Botan. Rev. 6: 25-47 (1940)
Vernalization and the growth phase concept
H. H. McKinney
Bureau of Plant Industry


The term vernalization is the anglicized form of the Russian word iarovization. It signifies that certain winter annuals and biennials can be induced to follow the spring-annual habit by suitable treatment of the germinated seed or the active bulbs before planting, thus making it possible to obtain sexual reproduction the first season from spring plantings. However, as the term is now used by some workers it embraces practically all of the environmental factors and all the methods applied at any time in the plant's development and which are capable of accelerating sexual reproduction in any species of plant. As the term iarovization was first used it referred to treatments which altered certain germinated seeds physiologically, these alterations in turn changing the direction of growth in suitable subsequent environments. The term was not applied to the breaking of a rest period as in the potato and many seeds. However, there has been a tendency among some workers to apply the term vernalization in this connection. In fact, it has been applied by Shchernetsky (63) to the process of soaking sugar beet seed in water for 4 days at 17.2° to 21.1° C. before planting with the intention of increasing the tonnage and sugar content of the roots.

In general it may be stated that the fundamental concept or principles of vernalization are based on the following facts (1) that species and varieties possess different optimum environmental requirements during their several critical growth phases, (2) that these optimum requirements must he met within certain limits, otherwise sexual reproduction will not occur or it will be delayed, and (3) that certain of these environmental requirements which are naturally supplied during the first developmental phases in certain plants can be supplied artificially to the slightly germinated seeds or to bulbs before planting.


In the present article reference will be made to those papers which appear the most pertinent, and no attempt will be made to cite all of the many titles in the literature. However, an attempt is made to cite papers which have good literature lists. These taken together with the present list should enable any student to become acquainted with the entire field very easily. Papers having rather large literature lists are so indicated in the literature list of this paper.

During recent years, Lysenko (42, 43, 44, 46), a Russian investigator, has devoted much time to the study of vernalization and growth phases in many crop plants. In Russia this line of physiology is being carried forward on a very large scale; however, it seems only fair to point out that the basic concepts (54, 70, 71) involved in these studies have been known within certain circles for many years. They simply have not been recognized in all circles of plant science until recently.

The older horticultural and agricultural journals and the older scientific journals and text books show that some growers and some botanists recognized part or all of these concepts. To take the specific phenomenon, vernalization of wheat, on which Lysenko developed his views, it is found that in 1857 Klippart (35), an early observer and close student of crops who was associated with the Ohio State Board of Agriculture, published a rather clear elucidation of this phenomenon in one of his annual reports to the State Board. However, 20 years previous to Klippart's record it was recorded that a grower in New York had produced a crop of grain from spring-sown winter-wheat seed which had been subjected to low temperatures before seeding. In 1850 Allen (1), an agricultural expert of that day, recorded a similar observation. It appears, therefore, that in Klippart's time the idea of vernalization was more than local in the United States. Since Klippart's record is so clear and to the point it is quoted as follows:

"To convert winter into spring wheat, nothing more is necessary than that the winter wheat should be allowed to germinate slightly in the fall or winter, but kept from vegetation by a low temperature or freezing, until it can be sown in the spring. This is usually done by soaking arid sprouting the seed, and freezing it while in this state and keeping it frozen until the season for spring sowing has arrived. Only two things seem requisite, germination and freezing. It is probable that winter wheat sown in the fall, so late as only to germinate in the earth, without coming up, would produce a grain which would be a spring wheat, if sown in April instead of September. The experiment of converting winter wheat into spring wheat has met with great success. It retains many of its primitive winter wheat qualities, and produces at the rate of 28 bushels per acre."

In 1883 Hellriegal (26) concluded that barley has a lower optimum temperature during the tiller-formation phase than during the stem-elongation phase.

In 1893 the phenologists, ecologists and horticulturists met with the International Meteorological Congress in Chicago and their several papers were published by the U. S. Department of Agriculture (68). At that meeting Paul Schreiber from Chemnitz, Germany, presented an ecological paper in which he stated:

"The growth of vegetation requires heat, water and sunshine; but of each the proper measure, as every excess or deficiency acts injuriously. It should, therefore, be the object of our investigations to determine how much of heat, water and sunshine is required by different plants, and how these influential factors are to be distributed during the various phases of plant life.

"Our information concerning the duration and power of sunshine is increasing so rapidly that we may hope for early and important additions to our knowledge concerning these elements of our investigations. 1f our labors in this direction are to be of practical value to the husbandman, they must include careful notations of the successive phases of plant life or, at least, of the main phases of growth -the so-called phenologic observations. If, in this manner, we discover the laws governing each separate phase or phenomenon, and from them the joint result of their reciprocal influences, our object will have been accomplished."

