The Theoretical Principles of Vernalization (1935)
T D Lysenko

SIGNIFICANCE OF THE INDIVIDUAL FACTORS IN THE SET OF EXTERNAL
CONDITIONS PLANTS NEED FOR TRAVERSING THE VERNALIZATION PHASE

For passing through the vernalization phase, as well as for passing through other phases of development, plants require not individual external factors, such as temperature, air, moisture, light, darkness, etc., but a set of factors. The composition of the set and the correlation of the factors within it are determined by the natural properties of the plants. Depending upon whether the plants are wheat or millet, they will require different environmental conditions for passing through the same phases of development phases that are the same, yet specifically their own, peculiar to their particular natures. Moreover, for traversing different phases of development, the same plant may require different sets of external conditions.

Many investigator (physiologists and others) often confuse the significance of individual external factors with the significance of the entire set of external conditions that plants require for traversing the vernalization phase. This confusion often brings it about that in practice, when employing presowing vernalization, these investigators fail to achieve the anticipated results. It often happens that so-called "vernalized" winter wheat does not ear, or ears slowly and not uniformly. Hence, the incorrect, although in the opinion of these investigators "legitimate," conclusion that not all varieties of winter wheat can be induced by vernalization to ear when sown in the spring.

1The spring low temperatures, which nearly every year are to be observed only for a short period of time, do not last long enough to enable the plants to complete their passage through the vernalization phase and therefore the latter do not ear.

In the majority of cases, the plants of many of our winter varieties of wheat, rye and other crops, when sown in the fields in the spring, cannot completely pass through the vernalization phase (hence the failure to ear) solely because of the relatively high temperatures of the sowing and postsowing periods.1 From this some investigators drew the incorrect conclusion that for vernalization "in general," including vernalization of winter and spring cereal seeds, the only thing needed is a temperature suitable for the vernalization of the given variety. Hence, we often read in the literature on the subject that vernalization is [42] the thermal stimulation of the sowing material, or that the vernalization of grains is the result of thermal treatment, whereas the vernalization of millet is the result of the operation of the darkness factor, etc. These investigators link the plant's passage through its phases of development with separate factors. It must be borne in mind, however, that although the high temperature of spring under field conditions is the sole reason for the absence of vernalization in winter varieties, nevertheless, it is impossible to vernalize wheat or other crops by temperature alone (any temperature—high, low or variable).

To pass through the vernalization phase, the plants of winter and spring crops require not only the thermal factor, but the thermal factor within a set of other factors. The components of this set known to us at the present time are: temperature, moisture, air. A certain quantitative combination of doses (depending on the variety) of these factors makes it possible (and in all the cases we know of, this possibility becomes actuality) for any variety of cereals, and for many other plants, to go through the vernalization phase.

Some investigators agree that plants need a set of factors and not only the thermal factor in order to pass through the vernalization phase; but they assign the principal role to the thermal factor. Their grounds for doing so are that the plants of spring-sown winter varieties cannot traverse the vernalization phase exclusively because of the high temperature in the spring and beginning of the summer.

If it were possible, in practice, artificially to create the conditions for the vernalization of plants on wide field spaces, the thermal factor would indeed become the principal factor. In the main, it would be the only thing needing regulation to suit the requirements of the plants. In plant-breeding practice, when growing several generations of winter varieties of plants in one year, we recommend the vernalization of plants in pots, and not of seeds. The potted plants should be kept for 6 weeks or 2 months in a temperature of 2°-6° C. under ordinary daylight. In that case, the vernalization of the plant will be more advantageous than the vernalization of the seed, for simultaneously with its passage through the vernalization phase shoots will be obtained which will have had time to gain vigour. In ordinary practice, however, when plants are grown under field conditions, it is impossible to regulate the thermal factor for the purpose of vernalizing the plants. Therefore, for field conditions, it is necessary to vernalize the seed. For plants in the field, or in pots, the chief factor that determines passage through the vernalization stage is temperature. But when these plants are still in the seed form, not yet sown in the ground, their passage through this phase of development, i.e., vernalization, depends, in practice, mainly on the moisture factor. Plants in the field or in pots nearly always possess sufficient moisture to enable them, given the suitable temperature, to go through the processes of vernalization. With presowing vernalization, however, the embryos in the seeds often lack sufficient moisture to enable [43] them to go through the vernalization processes even under the optimal temperature for the given variety.

The main object pursued by presowing vernalization is to compel the plants (still in the seed form) to go through the vernalization phase and at the same time to be economically suitable sowing material (i.e., to prevent the complete germination of the seeds). Therefore, in determining the doses of the separate factors comprising the set of conditions for the vernalization of the seed, the aim must be to enable the embryos just beginning to grow in the seed to pass through the vernalization phase and at the same time to make the given conditions least suitable for the complete germination of the seed. Hence, in state- and collective-farm practice, the moisture factor is at the relative minimum in presowing vernalization more often than the thermal factor. The amount of water with which the seeds are soaked when preparing them for vernalization is only enough to make them just begin to germinate. The further growth of the seeds should almost cease automatically owing to the shortage of moisture for the growth processes when the temperature is such that the seeds of the given variety will vernalize. For the vernalization of our winter varieties on our collective and state farms we recommend from 0°-2° C., for our spring, late-ripening varieties from 3°-5° C. and for our spring, early-ripening varieties from 10°-12° C.; in other words, for the vernalization of different groups of varieties, different temperatures are recommended. When preparing the seeds for vernalization, they must be moistened with a suitable quantity of water, depending on the temperature at which they are to be vernalized. The degree of moisture in the seeds of winter wheat must be brought up to 55% of their absolute dry weight. To bring the moisture from 12% (the usual normal degree of moisture in seeds) up to 55%, it is necessary to pour 37 kg. of water on every 100 kg. of seed. The moisture in the seeds of spring, late-ripening wheat must, during vernalization, be brought up to 50% of their absolute dry weight. With 12% of moisture in the seeds, 33 kg. of water for every 100 kg. of seed is required. The moisture in the seeds of spring, early-ripening wheat must be brought up to 48%. Every 100 kg. of seed requires 31 kg. of water. Different percentages of moisture (55 for winter, 50 for spring late-ripening, and 48 for spring early-ripening varieties) are given when preparing seeds for vernalization, not because the seeds of winter varieties require more moisture for swelling than the seeds of spring varieties, but because the seeds of different varieties must be vernalized under different temperatures. If the moisture in the seeds of spring varieties, which must be vernalized in a temperature of l0°-l2° C., is brought up to 55% (instead of the 48%) the vernalization processes in the embryos of these seeds will be traversed better than with a 48% moisture content; but at this temperature and with 55% of moisture, the seeds will sprout very considerably. The lower the temperature at which the vernalization of the sowing material of the particular variety is to proceed, the higher must be the percentage of seed moisture. [44]

