Journal of Heredity 24:169-179 (1933)
EARLINESS AND SEASONAL GROWTH HABIT IN WHEAT
As Influenced by Temperature and Photoperiodism
H. McKINNEY and W. J. SANDO*
Division of Cereal Crops and Diseases, United States Department of Agriculture
*Senior pathologist and associate agronomist, respectively.
|HEADING-TIME HALVED BY CHILLING
|Harvest Queen winter wheat grown in a greenhouse at 55 to 65°F., with the full winter daylight and light from a tungsten lamp (25 foot candles at the soil line) from sunset 10:30 p. m. daily. The plants on the left were from germinated seeds chilled at 30 to 35° in the dark for 67 days. Those on the right were from unchilled seeds germinated to a point equal to that of the chilled seedlings at room temperature just before all the seedlings were placed in the greenhouse. The plants from chilled and unchilled seeds headed 66 days and 128 days, respectively, after planting.|
During recent years plant breeders in the United States have been making practical use of Garner and Allard's1 findings on effect of length of day on plant development. By adjusting the length of the daily photoperiod it is possible to bring about sexual reproduction simultaneously in early and late types in several crop plants. In spring cereals it is also possible to obtain two generations in one year by growing the winter crop in a long day consisting of winter daylight and electric light, thus greatly facilitating hybridization and the increase of progenies.
|ćA brief summary of this work was given in the preceding issue of the JOURNAL (Page 165). References are given there to other Russian publications on this subject.|
Recently, reports of Russian work on the acceleration of sexual reproduction, the so-called "larovization" of plants, have attracted considerable attention in the United States.† Owing particularly to interest in this method as it applies to winter wheat, the writers have been requested to summarize their own work, as well as certain early American work, on the acceleration of sexual reproduction (earliness) in wheat.
Earliness and seasonal growth habit will be considered from a rather broad point of view. Several methods for inducing earliness of sexual reproduction in wheats will be discussed, together with certain plant characteristics that it is thought play a part in determining earliness and seasonal growth habit.
In the present work the effects of environment are based to a large extent on the time of heading which in the varieties used preceded fertilization from two to five days.
Inducing Earliness in Certain Types of Wheat by Chilling the Germinated Seed or Seedlings
The earliest work on the chilling of germinated wheat seeds that has come to the attention of the writers is that of Klippart3, an American investigator in Ohio, who obtained his results previous to 1857. Apparently this work bas been unknown to all recent workers who have studied this problem, and, because of the historical interest the following is quoted from Klippart:
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 and 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.
When the autumn sowing is delayed temperatures sometimes drop very quickly, preventing the emergence of the seedlings until the following spring. If there is enough moisture during the winter to start germination the resulting plants will produce a satisfactory commercial crop the following spring. This has been observed for many years in the Pacific Northwest and in parts of the Southwest. The writers of this paper have observed this relationship independently in date-of-seeding nurseries at the Arlington Experiment Farm (near Washington, D. C.). The senior writer observed the phenomenon in 1920 in date-of-seeding nurseries at Granite City, Ill., and stated that it seemed evident that "the effect of soil temperatures in relation to the stages of plant development is largely responsible."5
The studies of the writers on the chilling of germinated seeds were started without knowledge of the early American work, though it was known that German investigators had studied the effects of low temperatures on older seedlings. In 1924 the junior writer conducted an experiment with germinated winter-wheat seed held in a dark chamber for various periods up to six days at temperatures of 24, 32, and 40°F., respectively. However, none of these treatments was of sufficient duration to accelerate heading.
The results of the writers' first test in 1928-29 have been briefly summarized6. Typical plants from a part of this test are illustrated in the Frontispiece. The seeds of Harvest Queen winter wheat were moistened and soon as the plumules burst the pericarp tissue they were placed in a dark, mechanical refrigerator with the temperature ranging from 30 to 35°F. After 67 days at this temperature range the germinated seeds were planted in pots of soil in a greenhouse at 55 to 65°F. with full winter daylight and light from a Mazda tungsten lamp (25 foot candles at the soil line) from sunset to 10:30 p. m. daily. The duration of the daily photoperiod ranged from 16 to 18 hours. The unchilled seeds were germinated at room temperature until they reached the same stage of germination as that of the chilled seedlings. Plants from these seeds were used as controls.
Plants from the chilled seeds headed 66 days after planting, whereas plants from the unchilled seeds did not head until 128 days after planting.