W. Detmer of Jena, Germany, presented a physiological paper in which he makes the following statement:

"The position of the three cardinal points, the minimum, optimum, and maximum, is by no means the same for the various physiological processes which take place in a plant or an organ; it also varies for a given process in different species of plants, and is even influenced by the degree of development of an organ."

Dr. Egon Ihne of Friedberg, Germany, made the following statement in his paper on plant phenology:

"Although the same vegetal phase may set in on a date varying from year to year, the date depending primarily on the climate of each year, yet to reach this phase the plant requires an amount of heat that is constant from year to year. A plant may, therefore, be considered as a means for measuring heat; and the beginning of a certain vegetal phase is also a standard for measuring a certain sum total of heat supplied up to that date, and this sum total expresses the measure of heat required by the plant to reach The phase in question.

"Further, as has been demonstrated by physiological investigations, it is not every temperature above zero (centigrade) that is effective in vegetation, but the degree at which the temperature begins to be effective varies for different plants and phases. In the simple addition of positive temperatures, be they shade means, shade maxima, or insolation maxima, this variability finds no expression. The zero point of effective temperature should first be determined for each phase and plant in question."

Klebs (34) in 1918 emphasized the importance of recognizing the different requirements of plants during the several phases of development. In Sempervivum Funckii he distinguished three distinct phases, each having different environmental requirements. The same year Gassner (17) reported that earliness in winter wheat is favored when low temperatures obtain during the seedling stage and high temperatures obtain during the later stages of development.

In 1923 (48) the writer called attention to a field test in which a good crop of winter wheat was obtained in Illinois from seed sown late in November but which did not emerge until spring. In another paper McKinney and Sando (52) deal with additional literature on the subject.

From the records cited there can be no doubt that the fundamental concepts relating to vernalization and to growth-phase requirements are of long standing in America and in Europe.


The initial steps in vernalization may be summarized as follows:

Seeds are first soaked in a suitable quantity of water to just start germination, after which they are subjected to suitable temperatures for periods ranging from about 5 to 60 days, depending on the species and variety. Or in the case of some plants as beets, cabbage, cauliflower, carrots and turnips, the young seedlings are subjected to low temperatures for suitable periods.

In general there are two ranges of temperature in use, low temperatures ranging from slightly above freezing to about 10° C. and high temperatures ranging from 20° to 30° C. It is claimed that darkness is an essential feature during exposure in the high-temperature method, but darkness is of no importance in the low-temperature treatment of germinated wheat (52) nor in the high-temperature treatment of germinated corn (64).

In all cases germination is started but retarded by holding the moisture content of the seed at suitable percentages ranging from about 45% to 60% of the dry weight of the seed, depending on the species. In some cases salts have been added to the water to prevent excessive germination during the treatment (5). At the completion of the treatment, seeds may be dried for a short time before sowing or they may be sown in a slightly moist condition.

The low-temperature treatment is used for the seeds of wheat, barley, rye, oats, timothy, rice, meadow foxtail, vetch, rape, lentils, white lupines, crimson clover, red clover, Austrian winter field peas, carrots, beets, turnips, and cabbage, and it has been applied in some modified form to such bulbs as those of the Easter lily, daffodil, Dutch and Spanish iris.

The high-temperature treatment has been advocated for corn, cotton, soybeans, millet sorghum, Sudan grass, and rice.


In this review it seems unnecessary to consider the results of more than a few experiments which bring out principles. In the cereal plants, emphasis will be placed on wheat and rye because these plants have been given the most intensive study and because the principles established in the studies of these plants seem to apply to other winter annual members of the Gramineae in the tribes Hordeae and Aveneae.

Cereal Plants


In all types of wheat tested thus far by the writer and Sando (50, 52), sexual reproduction is not dependent on a critical temperature or a critical photoperiod, because this process occurs over very wide ranges of both factors. However, the time when sexual reproduction occurs and also the yield of seed are influenced by temperatures and the photoperiod (32, 33, 52) and by light intensity (52). Furthermore, the optimum conditions for the earliest sexual reproduction are not the same as the optimum conditions for the highest seed yield (5).

When winter wheats such as Harvest Queen and Turkey are grown at high temperatures in a long day throughout the life cycle, they, produce many side shoots or tillers, each of which has many internodes and leaves; the elongation of the internodes, head formation, and sexual reproduction are retarded. On the other hand, when the seed is germinated slightly and chilled at temperatures slightly above freezing for 60 days before it is sown at high temperatures in a long day, the resulting plants behave as spring wheats. They develop relatively few tillers which produce few internodes and leaves, the internodes elongate rapidly and heading and sexual reproduction are accelerated. In other words, the chilling treatment has not broken a dormant period such as occurs in perennial temperate-zone fruits, but it has stimulated a directional change in the plant's activities when growth continues in a suitable environment. On the one hand, the plant vegetates excessively and sexual reproduction is delayed, whereas on the other hand vegetation is reduced and sexual reproduction is accelerated.

