THE IMPROVEMENT of farm and garden crops undoubtedly began in prehistoric times after man abandoned his nomadic habits and settled in more or less permanent quarters.

Evidence has been found in graves and caves in many parts of the world of a slow, steady improvement in the quality of crops people have grown since the dawn of history.

We feel sure that they have always carried forward some sort of selection of the better plants for seed.

Man's native curiosity must have led individuals in every group in every age to save seed from the plants in their plots for planting the next year, even though they knew nothing as to how seeds are formed and how plants inherit their features.

The Arabs were the first people to recognize sex in plants. They knew they had to plant a few male trees in their date gardens in order to get dates. After all those centuries of living, when crops were good, and starving, when they were not, and not doing very much about it, the breeding and improvement of plants came to have a scientific basis when the experiments of a monk in Austria became known.

Gregor Johann Mendel (1822-1884), a botanist, entered the order of Augustinians at Brunn when he was 21 years old. In the monastery garden he grew peas year after year and made crosses between the different types (tall X dwarf, yellow seed X green seed, and such).

He noted and recorded the characteristics of size and color and how each persisted or failed in each succeeding generation of peas. After 22 years of experimenting and observing, he read a paper before the members of the Natural History Society of Brunn, in which he told how traits of peas were transmitted.

His discovery, known as Mendel's Law, explains the inheritance of many characters in animals and plants from their parents. His hearers failed to understand the importance of his findings. This small group should not be censured too severely—an outstanding botanist of the period also failed to appreciate Mendel's epochmaking work.

Father Mendel's paper collected dust on the shelves of the Brunn library until 1900, when three botanists in other countries discovered his paper, confirmed his results, and opened a new world of knowledge.

A PLANT produces seed in one of four ways.

The flowers of some plants are so formed that the stigma is never exposed and receives only its own pollen. Such plants, called self-pollinizers, include peas, beans, and wheat.

A second group of plants set no seeds or only a few seeds unless pollen from an unrelated plant reaches the stigma. They are called self-incompatible and include members of the cabbage family and some species of tobacco and lilies.

A third type sets seeds from its own pollen (self-pollination) or pollen from another plant (cross-pollination). Corn, onions, and celery are examples.

The fourth type has some plants that are male and some that are female. Seeds are formed only when male plants are present to furnish pollen. Spinach, asparagus, hops, and hollies are examples.

It is essential in all plant breeding work to control pollination.

Self-pollination is no problem with wheat, beans, and peas, in which the anthers and stigma are in close contact and are shielded within the flowers so that only their own pollen can function.

If they are to be cross-pollinated, however, their own anthers—the male organ—have to be removed before they shed pollen, and the stigma—the female organ—has to be protected for some days after pollen from another plant is applied to them.

The same procedure is necessary—in plant breeding, that is—with plants that have open flowers and are naturally cross-pollinated.

Pollination by hand is slow and laborious. Small, delicate flowers frequently are injured, and seeds are not obtained.

In nature, the pollination of many flowers depends on insects, which carry pollen on their legs and bodies as they move from flower to flower. Plant breeders now use insects to make cross-and self-pollinations by placing flies and bees in cages where plants are isolated to protect them from undesired pollens.

In the Netherlands I saw breeding work with cabbage, in which the plants were grown in small, screened cages with bumble bees as pollinators. The Dutch investigators discovered there were two types of female bees. One soon tried to escape from the cage and did not live long in captivity. The other lived contentedly in the cage. The contented bees were found to be diseased; the disease made them sterile and lacking in homing instinct. They flew low over the ground for short distances, settling and creeping in between plants or into little holes. They were easily captured in insect nets and were much calmer than healthy individuals.

THE PLANT BREEDER has available several methods of obtaining seeds for the improvement of crops: Mass selection, self-pollination and progeny testing, the combining of desirable plant characters by hybridization, and the use of hybrid vigor.

Improvement by means of induced polyploidy also has been tried but needs to be explored more thoroughly.

Mass selection was in general use during the 19th century and had considerable value in developing varieties of some plants. It consists in selecting uniform plants of the desired type for seed production. The selected plants are planted in an isolated plot or under cloth or screened cages to insure that undesired pollen will not reach them. When a crop is grown from seed of the selected plants, the best individuals are again selected, and the process of isolation is repeated.

Successive repetitions of this procedure result in a gradual improvement of the variety, since only superior plants are used each year to produce the succeeding generation.

The method is no longer in general use in plant breeding, but seedsmen use it to maintain a high degree of uniformity of stock seeds from which the main crop of seed will be produced.

Self-pollination with progeny testing is a vast improvement over mass selection. Mendel showed that a plant, although it seems identical to others, may carry hidden characters that show up only in some of its seedlings.

Self-pollination of a plant is inbreeding. It reveals the hidden characters of the parent plants and demonstrates their value for breeding.

