Wide Hybridization in Plants (Trans. 1960)
WIDE HYBRIDIZATION IN SILVICULTURE AND GREENBELT WORK.
SURVEY AND PROSPECTS

A.S. Yablokov

Acting Member of the All-Union Academy of Agricultural Sciences im. V. I. Lenin
All-Union Research Institute of Forestry and Mechanization of Forest Economy*

*[Vsesoyuznyi nauchno-issledovatel'skii institut lesovodstva i mekhanizatsii lesnogo khozyaistva— sometimes given in text as All-Union Research Institute of Forestry and Mechanization —Vsesoyuznyi nauchno-issledovatel'skii institut lesovodstva i mekhanizatsii— VNIILM.]

The term "wide hybridization" may be defined in two ways: a) it is intercrossing individuals taxonomically remote from each other (species, genera, etc.); and b) it is crossing within the limits of the same species, between individuals which are remote from each other ecologically—between ecotypes from different soils and climates, or between geographically distinct lines or varieties.

In forest-tree and shrub breeding, various problems have been solved according to the biological peculiarities of the species concerned and their economic utilization.

One of the most interesting problems was the acclimatization in the central and northern belt of the European part of the U.S.S.R. of lines of valuable trees like walnuts, Juglandaceae, and pyramidal poplars, or shrubs like filberts of the genus Corylus. Another very important task was to obtain more fast-growing and extra-productive, vigorous new tree forms for use in forests and for greenbelt planting. We successfully solved this problem by wide hybridization involving many coniferous and deciduous tree species: aspen, poplar, birch, larch, pine, oak, walnut, etc.

We paid special attention to obtaining new forest-tree forms with hybrid vigor, and we propagated these hybrid forms quickly and in large quantities by cuttings, root suckers and grafting. Moreover, in forestry and greenbelt tree raising, as in agriculture, it is most expedient to produce large quantities of F1 hybrid seed for direct use, with the object of growing heterosis forms.

The productivity, longevity and hardiness of forest and greenbelt trees grown from such seed is greatly increased. Our research on aspen, larch, birch, and other trees confirms this assumption.

The third problem in the production of forest breeds by wide hybridization was that of raising the immunity to diseases. In many cases this problem has been successfully solved, e.g. aspen immune to core rot and birch immune to rust have been produced.

Finally, wide hybridization trials are made in search of new crossing methods and techniques. Using these new means we may produce more valuable forms and even species of trees and shrubs, hitherto nonexistant, for use in afforestation and, in particular, for greenbelt planting.

Our forest-tree breeding projects were carried out on the theoretical basis of the materialistic, Michurin doctrine of heredity and its modifications. Our aim was not only to obtain practical results, but also to elaborate the theoretical aspects of breeding, particularly in relation to its application in forestry. This work should lead to a fuller satisfaction of the demands of the forest economy.

Our basic assumption was that in forestry, as distinct from horticulture, vegetable growing or field-crop growing (and also animal breeding), the aim is not the production of stable varieties of trees and shrubs. On the contrary, experience convinces us that complex species populations utilize the variety of microconditions of the external forest environment more fully. The various forms obtained by crossing different species adapt themselves more easily to these microconditions. Such complex forest populations can be raised from hybrid seed obtained from wide intraspecific and interspecific crosses.

*[Apparently types adapted to a particular micro-environment.]

In greenbelt planting it is equally important to obtain different microforms* through hybridization. Tree and shrub forms which will adapt themselves more easily to the peculiarities of existence under urban conditions may result from hybrid seed. These forms may often prove to be more stable and long-lived than nonhybrid forms.

The Michurin doctrine of heredity gives a correct interpretation of the dominant influence of the mother plant in transmitting distinguishing characters to the progeny. Bearing this in mind, the preservation and dominance of a majority of the desired maternal characters can be safeguarded in the seedlings. Moreover, with a careful choice of the pollen parent to enter the artificial cross, most of the valuable parental characters can be used.

We understand heterosis as a feature arising in crosses (in sexual or adequately composed graft hybrids) which benefits the hybrid plant by enabling it to react to the various elements of the external environment in a more versatile and highly valuable manner; by experiment we can find parental pairs yielding plants with decided heterosis. Such pairs can be utilized in crosses for many decades; an enormous quantity of hybrid seed can be obtained from them, and millions of trees with intense hybrid growth vigor can be raised.