So far as they have been studied, winter barley (6, 7), winter oats (6, 8) and winter rye (59) have given ample evidence of reacting essentially in the same manner indicated for wheat.

Some varieties such as Purplestraw wheat are intermediate or facultative with respect to seasonal growth habit. They require a moderate amount of low temperature during early growth for early sexual reproduction. Some late spring wheats such as Kinney will head earlier if low temperatures are supplied during early growth.

In order to determine the commercial value of vernalization in the United States field trials were carried out with winter and spring wheats at Rosslyn, Va.; Lincoln and Alliance, Nebraska; Hays, Kansas; Mandan and Langdon, North Dakota; and Moccasin, Montana (51).

The most satisfactory yields were obtained at Langdon, North Dakota, the most northerly station. At that station Kanred winter wheat from seed chilled 50 days outyielded all the unchilled Marquis spring-wheat controls, though the difference was slight. Slightly greater yields were obtained from certain chilled spring wheats than from the unchilled spring-wheat controls, but the same spring varieties at Mandan gave the highest yields when the seed was not chilled.

Although rapid acceleration of sexual reproduction reduces the yield of seed per plant because of a reduction in the number of tillers and in the number of fertile forets per head (52), yields have been higher when the plants were cultured under suitable short days and low growing temperatures during the initial growth phase (50, 52) than when the germinated seeds were vernalized in the usual manner.

Harvest Queen plants from germinated seeds vernalized for 40 days near 33° F. and grown with uninterrupted light at summer temperatures, headed 89 days from the time the seed was put to soak (50). This is the most rapid heading time yet obtained in Harvest Queen winter wheat from chilled germinated seed. However, only 10 to 30 seeds per plant were produced.

Yields of 75 seeds per plant have been obtained in Harvest Queen when heading took place 88 days from the time the seed started to soak and when the seedlings and plants were grown according to the following schedule of temperatures, photoperiods, and times (50):

21.1° to 23.9° C. on moist filter paper for two days to start germination.

10.0° C. for 36 days with a photoperiod of 8 hours in culture chamber.

15.6° C. for 18 days with a photoperiod of 8 hours in culture chamber.

21.1° to 23.9° C. to end of test with a photoperiod of 18 hours in greenhouse.

These plants actually headed one day earlier than those from vernalized seed, and it is likely that still earlier heading is possible without reducing the yield to 30 seeds per plant.

Harvest Queen plants from chilled germinated seeds will yield 75 seeds if grown at 21.1° to 23.9° C. with a daily photoperiod of 16 to 18 hours. However, 100 or more days are required for plants to head when computing time from the beginning of the soaking process (50).

Although varieties of winter wheat differ in their temperature and length-of-day requirements for earliness (50), tests indicate that all the winter varieties tested complete their life cycles quite rapidly and produce good yields of seeds when grown near 7.2° to 10.0° C. with a daily photoperiod of 8 to 10 hours for 45 days, followed by temperatures near 21.1° to 23.9° C. with a daily photoperiod of 16 to 18 hours. Exposure of 20 days to the low temperatures and short days has been satisfactory for the facultative varieties such as Purplestraw.

In hybridizing work Sando and the writer have obtained essentially simultaneous flowering in early, intermediate and late varieties of both winter and spring wheats, thus making possible all combination crosses.

In addition to hastening maturity it is claimed by Russian workers that vernalization increases drought resistance in spring cereals, and it is now claimed that drought resistance is increased when the seeds are moistened and dried intermittently three times at moderate temperatures, 20° to 22° C., allowing germination to proceed slowly during the moist stages (27, 28, 29, 30, 57, 58). Germination is started by adding water amounting to 30 per cent of the dry weight of the seed. The seed is then dried, remoistened with water amounting to 20% of the dry weight of seed, dried again, remoistened with water amounting to 15% of dry weight of seed, dried and sown. The object is to allow very slight progress of germination at each moistening and to keep the plumules and roots from developing to a point which prevents easy drilling. The method is referred to as prehardening.

Data cited by Zuhr (77) indicate that vernalization reduced bunt and loose smut in a spring variety and in a facultative variety of wheat grown in the field. Loose smut was reduced 45.3% to 68.3% in comparison with non-vernalized controls.