Selfed—self-pollinated—seed can be obtained only on plants that are self-compatible. Wheat, rye, and barley are examples. They produce seed without pollen from other flowers because their flowers never fully open and expose the stigma to other pollens.

Onions, corn, celery, and carrots bear fully opened flowers and cross-pollinate readily. If their own pollen reaches the stigma, they also set selfed seed. These plants, unless isolated, set a mixture of selfed and crossed seed.

Plants may be isolated in a number of ways. A cloth or a cage of wire screen may be placed over the plant. The flower may be enclosed in a paper bag or small cloth or wire cage. The object is to keep the flowers protected from visits of pollen-bearing insects and from windblown pollen.

Lettuce is both self- and cross-pollinated. Selling is assured by using cloth bags tied to the stem of the plant below the flowers and to a stake above them.

A variety of any seed-propagated plant is valuable only when it comes true to type. A breeder can determine trueness only by growing a number of seedlings from selfed seed in a progeny test and observing them for uniformity. A plant that is fairly pure (homozygous) for many characters, will produce uniform progeny. A plant not pure (heterozygous) will produce variable progeny.

The breeder selects the best plants from the most uniform progeny and self-pollinates them in succeeding years until he gets the degree of uniformity he wants. When that point is reached, he isolates all the plants of the selected progeny in a large cage or in an isolated plot far from other similar plants so they can form seed only from selfing and interpollinations. If the progeny obtained from this mass planting is satisfactory, the line may be considered established.

Complete self-incompatibility in crop plants is not common. Some groups, such as cabbages and radishes, are only slightly self-compatible, and pure breeding lines of such plants are difficult to obtain.

O. H. Pearson found that seeds were produced if flowerbuds of cabbage were artificially opened and their pollen applied to the stigma. Seed is produced also on self-incompatible radishes by bud pollination.

Cabbage plants of the same variety will cross-pollinate if they are not too closely related.

Onions and some crops lose vigor after a few generations of inbreeding. With such crops it is probably unwise to continue self-pollinations beyond a few generations. Varieties made uniform by inbreeding can then be mass planted for seed production.

Plants, such as asparagus and holly, that have separate male and female individuals present a problem.

With asparagus, where yield is of primary importance, it is necessary to obtain production records of male and female plants for a number of years if the best potential parents are to be chosen. It is also desirable to test the combining ability of each male with all the female plants. Such a program takes a long time. It probably explains why there are relatively few named varieties of asparagus, compared to the many in most other crops.

Several plant breeders have attempted a different way to breed asparagus. Various investigators discovered that male asparagus plants outyield female plants.

W. W. Robbins and H. A. Jones, of the University of California, learned that an occasional perfect flower, with both male and female organs present, was borne on a male plant. Male plants of this type seldom occur. Later, C. M. Rick and G. C. Hanna determined the mode of inheritance of sex in asparagus and showed it was controlled by a single genetic factor.

The factor for maleness was found to be dominant and was designated by M. Femaleness was designated by m. When females were crossed with males, one-half of the progeny was male (Mm) and one-half was female (mm). It was reasoned that selfing perfect flowers on male plants should produce one-fourth MM to one-half Mm to one-fourth mm. This has proved to be the case, and J. Sneep of Holland obtained all male plants that are MM and these can be planted with female mm plants to produce hybrid seeds that will all be Mm. All seedlings therefore have the male characteristics. By testing with various female plants, breeders hope to develop all male plants of superior quality.

Hybridization makes it possible to combine the desirable characters of two plants. Hybrids are produced by the crossing of plants of different genetic constitution. Hybrids may be made between varieties, and some can be made between species. When cross-pollinations are made, the seed parent should be protected from receiving undesired pollens. Removing the anthers is not necessary if the plant is completely self-incompatible. The anthers of self-compatible flowers, in order to eliminate self-pollination, should be removed before pollen is shed. The stigma of each flower should be protected from contact with undesired pollens.

Plants that are naturally cross-pollinated by insects or windblown pollen are usually heterozygous for some characters. It is hard for the seedsmen to keep such varieties pure. It is not feasible to isolate the plants within any sort of cage when the seed crop is grown on a large scale. Seedsmen resort to isolation by separating cross-compatible varieties at distances too far for windblown pollen and insect transference of pollen. Such precautions are not necessary for crops that are naturally self-pollinated.

When two heterozygous plants are crossed, the seedlings will be variable, depending on how heterozygous the parents were. Much time can usually be saved in plant breeding if relatively homozygous plants are used as parents.

Plants may be made homozygous by making successive self-pollinations. Some plants are made fairly true after a few selfings. Others may require seven successive self-pollinations—or even more.