Germinated wheat seeds and seedlings have been chilled at different temperatures, as indicated in Table I. These data show that temperatures from 37 to 44°F. were more favorable than the lower temperatures for stimulating earliness in the winter varieties tested. Sol and Currell varieties were especially favored by the temperatures above 35°F. However, the advantage became less at 42 to 44° F.
TABLE 1.—Number of days from the end of the chilling treatment to heading In three winter varieties and in Marquis and Prelude, spring wheats. Seedlings were chilled 65 days in the dark at different temperatures. After chilling, the plants were grown in a greenhouse with uninterrupted light at 70 to 75°F. at night. During the day temperatures went above 75°F., especially when the sun was bright. The chilling treatment terminated March 2, 1932.
|Variety||Number of Days from End of Chilling to Heading|
|30 to 35°F.||37 to 39°F.||39 to 41°F.||42 to 44°F.||No chilling|
a Corrections were made in heading times to allow for differences in the
development of the seedlings held at the different chilling temperatures at
the time chilling was discontinued. The time comparisons were computed from
plumules one-sixteenth of an inch in length.
The low temperatures retarded earliness in Prelude and Marquis spring wheats. Other tests show that the heading time is reduced six days in Kinney, a late spring wheat, when the germinated seeds are chilled at from 37 to 39°F. for 28 days and the subsequent plants are grown at summer temperatures in continuous light.
The influences of the duration of chilling, temperature, and daily photoperiods subsequent to chilling are shown in Table II for several varieties.
It will be noted that more varieties failed to head at 70 to 90°F. in a natural day than at the lower outdoor temperatures with a natural day, and the number of failures were least with uninterrupted light at the outdoor temperatures. lt is evident from these data that the proper time required for chilling the germinated seed will be determined not only by the characteristics of the variety but also by the temperature and light conditions that obtain during the subsequent growing period.
|TABLE II.—Influence of different growing conditions on the time in days from the end of the chilling period to heading. Slightly germinated seeds of the varieties were chilled in the dark at 30 to 35°F. for different periods. Plants from unchilled seedlings were grown for comparison. Plantings made April 15, 1932, at the Arlington Experiment Farm, Rosslyn, Va.|
|Variety||C.I.a No.||Growing conditions after chilling|
|24-hour day outdoors||Natural day outdoorsb||Natural day in greenhouse 70 to 90°F.c|
|Number days chilled||Number days chilled||Number days chilled|
|aC. I. denotes accession number of the Division of Cereal Crops and Diseases,
formerly Office of Cereal Investigations.
bThe The mean temperatures outdoors for April, May, June, and July were 52.6, 64.2, 73.2, and 77.4°F., respectively. The departures of the daily maximum and daily minimum temperatures from the mean temperatures averaged near 10° for these months.
cAt night and on dark das the temperature in the greenhouse was controlled between 70 to 75°F. until May 10 when the steam was turned off. The temperatures at midday averaged 90°.
dThis wheatcame to the senior writer unnamed from the Kansas Agricultural Experiment Station. It is said to be from a cross between Kanred and Hard Federation. It has the beak of Kanred and the chaff is white. It was of particular interest because of its extreme earliness in the mosaic-disease nursery at the Arlington farm and because it gave evidence of having a rather extreme low-temperature requirement during its initial growth phase for earliness.
eZeroes indicate that no heading occurred during the summer.
From these results it appears that the lower temperatures and longer days that prevail in Russia may be more favorable for the use of springsown chilled (iarovized) winter wheat, especially certain varieties, than the higher temperatures and shorter days obtaining in the principal winterwheat areas of the United States. So far as the United States is concerned it remains to be demonstrated that spring-sown chilled winter wheats have any advantage over suitable spring wheats in the cropping system.
By chilling germinated winter-wheat seeds it is possible for the plant breeder to study spring-sown winter varieties and spring varieties simultaneously in northern regions where winter wheats cannot be grown when fall sown. It is also possible to obtain two or more generations of winter wheat in one year, depending on the characteristics of the genotype. In practice the method has been used to advantage in producing one crop of winter wheat outdoors and a second crop in a greenhouse. Germinated seed is chilled at 37 to 39°F. for 50 to 65 days preparatory to sowing early in the spring. Seed from the resulting field crop is germinated and chilled. The plants from these chilled seedlings are then managed so as to avoid heading during the shortest days of the year, as low light intensity frequently induces sterility even when the days are lengthened by means of [azda tungsten lamps. The seed from this crop may be germinated and sown in the field in regions of moderate winters before February 15, or it may be germinated and chilled in a refrigerator preparatory to sowing in the field in the spring.