These aspects of the vernalization problem have not been tested to any extent in other parts of the world.

Vasiljev (69), Timofeeva (66) and others found that winter hardiness in winter wheats is reduced by vernalization. It appears that vernalization of the germinated seed before sowing prevents the normal hardening which takes place in the field during the autumn and early winter.

Buĭlina (9) found that the optimum time for vernalization tends to increase with the degree of winter hardiness characteristic of a given variety of winter wheat. The varieties having low winterhardiness required 45 to 55 days, those of medium hardiness required 55 to 60 days, whereas the most winterhardy required 60 to 65 days.

Rice was vernalized at 3° to 5° C. by Ossewaarde (55) in Holland. The time periods were 2 weeks and 5 weeks. The vegetative period was shortened 2 to 7 days and the yield of straw and grain were increased by the two-weeks treatment.

The high-temperature method has been used also for vernalizing rice as indicated below.


It has been claimed that certain summer annuals have a short vegetative period when the slightly germinated seeds are held for 5 to 15 days at temperature ranging from 20° to 300 C. in darkness (45).

Seed of several cereal crop plants has been treated according to the methods recommended and comparisons have been made in the field between the plants from the treated and the nontreated seed.

Sprague (64) working with several hybrids, inbreds and one variety of corn Zea mays gave the method a rather thorough trial at the Arlington Experiment Farm.

Although the seed was soaked in a 0.5% solution of Uspulun to cut down molds, vernalization reduced the germination greatly over the controls and caused delay in emergence in most cases.

Vernalization reduced the number of internodes very slightly and hastened sexual maturity. However, the differences in time of maturity between the vernalized and the control series were so slight as to be of no agronomic importance. Yields of grain were reduced in the vernalized series. In the five comparisons which were significant the reduction was approximately 15% to 30%. As in the case of wheat the day length following vernalization influences the time of maturity of the corn plant. Sixteen hours of light daily during culture hastened sexual maturity from 10 to 15 days over continuous light. Vernalization was just as effective in continuous light as it was in the dark.

Tests with vernalized corn were conducted by Elema (15) in Holland and the results agree in general with those reported by Sprague.

Martin reports (53) that seed of 41 varieties of sorghum were vernalized by the recommended high-temperature method and all varieties failed to mature earlier than the controls. In most varieties vernalization greatly reduced or completely destroyed germination. Similar results have been reported from Holland by Elema (15) and van Hock (31).

Haigh conducted tests with rice in Ceylon, and his results were reported in a personal communication to the Imperial Bureau of Plant Genetics (2). Seed was soaked for 24 hours, then held for 6 days at three different temperatures, 25°, 30°, and 35° C. Sowings were made in the field. Vernalization favored flowering by 5 to 10 days in comparison with the dry-seed controls, and by 1 to 4 days in comparison with the controls sown with germinated, nonvernalized seed. The method has no practical value, according to the report.

Forage Plants

Čepikova (11, 12) reports that Alopecurus pratensis (meadow foxtail) and Phleum pratense (timothy) produced seed the first season when the germinated seeds were first vernalized near 3° C. Trifolium pratense (red clover, double-cut type) produced seed the first season when the seed was vernalized at 10° to 120 C. for 10 days.

Zerling and Čepikova (75, 76) carried out additional tests and found that Trifolium pratense (red clover, single-cut type) seeded the first season when vernalized 20 to 40 days at 3° to 8° C. These investigators report that vernalized meadow foxtail, timothy and red clover were earlier and gave higher yields than the controls the second season. In other words, the autumn, winter and spring conditions seemingly did not have an equalizing effect on the controls when compared with the vernalized lots. The timothy seemed to show the greatest benefit the second season from vernalization of the year previous.

Kostov (38) reported favorable results with vetch vernalized for 10 days at 8' to 10° C.

Žvanskii (73) and Žvanskii and Spilevskii (74) report that seed was obtained from winter rape the first season when the germinated seeds were kept in snow for 16 days and in a cool cellar for 15 days before sowing on April 23.

Soybean seeds were vernalized by the high-temperature method in Holland by Elema (15) and by van Hoek (31) and in England at the Long Ashton Station (2, 118). All reports indicate that plants from vernalized seed behaved as the plants from the nonvernalized seed. Negative results were reported from Long Ashton with respect to peas vernalized by the low-temperature method.

In the United States McKee (47) tested the effect of low-temperature vernalization on White lupine (Lupinus albus), crimson clover (Trifolium incarnatum) hairy vetch (Vicia villosa), Austrian Winter field pea (Pisum arvense), double-cut red clover (T. pratense), and white sweet clover (Melilotus alba). Germinated seeds were held at 0° C. for 40 days before sowing. All species except the red clover and sweet clover came into flower and fruit when vernalized. Nonvernalized lots either did not bloom or were late. The temperatures were probably too low and the time too long in the case of the clovers.

