bushes had been destroyed and about four-fifths of the area in eighteen

states had been cleared of barberry. About 200,000 square miles are

still partially infested, mostly in areas that are not easily accessible.

Barberry eradication has been accompanied by a marked reduction

in losses due to stem rust. This period has coincided with the increasing

and widespread use of resistant varieties in the Great Plains. It is this

area i n which the most severe losses occur. Damage to susceptible common wheat varieties in experimental plots in the spring wheat region as

late as 1945 and to d u r n wheat in 1951 and 1952 showed very clearly

that barberry eradication alone is not sufficient to control stem rust in

this area. The reason is now known to be the wind-blown spores coming

from the overwintering areas in south Texas and northern Mexico. Infection from barberry bushes usually occurs earlier in the spring than

that from wind-blown spores, and in some areas these bushes are the

principal source of infection. These facts, plus the production of new

races of stem rust by hybridization on barberry bushes as discovered by

Craigie (1927, 1928), provide sound reasons for continuing the eradication campaign.

2. Leaf Rust

Leaf rust of wheat, although more generally and uniformly distributed throughout the humid wheat-growing areas of the world, causes less

damage than does stem rust. Formerly, it was most severe in the winter

wheat areas of the United States, especially in the East and Southeast.

I n more recent years, damage has been considerable in the central United

States and in Canada on hard red spring wheats. In 1938 it occurred

in epidemic force in this area and caused losses u p to 30 per cent in several states from Texas to Canada.

Leaf rust does not require infection of a n alternate host in its life

history, and unlike stem rust it survives the winter on wheat plants, as

far north as Maryland. Although the rust can resist cold, it cannot endure high summer temperatures, and, consequently, it often dies during

the hot summers of the southern wheat areas. Chester (1939) showed

by me:i.ns of airplane spore traps that viable spores are blown back from

the North by the northerly fall winds to infect the winter wheat in the

South. A t times this infection is so severe that it injures the wheat for

winter pasture and causes a poor survival of the crop. Leaf rust thrives

during periods of damp weather with temperatures ranging from 50" F.

to 60" F. Little or no infection occurs above 80" F.

The most practical method of combating leaf rust is by breeding re-

Tài liệu bạn tìm kiếm đã sẵn sàng tải về

V. Improvements in Methods of Breeding Wheat

Tải bản đầy đủ ngay(0 tr)