If hybrid seedlings are both self- and cross-compatible, seeds may be obtained by self-pollinations, by inter-crossing, or by backcrossing to each parent. If the hybrids are self-incompatible, seeds are produced only from cross-pollinations and backcrosses.

The seedlings obtained by crossing two plants are the F1 hybrid generation. If F1 plants are self-pollinated or two of them are crossed, the seedlings are the F2 generation. The progeny of a self-pollinated plant is the first inbred generation. When hybrids are crossed to either parent, the seedlings obtained are called backcrosses.

The hybrids obtained by crossing two plants may be of little value even though the parents possessed desirable characters. Subsequent generations from the hybrids must be obtained in order to get new desirable combinations of the parental characters. The plant breeder should select as parents for each succeeding generation the plants that are closest to his objective.

A plant may be undesirable in every respect except in resistance to a certain disease. If it is crossed to a more valuable, although susceptible plant, the hybrids may be susceptible or resistant. It is advisable in such instances to use the backcross method of breeding.

If the hybrids are resistant, the best plants among them should be back-crossed to the susceptible parent. The resulting progeny will contain both resistant and susceptible plants.

By making successive backcrosses to the susceptible parent—using in each generation only the superior resistant plants as one parent—the breeder can transfer resistance from one variety to another.

If the first-generation hybrids are all susceptible, backcrossing to the susceptible parent is advisable even though resistance has not appeared in the hybrids. Some of the plants of the first backcross generation should then be self-pollinated if possible or crossed with sister plants. The progeny obtained will produce both resistant and susceptible plants. The best resistant plants should be backcrossed to the original susceptible parent. This procedure will eventually yield desirable plants that are resistant to the disease in question.

Hybrid plants often are more vigorous than either parent. Hybrid vigor is utilized in the production of some crops. The outstanding example is hybrid corn.

Hybrid seed is available in cabbage, cantaloup, castorbean, corn, cucumber, eggplant, onion, pearl millet, petunia, snapdragon, sorghum, spinach, squash, sugarbeets, tomato, and watermelon.

HYBRIDS, to be useful, must be uniform in growth, quality of product, and high yield.

When two varieties of any crop plant are crossed, the hybrid may or may not exhibit hybrid vigor. If the varieties are homozygous, the hybrids will be uniform, the degree of uniformity depending on how pure the parents were. The vigor of hybrids between different plants of the same variety or species varies greatly.

The method now used generally to develop hybrids is to obtain homozygous lines by inbreeding and then to make all possible crosses between them to test their combining ability. Desirable inbred lines that are obtained are kept isolated to maintain their genetic purity. The producer of hybrid seed then makes the appropriate cross-pollinations, and the seed he sells, although it produces a good crop, will not produce uniform seedlings because of its hybridity.

Only corn and onions are now grown on a large scale from hybrid seed. The others are used so far by home gardeners, but as better methods of obtaining hybrid seed are developed, other crops are certain to be produced by this method.

Hybrid seed is more difficult to produce than ordinary seed. The method of making cross-pollinations varies with each crop, and the considerable hand labor required makes the seed rather costly.

Hybrid corn is relatively easy to produce because the male and female flowers are on separate parts of the same plant, and one can be removed without disturbing the other. The corn tassels, borne on the top of the corn plant, produce the pollen. A plant that is detasseled can set seed only if pollen is supplied from another source.

Hybrid corn seed is produced by planting six rows of the seed-bearing parent bordered on each side by a row of the pollen-bearing parent. A ratio of one row of pollen parent to three rows of seed-bearing plants is in general use. The tassels of the six rows are removed, and all seed set on them will be hybrid.

A large seed field may have many blocks of plants arranged in this manner. Such fields are kept isolated according to regulations established by the States where hybrid seed is produced. The distance required between plantings of different lines varies with the size of the field. In all instances, a certain number of rows of the pollen parent must be planted around the entire field. These rows intercept pollen blown in from other sources and also the pollen from flying insects that work over these plants.

Most hybrid seed is produced by hand crossing. This involves removing the anthers (emasculation) of all flowers to be crossed before pollen has been shed and keeping such flowers protected from insects and windblown pollen. The small, fragile flowers of some plants are easily injured and are difficult to isolate under bags. Emasculation and pollination are carried on in greenhouses for some plants, such as snapdragon and petunia.

The utilization of male sterility to produce hybrid onion seed has stimulated a search for the same character in other plants. Plants that are male sterile do not need to be emasculated.

A male-sterile plant of the variety Italian Red was found by H. A. Jones in the onion-breeding plots of the University of California at Davis in 1925. Viable pollen was not produced on the flowers of this plant, and all seed set on it originated from pollen from other plants.

The male-sterile character had to be introduced into inbred onions in order to produce a wide range of onion hybrids, uniform in shape, color, and other characters.