The same investigator tested foxtail millet (Chaetachloa italica), Sudan grass (Sorghum vulgare sudanense), soybean (Soja max) and crotalaria by the high-temperature method of vernalization. In most cases vernalization decreased vigor and in no case did the treatment hasten the time of maturity.

Miscellaneous Plants

Burr and Turner (10) vernalized seeds of tomato and cucumber at 10 to 3° C. for 7 to 44 days before sowing. Vernalization reduced germination, stunted the plants and reduced fruit yields. In later tests these investigators (Turner and Burr 67) found that vernalization at -0.28' C. followed by continuous illumination of the seedling and young plants for 2 to 24 days hastened maturity and increased the yield of fruit in tomato. The failure of the first test is attributed to the fact that the vernalized seed was kept too long before planting.

Yamamato (72) reports that seed production is favored in radish (Raphanus sativus L.) when the small seedlings are first subjected to 00 to 50 C. for 10 to 15 days before placing at warm temperatures.

Van Hoek (31) vernalized potato tubers in a lighted box at 18° to 20° C. for 26 days. Plants from vernalized tubers grew more rapidly, were more vigorous and matured 6 days sooner than the controls. Twenty-three hills from vernalized and from nonvernalized tubers yielded 22.3 and 20.6 kg tubers, respectively. Tubers from the vernalized series were larger than those from the control.

Chesnokov (13) tested beet seeds (Beta vulgaris var. Egyptian and Bordeaux) vernalized 43 days and 68 days at 3° to 5° C. The longer time of exposure increased the amount of bolting the first season. This treatment induced bolting in almost 2/3 of the plants in both varieties and seed was produced. Similar results have been reported by others (4).

In southern New Mexico (56), high yields of sugar beet seed are obtained in late June and July from seed sown in the field duing late August or early September. Here vernalization takes place naturally during the mild winter.

In a later paper Chesnokov (14) states that 80% to 90% of the beet plants bolted the first season when the young seedlings were chilled for 50 days. He claims turnip, cabbage and carrot seeded more freely the first season when the young seedlings were chilled than when slightly germinated seeds were chilled.

Ta~slanov and Pudovkina (65) vernalized germinated cotton seeds at 13° to 23° C. for 3 days followed by 25° to 30° C. for 13 days. Vernalization hastened flowering and maturity 9 to 11 days in the Egyptian varieties; acceleration was especially evident in the late varieties. The American varieties showed less acceleration and more variations between themselves than was the case in the Egyptian types. It is claimed that the yields of all Egyptian varieties and part of the American varieties were increased. Tests carried out with cotton in Indore (2, 134) led to the conclusion that vernalization has no commercial advantage in that locality.

The acceleration of blooming in ornamental plants has been practiced for many years by growers and by amateurs. A recent book by Lawrie and Poech (40) covers this field rather fully. Some of the methods used involve the breaking of a true rest period previous to the final treatments which accelerate blooming. High and low temperatures during storage and after potting or planting are used.

The acceleration of blooming in daffodil and bulbous iris was studied by Griffiths (25). He found that daffodil bulbs should be kept under ordinary shed storage conditions. If temperatures go below 21° C. additional heat should be supplied. When the buds are visible—about August 1—they are stored at 10° C. for about one month, then potted and held at 10° to 18° C. for about a month in a cellar. If the bulbs are then benched in a greenhouse with night temperatures gradually increasing from 10° to 15° C., flowering begins just before Christmas, depending on the variety.

Early blooming in Dutch and Spanish iris is accomplished by storing at 26.7° C. from the time of digging to August 1. After this treatment they are stored at 100 C. for a month, then planted in a well ventilated greenhouse. Blooming occurs in December and January, depending on the variety.

In Bermuda (3) growers and experimenters accelerate blooming in the Easter lily when bulbs are stored at 2.22° C. for a month before planting October 15.


McKinney and Sando (50) reported that earliness in wheat is correlated rather closely with a number of leaves and internodes produced by the culms or stalks. A number of early, intermediate and late spring varieties were studied with different daily photoperiods in two ranges of temperature. Early varieties have fewer leaves and internodes than late ones, also temperature and light conditions favoring few leaves and internodes in a given variety tend to favor earliness except that change in time of flowering is somewhat more sensitive to environmental change than is the internode number. For example, Prelude headed from 32 to 43 days after planting with a leaf number of 5. When conditions induced 11 leaves per culm, heading was extended to 140 days. The variety Reward headed from 33 to 53 days from planting with a leaf number of 6. When 13 leaves per culm were produced, heading was extended to 104 days.