A moisture content of winter wheat seeds not exceeding 50% (of absolute dry weight) is a guarantee that, when being vernalized, the seeds will not germinate at a temperature of 0°-2° C., nor even at a temperature of 3°-5° C. But with a moisture content of 50% and the temperature recommended for the vernalization of winter wheat (from 0-2° C.), the vernalization processes will not take place, or will take place at an extremely slow pace. A higher temperature (from 3°-5° C.) and a seed moisture content of 50% will be sufficient to enable the vernalization processes to take place. But at a temperature of 3°-5° C., the sowing material of our winter varieties may, during the 40-50 days of vernalization, suffer to some extent from fungus and other diseases. Therefore, notwithstanding the fact that a temperature of from 0°-2° C. is in itself less favourable for the vernalization of winter varieties than a temperature of 3°-5° C., it is, in practice, more profitable to vernalize these varieties at a temperature of 0°-2° C., and to bring the moisture content of the seeds up to 55%. With such a moisture content, a temperature of 0°-2° C. will be no less effective for winter varieties than a temperature of 3°-5° C. with a smaller moisture content (50%). The development of fungous microflora, however, will be ever so much less.

Thus individual factors of the set of conditions necessary for passage through the vernalization processes, when in the minimal state, i.e., when they are, as it were, limiting factors, can be brought out of the minimal state by altering the doses of other factors of the set.

For some plants the combination of doses of the individual factors composing the set of conditions necessary to enable the seeds to go through the vernalization processes does not fully coincide with the combination of doses of the same factors for the germination of the seeds of these same plants. For different plants, and even for different varieties, the doses of the factors for growth and for •passing through the vernalization phase differ. The bigger the difference in the doses, the easier is it, in practice, to carry out the presowing vernalization of the given plant. A wider range of variation in the doses of individual factors may be permitted when vernalizing seeds.

In cereals, the discrepancy in the doses of moisture factors for the vernalization and the growth processes, respectively, is not very great. In the vernalization of spring wheat, a moisture content of over 50% (of the absolute dry weight) leads to the germination of the seeds; with a moisture content of less than 45%, the vernalization processes take place at an extremely slow pace (almost come to a stop). Therefore, in practical farming, when vernalizing spring wheat, and more so when vernalizing winter wheat, the collective and state farms must pay most attention to the moisture in the seeds. In the presowing vernalization of cereals under collective- and state-farm conditions, the passage through the processes of vernalization depends mainly on the moisture factor. On the other hand, the vernalization of cereals and of many other plants in the fields (in the open) depends in the main, on the thermal factor of spring and beginning of summer. In general, however, to go through any phase of development, including the vernalization [45] phase, a plant requires not individual factors—temperature, moisture, light, darkness, mineral nutrition—but a set of factors, in a certain combination. By altering the doses of some factors it may become possible to alter the doses of other factors, without reducing the effectiveness of the set as a whole. By altering the dose of one factor it is possible to make an ineffective set effective.

SEQUENCE OF PHASES IN THE DEVELOPMENT OF PLANTS

It was stated above that the changes that take place during the vernalization of winter cereals and other plants constitute necessary phases of their development. Until plants have undergone these changes (under artificial conditions when the seed is vernalized, or under natural conditions after sowing), they cannot proceed with their further development and, as a result, cannot fruit.

To be able to fruit, plants must go through other qualitative (phasic) changes besides the vernalization phase. The qualitative changes that take place in passing through the vernalization phase are not enough to enable plants to fruit. For example, after completing the vernalization phase, the plants of any variety of wheat, rye and other winter crops can normally continue their development, ear and fruit, only if sown under the conditions of spring or the beginning of summer. If, however, vernalized winter or spring plants are sown in the latter half of the summer, or in greenhouses in the winter (up to February), such vernalized plants do not fruit and in external appearance differ in no way from unvernalized plants. This example shows that under the conditions of spring or of the first half of summer, these plants, after passing through the vernalization phase, undergo certain other changes which they cannot experience under the conditions of the second half of summer, autumn or winter, and without which they cannot fruit. Thus, under the conditions of the latter half of the summer, autumn or winter, notwithstanding the favourable artificial or natural temperature created for the plants, certain conditions are lacking for the normal development even of vernalized plants. The deficient factor in this case for vernalized plants is inadequate duration of daylight.

By artificially lengthening (by means of electric light) the short autumn and winter days, and by suitably raising the temperature (to 15°-25° C.), it is possible to induce cereals to develop normally when sown in the autumn and winter. In such cases it is only necessary to vernalize the seed of winter varieties before sowing, or to vernalize the plants after sowing. For traversing the vernalization phase, the photo factor and the duration of daylight play no role at all. Plants can go through the vernalization phase with equal success under long-day conditions and short-day conditions, no matter how short the day may be (even in continuous darkness), as long as the [46] seeds are sufficiently moist and the temperature does not exceed the range permissible for the given moisture of the given plant. At a temperature below 0° C., none of the varieties of cereals known to us can pass through the vernalization phase; at a temperature exceeding 10° C., it is practically impossible to vernalize the majority of winter varieties of cereals known to us: they vernalize too slowly.