Male sterility is inherited as a recessive gene. Two types of cytoplasm are involved. When the gene for male sterility is present in a plant with normal cytoplasm, the plant produces normal pollen. The male-sterile gene functions only in the presence of a sterile type of cytoplasm, and viable pollen is not formed. Hybrid onion seed can be produced by planting a male-sterile variety with a normal variety that furnishes pollen.

The male-sterile gene has now been incorporated into many onion varieties, and hybrid onions are widely grown. The male-sterile bulbs are planted in four rows with a row of bulbs of the pollen parent on each side. Bees and flies carry pollen to the male-sterile plants, and all their seed is of hybrid origin.

The time of blooming varies with different onions. The pollen parent bulbs are planted a week or two earlier, depending on their flowering date. This insures that pollen will be available when the male-sterile plants come into flower.

The same general type of cytoplasmic male sterility as occurs in onions has been found in sugarbeets, carrots, corn, millet, orchardgrass, pepper, petunia, sorghum, tobacco, and wheat. Hybrid petunias are produced with male-sterile lines. Research has begun with beets, carrots, and other crops.

Plant breeders are aware of the potential possibilities of this method of producing hybrid seed. Undoubtedly male sterility will be discovered in other crops.

Polyploid plants are ones that have multiples of their basic chromosome number. Plants with twice the basic number are called diploids. Those with three sets of chromosomes are triploids. Those with four sets are tetraploids.

There are known instances of spontaneous doubling of the chromosome numbers of a primrose and of the poinsettia.

Some of our finest ornamental plants and many of our fruits are polyploids. Until about 1910, practically all varieties of garden iris were diploids. Larger flowered seedlings began to appear about this time in the plots of iris breeders. These large-flowered seedlings were named, and in 1943 the chromosome number of 109 of these new varieties was determined; 108 were tetraploid. One was a triploid.

The new varieties of poinsettia originated as bud sports and were propagated because of their superior characters. These new varieties have been found to be tetraploids.

The discovery in 1937 that colchicine would double the chromosome number of plants gave plant breeders the opportunity of exploring the possible usefulness of tetraploidy in improving crop plants.

Research workers found that the immediate results were not promising. Tetraploids of seed-propagated crops are highly sterile, especially those of normally self-pollinated crops. The tetraploids of many plants were inferior to the diploids, and some of the early enthusiasm regarding induced tetraploidy subsided. It was not generally recognized that induced tetraploids should be considered as raw material for continued breeding and selection.

MANY ORNAMENTAL plants are propagated from cuttings (asexual propagation), and fertility is not important in them. In fact, practically all asexually propagated ornamentals do not come true from seed because of their heterozygosity.

Induced tetraploids of such plants have shown some promising results. Several species of Lilium have been made tetraploid and are coming into use in gardens. A tetraploid forsythia produced by colchicine treatment at the Arnold Arboretum, Boston, Mass., bears larger flowers of a deeper golden color.

Tetraploid carnations from colchicine have sturdier stems and larger flowers. A few tetraploid carnations are on the market.

Induced tetraploids usually flower later than their diploids and produce fewer flowers per plant.

This was the situation in lilies, but after 15 years of continued breeding and selection, early blooming and floriferous tetraploid lily seedlings were developed. There has been a rearrangement of the genes in these lilies, and late blooming and lower flower production have been eliminated.

Colchicine-induced tetraploids may be useful in breeding when one parent is diploid and the other is tetraploid.

In cranberry, there are three species: two are diploid and one is tetraploid. All attempts to cross the tetraploid with the diploids failed. The chromosomes of the diploid varieties were doubled, and some of these induced tetraploids were crossed with the tetraploids. These crosses are leading to new and improved varieties of cranberries.

Hybrids between species are usually sterile. The chromosomes of the two species, although able to form a new plant, cannot form functional pollen and egg cells. Doubling the chromosome number of such hybrids usually produces fertile plants that may lead to developing new varieties. Such sterile hybrids have spontaneously produced branches with fertile flowers. The branches have the double number of chromosomes.

Polyploid sugarbeets have been produced in Sweden and Japan. The tetraploids with 36 chromosomes were inferior to the diploids with 18 chromosomes, but when they were crossed with diploids, they produced useful triploids with 27 chromosomes. Swedish plant breeders have reported that after a long period of breeding and selection, useful strains of tetraploid sugarbeets are being obtained.

A triploid watermelon from Japan is available. It was developed by crossing colchicine-induced tetraploids with diploids. The cross is successful only with the tetraploid as seed parent. The triploid watermelons are seedless. They have a thin rind and high content of sugar.

THE PLANT breeder now has many techniques and tools to work with. The accomplishments of the past half century are many, but will probably be far exceeded as newer and more improved methods are devised.

Alfred Stefferud

Copyrighted material; do not reprint without permission.

 

View all articles by Alfred Stefferud