Under conditions favoring the earliest heading—continuous light at summer temperatures—the earliest varieties produced 5 leaves and the latest produced 11 leaves per culm.

Purvis (59) working with winter rye found that earliness is favored by the conditions which favor few leaves per culm. The same results were also observed and recorded by McKinney and Sando (52) for winter wheat. Using Petkus spring and winter rye Purvis and Gregory (60) found that approximately the first seven of the lateral primordia of the main axis are obligate leaf primordia, the subsequent 18 lateral primordia are labile or facultative in that they may become leaves or spikelets, depending on the temperature and photoperiod, and the subsequent ones are obligate spikelet primordia.

Harvest Queen winter wheat (52) has approximately seven obligate leaf primordia and approximately fifteen labile primordia. These numbers are inherently less in the early spring and winter wheats and greater in the late varieties.

The number of stalks per plant (tillering) (6, 16, 32, 59) and the number of seeds per plant are intimately connected with earliness. The writer and Sando (50) reported that few tillers and small numbers of seeds result when winter wheat is forced to complete its life cycle too rapidly, but that seed yields are higher when forcing is less rapid and is accomplished by means of low growing temperatures with short days, followed by the higher temperatures and long days, than is the case when the germinated seeds are vernalized before seeding and the subsequent plants are forced by high temperatures and long days.

The writer with Sando (52) found that immature germinated seeds of Harvest Queen winter wheat vernalized as efficiently as mature germinated seeds in a refrigerator. However, they did not test seeds earlier than the soft-dough stage. Kostjucenko and Zarubailo (36) report that the maturing seeds of winter wheat are vernalized naturally in the field in northern areas when the temperatures are sufficiently low before the seed is mature. Gregory and Purvis (19, 22) report the same phenomenon in winter rye but their tests with wheat failed. They found that winter-rye heads chilled during the middle period of ripening vernalized, whereas those chilled before and after this period did not. They (18, 22, 24) also succeeded in vernalizing excised germinated winter-rye embryos on nutrient agar containing carbohydrate at 1° C. In later tests Gregory and de Ropp (24) found excised embryos grown on nutrient agar containing no carbohydrate failed to vernalize whereas those on agar containing 3% sucrose vernalized.

Kostjucenko and Zarubailo (37) in summarizing their work conclude that the milk-ripe stage responds to vernalization because the physiology of the seed at this time is nearly comparable to the seed during germination. They cite data relating to the carbohydrate content, peroxidase and catalase activities in support of their conclusion. These investigators conducted field tests in northern and more southern areas in Russia and found that natural vernalization during maturity in the north shortened the vegetative period the following season when the seed was sown at more southern stations in comparison with plants from seed grown at the southern station the previous season. They point out that natural vernalization must be taken into account when seed of winter and late spring cereals is taken from northern to southern areas, otherwise genetic and agronomic results may not be properly interpreted. They indicate that winter hardiness is greatly reduced by natural vernalization and they recommend that seed of winter cereals intended for fall sowing in north Russia come from the more southern regions where natural vernalization does not occur. Temperatures at 15° C. and below during the milk stage of the seed are regarded as favoring natural vernalization.

In the United States, the daily mean temperature may reach 150 C. during the midperiod of seed development at some points near the Canadian border. However, practically no winter wheat is grown in that latitude and it is still a question as to the vernalization response in the Durum wheats which are later than most of the common spring varieties.

In their studies on winter rye Gregory and Purvis (20) found that devernalization can take place. They ran a test in 5 parts, each for a period of 6 weeks. Germinated seeds were subjected to 1° C. and to 20° C. in darkness for alternating periods. The 1° C. treatments were for 1, 2, and 3 and 6 day intervals, respectively. After each of these intervals there was in each case exposure to 20° C. for 1 day, followed by the respective schedules above at 1° C. While at 1° C. the seedlings had access to the ordinary or normal atmosphere but while at 20° C. the seeds were in an atmosphere of nitrogen. A control received 1° C. every day, but the nitrogen and ordinary atmosphere were alternated daily. After the treatments the seeds were planted and cultured at suitable growing temperatures and photoperiods. The seed lots receiving the 1-degree treatments for 1, 2, 3 and 6 consecutive days and the control seed lot receiving 1° C. continuously had 0, 20, 60, 100 and 100 per cent of the seeds vernalized, respectively. It is concluded that high temperature nullifies the effect of low temperature.

