Problems of modern hemp breeding, with particular reference to the breeding of varieties of hemp containing little or no hashish

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Author: G. Bredemann, Fr. Schwanitz, R. von Sengbusch, Hamburg
Pages: 31 to 35
Creation Date: 1956/01/01

Problems of modern hemp breeding, with particular reference to the breeding of varieties of hemp containing little or no hashish

G. Bredemann
Fr. Schwanitz
R. von Sengbusch, Hamburg*

Hemp ( Cannabis sativa L.) is one of the oldest cultivated plants known to man. Its home is in Central Asia. From there the plant came at an early date to China, where it is supposed to have been cultivated for more than 4,500 years. Wild hemp is said still to be gathered to-day, and occasionally also cultivated in its original home. Wild hemp ( Cannabis sativa) of the variety called Ruderalis Hanisch, is distinguished by its strong ramification, the loose build of the flower and fruit stems and its small fruit. The fruit covers are also strongly developed and the fruit falls off easily. Wild hemp has a fibre content similar to that of the varieties of hemp found in central Russia, but its content of valuable long fibres is smaller, probably because of its stronger ramification. There are a great number of varieties as well as transitional forms of wild hemp; but since hemp easily becomes wild, it is difficult to tell whether these plants with the characteristics of cultivated plants are really good varieties of the wild plant, or descendants of cultivated plants which became wild, or even the result of natural cross-breeding between wild and cultivated forms.

It is said that in the 7th century B.C. hemp was brought by the Scythians from Central Asia to the regions north of the Black Sea and the mouth of the Danube, and from there by way of Russia to north and central Europe. Russia and the Baltic States became at an early date one of the most important areas in which hemp was cultivated for fibre. These northern varieties of hemp are distinguished by their low growth and early maturity.

Hemp also spread by way of Asia Minor to the Mediterranean countries, where it has been known since about 450 B.C. A second centre where hemp was cultivated for fibre developed in Italy and France. These southern varieties of hemp are of large growth, and mature late.

The form and yield of the hemp plant are influenced not only by the climate, but also by selective breeding for a specific use. There are special fibre varieties which are cultivated exclusively or mainly for their fibre. These are relatively long or even very long, are not ramified, and yield only small quantities of seed. In addition there are varieties of oil hemp which are of low growth, mature early and yield large amounts of seed. These are grown to obtain fat oils. Finally, some varieties of hemp are cultivated for the sake of the drug hashish. This variety of Cannabis sativa, incorrectly referred to as a separate species of Cannabis indica Lam. or Indian hemp, is distinguished by its low growth, strong ramification, and small dark-green leaves. Between these three main varieties there are various intermediate forms.

The following worked as a team in the treatment of all the hemp problems discussed in this paper: Bredemann (Hamburg), Garber (Hamburg), Huhnke (Hamburg), Neuer (Krefeld), Schwanitz (Hamburg), and von Sengbusch (Hamburg).

The oldest use of the plant is the production of bast-fibres from the stem. This is said to have been known to the Chinese from about 2800 B.C. The seeds were used for culinary purposes only at a later date. The use of hemp as the source of a drug was apparently unknown to the Chinese. This use seems first to have been discovered in India, where hemp was cultivated for medical purposes as early as 900-800 B.C. In mediaeval times hemp was brought to North Africa, where since then it has been cultivated exclusively for hashish.