Fig. 15. Winter wheat Novokrymka 0204
Sown in greenhouse on August 6. Photographed on September 26. The plants in the first pot on the left (sown with ordinary seeds) were grown in continuous light. The plants in the second pot from the left (sown with vernalized seeds) were also grown in continuous light. The plants in the third pot from the left (also sown with vernalized seeds) were grown in continuous light for the first 17 days and for the next 32 days under 10-hour-day conditions. The plants in the fourth pot from the left (sown with ordinary seeds) were grown under 10-hour-day conditions. The plants in the fifth pot from the left (sown with vernalized seeds were also grown under 10-hour-day conditions

In greenhouses, under the conditions of artificially shortened spring or summer days, or under those of natural short autumn or winter days, the growth of the vernalized plants of cereals does not cease. Given suitable mineral nutrition, these plants produce a fairly large amount of green mass, but they do not fruit. In our experiments we have been able to keep barley plants under the conditions of a 10-hour day for as long as two years. These plants produced new leaves all the time but did not fruit. The same variety of plants can be forced to ear and flower in 25 to 30 days if given the same thermal conditions but placed in continuous light instead of getting light only 10 hours a day.

These observations indicate that cereals can continue to grow under short-day conditions; but then their further development (after the vernalization phase), their advance towards fruiting, is inhibited, or else proceeds very slowly. [47]

To solve the problem of whether short-day conditions are unsuitable for the whole of a plant's further development after the vernalization phase or only for a part of it, only for some of its phases of development, E. P. Melnik conducted the following experiment in our laboratory. In 1932, she sowed the winter wheat Novokrymka 0204 in a vernalized and in an unvernalized state. The experiment was conducted under high summer thermal conditions, which made it impossible for the plants from the unvernalized seeds to vernalize. Therefore, in spite of the continuous light that was given to the experimental plants, i.e., in spite of conditions most favourable for the fruiting of vernalized wheat, the plants of the unvernalized sowing material grew for a long time, developed numerous leaves, but did not fruit. The plants of the vernalized sowing material, however, under these very same conditions, proceeded to ear and fruit fairly quickly (on the 35th day after sowing). Another series of experimental plants from vernalized and unvernalized sowing material was grown under the same thermal conditions, but with a 10-hour day instead of continuous light. Under these conditions, the plants of both the vernalized and unvernalized seeds failed to fruit. In external appearance, the vernalized plants in no way differed from the unvernalized plants.

Some of the potted plants from vernalized and unvernalized seeds were put in continuous light after having been kept under 10-hour-day conditions for different numbers of days. When this change had been effected all the vernalized plants, irrespective of the length of time they had been kept under the short-day conditions, fruited rapidly. The plants of the unvernalized sowing material, however, regardless of the number of days they had been subjected to short-day conditions, failed to ear when put under continuous light, as was the case with the unvernalized plants which had been kept under continuous light all the time. This is 'further proof that passage through the vernalization phase does not at all depend upon variation in the duration of exposure to daylight.

The following question naturally arises: Are plants which fail to fruit under short-day conditions qualitatively the same whether they are derived from vernalized or unvernalized seeds? In external appearance, plants from vernalized sowing material grown under short-day conditions differ in no way from those derived from unvernalized seeds grown under the same conditions. Both the former and the latter tiller fairly vigorously under these conditions but they do not develop stalks. Placed under long-day conditions, or still better, in continuous light, the plants from vernalized seeds rapidly began to differ in external appearance from those grown from unvernalized seeds. Consequently, the plants from vernalized sowing material which had grown for a long time under short-day conditions and had been unable to fruit, although not differing in external appearance from the plants of Unvernalized seeds, nevertheless differed from them qualitatively. This leads us to the conclusion that the changes taking place in the cells of the just slightly grown embryos during presowing vernalization are not lost, but [48] are transmitted to the plant's new cells formed in the process of growth, no matter how long its further development is retarded.

The chief object of the experiment we are discussing was to ascertain whether a long day or continuous light is a necessary condition for a plant's entire further development after the vernalization phase, or only for separate phases of development. Therefore, in the experiment referred to, variations were made in the treatment of the vernalized sowing material. In the first postsowing period, which was of different duration for different plants (from 2-6 to 40 days), some plants were grown under continuous light and then transferred to a 10-hour day. After being kept under continuous light for 20 days and then transferred to a 10-hour day, the plants from the vernalized sowing material formed stalks, eared and completed their development just as quickly as those plants which had been kept under continuous light all the time. This shows that wheat plants do not need a long day or continuous light for the processes of development and the growth of stalks, although the latter do not appear when the plants are grown (from the moment the seeds sprout) under short-day conditions. Moreover, it also shows that long-day or continuous light conditions are not essential for the plants' entire cycle of development after the vernalization phase. Wheat requires a long day, or more correctly, continuous light, only for a part of its cycle of development. This stage of development has been named the photo phase. It has been ascertained that cereals pass through the photo phase, which follows immediately after the vernalization phase, best of all in continuous light or at least under a long day, in combination with the other factors (temperature, moisture, air).

On the basis of this experiment, we believe, by analogy with the vernalization phase, that when passing through this (photo) phase of development, which requires prolonged light, the plants undergo qualitative changes which are later transmitted to all the new cells formed in the plants when subsequently grown under short-day (10-hour) conditions. Under the influence of a suitable set of external conditions, which includes continuous light, or a long day, changes take place in vernalized plants that are transmitted to the new cells formed in the process of the plants' growth, in the same way as are the changes characteristic of the vernalization phase.