The Michurin doctrine of heredity established the preservation of heterosis beyond the first hybrid generation, in which its occurrence had been confirmed earlier, to the second, third, and more advanced generations. It is preserved by backcrossing and complex hybridization of progeny from chosen parent material.

Our experiments confirm these assumptions and indicate the scope for creating heterosis forms by wide hybridization. Many tree hybrids which exhibit heterosis can be easily propagated vegetatively for long periods.

Certain hereditary characters which accumulate in wild plant species over the centuries must be shaken loose before valuable trees and shrubs can be acclimatized successfully in new regions of the U.S.S.R. At the same time, the hereditary stock of the plants must be enriched by new characters enabling them to adapt themselves more easily and firmly and to grow and develop normally in the new environmental conditions.

In this problem, interspecific and intergeneric hybridization are of particular significance for many forest trees and shrubs. Periods of acclimatization, in particular of breeds such as walnuts, filberts and Lombardy poplars, can be considerably shortened by hybridization.

One aim of forest-tree breeding is the production of forms with an increased longevity and a fuller capacity to adapt themselves to conditions of the external environment.

These aims can be achieved through wide hybridization. The hybrids derived, with increased longevity, productivity, and resistance to unfavorable living conditions, are an asset to forestry and other tree planting.

If one considers the sexual process and the formation of the zygote not only a physical union of female and male gametes, but also a physiologicalbiochemical process in which the two gametes are mutually assimilated, one may theoretically expect wide hybridization, in combination with the rational training of hybrids, to result in new species of trees and shrubs. Without man's help such species would never have arisen in nature. Thus, it seems possible for man to exert his influence consciously and effectively to determine, in some degree, the direction of evolution in the vegetable world.

Summary of Experiments on Wide Hybridization in Forest Trees and Shrubs

*[Moskovskii lesotekhnicheskii institut — MLTI.]

To confirm what has been said, we cite some of the main results of experiments applying wide hybridization to forest trees and shrubs. These experiments were carried out on the basis of the Michurin doctrine of heredity and its modifications by teams of scientific staff members and candidates of the Breeding Section of the All-Union Research Institute of Forestry and Mechanization (VNIILM) between 1934 and 1957, and by the Department of Breeding and Dendrology of the Moscow Institute of Forest Technology*.

Walnuts. Work on interspecific hybridization of walnuts was begun by us in 1934 and has continued with some interruptions until the present day. In this period a technique of intercrossing Juglans species has been worked out, and crosses have been made involving the Manchurian walnut (J. mandshurica), the butternut (J. cinerea), the black walnut (J. nigra), the Persian walnut (J. regia), the Japanese walnut (J. sieboldiana) and the flat Siebold heartnut.

At the beginning, in Moscow, we found only seven trees of three species suitable for crossing. In the first years, some hundreds of hybrid walnuts were obtained, and from them a little over a hundred interspecific hybrids were grown.

*[Vsesoyuznyi nauchno-issledovatel'skii institut lesnogo khozyaistva — All-Union Forestry Research Institute.]

In 1937, the first stock-plant collection of these interspecific hybrids was planted in the breeding nursery of VNIILKh* near Ivanteevka, Mytishchi District, Moscow Region.

At the same time, some interspecific hybrids were obtained between the Manchurian walnut, hickory (Carya), and Pterocarya. Two of these are growing in the stock-plant collection, where they develop normally and bear fruit.

Among the interspecific hybrids, growing in the stock-plant garden at Ivanteevka, there are numerous valuable trees, marked by fast growth, fertility, dense foliage and the decorative qualities of their seedlings. Hybrids between the Manchurian and Japanese walnut (J. mandshurica × J. sieboldiana and the reciprocal cross) are characterized by strongly pronounced heterosis. In spite of the fact that the stock garden is located on poor, quite dry, loamy, strongly podzolic soil, and overlies sand deposits which reach a depth of 20 m below the soil surface (i. e. the ground water level), the growth exhibited by some of the hybrids exceeds that of the control plants of Manchurian walnut and butternut by five to six times (Figure 1). Marked heterosis is also exhibited by hybrids between butternut () and Manchurian walnut (), and also by hybrids between the Manchurian and black walnut.