The cultivation of the oil-bearing varieties of hemp is of no economic importance. The two main varieties of fibre hemp mentioned above developed as a result of natural selection, which for many centuries went on under varied climatic conditions, and was primarily influenced by very different lengths of day and vegetation periods. There are, of course, all kinds of traditional forms. The northern varieties of fibre hemp need in general-as has been mentioned-a comparatively low temperature and a short period of vegetation. They are accordingly of low growth. The southern varieties require a high temperature and a long period of vegetation, and consequently grow taller. In both types, short days during the principal period of vegetation generally reduce the time of growth and the development of the plant, and promote the reproductive phase (a few exceptions to this rule were reported by Bredemann in 1952). Shorter days impede the growth and advance the blossom. In regard to their photoperiodic reaction, the varieties of hemp which grow in northern and central Europe are adjusted to the length of summer days in that part of the world, and so they complete their development and form mature fruits. Hoffmann (1943) described northern hemp as a "generative type with a strongly developed production of seed ". If these northern varieties are planted in southern latitudes in which the summer days are shorter, or if they are cultivated in our part of the world (e.g., in Germany) in the greenhouse in winter, the plants develop much faster from planting to blossom; but they blossom very quickly, and complete their growth so soon that they yield little straw and fibre; consequenty, the cultivation of these varieties under such conditions would not be a paying proposition.

Most of the southern varieties, on the contrary, are adapted to the short days of the southern summer. Hoffmann refers to them as "plant types with a low seed yield ". If they are cultivated in the long days of central Europe, they develop and grow very well, but they blossom and bear fruit so late that it never ripens. In special cases, if varieties adapted to the length of day in very low latitudes are brought to the north, even the bast-fibre does not ripen, the fibre content of the stem remains low, and the fibre is of inferior quality.

When there were only very limited international economic relations, each country obtained the seed it needed from its own plants. Therefore, particularly in central and northern Europe, the seed harvest often failed, as a result of bad weather, because the fruit did not ripen sufficiently. The present economic development towards the use of large spaces, the result of international co-operation, makes possible a much better cultivation of hemp. Since as a rule shorter days help and accelerate the blossom and the fruit, it seems desirable that the seed should be produced in the south. This has another advantage, for southern varieties of well-developed growth can be cultivated in central and northern Europe, and although it is true that the seeds of these varieties do not ripen in that part of the world, they can still yield large amounts of high-quality fibre.

Although hemp is a very old cultivated plant in great parts of Europe-except Russia-it has only recently been grown on a sizeable scale, and consequently has not been changed much by breeding. Only its time of maturing and its length of growth have been adjusted to different climates by the joint effect of natural selection and very limited artificial breeding.

The lack of systematic work on hemp has had the result that until very recently hemp could only be described as a wild plant cultivated by man, rather than as a genuine, cultivated plant.

Accordingly, hemp has, or at least until recently had, a number of the unfavourable characteristics of wild plants.

They are

  1. dioeciousness, which owing to the early death of the male plant results in the fibre becoming uneven and of considerably lower quality;

  2. low fibre content, as a rule around 10%;

  3. unevenness of the fibre, from the existence side by side of primary and secondary fibres of very unequal quality;

  4. the presence of extraneous matter, principally resins exuded by the glandular cells, more especially of the female inflorescence. These resins contain narcotic substances, which are misused in the form of the drug called hashish (marihuana).

These unfavourable properties, especially the low fibre content and the loss of fibre and the poor quality caused by dioeciousness, have led, despite its advantages, to a worldwide decline in the cultivation of hemp at the very time when agricultural production has been on the increase. As explained, this is due to the low yield of the wild hemp plant.

Bredemann (1924) made experiments designed to increase the fibre content of the plant by means of a special breeding process-namely, the elimination before flowering of all male plants of low fibre yield. In this way he succeeded in breeding a plant which had a fibre content twice that of the original plant-i.e., about 24% instead of 12%.

Experiments were started by Neuer in Germany in 1934 to eliminate the dioeciousness found in wild plants. These experiments were later continued in collaboration with von Sengbusch. A monoecious variety of hemp was bred-that is to say, a plant always possessing both male and female blossoms. In these new monoecious varieties of hemp, all plants blossom simultaneously. A plant has been bred which no longer has the decreased yield and low fibre quality found, owing to the early death of the male plants, in dioecious hemp.

These two new strains did not, however, prove at all satisfactory in their yield. This applies equally to the strain with an increased fibre content (bred by Bredemann) and to the monoecious hemp. In quality as well as in quantity of yield, they still showed a number of weaknesses which had to be overcome if the cultivation of hemp was to be worth while. The two groups of workers combined in 1950, therefore, to continue jointly the work on the breeding of hemp.