The qualitative changes characteristic of the photo phase can take place only after the vernalization phase. This has been proved by numerous experiments conducted in the vernalization of winter cereals and other plants. Late spring sowing conducted not only with unvernalized, but even with slightly undervernalized winter crop seeds, results in failure to fruit and. ear. Hence, in spite of favourable external conditions (long spring and summer days) for the plants' passage through the photo phase, such plants cannot proceed to that phase. The reason for this is that the qualitative changes that take place with presowing vernalization have not been completed (the sowing material was undervernalized), and vernalization fails to continue after sowing because of the high temperature. If such plants are [49] vernalized, or if their vernalization is completed, in the autumn, and they are then placed under short-day conditions, they will neither ear nor fruit. Hence, during the long days of spring and summer the undervernalized plants do not traverse the photo phase. After vernalization these plants will need long-day conditions to enable them to undergo the qualitative changes that are characteristic of the photo phase.

Fig. 16. Winter wheat Erythrospermum 1325/5
The plants in all the sheaves were sown simultaneously on March 30, 1930, at the Ukrainian Institute of Selection and Genetics (Odessa). The plants in the first sheaf on the left were grown from unvernalized seeds. The plants in all the other sheaves were grown from vernalized seeds. The respective periods of vernalization were as follows, from left to right: 7, 11, 17, 21, 26, 31, 36, 41, 46, 52, 57, 62, 67, 72 and 77 days. 41 days (9th sheaf from left) of presowing vernalization were enough for the fruiting of the given variety. If vernalized for fewer days, this wheat does not ear

The photo phase cannot be traversed before or during the vernalization phase. It can be traversed only after the vernalization phase has been passed through.

Figure 16 shows sheaves of Erythrospermum 1325/5 winter wheat that was sown in the spring of 1930. The plants in the first sheaf on the left are [50] from ordinary seeds. The plants of all the other sheaves are from sowing material that had been verna1ized for different numbers of days (7, 11, 17, up to 77 days). It can be seen from the illustration that ears were produced only by the plants grown from seeds that had been subjected to presowing vernalization for 41 days and longer. The plants of all the other variants grown from seeds that had been vernalized for a smaller number of days failed to ear. The external appearance and behaviour of these plants (from undervernalized seeds) differ in no way from those of winter plants grown from ordinary spring-sown seeds. (In the illustration, first sheaf on the left.) These same plants, obtained from undervernalized sowing material, can be brought to fruiting under spring and summer conditions. For this purpose it is necessary to complete their vernalization, which is done by giving them an additional vernalization period corresponding to the deficiency they suffered before sowing. After this they will be able to pass through all the other phases of development under spring and summer conditions.

The time needed for completing the vernalization of plants does not depend upon the length of the interval between the first period of vernalization and the beginning of the second. Plants from undervernalized seeds can continue their vernalization in the field immediately after sowing, bearing in mind the low temperature conditions of early spring. If, however, the external conditions are unsuitable for passing through the vernalization phase it will not be completed. It will be completed only when those conditions are forthcoming. The method of completing the vernalization of undervernalized seeds is often practised in our laboratory. We vernalize the seeds of many varieties of wheat that require 56-57 days to go through the vernalization phase in 5, 10, 15 and up to 40 days. Such sowing material is kept in a semidried state (up to 15 or 20%) and is used for various experiments as required. To obtain fruiting plants from such sowing material it is necessary to vernalize it for the additional length of time it would have required to complete the vernalization during the first period.

Consequently, during the vernalization of seeds or plants, an accumulation of changes takes place. These changes remain in the cells in which they have taken place and are also transmitted to all the new cells formed from them. If these changes in the cells are not completed, i.e., if the given phase of development does not reach its end, the accumulation of changes may continue in the newly-formed cells up to a certain limit, which is a sign that the passage through the given phase of the development has come to an end. After this, changes in the cells in this direction no longer take place, no matter how long the plants are kept under the influence of the external factors which caused these changes to take place earlier. In addition to different dosages of external factors for passing through the vernalization phase, the winter plants of different varieties of wheat, rye, barley, etc., also require different lengths of time during which these factors operate. Thus, for vernalization at 55% of moisture and 0°-2° C. temperature, the [51] various varieties of winter wheat require the following: Erythrospermum 808 1/26 needs 18 days; Kooperatorka, 40 days; Stepnyachka, 45 days; Ukrainka, 50 days. If vernalized for shorter periods under these conditions, and if sown under high-temperature conditions, these varieties fail to ear. Longer periods of presowing vernalization do not, however, produce earlier earing in plants from seeds of these varieties than occurs in plants from sowing material vernalized for only the number of days required for the given variety.

Thus, when passing through the vernalization phase, the accumulation of qualitative changes in the just slightly grown embryo, or in the green plants, takes place only up to a certain limit. Beyond this limit, the accumulation of changes in the given direction ceases entirely. If, however, this limit has not been reached, the plants cannot pass to the next phase of development, i.e., cannot undergo the other changes that characterize the next phase, even though the external conditions are favourable for these changes. In the development of plants, a sequence of phases (stages of development) is observed. Normally developing (i.e., hereditarily unaltered) plants cannot skip any phase of development.

PHASIC CHANGES IN PLANTS TAKE PLACE AT THE GROWING POINTS OF THE STEMS

1A collaborator at our laboratory.

The phasic changes that take place in a plant, or in its separate organs, are irreversible, i.e., cannot be turned back. Our numerous experiments show that undervernalized plants can always be brought to. complete vernalization. This applies not only to sowing material, but also to, plants grown from undervernalized seeds. The vernalization of such plants can be completed at the time of sowing or whenever thereafter suitable external environmental conditions are created for these plants. The possibility of completing the vernalization of the sowing material and plants of wheat has already been mentioned above. Other plants behave in the same way as wheat in this respect. Let us cite the case observed in the work of D. A. Dolgushin.1 In 1930, at the Ganja station, he vernalized cabbage seeds for sowing with the object of obtaining flowering and fruiting plants. Not one of the experimental plants developed flowering shoots in the first year of life, neither did the control plants obtained from ordinary seeds of the same variety. In the autumn, a few score of experimental and control plants were transplanted to sand in the laboratory, where they passed the winter, and in the spring were planted in the field. All the plants from the vernalized seeds which had not developed flowering shoots in their first year produced a flowering shoot in their second year. The plants from the ordinary seeds failed to develop flowering shoots even in their second year. In, our opinion, [52] two explanations are possible of the failure of the cabbage plants grown from vernalized seeds to develop flowering shoots in the first year of life.