FIGURE 1. Interspecific hybrid No. 97 (J. sieboldiana × J. cinerea) which exhibits marked heterosis; it bears abundant fruit, is very winterhardy and withstands frost of -40° C easily

Manchurian × black walnut hybrids have a markedly improved trunk shape; the pronounced upright trunk is reminiscent of that of the paternal type, the black walnut.

Manchurian × Persian walnut hybrids also exhibit growth heterosis, but toalesser extent than J. mandshurica × J. cinerea or J. sieboldiana × J. nigra hybrids.

Moreover they are marked by a peculiar trunk and crown development. They tend to form several trunks; already at a height of 0.5-1 m, they often form two or three trunks, or a number of thick branches.

According to our observations, this peculiar branching habit is characteristic of the two Old World walnut species, J. mandshurica and J. regia.

In later large-scale experiments with interspecific walnut crosses, we observed many young hybrids characterized by a high frost resistance, fertility, and high yields. Some dozens of these different interspecific hybrids were taken as seed parents and were very successfully used for largescale crosses with the best young interspecific hybrids obtained in the prewar period, and with the most valuable southern walnut species, such as Persian walnut and black American walnut.

Intergeneric hybrids have now been obtained from crosses between young interspecific hybrid walnuts and pecan (Carya oliveformis) or hickory (C. alba).

During the years 1947 to 1949 we collected 1,897 walnuts from repeated interspecific crosses, and 429 walnuts from intergeneric crosses (involving pecan and white hickory). In 1951 to 1952 a new stock-plant collection was planted, with 1,851 backcrosses and complex interspecific and intergeneric walnut hybrids. This collection is of great scientific interest and is almost unique among stock-plant collections for the variety and value of the wide-hybrid walnuts it contains. It provides convincing proof of the preservation of hybrid vigor and fertility in generations other than the F1 in remote hybrids; many of these hybrids are extremely strong growers, very winter-hardy and vigorous; seedlings of wild walnut species can in no way compare with them.

There are hybrids among the backcrosses and complex crosses which resemble the Persian or black walnut in their external appearance (leaves, shoots, buds, bark of trunk), but which are absolutely not comparable with these species for winter hardiness and frost resistance. These should find use as new pioneer walnut species in the central belt of the European part of the U.S.S.R.

Thousands of backcross and complex hybrid walnut seedlings are still growing in the seedbeds, pending their being planted out on a permanent site in the stock-plant collections.

I.A. Kazartsev continued the experiments started by us in 1954 to 1957, and carried out large-scale crossings on our hybrid walnuts in the stockplant gardens at Ivanteevka. He obtained 2,758 hybrid plants from them and raised 2,202 new complex hybrid and backcross seedlings. Also, during the same years he successfully cross-pollinated the Persian walnut with Manchurian and black walnut in the Veselo-Bokoven'kovskii stock-plant walnut collection in the Ukraine. From these crosses he obtained 432 hybrid fruits of the Persian walnut and raised 336 hybrid seedlings in the breeding nursery of the All-Union Institute of Forestry and Mechanization at Ivanteevka, near Moscow. Already in the first years of their life, some of Kazartsev's hybrids exhibit notable characteristics, and their scientific value for further studies in the acclimatization of Persian walnut trees in the Moscow area is beyond doubt.

Our experiments in applying wide hybridization to walnuts confirm again the correctness of the Michurin doctrine and of his methods of applying wide hybridization to walnuts. Hybrid walnuts adapt themselves much more easily to a new environment and exploit it more fully, as a rule, than nonhybrids. As a result, hybrid walnuts are frequently faster growing, more fruitful, hardier and healthier than seedlings of wild walnut species) growing under the same conditions.

Giant filberts. The very first attempts to acclimatize subtropical filberts near Moscow, by sowing the seed of a number of varieties obtained from the Caucasus, did not give positive results. The seedlings perished rapidly as a result of the severe winters; only individual plants persisted somewhat longer; but these, too, led a pitiful existence and eventually died.