The next aim in the breeding process was to try to combine monoeciousness and ample fibre content, as well as to create useful monoecious and dioecious varieties of ample fibre content for the different districts in which hemp is cultivated. First, the special characteristics of existing varieties were studied, primarily to find weaknesses which should be overcome by breeding. After this preliminary examination, cross-breeding was started as follows:

  1. monoecious plants of low fibre content with dioecious plants of high fibre content;

  2. dioecious plants of low fibre content with dioecious plants of high fibre content;

  3. dioecious plants of low fibre content and late maturity with dioecious plants of high fibre content.

In each case considerable progress was made. Field work in Germany, France, the Netherlands and Sweden on the fibre yield of the new monoecious and dioecious stems showed that an average net fibre yield of 2,000 to 2,500 kilograms per hectare could be reached from a straw harvest of 10,000-12,000 kilograms per hectare, and a fibre yield of 20-22% by chemical process, which means a fibre yield of 24-25% by natural retting.

In the course of this work, methods were also developed for the separation of the valuable primary fibres from the less valuable secondary fibres. These processes facilitated the selection of varieties with a particularly high content of primary fibres. By continuous selection carried on for a period of three years it became possible to increase the content of primary fibres to about 70% of the total fibre content. Through this work the hemp plant was changed in about 25 years from a cultivated wild plant to a genuine cultivated plant and its cultivation thus made profitable.

There remains the problem of improving the quality of the fibre by breeding. It has already been attacked by the working group referred to above and, in view of the success already achieved, its solution can be expected in the predictable future.

Another remaining problem is that of improving the process by which the fibres are produced, and more especially of changing them to match the increased fibre content of the new strains.

Methods of cultivation suitable for the new strains must also be developed. The question of which districts would be most suitable for such cultivation has to be studied, and also the economic advantages of increasing the cultivation of the new strains compared to those of other important cultivated plants.

Lastly there remains another very important problem which can be solved only by actual breeding: the elimination or reduction of the hashish content, which has remained the same in the cultivated as in the wild varieties. The breeding of varieties of hemp which contain little or no hashish is essential, because even the fibre hemps can produce so much hashish on their female inflorescence that they can be misused for producing narcotics (Adams, Pease and Clark 1940, Ball 1910, Hamilton 1915, Hitzemann 1941, Matchett, Levine, Benjamin and Pope 1940, Paris and de Merac 1948, Pascal 1917, Pulewka 1950, Robinson and Matchett 1940, Sabalitschka 1925, 1927, Straub 1928, Walton, Martin and Keller 1938). This danger can be overcome only by breeding varieties which contain little or no hashish.

Hashish is the resin produced by the glandular hairs of the female inflorescence. This resin contains at least three che-mically-related substances: Cannabinol, Tetrahydrocann-abinol, and Cannabidiol.

Of these, cannabidiol, found in the greatest quantity, has no narcotic effect; cannabinol is weak or inactive; but tetrahydrocannabinol, a derivative of cannabidiol, is the principal narcotic agent (Todd 1942, 1940; Bergel 1938; Adams, Pease, Cain, Clark 1940; Adams, Pease, Cain, Baker, Clark, Wolff, Wearn, 1940; Bose 1943; Wollner, Matchett, Levine, Loewe 1942).

