Proceedings of the Eighth International Grassland Congress
held at the University of Reading, England, 11-21 July 1960. p. 51-53
Michurin Methods in Breeding Forage Crops by Means of Hybridization
A. M. Konstantinova
(Williams Research Institute of Fodders, Lugovaja, Moscow Region, U.S.S.R.)
Summary. The investigations were carried out on the problems of transformation of wild lucerne into highly productive cultivated hybrid varieties. The Michurin theory of plant hybridization was used as the theoretical basis of the work. The main attention was paid to problems of selection of parents for crossing and to 'training' of hybrid progenies.
The development of methods for transforming wild populations into valuable highly productive varieties is of great importance in breeding perennial forage grasses. This is connected with the fact that in many regions of the USSR grass-breeding is based on the introduction of wild native forms. Wild populations of grasses are usually well adapted to local conditions, but they have many undesirable properties such as low productivity, poor aftergrowth, uneven ripening of seed, seed-shattering, etc. Wild grasses and legumes with entirely desirable characteristics are very seldom found. In most cases considerable changes of the wild forms are necessary before they can be introduced into culture.
The Michurin methods of plant breeding offer great possibilities in this respect. These methods are based on the understanding of heredity as a property of the organism to require certain conditions for its development and to respond to them adequately. Heredity is formed by the influence of environment on the preceding generations. Therefore changes in heredity are due to environmental changes only.
The foundations of the Michurin theory of plant breeding by means of hybridization are:
(1) Ecological-geographical principles in choosing parents with due consideration to the soil and climate differences in areas of their origin. More vital and productive progenies can be obtained as a result of crossing those forms within species which are the most remote, from the ecological-geographical point of view.
(2) The directed 'training' of the hybrid plants of the first generations by growing them under such environment and treatment as to promote the development of the desirable characteristics of the strain.
The following are the most important laws of the Michurin theory: (a) The ability of plants in the first stages of their development (at germination, in the seedling phase) to respond greatly to the influence of environment; (b) Higher adaptability of the first generations, especially if geographically distant parents were crossed; (c) The most pronounced conservatism in heredity of wild species and old local varieties, especially when they are grown in conditions corresponding to those of their native place.
The important methods of plant breeding (overcoming and weakening this conservatism by hybridization, and placing parents which show conservatism in heredity in an unusual environment) are based on these laws.
Owing to selectivity in fertilization it is possible to apply free pollination and artificial crossing without the emasculation of the flowers in the breeding of crosspollinating perennial grasses.
Following these principles we have succeeded in transforming the northern wild yellow lucerne (Medicago borealis Grossh.) into a highly productive hybrid variety in the moist and cold conditions of the northern regions of the USSR, on heavy, loamy, podzolized soils. The Mologian, Moscowian and Angarian (from East Siberia) varieties of yellow lucerne were chosen. These forms are of no value as forage plants; they have low productivity and very low aftergrowth, but they are well adapted for growing in the North.
An important hereditary capacity had been found in the Mologian lucerne. This variety, when transferred from its natural environment to arable soil in the same natural area, does not change its characteristics. Due to the pronounced hereditary capacity inherent in the wild forms, Mologian lucerne behaves similarly in the conditions of the Moscow region. Even after crossing with the common lucerne (Medicago sativa L.), its characteristics invariably dominate in the hybrid progenies. Similar results were found in our experiments with the Dedinovian, Ilovenian and Pskovian wild yellow lucernes.
The conservatism of wild forms has not always been taken into consideration in attempts to transform them into a cultivated type, or in their hybridization. This was often the reason for failure in their introduction.
In order to weaken the conservatism of the wild Mologian lucerne this variety was introduced into conditions that did not correspond to its heredity. It was grown in the Omsk Region of Western Siberia with a dry soil and climate that contrasted sharply with the conditions of the native land of the variety. Reproduction in this unusual environment forced it to assimilate the new conditions of existence. The Mologian lucerne weakened its hereditary capacity during 2 generations. Experiments in crossing it with common lucerne particularly stressed this fact. Mologian lucerne taken for crossing after 2 generations in the unaccustomed conditions of the Omsk Region, produced up to 20% pronounced hybrid plants, in the hybrid progenies, while when crossed with the same common lucerne in its native conditions in the Moscow Region it retained its conservatism and its characteristics dominated.