Accordingly, we resorted to wide hybridization and crossed the common hazelnut from the Moscow area with cultivated varieties of filberts from Sochi; but we found that the wild hazelnut characters are dominant over those of the southern filbert. In external features and fruit characters the hybrids hardly differed from the local wild hazelnut. Our method was based on I.V. Michurin's doctrine of heredity, application of his rules on the choice of parental pairs, special consideration for the part played by the maternal plant in the cross, and subsequent training of the hybrids. We proceeded as follows:

We chose a suitable hazelnut specimen in the Moscow area as pollen parent and, in the year 1935, used its pollen on the racemes of a number of cultivated filbert varieties growing in the stock-plant collection of the Sochi Fruit Station. As a result, a few hundred hybrid seeds were obtained on maternal filbert trees of the Barcelona variety. Our assumption that the properties of filbert variety Barcelona must be predominant in the hybrid embryos in these seeds was later confirmed by the growing seedlings We trained these seedlings (filbert variety Barcelona × common hazelnut) under Moscow area conditions, in the Ivanteevka breeding nursery. Seedlines of filbert variety Barcelona grown simultaneously from seeds obtained by open pollination of the same filbert shrubs which had been used in the crosses with common hazelnut served as control plants.

The results of this experiment were more striking. All openly pollinated control seedlings of the filbert Barcelona variety showed poor growth and were killed by frost in the very first years of their life. About half the hybrid plants (filbert Barcelona × common hazelnut) were ultimately also destroyed by frost, but they persisted a little longer than the former. Some of the hybrids that survived and that are growing until now are frostdamaged every year, and bear almost no fruit; these are plants that have largely inherited the maternal characters for temperature response. The remainder are hybrids that are highly winter-hardy, grow vigorously and bear good yields (for example, hybrids Nos. 42, 31, 14). Moreover, their nuts resemble those of filbert variety Barcelona in external features and taste.

FIGURE 2. The significance of the seed parent in the inheritance of nut size and shape, in crosses of filbert and common hazelnut
1—filbert Barcelona; 2—hybrid No. 42: filbert Barcelona (seed parent) × wild hazelnut from Moscow area (pollen parent), crossed at Sochi; 3—hybrid No. 80: wild hazelnut from Moscow area (seed parent) × filbert Barcelona (pollen parent); i.e., same parent stock as 2, and crossed in same year, but in Moscow area.

In the same year, a cross was made between the common hazelnut (seed parent) and filbert variety Barcelona in the Moscow area. Here, the common hazelnut characters in the hybrids predominated over those of filbert variety Barcelona, and the fruits are very small and resemble those of the hazelnut in external features (for example, hybrid No. 80, common hazelnut × filbert Barcelona) (Figure 2).

New filbert varieties were also obtained by crossing the purple-leaved hazelnut (Corylus avellana var. atropurpurea) with the filbert varieties Cherkesskii II, Kudryavchik [Curly], and Barcelona. They are characterized by considerable ornamental qualities, fertility, thin hulls, and tasty nuts. They are now used for pollinating the new filbert Barcelona × common hazelnut hybrids and, as a result, new purple-leaved hybrids are obtained.

Large-fruited Barcelona × common hazelnut hybrids were also crossed with southern filberts, I.P. Sikor's filberts, and purple-leaved hazelnut hybrids.

A few thousand such hybrids are being grown in a young nut garden. Many of them have already begun bearing fruit; they are marked by luscious growth and are highly decorative and winter-hardy. In future years new varieties of northern frost-resistant filberts will be chosen from them.

In 1957, R.F. Kudasheva, at the "Zakatal'skii" State Nut Farm, in the Azerbaijan SSR, made some new, very promising crosses between the best, large-fruited varieties of cultivated filbert (Konig, Nottingham, etc.) and our Moscow area hybrid filberts or common hazelnut. She obtained 1,920 hybrid fruits, which were subsequently brought to Ivanteevka and sown in the nursery in the fall.

Thus, the use of wide hybridization in the breeding of filberts will aid the establishment of filbert cultivation even in the districts of the Moscow area.

Poplars. Trees of the genus Populus are reared in various parts of the world, in countries where they can exist normally.

Poplars have become one of the favorite objects of breeding because of certain biological properties and because of their ever-increasing importance for the national economy. Poplar breeding aims at character improvements in various directions: fast growth, resistance to diseases and pests, frost-resistance, increased productivity (increased lifespan), and improved ornamental qualities.

At the present time, a number of valuable new poplar varieties have been obtained by wide hybridization in Europe and America; they surpass wild poplar species considerably in the qualities mentioned above.

In the U.S.S.R., hybridization studies in an attempt to produce new, valuable poplar varieties have been under way for some decades in a number of scientific institutions in Moscow, Leningrad, Ufa, Kiev, Khar'kov, Tashkent and L'vov.