Modern plant research suggests that it must be possible to eliminate these undesirable narcotics by breeding. Von Sengbusch succeeded, decades ago, by means of appropriate chemical selection processes, in finding mutants in five varieties of the lupin plant, which had a low alkaloid content and were therefore not poisonous, and in developing from these mutants corresponding varieties of sweet lupins. By succeeding in finding three different mutants of the yellow lupin ( Lupinus luteus) and two different mutants of the blue lupin ( Lupinus angustifolius), he proved not only that it is possible by systematic research into a great number of plants to find individual strains which do not contain substances once considered characteristic of the species, but also that individual plants free of such substances can be produced by the mutation of different genes. Von Sengbusch also succeeded, by systematic selection, in breeding a strain of tobacco ( Nicotiana tabacum) which contained little nicotine. Furthermore, Von Schwanitz and Von Sengbusch have already obtained good results in their most recent work in selecting varieties of spinach containing a low content of oxalate and saponine. Other research workers have succeeded in finding varieties of Sudan grass with a greatly decreased amount of glucosides containing hydrocyanic acid, and varieties of melilot which contain no coumarin. This all goes to show that it is certainly possible to eliminate, through selection and breeding, secondary plant substances characteristic of a particular plant species.

When von Sengbusch bred the sweet lupin he succeeded not only in reducing the high alkaloid content by finding suitable mutants, but also in making cultivated plants from the wild varieties of the lupin by a systematic search for other characteristics of cultivated plants, such as firm adherence of the pods to the stems, soft covers and white colour of seeds, fast development of the young stages of the plants, etc. (von Sengbusch: "20 years' sweet lupin research and breeding in Germany", Forschungen und Fortschritte 21/22, Jg. Nr.22/23/24, Nov. 1947). As mentioned above, he has successfully started similar work on hemp with other research workers. He is therefore particularly anxious to attempt to eradicate the last important wild characteristics of hemp by breeding methods similar to those with which he succeeded in reducing the alkaloids of lupins and tobacco.

In the last few years, preliminary experiments in breeding hemp plants with a low hashish content have therefore been started by the authors of this paper in collaboration with W. Huhnke in the Department for the Breeding of Cultivated Plants of the Max Planck Institute for Breeding Research.

The starting-point was the knowledge gained in examining, by paper chromatography, a large number of individual planys, clones and varieties of fruit and vegetable species. It had been clearly shown that important factors in the quality of the fruit - such as sugar and acid content - were complex attributes which themselves had several different characteristic elements. In several edible fruit species, glucose, laevulose, saccharose and other kinds of sugar are found side by side. In some species considerable variations are found in the proportions of the different kinds of sugar; and indeed some of the varieties of sugar are sometimes absent. Such differences can also occur within one and the same species; varieties and clones of strawberries are distinguished not only by the amount of the different kinds of sugar they contain, but by the entire absence of some of these sugars, such as glucose. This is because the various types of sugar are formed, or the more complex sugars are synthesized from basic sugars in the plant in genetic and biochemical independence of one another; consequently, saccharose is lacking on account of one particular mutation or combination of genes, while quite different mutations are required to prevent the production of laevulose.

Three different components can also be found in hashish. Judging by experience with the lupin and other cultivated plants, it will be possible to breed varieties of the Indian hemp plant which have no - or at least very little - hashish. The fact that hashish contains three different substances shows the way that has to be followed in breeding. It would not be advisable to search only for mutants in which all three components of hashish are missing. Our findings concerning the different types of sugar in the clones of strawberries suggest that mutants which lack only one of the hashish components will be found more frequently than mutants or combinations which lack all three. The best method is therefore to look first for mutants which lack only one hashish component. By cross-breeding these it should be possible to produce strains which lack all three.

In examining large quantities of plant material, the Beam test can be employed for the determination of cannabinol and the Diazo test for that of cannabinol. If these tests are supplemented when necessary by paper chromatography, and their results are statistically treated, they should enable the required mutants to be found. A suitable method of quick determination of tetrahydrocannabinol has still to be developed.

It was known from the beginning that, in addition to the breeding of varieties of hemp which do not contain narcotic substances in the resin exuded by the glandular hairs of the blossom region, there must be yet another possibility of breeding hashish-free hemp. If the glandular hairs are the only organs which produce hashish, it must be assumed that mutants which do not have these glandular hairs are completely hashish-free. By a systematic search for morphological mutants of this nature it may be possible to find varieties of hemp which are completely free of hashish.