The possibility of weakening the conservatism of wild forms by radically changing their environments made it quite possible to take for breeding purposes in the Moscow Region, not only the local wild lucerne but also forms from more distant regions where conditions are the same or even more severe. Therefore the wild Siberian northern forms of lucerne were chosen. These forms are characterized by high conservatism in the conditions of their native land. In the conditions of the Moscow Region the hereditary capacity of these wild forms was weakened and was no longer predominant when they were crossed with common lucerne. Typical hybrid plants with intermediate characteristics predominated in F1 and amounted with Angarian to 93%, Yakutian 87%, Khakassian 42%, West Siberian 100% and with the Volga-Akhtubian Medicago coerulea 96%. These results were obtained with both artificial and free pollination.
It has been found that when wild forms are placed in unaccustomed environments the type of their development is changed and the hereditary capacity weakened even within one generation. For example, when grown under free pollination conditions with common lucerne the progeny of two-year plants of wild Angarian lucerne produced up to 20% pronounced hybrid plants, the progeny of the three-year plants up to 58% and of the four-year plants up to 89%.
These results show that the weakening of hereditary capacity obtained by placing wild forms in changed and unusual environments is decisive in transforming them into cultivated varieties. Without this preparation hybridization cannot give the required changes in heredity of the wild forms.
During our experiments with lucerne it was also established that the Michurin method of selection of geographically-distant parent forms is of primary significance. The hybrid progeny originating from the crossing of forms of distant origin were considerably more productive than the progeny of nearer ones. For example, the productivity of the progeny obtained by crossing the Mologian lucerne with another northern form, the blue Kazanian lucerne 36, only slightly exceeded that of the low-productive Mologian and did not exceed the productivity of the Kazanian 36. On the other hand, the hybrid obtained by crossing the same Mologian variety with the geographically-distant southern (West Chinese, Asia Minor, Armenian and other) varieties of common lucerne from districts of irrigated farming, exceeded the original wild Mologian almost 8 times in productivity. The hybrid plants obtained by crossing such distant forms were taken as initial material for creation of the northern hybrid lucerne varieties.
It would be incorrect to say that the creation of the northern lucerne varieties depended only on the selection of geographically-distant parents with desirable characteristics. Its further development, or 'training', beginning with the first, and most plastic, hybrid generations was of even greater importance.
One of the main factors of 'training' was the cultivation of hybrid progeny under the influence of the soil and climatic conditions of the district for which the variety was being created. At the same time a number of agrotechnical measures were also applied. Among them were: a system of fertilizing, both initial and topdressing, that ensured the fertility and neutral reaction of the soil; spring and autumn sowing; early seed-harvests; repeated cuttings and other measures. It was also found that the hybrid progeny in the main was responsive to the influence of environment in the first and second generations only; beginning with the fourth and fifth generations the hybrid becomes conservative to a certain extent.
While working out methods of 'training', the significance of different external factors in controlling the dominating influence of the parental characteristics on the development of the hybrid progeny was ascertained. It was established, for example, that growing in fertile soil with neutral reaction can intensify and develop in the hybrid progeny the properties of cultivated lucerne, and weaken the properties of the wild variety. Under the influence of fertile soil, plants with variegated violet flowers, more twisted pods and rapidly-growing herbage (characteristics typical of the cultivated lucerne), begin to predominate in the hybrid progeny. Under the influence of comparatively poor and more acid soil, the number of low-productive plants with the variegated greenish flowers and unfolded pods that are more typical of the wild lucerne, increases in the progeny.
Sowing in spring also helped in making the variety of a more cultivated type, while early seed harvest improved the seed productivity of the variety. Frequent mowing promoted the capacity for quick aftergrowth.
The results of these experiments made it possible to limit the dominating effect of the wild form, which could already be seen in the first generations of the variety Northern Hybrid. The development of the hybrid progeny was directed towards the increase of yield and acquisition of cultivated characteristics by growing them on the fertile soil with neutral reaction for 5-6 generations. The original wild form grown in the same conditions remained almost unchanged.
The effectiveness of this work has been proved by the development of the lucerne variety Northern Hybrid 69 for the zone of podzolized soils of the USSR. It yields from 7000 to 10,000 kg of hay/ha in 2 or 3 cuttings and 200-300 kg of seed/ha, surpassing in this respect all other varieties in the zone of podzolized soil. Stands of this variety can remain productive for 5-10 years without resowing.
The principles of Michurin theory concerning the selection of parents and the 'training' of the hybrid progeny can be applied on a wide scale in the creation of the so-called complex-strains (polycross).