Firstly, that the cabbage plants grown from the vernalized seeds may not have been completely vernalized. Therefore, they were unable to develop flowering shoots, but formed heads. In the winter these plants completed their vernalization in the laboratory, in spite of the relatively high temperature at which the processes characteristic of the vernalization phase could only take place slowly. The plants from the ordinary seeds did begin to go through the vernalization phase in the laboratory in the winter, but owing to the relatively high temperature, the process was slow, and therefore they failed to complete the passage in the course of the winter. For 'this reason these plants failed to develop flowering shoots also in the second year of life.

Secondly, the cabbage plants from the vernalized seeds may have been fully vernalized before sowing, but owing to the high temperature and the late emergence of the shoots above ground in the spring, these plants were unable to pass through the photo phase, without which such plants cannot develop flowering shoots and reproductive organs.

A number of other special experiments that have been conducted show that the processes characteristic of the vernalization phase cannot be reversed. Whereas incompletely vernalized plants can be completely vernalized if placed under suitable conditions, we know of no case of the unvernalization of vernalized plants. Plant cells possessing the qualities of the vernalization phase cannot be turned back to the initial (prevernalization) state.

At the same time, we know of a number of perennial plants that require vernalizing every year. If the conditions for passing through the vernalization stage are absent, some perennial plants that have already fruited cannot fruit again. For example, many varieties of perennial rye or barley, transplanted in the winter or spring from the ground into pots to grow in the greenhouse, proceed to ear and flower at the end of the spring or the beginning of the summer. Later, these plants produce ripe grains, i.e., they complete their cycle of development. Simultaneously (or later), as the old straw below withers, these plants form new shoots, which do not develop into stalks that year (the straw does not develop). The further behaviour of these plants differs in no way from the behaviour of the ordinary annual unvernalized winter plants of rye, barley or wheat. Until these plants are placed under low temperature conditions (from O°-10°C.) for the purpose of passing through the vernalization phase, they cannot proceed further in the direction of developing new stalks and reproductive organs. Behaviour similar to that of the plants of perennial rye and barley is observed among the summer shoots of some annual plants of ordinary winter varieties of wheat sown in the autumn, 'which in the spring normally proceeded to ear. In the summer, the shoots that grow from the roots of these plants behave like typical unvernalized winter plants.

All these examples of the behaviour of the plants of perennial rye and barley, or of the behaviour of shoots that appear in the spring from the [53] lower parts of winter wheat that has passed through the winter, seem to contradict the above-formulated proposition that a plant's passage through its phases of development is not reversible. On the one hand, cells that possess the properties of vernalization cannot be turned back to the initial (prevernalization) state-in their individual development plants can only move forward. On the other hand, the plants of perennial rye and of annual winter wheat which have produced ripe grains and, therefore, have passed not only through the vernalization and the photo phases, but also all other succeeding phases of development, can form shoots in the lower parts, which (in the phasic sense) begin their development anew. First they must pass through the vernalization phase, then the photo phase, and so forth. All this seems contradictory; but the contradiction is only apparent. To be able to understand this seeming contradiction it is first of all necessary to determine: a) in what parts of the plant the qualitative changes characteristic of the separate phases of the given plant's development take place; b) how these changes are transmitted from cells to cells.

To answer the question as to the parts of a plant in which the qualitative changes related to the phasic changes take place—whether, under given external conditions, in the entire plant or only in definite parts of it-we conducted a number of experiments, chiefly with soya bean and cotton. Plants obtained from cuttings taken at successive points along the stem of a soya bean behave differently as regards initiation of fruiting (flowering). All the plants from cuttings taken above the point of insertion of the first flowering shoot of the mother plant come into flower with extreme rapidity (as soon as the cuttings take root), whereas the plants from cuttings taken below the point of insertion of the first flowering shoot (of the mother plant) are late in flowering. The lower the point of the main stem from which the cutting was taken, the later was the flowering. The same was observed in our experiment with cotton. After passing the winter with fallen leaves in a cold greenhouse (from 0°-5° C.), the old tufts of cotton plant that had already fruited began to develop new young leaves in the spring, when warm days arrived. Simultaneously, sympodial (fruiting) branches began to appear in the axils of the young leaves. The sympodial branches did not appear in all the axils, but only in the axils of the leaves situated above the point of insertion (along the main stem) of the previous year's first sympodial branch. No new sympodial branches appeared in the axils of the leaves below the first, old sympodial branches; in them monopodial (vegetative growth) branches appeared.

Usually, there is a greater flow of nutritive substances to the upper buds of the stem than there is to the lower ones. Therefore, to determine whether this alone, in the case in question, explains why fruit buds appear in the upper parts of the cotton stem and growth buds in the lower, the tops of a number of plants were cut off at the point of insertion of the previous year's first sympodial branch. This caused an increased flow of nutritive substances to the remaining parts of the plants. Nevertheless, all the [54] buds of these truncated plants gave rise only to vegetative growth shoots (monopodial branches) but not to flowering shoots. Consequently, in this case, the appearance of vegetative growth or fruiting branches depended not upon nutrition, but upon the tissue cells from which buds were formed.