The results obtained have been fair enough. New valuable varieties of white, black and balsam poplar (Populus alba, P. nigra, and P. balsamifera), and particularly of aspen (P. tremula), have been produced by wide hybridization.

Concrete examples can be cited of what has been done by the Breeding Division of the All-Union Research Institute of Forestry and Mechanization. In 1936, the first crosses between the white poplar or aspen and Populus bolleana Zauch were made to obtain new varieties of pyramidal poplars for the Moscow area, Siberia, and the northern districts; this purpose was successfully fulfilled.

New lines and varieties of the pyramidal silver poplar were obtained from crosses between the white poplar and P. bolleana: Moskovskii serebristyi [Silvery Moscow], Sovetskii piramidal'nyi [Soviet Pyramidal] (Figure 3) and Ukrainskii serebristyi [Silvery Ukrainian]. The fast-growing Yablokov's pyramidal poplar, which is as winter-hardy as aspen, was obtained from crosses of rot-resistant aspen with P. bolleana. New hybrid forms, distinguished from all other pyramidal poplars by their ornamental qualities and modest requirements, were derived from these crosses.

We obtained a number of new pyramidal black poplar varieties from crosses between the Lombardy poplar (P. italica) and the ordinary black poplar (Populus nigra) from the Ufa area. The hybrids are outstanding in their fast growth, winter hardiness, and ornamental qualities; they grow successfully and very rapidly in the Moscow area, and possess an incomparably higher frost resistance than the Lombardy poplar. Examples are the Russkii [Russian], Stalinets [Stalinist], Pioner [Pioneer], Maksima Gor'kogo [Maxim Gorki], Privolzhskii [Volga Area], Lesovod [Forester] and Sovetskii pisatel' [Soviet Writer] varieties. They are easily propagated from hard wood cuttings. In contrast to the Lombardy poplar, they develop a well-marked basal trunk portion, have a compact pyramidal branching habit and a very high growth rate (in the Moscow area the Russkii variety has a yearly growth increment of up to 3 m). Accordingly, varieties such as Russkii, Stalinets, and Maksima Gor'kogo will be as valuable in shelterbelt and greenbelt planting as in forest planting for timber production, in which good straight trunks of even width are required.

FIGURE 3. Group of selected seedlings of the poplar variety Sovietskii piramidal'nyi, in the arboretum near the "Forest Economy" pavilion of the U.S.S.R. Exhibition of National Economic Achievements; they are marked by high winter hardiness and quick growth

Two hybrid varieties of poplar obtained by crossing Mongolian poplar (P. suaveolens) with Berlin poplar (variety Ivanteevka), and Mongolian poplar with aspen (variety Podmoskovnyi — Moscow area) are fast growers on our podzolic soils.

Fast-growing, rot-resistant hybrids were obtained between the aspen and Populus canescens (aspen hybrids Nos. 2832, 2834, 2835, 2836), between aspen andAmerican aspen (Populus tremuloides and P. grandidenta), aspen and black poplar, and aspen and the white poplar.

Continuing the work started by us, I. A. Kazantsev obtained valuable poplar — subgenus Turanga hybrids by crossing the hybrid variety Moskovskii serebristyi with Populus diversifolia subgenus Turanga. Some of these new poplar — Turanga hybrid varieties have a very decorative pyramidal branching habit, dense, beautiful, dark green foliage, and are fast-growing and winter-hardy. They should find wide use in greenbelt and shelterbelt planting on the saline dark and light chestnut soils of the southeast, as soon as a technique for their large-scale vegetative propagation has been worked out.

S.P. Ivannikov used a clone selected from a female fast-growing, rotresistant giant aspen (triploid) from the Oboyan' Forest of the Kursk Region, in various large-scale intraspecific and interspecific crosses with the Shar'in triploid aspen, with aspen form No. 15 from the Shar'ya Forest, and with other aspens. On cross-pollinating the resulting F1 hybrids he obtained more than 100,000 seedlings which he planted in trial and stock plantations. He succeeded in crossing this valuable triploid aspen with P. canescens, white poplar, and P. bolleana; fastest growing in the first years were the Oboyan' aspen × Populus bolleana hybrids

S.P. Ivannikov repeated the hybridization between white poplar and P. bolleana using white poplars from Kursk and Oboyan' (where huge straight-trunked trees of this species grow in the streets). He obtained a large number of hybrids with a clearly marked pyramidal type of branching. Thus, a new line of pyramidal silver poplars suitable for the Kursk Region and adjacent regions of the central forest-steppe was produced.

