USDA Yearbook of Agriculture (1937)
Improvement of Flowers by Breeding
S. L. Emsweller, Principal Horticulturist,
Philip Brierly, Associate Pathologist,
D. V. Lumsden, Associate Physiologist,
F. L. Mulford, Associate Horticulturist,
Division of Fruit and Vegetable Crops and Disseases, Bureau of Plant Industry
MASS SELECTION AND LINE BREEDING
Mass selection does result in a gradual improvement even if some of the selected plants transmit poor germ plasm to the next generation, since the good ones have a better chance to be selected and in time predominate, gradually building up the variety. In this way desirable results are often secured, but frequently many years are required before the effects are noticeable. It is not true, however, even though commonly believed, that the selection process in itself is responsible for the gradual changing or improvement of a flower. There is little reason, in the light of our present information on plant breeding, to continue using this method. But even today it is the general practice among many flower breeders.
Following mass selection came so-called line-breeding. Here the breeder selected single plants and grew the seedlings from each selection separately. In this way the descendants of an individual plant could be studied, and the method was soon recognized as a considerable improvement over the earlier practice. It was possible to determine more readily whether an observed variation was hereditary (in the germ plasm) or simply an environmental modification. It also disclosed time status of a given characteristic in the heredity of a plant. If all the seedlings possessed the characteristic, it might be assumed that the plant was "pure" or homozygous for that characteristic and that it would be passed on to all the descendants; if some seedlings had it and some did not, the plant was heterozygous or mixed in its inheritance.
Unfortunately much of the single-plant-selection work of both amateur and professional flower breeders is done with open-pollinated plants—that is, no provision is made to protect from cross-pollination with other nearby plants. Of course, if a flower is normally self-pollinated, it may not be necessary to have this protection, but there are not many flowering plants that are entirely self-pollinated. The outstanding one supposed to be self-pollinated is the sweet pea. Here the stigma, completely surrounded by a group of anthers, is enclosed in that portion of the flower called the keel (fig. 9, D). When the flower matures the pollen is shed and falls on the stigma, which normally does not protrude from the petals forming the keel. In this manner the flower is supposedly protected from all pollen except its own.
Yet conditions in sweet-pea seed fields make this doubtful. If the sweet pea is entirely self-pollinated, it should be a simple matter to keep varieties pure (homozygous), since they could only acquire new genes by way of mutations. While the mutation rate in sweet peas has not been studied, the very large number of offtype plants (rogues) in sweet-pea seed fields certainly cannot be explained on this basis. In some instances these rogues are simply the result of accidental seed mixing, but this is not likely to occur in the case of the more careful growers, who keep their stock seeds separate. Very frequently, also, sweet-pea breeders encounter considerable difficulty in making a strain pure. An examination of several such cases showed that many of the flowers were not fully protected from cross-pollination, as had been assumed. The keel was not fully developed and was too short for the pistil and anthers. As a result the stigma very frequently protruded, even beyond the anthers, so that cross-pollination could easily have occurred.
There are some flowers in which the single-plant-selection method, depending on self-pollination, cannot be used. This may be due to self-sterility, as in some lily species, especially most forms of Lilium longiflorum Thunb., and in some iris, petunia, and Nicotiana species. Sometimes reasons other than self-sterility are responsible for failure of flowers to set seed when single plants are isolated. The pollen may be discharged before the stigma is receptive, or the reverse condition may occur. The structure of the flower may be such that self-pollination is impossible without the aid of some agency such as insects. Sometimes, as in species of Lychnis, the plants are either male (staminate) or female (pistillate). Such types obviously cannot be self-pollinated.
Single-plant selection or line breeding has many advantages over mass selections. When accompanied with self-pollination it quickly gives strains very uniform for habit of growth and other characters. All undesirable qualities are disclosed rapidly and may be eliminated within a comparatively few generations. Unfortunately, continued self-pollination frequently results in a loss of vigor. As a rule this weakened condition may be corrected by making cross-pollinations between inbred strains. This situation has received considerable attention from corn breeders and is discussed at some length in the 1936 Yearbook of Agriculture.
Compare the opinions expressed above with those of Brunson (1937).