Figure 17 shows two soya bean plants grown from cuttings in continuous light. The plant on the left is from a cutting taken from the top of a sterile plant; before the cutting was taken it had grown in continuous light, which usually prevents soya beans from fruiting. The plant on the right is also from a cutting taken from the top of a plant, but one that had fruited. Before the cutting was taken the plant had grown under the ordinary conditions of alternate day and night. Before the cuttings were planted, their leaves and all their buds were removed. After taking root, the plant from the cutting taken from the sterile plant (Fig. 17, left) did not flower. Under the same continuous-light conditions the cutting taken from the plant that had fruited formed buds as soon as it took root and later flowered and produced beans. Thus, on the basis of our experiments we arrived at the conclusion that the failure to form reproductive organs (fruit buds) may not depend in many cases upon the flow of nutritive substances to the section of the given tissue (experiment in cutting the tops of cotton plants). The failure to form reproductive organs may also not depend upon the location of the given buds. It may be immaterial whether they are situated on lower, middle or upper parts of the stem (experiment in taking cuttings from fruiting and sterile soya bean plants). The formation of reproductive organs depends first of all upon whether the cells of the given tissue have gone through those qualitative, phasic changes without which they cannot assimilate the external conditions necessary for the formation (development) of reproductive organs. At the same time the above example of the growing of soya bean plants from cuttings taken at successive points along the stem of the mother plant appears to show that the time of fruiting depends upon the location of the tissue along the stem (the part of the stem from which the cutting is taken). In this experiment the lower, and hence the older, the part of the stem of the mother plant from which the cutting was taken, the later the plants proceeded to flower. In this case it was found that the tissue of the lower part of the stem was less ready to form fruit buds than that of the younger upper part.

Thus at different points along the stem the tissue cells may possess different phasic qualities. Different parts of the stem tissue may be in different phases of development. The tissues of the lower part of the stem are in a younger phase of development than those of the upper parts. The lower part of the stem may possess the properties of the vernalization phase; the upper parts may possess the properties of the next phase, the photo phase, etc.

The proposition that the tissue at different points along the stem may possess different properties as regards readiness for fruiting is supported not only by our experiments but also by a number of facts met with in practical farming and in the literature on the subject. The lower down the main [56] stem a cutting is taken from a fruit tree (apple or pear) grown from a seed and not from a graft, the younger will be the phase of the new shoots, and the more years will pass before these shoots proceed to fruit. Low-cut stumps of forest trees produce shoots as young (as regards readiness for flowering) as one-year shoots grown from seeds. The fact that such shoots, possessing a strong old root system, will grow more quickly and vigorously than a one-year seedling is another matter; and therefore, of course, the quality of the wood of such trees cut for economic purposes will also be different.

Fig. 17. Soya Bean
Both plants were grown n continuous light; the plant on the left was grown from a cutting taken from a sterile plant; the plant on the right was grown from a cutting taken from a fruiting plant. The plant on the left, grown in continuous light, did not flower. The one on the right, after throwing out a new stem 2 mm. long, formed a flower bud and fruited. Hence, the tissues of the two cuttings were not of like quality

N. P. Krenke (Plant Surgery, pp. 264-78, 1928) deals in considerable detail with the different qualities of cuttings taken from different parts of plants. It will not be amiss to cite the example of the cuttings of ivy (Hedera helix) that Krenke mentions in his book. "Here it is appropriate to mention," writes Krenke, "the planting of cuttings taken from the flowering shoots of ivy and of several varieties of climbing ficus. The strands of ivy, when trailing on the ground, easily take root. Usually such strands do not develop flowering shoots. But, under the southern conditions that are normal for ivy, if the strand climbs a support, flowering shoots will form on such branches. A special feature of the latter is their leaves. Their margins are entire and they are of acuminate-ovate shape, whereas all the rest (except the first leaves of the seedling) are palmately, lobed. If a cutting from the flowering shoots is planted (preferably before the formation of flowers), it will grow into an erect tree, whereas any cutting from a climbing branch will reproduce the climbing form. Moreover, on the tree that develops, all the leaves will be like those on the flowering shoot of the cutting from which this tree had grown. True, with increased nutrition and water supply, individual shoots with palmately lobed leaves will appear on this tree. At the same time, the seeds from such trees will produce the ordinary climbing form of ivy. The same applies to the ficuses we have mentioned. Consequently, this phenomenon belongs to what is called "Dauermodificationen." In our opinion, this, of course, is not a question of Dauermodificationen, but of phasic changes which are irreversible in vegetative propagation of plants.

Thus, 1) the individual phases of development proceed in strict sequence; 2) each developmental phase can be passed only after the preceding phase has ended, provided the external conditions suitable for it exist; 3) the tissue cells may be passing through different phases of development at different points along the plant stems. If a plant has grown from a seed, the lower part of the stem, although the oldest as regards age, possesses the properties of the youngest phase of development. And vice versa, the upper part of the stem, although the youngest as regards age, may be in an older phase of development.

From this we draw the conclusion that when a plant, or individual parts of it, go through the vernalization or other phase of development, changes take place only in the cells at the growing points of the stem. The changes that take place in the cells at the growing points as a result of cell division are transmitted to the newly-forming cells. Given the suitable external conditions, [57] the young cells go on changing until these changes reach their limit, i.e., until the given phase of development is completed. Thereafter, under other necessary external conditions, they begin to pass through the next phase of development. This explains the different degrees of readiness for fruiting manifested by the tissues at different points along the stem.

When external conditions are suitable for a plant's rapid progress through its phases of development and unsuitable for its rapid growth, cases may often be observed of a plant's rapid progress through its phases of development although its growth is extremely restricted. When the plant has completely passed through its first phase of development it passes on to the next phase if the external conditions are suitable for this, and so on until the seeds ripen. The quicker a plant, under the suitable external conditions, passes through its phases of development and the slower it grows under these conditions, the lower will be the point on the main stem at which the tissue will be ready to form (develop) fruit buds. In our experiments, the height at which the first fruit buds appeared in the axils of the leaves on the main stem (counting from bottom up) of plants like cotton, brown hemp and soya bean, varied greatly, depending upon the conditions under which the plants were grown. In the practical farming of cotton (upland) the first sympodial branches appear in the axil of the fourth or fifth leaf. In our experiments with plants of this variety of cotton, some variants had the first sympodia in the axil of the second leaf. The plants of other variants developed 25-30 regular leaves and were unable to form sympodial branches. The tissue cells of the stems of these plants did not undergo the corresponding changes. The same may be easily observed in brown hemp, soya bean, and other plants.