Special investigations undertaken in the winter of 1954/55 on about 20,000 individual plants, a very small quantity for such experiments, have already indicated that this assumption is correct and that the selection process developed is suitable. Out of 20,000 plants examined by the Beam test, about eighty showed no reaction. These plants were then examined by the Diazo test for lack of cannabinol. It was found that about a quarter of the plants which gave no reaction under the Beam test also gave none under the Diazo test. It is therefore clear that, even among such a small number of plants, some lack several components of hashish. It is not yet possible to say whether plants which show no reaction under the Beam and Diazo tests are also free of tetrahydrocannabinol. It seems likely, however, that further experiments will detect such plants.

The selection of plants which lack glandular hairs has also been successful. A sort was found with a sector, obviously changed by somatic mutation, free of glands and giving no reaction under either the Beam or the Diazo tests.

The results achieved in selection clearly show that the method offers hope that hashish-free varieties of hemp can be bred. Whether this can actually be done depends solely on means. Although it has been shown to be possible to develop effective large-scale methods of finding hashish-free plants, their further development and improvement is necessary. Finally, the hereditary stabilization from and the breeding of hashish-free varieties still involve a number of technical problems, the solution of which will be time-consuming and expensive. These difficulties principally arise from the fact that the hashish content can only be determined on the flowering plant and, secondly, that the hemp plant is heterogamous. Since male plants do not produce hashish, it cannot yet be determined whether the male plant inherits the character of freedom from hashish. It will therefore be necessary to develop longer and more complicated breeding processes in order to obtain dioecious hemp strains in which all female and male plants have this desirable character.

The process of selecting and breeding monoecious varieties is somewhat simpler, since the hereditary character of the hashish content of each plant can be determined without difficulty. Here the difficulty in breeding is that, once a plant has been ascertained to be hashish-free, it can be assumed that the seeds of this plant have already been fertilized by the pollen of another plant with the hereditary character of containing hashish. If it is desired to obtain hashish-free varieties as quickly as possible, it will be necessary to employ the pruning method developed by von Sengbusch, in order to make the same plants flower twice - once to select the hashish-free varieties, and again to cross-breed hashish-free plants.

To carry out this breeding work would require considerably more effort and means than were necessary for the first preliminary experiments so far made. It is impossible to carry out the further necessary experiments - like the previous ones - alongside and at the expense of other important tasks.

The selection of monoecious plants with high quantity and quality of fibre has cost about 30,000 marks par annum for twenty-five years. This means that so far about 750,000 marks has been spent on transforming the wild hemp plant into a genuine, cultivated plant. It may be assumed that ten more years of breeding will be necessary to improve the quality of the hemp fibre, at a probable additional cost of 200,000 or 300,000 marks.

There will also be the expense of breeding hashish-free hemp plants. It will be necessary to work out or develop methods quickly determining the three substances to be eliminated. These methods must be suitable for breeding purposes. It will probably be necessary to examine several million individual plants. It will also be necessary to develop processes of genetic breeding in order to stabilize the mutants found, and to breed by these methods, which will probably be very complicated, new varieties which will yield large amounts of fibre and at the same time be hashish-free. This breeding work will probably require ten to fifteen years, and cost about 30,000 marks a year.

The transformation of Lupinus luteus from its wild form into a cultivated plant has taken place before our eyes in the last thirty years. A great number of undesirable qualities of the wild form have been eliminated by the selection of suitable mutants. Similarly, important bad qualities of the wild form of hemp have been eliminated during the last twenty-five years; and as a result we have today monoecious as well as dioecious varieties with a very high yield of primary fibre. These high-quality varieties of hemp can, however, only become real cultivated plants through the breeding of varieties which could not be misused by producing narcotics. The results obtained with these two plant species (lupin and Indian hemp) justify an optimistic belief that the breeding of hemp plants which have no or very little hashish is possible. It is desirable from the standpoint of public health that ways and means should be found for achieving the aims of the research outlined in this paper.

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