*[No indication is given in the text of number of trees in plantation.]

A number of very fast-growing new poplar varieties were bred at the Bashkir Forest Experimental Station of the All-Union Research Institute of Forestry and Mechanization by A. M. Berezin. Among them, his hybrid poplar (P. Balsamifera × laurel-leaved poplar (P. Laurifolia) is marked by exceptional productivity; certain twenty-year old plantations* of these hybrids in Bashkiriya have a yearly growth increment of 40 to 50 cubic meters and a total timber reserve of up to 900 cubic meters.

During many years of experiments, we have worked out a sound technique for such hybridization and, by using it, we can produce valuable new varieties of hybrid poplars for every zone or region of the country. Theoretically, this technique of crossing poplars is firmly based on the Michurin doctrine of heredity and its modifications and his theory on plant breeding.

Birches. Experiments on wide hybridization involving birch (intraspecific and interspecific) were carried out by the Department of Breeding and Dendrology of the Moscow Institute of Forest Technology (Cand. A.Y. Lyubavskaya). Birch proved to be a very gratifying object for breeding by selection and, in particular, by wide hybridization. Seedlings with marked heterosis were obtained by intraspecific hybridization between the European white birch Betula verrucosa from the environs of Moscow and B. verrucosa from the Orel Region. In growth these seedlings exceeded the ones obtained from the same European white birch specimen by open pollination, by at least three to four times.

Crosses between the European white birch and various Far-Eastern birch species (with the exception of B. schmidtii) succeeded quite easily; particularly numerous and of good quality were hybrid saplings from the European white birch, B. ermani, and B. japonica. Crosses between the European white birch and B. papyrifera (U.S.A.) proved unsuccessful. Hybrids between the European white birch and Alnus incana were obtained for the first time, but in small numbers and without heterosis appearing in the hybrids.

A large number of successful crosses was made between various climatypes and forms of the Karelian birch (from Karelia, Belorussia and the Ukraine) and between the Karelian birch and various other birch species.

Maple and lilac. Experiments in obtaining particularly decorative forms of maple for ornamental gardening by wide hybridization were carried out in the Moscow area. Interesting interspecific hybrids were obtained from crosses involving the red maple (Acer rubrum), the Norway maple (A. platanoides), the box elder (A. negundo), and others (Cand. I.N. Zaikina). During these experiments, a method of intercrossing maple species was worked out, and the peculiarities of the floral biology of the genus were studied. For example, it was established that in the Norway maple the same branch can produce flower racemes of different sexes in different years; in one year all flowers in the racemes on a particular branch are female, and in another year the same branch carries only male flower racemes. This shows that the sexual characters in trees can change. We are convinced that they depend basically on the food supply to the branches on which the sex organs (flowers, cones) are formed.

Experiments on wide intraspecific and interspecific hybridization in lilac proved even more successful. Candidate O.E. Nikolaeva obtained interspecific hybrids by crossing lilac species, such as Syringa vulgaris, the Hungarian lilac (S. josikaea), the Amur lilac (S. amurensis), and also numerous intervariety hybrids of common lilac. In addition, O.E. Nikolaeva carried out interesting and successful experiments in vegetative hybridization between various lilac species. For this purpose she worked out an original method of grafting lilac at a very early age; by transplanting the embryos of one species onto the endosperm of another.

The joint team of the All-Union Research Institute of Forestry and Mechanization and of the Department of Breeding of the Moscow Institute of Forest Technology carried out successful experiments on wide hybridization between conifers: larches, pines, spruces, and sequoia.

Larches. Already in 1933 to 1936, the author worked on interspecific crosses involving European, Siberian, and Japanese larches, as well as the golden larch (Pseudolarix kaempferi). The resulting hybrid trees are already 20 years old. All of them grow well, exhibit hybrid growth vigor, are abundantly fertile and yield good-quality seed even when selfed. Contrary to indications in the literature, no fruit-sterility or pollen abortion have been observed in these hybrids.