When vernalizing seeds artificially, special conditions are created which inhibit the growth of the seeds and favour the rapid progress of the embryos that are just beginning to develop through the vernalization phase. We can already induce several plants (millet, soya beans) before they are sown, but after passing through the vernalization phase, to go through the next phase of development, the photo phase.

In soya bean plants grown from vernalized seeds, the slightly-developed embryos of which have gone through the vernalization and photo phase processes, it is possible to observe not only early flowering, but the appearance of the first fruit bud at a low level of the stem.

Cases are not rare in soya bean plants vernalized before sowing of the first buds appearing in the axil of the first leaf. In such plants, the stem tissue, even before sowing, may be quite ready (as regards phasic qualities) for fruit buds to form (develop) from its cells if the conditions for this are suitable.

The readiness of plants, as regards phase of development, for fruiting does not necessarily mean that these plants will bear fruit. It merely means that out of the qualitatively, phasically ready cells reproductive organs may develop. The development of these organs, like everything else in a plant, [58] calls for specific external conditions. By mineral feeding or light alone it is easily possible to create such an environment for cotton (and for many other crops) that this plant, although phasically ready for fruiting, will not only he unable to develop fruit buds, but will lose the buds, flowers and ovaries (bolls) that it may already have developed.

THE LOCALIZATION OF PHASIC CHANGES

From the foregoing we arrive at the conclusion that the phasic changes in plants taking place in the cells at the growing points of the stem are transmitted by division to all the new cells that are formed from them. Can the phasic changes be transmitted in some way other than cell division, i.e., can the phasic changes that take place at the growing point of a stem be transmitted to the cells of a lower part of the same stem, and also to the cells of other stems or branches above them?

A number of observations, and also special experiments, show that phasic changes are localized in the cells in which they take place. They can be transmitted only to the new cells that are formed from them, i.e., these changes are transmitted only, from the mother cells to the daughter cells.

When vernalized seeds of winter varieties are used in practical farming, cases occur when the plants from such material produce one or two (usually central) flowering shoots and a rosette of winter, nonflowering shoots. The explanation of this phenomenon is that the cells of the growing point of the central bud in the embryo were vernalized, whereas the cells of the growing points of the other (not central) buds were not vernalized (or were undervernalized). They cannot acquire the quality of vernalization from the neighbouring vernalized cells (Figs. 18 and 19). This, too, explains why perennial rye requires annual vernalization of the stems that grow below out of dormant buds.

In proof of the proposition that phasic changes are localized and that they can be transmitted only to the cells that are formed out of the changed cells (i.e., by their division), a number of facts can be cited.

Some of the stems of the same winter wheat plant (Fig. 20) that had wintered in the field (and consequently had gone through the vernalization phase) did not fruit in the spring, after being put under short, 8-hour-day conditions. Under these conditions, the changes characteristic of the next, the photo, phase cannot take place in the growing points of these stems. Under 24-hour-day conditions, however, the other stems in the same cluster fruited without exercising any phase-changing influence upon the neighbouring stems.

1For this phase of development, many of the so-called short-day plants require prolonged darkness (long nights).

The perennial cotton plant No. 01632/2 (of Abyssinian origin) cannot fruit under the conditions prevailing in Central Asia, and still less under those prevailing in the Ukrainian S.S.R. Owing to the excessively long day [60] in our districts, the plants of this variety of cotton cannot pass through the phase that follows the vernalization phase.1 Figure 21 shows a cotton plant of this variety that has been grown for two years in continuous light.

     
Fig. 18. Winter rye Tarashchanskaya, autumn sowing:
a—sterile shoots grown in late spring from dormant unvernalized buds at the tillering nodes of normally fruiting rye
  Fig. 19. Winter wheat sown in autumn
In the spring has at the lower parts of its stem winter-grown unvernalized shoots from the dormant buds at the tillering nodes
  Fig. 20. Winter wheat Byelokoloska Ostistaya 0719, autumn sown
Taken from the field on April 23 and grown in greenhouse. The left half of the tillers was grown in continuous light; the right—under 8-hour-day conditions
  Fig. 21. Perennial cotton plant from Abyssinia, No. 01632/2
Sown on May 15, 1932. This variety cannot go through the photo phase in view of the long day in Odessa

In the first year of this plant's life, one of its branches (branch a) was kept in the dark for 14 hours during 30 consecutive days. After that, this [61] branch was kept under the same conditions as all the others. Later, buds formed on this branch. It was the only one on the whole plant that fruited in the second as well as in the first year of its life. The entire mass of nonflowering branches of this cotton plant could exercise no influence on the fruiting branch, nor could the latter transmit its properties to the neighbouring branches.

Fig. 22. Shabdar (annual clover)
The bush on the left was grown from spring-sown vernalized seeds; in the summer it produced flower buds, flowered and seeded. The bush on the right was grown from ordinary seeds; sown in the field at the same time as the first, it could not vernalize and therefore did not form reproductive organs. The middle bush was grown from undervernalized seeds. Before sowing, a group of embryo cells of this pliant was able to complete the vernalization phase. Out of some of the cells flowering shoots were formed, and out of the adjacent unvernalized embryo cells nonflowering vegetative buds were formed

All these facts show that phasic changes are localized in the cells and cannot be transmitted to the neighbouring, adjacent cells.

It is not infrequently stated in the literature on this subject that if cuttings from one- or two-year apple or pear seedlings are grafted on to the crowns of fruit-bearing trees, their own fruiting is greatly accelerated, i.e., a property of the fruit-bearing trees is, as it were, transmitted to the grafts, which, according to our interpretation, are phasically not ready to form fruit buds. If these statements were true, our interpretation of the phasic changes that take place at the growing points, and of the localization of these changes, would be wrong. There is every ground for doubting the truth of these statements on this question, the more so since they are merely repetitions (to be found in many textbooks) and contain no references to sources, no indication as to when and by whom it was observed that the fruiting of one- or two-year seedlings is accelerated by grafting on to the crowns of fruit-bearing trees. The authoritative view expressed by I. V. Michurin on this question contradicts these unsupported statements. In his [62] book Results of Half a Century of Work, I. V. Michurin writes: "But we get the very opposite from the erroneous assertion that it is possible to accelerate the inception of fruiting of a young hybrid seedling in its early stage of development by grafting cuttings from it on to the crown of an adult, already fruit-bearing, tree of any variety. The result of such action is quite the contrary—not an acceleration, but a retardation of the inception of fruiting, except in those cases when we perform this operation not with young hybrid seedlings, but with branches adult as regards age and time of fruiting."