In later tests these investigators also (21, 23) found that an atmosphere of nitrogen extended the vegetative period in spring rye. Rye seeds thus devernalized were revernalized when subjected to 1° C. for 3 weeks in the normal atmosphere.

Winter rye (22) which had been vernalized and then devernalized lost its ability to head early at high temperatures, but it produced more tillers at high temperatures than rye which had never been vernalized.

From the results cited it is to be expected that warm day temperatures will nullify the natural vernalization induced in maturing seeds by low night temperatures until a sufficiently low daily mean temperature is reached. The exact relative efficiencies of low temperatures for vernalization and of high temperatures for devernalization when these phenomena are working against each other apparently has not been determined in terms of time and degrees of temperature.

Germination, though progressed very slightly, and at least 50% moisture are essential for the successful vernalization of mature seeds (41, 43, 44, 52), when the temperature is at optimum.

Darkness and light had no apparent influence on the vernalization efficiency of low temperatures in the case of germinated winter-wheat seed (52).

Turkey winter wheat seed was held at 3.3° C. for 61 days. One sample was in total darkness during the test and another sample received daylight during the entire day each day of the test. After vernalization the seeds were sown outdoors during early summer with the natural photoperiod. The plants from seed chilled in daylight headed 47 days after sowing whereas the plants from seed chilled in darkness headed 49 days after sowing. A difference of two days is not significant thus indicating that low temperatures and not darkness stimulated the early heading. Turkey wheat sown during the summer without vernalization does not head in the vicinity of Washington, D. C.

Whyte (70) indicates that these findings are inconsistent with earlier findings published by McKinney and Sando (49, 50, 52). However, the writer fails to find such inconsistency. The earlier tests referred to relate to plants growing at temperatures considerably above 38° F. and in short days vs. long days, whereas the light and dark test referred to above was carried out at 3.3° C. with germinated seeds, essentially the conditions of seed vernalization, and the conclusion relates only to slightly germinated seed (vernalization), a point which seems clear enough from the chapter headings and descriptions in that paper (52, 630).

Tests and observations reported by the writer and Sando (52) ruled out the endosperm, the tips of the roots, coleoptile tip and tip of the first true leaf of seedlings as the sole active centers of sensitivity to the vernalization process.

Krasnoseljskaja-Maximova (39) claimed that spring cereals contain no detectable substance which favors early flowering but that the winter cereals contain an inhibiting substance which must be counteracted before the plants can proceed to sexual reproduction. Later Sereiskii and Sluckaja (62) claimed that winter wheat contains no such inhibitor.

Richter (61) in his report as director summarizes the results obtained in studies conducted under the direction of Cailahjan on vernalization and photoperiodisni as follows:

Vernalization shifted the iso-electric point of albumino-lipoids towards the acid end, increased the permeability of the protoplasm and the mobility of the albuminous complex, intensified photosynthesis, increased dry matter in insoluble proteins, and decreased soluble protein. It is claimed that sexual processes controlled by light occur in the leaves and are related to the formation of flower hormones (floregin) which moves to the promeristem. In grafting experiments the floregin was transferred from stock to scion. Floregin was found not to be specific for species or biologic forms. It is claimed that no substance inhibiting or retarding flowering is formed in the leaves.

Purvis and Gregory (22) have shown from studies with excised embryos and with ripening grain that vernalization in winter rye is localized in the embryo and is independent of changes in the endosperm or aleurone layer. They conclude that the growing embryo is able to synthesize hormones at low temperatures from a substrate containing glucose, and inorganic salts including nitrates. It is their idea that a precursor (A) in the embryo of winter rye is converted by autocatalysis at low temperatures into a substance (B) which in turn may be converted into a spikelet initiating substance (C) and a spikelet maturation substance (D) or into a vegetative leaf-promoting substance (E), depending on the subsequent photoperiod or temperature, and the system C ⇄ B is reversible. Devernalization due to drying is accounted for by a conversion of substance (B) to substance (E). Substance (B) is naturally in high concentration in spring varieties.


In general the chilling method of vernalization has been found to accelerate sexual reproduction with greater certainty than the high-temperature method in the particular species for which each method has been recommended. Many workers report that they have been unable to obtain acceleration with the high-temperature method and in those cases where acceleration has occurred the commercial advantage has not been evident.

For several years vernalization methods have been tested in many parts of the world and it is noteworthy that the majority of investigators outside of Russia fail to recognize any great commercial value to be derived from the methods as applied to the small grains, rice, corn, sorghum, forage crops and cotton in the regions where these crops are adapted. It seems to be the general consensus of opinion that the crop problems can best be solved through developing better adapted genotypes.