The first intergeneric hybrids between the Siberian and golden larch were obtained in 1935. One of the two plants resulting from this cross was normal in growth, but was slower-growing than ordinary seedlings from the same tree and than Siberian larch × Japanese larch hybrids also derived from pollination of the same Siberian larch tree. The other plant resulting from the intergeneric cross is a real midget and has not exceeded the height of 1 m in 20 years.

In recent years, extensive and very promising work on wide hybridization in larches has been carried out by R.F. Kudasheva in the Breeding Department of the All-Union Research Institute of Forestry and Mechanization. Working with Siberian, Dahurian, Japanese and European larches, as well as with crosses involving our interspecific hybrids, she obtained several thousands of interspecific backcrosses and polycross hybrids. Many of these hybrids exhibit marked growth heterosis and are very ornamental. Moreover, by means of pollinating a Japanese larch with pollen mixtures (Sequoia gigantea, S. sempervirens, Cupressus funebris, Cupressus lusitanica, and Cedrus deodara), she succeeded in producing a number of interfamily hybrids, which are varied in growth, vigor, and external characters.

Kudasheva's experiments prove convincingly that large amounts of hybrid larch seed can be produced by wide hybridization, and that the plants grown from such seed are marked by considerable heterosis. This heterosis is particularly pronounced in hybrids between European, Siberian and Japanese larches. These findings were confirmed by us in previous studies of larch crosses, and by A. V. Al'benskii on the basis of his experiments.

F1 hybrid seed from interspecific larch crosses should find practical uses in tree raising for plantations outside the natural distribution areas, and also for greenbelts and shelterbelts. The resulting plantations should be long-lived, productive and stable. Dr. S. Larssen has already successfully produced such seed in Denmark. Hybrid larches will adapt themselves relatively easily to new environmental conditions and thrive in them.

Pines and spruces. N.Y. Kotelova carried out experiments on interspecific hybridization between the Scotch pine (Pinus sylvestris) and American pine species: P. muricata, P. banksiana, and also Pinus mugo. She also made intraspecific crosses between climatologically remote Scotch pine types. These are interesting from a technical point of view when the production of large quantities of F1 hybrid seed of the Scotch pine is needed.

Considerable difficulties are encountered in experiments on wide hybridization between the Scotch pine and other species, because of the two-year cone-development period. Kotelova's P. sylvestris × P. muricata hybrids are markedly heterotic and differ favorably from ordinary Scotch pines by their luscious growth, and abundant development of long, dark green needles and strong, thick annual shoots. Some of the P. sylvestris × P. mugo hybrids, on the other hand, deviated markedly in growth vigor as well as in external aspect in the direction of the paternal species — P. mugo; hybrids of P. sylvestris × P. banksiana are intermediate in character and exhibit a lesser hybrid growth vigor than the P. sylvestris × P. muricata hybrids.

Z.I. Zabolotnova carried out successful experiments on the hybridization of the Colorado spruce (Picea pungens) with the Norway spruce (P. excelsa). The hybrids that were obtained had the ornamental quickgrowing, pectinate habit of the Norway spruce. Growth heterosis was pronounced in many of them.

Somewhat earlier we obtained a small number of hybrids from a cross between the Colorado spruce and the Norway spruce, and between the former and the Canadian spruce (Picea alba). Many of these hybrids exhibit characters of the seed-parent species, the Norway or Canadian spruce. This is particularly marked in Colorado spruce × Canadian spruce hybrid seedlings. A certain number of the Colorado spruce × Norway spruce hybrid seedlings are heterosis forms, while retaining the external features of Colorado spruce.

Sequoia. Experiments on sequoia hybridization were also carried out in the Breeding Department of the All-Union Research Institute of Forestry and Mechanization, though on a relatively limited scale. The object of these experiments was to study the floral and fruiting biology of the two sequoia species, giant sequoia and redwood (S. gigantea and S. sempervirens), and to seek methods of modifying some of their hereditary traits, to facilitate the better acclimatization of sequoia in the U.S.S.R. These trees used to grow in the Far East and on the Volyn'. Another object of the experiments was an investigation of the reasons for the bad quality of sequoia seed and its poor germination. On our initiative Cand. V.I. Ermakov (Breeding Department of the All-Union Research Institute of Forestry and Mechanization) carried out experiments in crossing giant sequoia with redwood in the Crimea and also worked on artificial intraspecific hybridization and pollination between close relatives. These experiments were continued afterwards by the Breeding Department team in the Caucasus.