BRIEF CONCLUSIONS CONCERNING THE PHASIC DEVELOPMENT OF ANNUAL SEED PLANTS

1. Not only do different plants require different conditions for their normal growth and development, but the same plants require different external conditions in the course of their lives from seed to seed. The fact that a plant requires different environmental conditions for its development shows that the plant's development from seed to seed is itself different in different periods. The development of annual plants consists of separate stages phases of development.

The phases of development of a seed plant must be understood to mean not the formation (development) of the different organs and parts, but the qualitative turning points and stages characteristic of and conditioned primarily by the changes in the environmental conditions the developing plant requires.

The need (including assimilation) of relatively definite conditions and also the changes in this need during the course of the individual life of a plant are conditioned by the entire generic, specific and varietal history of each individual case of a seed embryo examined by us.

The development of all preceding generations gives fairly definite direction to the development of a plant from a given, concrete seed. As special experiments and observations of plant life have shown, the relatively directed development characteristic of an embryo in the seed proceeds in stages, or phases.

By the growth of a plant we mean what this is usually taken to mean in practical farming, namely, the increase in the plant's weight and volume, irrespective of the organs and parts of the plant at whose expense this increase is obtained.

We apply the same conception to the growth of the separate organs and parts of a plant. For example, by the growth of the root of a sugar beet we mean the increase in the root's mass and volume.

The term growth does not characterize the qualitative state, the state of maturity of a plant, or of its organs. Growth is the increase in the mass of a plant in a given phase of development. The property of growth can be expressed [63] in different degrees, depending on the nature of a plant, on its phase of development, and also on its environmental conditions.

2. The sets of external conditions a plant requires for passing through its phases of development and for growth in a given phase of development often do not coincide. This incongruity applies not only to the difference in the dosages of factors required for growth and in those required for development. In the case of many plants there is a difference in the very factors that constitute the sets required for development and growth respectively. Hence, in the life of individual plants the following may often be observed: a) rapid growth and slow development, slow progress towards fruiting; b) slow growth and accelerated development; c) rapid growth and rapid development..

3. When seeds are vernalized under artificial conditions in the laboratory or in the collective-farm barn, conditions are created under which the plants (the just slightly-developed embryos) pass through one of their phases of development (the vernalization phase) with growth extremely slow, barely perceptible.

4. The changes that take place in the just slightly-grown embryos during presowing vernalization constitute one of the phases of a seed plant's development. Without these changes the plants of winter varieties (and of all spring varieties, it must be assumed) cannot fruit.

Given the suitable external conditions, a plant can begin to go through the vernalization phase immediately after the embryo barely begins to grow. If the external conditions suitable for passing through the vernalization phase do not exist, the plants do not traverse this phase of development until the necessary conditions appear; but the growth of such plants (in this case the development of leaves and roots) can continue.

Experimental data show that to pass through the vernalization phase plants in the shape of just slightly-grown embryos and 5- to 8-month-old plants of the same variety of winter wheat require the same environmental conditions and the same length of time. Hence, the speed with which a plant goes through the vernalization phase does not depend upon its size and age. The speed of this passage depends upon the nature of the plant and upon its environmental conditions.

The plants of winter varieties of wheat, rye and other crops sown in the autumn usually go through the vernalization phase not in the shape of just slightly-grown embryos but as green tillering plants.

5. To pass through the other phases of development as well as through the vernalization phase plants require not separate external factors such as temperature, air, moisture, light, darkness, etc., but sets of factors, the components of which are determined by the natural properties of the plants in question. Wheat and millet, for example, require different conditions for passing through the same phases of development, phases that are specifically their own and inherent in the nature of the respective plants. Moreover, to pass through the different phases of its development, the same plant requires different sets of external conditions. [64]

In the majority of cases, when the plants of our winter varieties of wheat, rye and other crops, are sown in the field in the spring, they cannot complete their vernalization (hence their failure to ear), solely because of the relatively high temperature in the sowing and postsowing periods. This does not mean that for vernalization "in general," including presowing vernalization, winter and spring cereals require only suitable temperature. To pass through the vernalization phase the plants of winter and spring crops require not simply the thermal factor, but that factor in a set of other factors. The known components of this set are: moisture, temperature, air (and also the plastic nutritive substances to be found either in the seed or in the green plant). A certain quantitative combination of doses of these factors (according to variety) enables any variety of cereals, and of many other plants, to traverse the vernalization phase.

If it were possible in practical farming artificially to regulate the temperature over wide field spaces, the thermal factor would be the chief factor in the spring sowing of winter plants. It would only be necessary to regulate it to suit the plants' requirements for going through the vernalization phase. All the other factors of the set required for vernalizing cereals under the spring field conditions prevailing in our districts are always present in the necessary doses. In practical farming, plants have to be vernalized in the seed state under artificial conditions before sowing. In artificial vernalization it is necessary to create not only the temperature required by the given plants, but also a number of other necessary conditions. In the presowing vernalization of cereals, as well as of other plants, the chief and decisive factor is usually moisture.

6. In the development of annual seed plants, a definite sequence is observed in their passage through their different phases of development.

Passage from one phase of development to the next can begin only after the one phase has been completed, and only if the environmental conditions suitable for the next phase are present. For example, wheat plants can begin to go through the photo phase only after the preceding phase of development, namely, the vernalization phase, has been fully completed, and only under long-day conditions (still better, in continuous light).

First published in 1935

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