Some commercial value is attached to the chilling method when used to force flowering in daffodils, Dutch and Spanish iris, and Easter lily. The method seems to offer commercial possibilities for speeding up seed production in certain biennials such as the garden beet and sugar beet, and it has considerable value in speeding up seed production in genetic and plant-improvement work with many crop plants. However, entirely aside from the stimulating influence of low temperatures during germination, it is strikingly evident that the temperature, the photoperiod, the intensity and the quality of light 'during the entire period of growth have marked influence on the time when sexual reproduction occurs and it is only when these factors are understood in relation to all the growth phases of the particular genotype that the most satisfacory results can be obtained from the initial chilling.

In view of the evidence, it seems justifiable to conclude that Harvest Queen, Turkey and similar winter wheats and other winter cereals are not typical long-day plants with respect to their earliest sexual reproduction, but are what may be termed short-day → long-day plants, and they may be considered as low-temperature → high-temperature plants. This method of expression indicates that low temperature or short days and low temperatures in combination must obtain in the germinating seeds or in the young growing plants, respectively, for suitable periods in order that the first labile primordium and subsequent ones will develop into spikelets instead of leaves when the high temperatures and the long days are introduced. A similar situation seems to apply to the facultative varieties and to certain late varieties commonly placed in the spring group, but in these the initial optimum temperatures are either not so low or the periods of exposure to low temperatures are shorter than is the case with the strictly winter varieties.

The typical spring cereals are high-temperature and long-day plants in the truest sense with respect to early sexual reproduction. However, as in the case of winter varieties the spring varieties differ with respect to their optimal environmental requirements for the earliest completion of the life cycle.

Earliness of sexual reproduction appears to depend on the interrelation of several plant characters, i.e., (1) characteristic number of obligate leaf primordia and attending internodes, (2) characteristics of the embryo which bring into existence or activate substances which in turn activate the labile lateral primordia into spikelets at characteristic (a) temperatures, (b) photoperiods and (c) periods of time, (3) growth characteristics of the stem internodes and (4) rate of maturity of the sex organs and seeds.

Conditions favoring the earliest sexual reproduction induce relatively low yields of seed per plant. In Marquis spring wheat and Harvest Queen winter wheat high seed yields obtain when five to seven of the labile primordia produce leaves and when the heads on 3 to 4 stalks have 14 to 15 well-filled spikelets. This is accomplished by gradual changes from the lower to the higher growing temperatures and from the short to the longer days. The total time requirement is greater than required for the earliest reproduction.

On the basis of the reversals in the order of heading in certain pairs of varieties when grown in different environments, it can be concluded that segregating populations from certain parent crosses will not give constant segregating ratios for earliness under all conditions of temperature and day length. As pointed out in a previous paper (50), populations which are segregating for earliness and lateness should be tested and classified as far as practicable under several conditions of temperature and photoperiod to facilitate the selection of genotypes homozygous for the several characters influencing earliness and lateness.

These conclusions do not vitiate the fundamental postulates of evolution or of genetics but they do indicate that a knowledge of growth phases and character expression in relation to environmental factors will facilitate the adequate planning of many genetic studies and the interpretation of the results. Such characters as yield, seasonal growth habit, recumbence, relative earliness, resistance and susceptibility to certain parasites, to extreme temperatures and to drought, serve to illustrate a few of the complexes which can be studied profitably under several conditions of environment for the purpose of determining at least some of the simpler characters of which they are composed. In critical genetic studies these simpler characters offer advantages over the complex ones.

The responses of the healthy or the diseased plant to temperature, the photoperiod, light intensity, light quality, humidity, soil moisture, soil fertility, etc. are most certainly determined by internal mechanisms which in turn constitute genetic characters, and although these characters cannot be measured directly at present, in some cases they can be measured indirectly and he dealt with as characters. As this procedure is followed, and as it becomes more generally recognized that dominance and segregation ratios in many characters must be considered within well defined environmental limits, some of the confusion regarding the environment and inheritance will disappear.

1 See Summary of Moscow Conference in Herbage Reviews 5: 118.-120. 1937.
2 The asterisk (*) following a citation indicates that the paper is abstracted in the bulletin listed in citation No. 2. A paper having a comprehensive literature list is indicated by the letter (L) following the citation.

The various experimental results discussed in this paper seem to shed no direct light on Lysenko's1 view that certain genotypes can be altered by systematic culture in an environment which differs from that naturally required by the plant. This so-called alteration of the genotype—referred to as "training" the plant-would virtually amount to directed mutation on a mass-production scale. To settle this point it would be necessary to plan and conduct the experiments in a somewhat different manner than was followed by the writer and the other investigators cited in the vernalization and growth-phase studies.



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