*[This should probably read "redwood".]

As a result, it has been established that: a) the two sequoia species, giant sequoia and redwood, set almost no seed capable of germination on self-pollination. When small numbers of viable seeds are set, the resulting seedlings and saplings are generally very weak and most of them die at an early stage. This was established in a particularly striking manner in the case of the giant sequoia*. One spring we sowed 1 kg. of redwood seed, set after open pollination. The seed had been collected in the Sochi and Loovskoe Forests from trees growing there. Only some hundreds of straggly seedlings developed, most of which died in their first year. Only some dozens of plants could be raised from 1 kg of seed.

b) redwood can be successfully crossed with giant sequoia, and also with Taxodium distichum, and less successfully with Cryptomeria japonica.

c) germinating seed can be obtained from redwood that has been pollinated with various pollen mixtures, even with the participation of larch pollen.

On artificial cross-pollination (even a single pollination) the germination capacity of sequoia seed increases markedly (up to 50%) and the germination energy is raised. The seedlings and saplings obtained from such seed are far more vigorous and grow more rapidly.

Now that some of the basic problems encountered in sequoia breeding and in growing its seed have been solved by the Breeding Department of VNIILM, the first seed-growing trial plots can be laid out. These should yield quality sequoia seed in large quantities and should serve the extensive and rapid propagation of the best forms of the two sequoia species.

Experiments in Vegetative Hybridization

In conjunction with R.F. Kudasheva and M.I. Dokuchaeva we have experimented on a limited scale in the Breeding Department of VNIILM with the vegetative hybridization of forest-trees. Among deciduous trees, oak and Spanish chestnut (Castanea sativa) were chosen for this purpose, and among conifers, Scotch pine and pines belonging to the section with five-needled fascicles (P. cembra, P. koraiensis, P. strobus).

Grafts of Spanish chestnut on oak (Quercus robur), Q. mongolica, and Q. borealis — and the reciprocal grafts — were successful, and at the present time the scion (Spanish chestnut) exhibits marked hybrid characters. On some chestnut shoots, individual leaves show clear oak traits or characters that are intermediate between oak and chestnut.

The frost resistance of the chestnut is considerably increased. The graft hybrid even stood up to the exceptional winter frosts of 1955, when the temperature dropped to -40°C and lower, without suffering much frost damage.

We have worked out a technique of grafting Spanish chestnut on oaks and also methods of raising the plants subsequently. Oak-beech grafs have so far not been successful. Experiments in producing vegetative hybrids between conifers are noteworthy. One-year-old seedlings or slips of Pinus cembra, P. peuce, and P. strobur have been grafted onto five to ten-year-old Scotch pine saplings. These experiments demonstrate the feasibility of extensive grafting of slips or young seedlings of all pines with five-needle fascicles onto Scotch pine rootstocks. The grafts take well, and scions and rootstocks grow normally. Moreover the growth vigor of P. cembra is considerably increased through the agency of Scotch pine.

Shoots of P. koraiensis grafted on young Scotch pine plants reach their flowering stage much earlier (male inflorescences so far), and yield abundant quantities of pollen at the age of only four or five years. P. cembra scions taken from fruit-bearing cedar-pine trees in the taiga near Krasnoyarsk develop female cones two to three years after being grafted on Scotch pine. In 1957, the first mature cones were collected. Ripening was evidently accelerated through the nutritional agency of the Scotch pine rootstock.

Slip grafts of P. strobur on Scotch pine grow normally, and the scion has so far not been subject to pine rust, which attacks a large number of P. strobur trees growing on their own roots.

Grafts of P. peuce on Scotch pine develop most vigorously and luxuriantly.

The prospects and the scientific value of these experiments done on deciduous trees and conifers are beyond doubt, and it is imperative that further efforts be made in this direction.

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Wide hybridization of forest cultures promises solutions for many afforestation problems. Forest-seed growing techniques may become radically modified and re-organized by wide intraspecific hybridization.

Large-scale use of intergeneric and interspecific hybridization can aid the acclimatization of valuable cultures and lead to the production of new forms or even species of trees and shrubs.

The present unsatisfactory approach of forest and greenbelt economy to research in seed growing, breeding, and acclimatization of forest cultures must be revised drastically if the promise of wide hybridization is to be realized.