When small numbers of seeds are being treated they may be germinated and placed in damp soil. At 37 to 39°F. the plumules may become 2 or 3 inches long when held for 50 to 65 days in darkness, but on transplanting in the light they become green and develop into strong plants. When large numbers of seeds are being handled it is not practicable to trans. plant seedlings. In this case it is ad. vantageous to retard germination during the chilling treatment. This is accomplished by maintaining the moisture content of the seed near 50 per cent of the dry weight of the seed. It has been found more satisfactory to soak the seed in an excess of water until it is soft, than to add only the quantity of water necessary to bring the moisture content up to the required percentage as recommended in the Russian methods. An excess of water seems to reduce some of the irregularities in germination,
Six to twelve hours soaking is sufficient for most hard-seeded varieties, but less time is required for the soft-seeded types. When soaking several bushels of seed on a floor, the water may be applied several times in order that the seed may be kept wet, the excess water being allowed to drain away. The pile should be covered with a canvas. Small lots of seed can be soaked in an excess of water in a container. After soaking, all the water is drained off and the damp seed allowed to germinate slightly at 60 to 75°F.; 65 to 70°F. has been found practical. When the early stages of germination are evident in the majority of seeds, the seed is spread out in a thin layer to dry to 50 to 55 per cent moisture. During the drying process the seed should be shoveled or stirred, depending on the quantity. Fans hasten the drying process.
Combining Temperature with Light During the Initial Growth Phase to Control Earliness
chilling quantities of seed as great as a peck or more, care must be exercised
to avoid too much rise in temperature due to the heat of respiration. Shallow
boxes or bins 3 to 4 inches deep have been found better than grain sacks for
containing the seed during the period
of germination and the chilling treatment. Seed should be stirred several times
while it is being chilled.
Smaller quantities of seed are handled in Mason fruit jars or in bottles throughout the soaking and chilling treatment. These containers should not be completely airtight and the moisture content of the seed should be watched to avoid undue loss of moisture during the chilling treatment. Some varieties and some lots of seed of the same variety germinate more rapidly than others at low temperatures. If germination proceeds too fast the seed should be freed from some of the water.
If germinated seed is to be drilled the plumules should not exceed one-thirty-second to one-sixteenth of an inch in length. Vigorous seeds with plumules one-sixteenth of an inch long can be dried and held for a short time if necessary, but if germination proceeds beyond this range irregularities and reduction in stand may result. When the plumules are broken further development is prevented in nearly all cases.
Seeds 12 days from fertilization have reacted to the chilling treatment as efficiently as mature seeds. Immature seeds are first germinated near 60°F.
Although the method of chilling winter-wheat seed in darkness has its value, it also has its limitations. Many investigations on the adaptation of the wheat plant, and on the expression of characters in physiologic, genetic, and breeding work must be conducted in the light during the initial growth phases when low temperatures obtain. Temperature is a major factor in controlling earliness in wheat, especially in winter wheats chilled in the dark, but the writers cannot take the view of Lysenko4 that temperature is the primary controlling environmental factor in earliness under field conditions.
Wanser7 was the first to point out the importance of photoperiodism in the acclimatization of wheat. His studies, those of Garner and Allard,2 and many others show clearly that the length of the daily photoperiod is of major importance in controlling earliness. Studies previously summarized6 and which will be published in full later show that within limits temperature and the length of the daily photoperiod compensate for each other in their influence on earliness. Other factors such as light intensity and quality, soil fertility, etc., also influence earliness and they must be taken into account. However, temperature and the photoperiod appear to be the two major environmental factors which at present warrant concentrated effort In studies on earliness.
When sexual reproduction is accelerated very rapidly the yield of seed or plant is reduced owing to a reduction in the number of tillers and in the number of fertile florets per head. However, yields have been less when extreme earliness is induced in plants from germinated seeds chilled in the dark than when the plants were grown under suitable conditions of light and temperature during the initial growth phase.
Harvest Queen plants from seeds chilled for 40 days in darkness and grown with uninterrupted light at summer temperatures headed 89 days from the time the seed started to soak. This is the most rapid completion of the life cycle vet 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:
These plants actually headed earlier than those from chilled seed and it is likely that still earlier heading is possible without reducing the yield to 30 seeds per plant.
OF TEMPERATURE AND LIGHT
graphs and table showing the influence of initial temperatures and of various
lengths of the daily photoperiod on the expression of earliness in different varieties
of wheat when all were subsequently allowed to grow at temperatures averaging
above 70°F. with 16 to 17 hours of light daily. Seed was germinated but not
chilled before sowing. The first seeding was made October 21, 1931.
Explanation of graphs.—Mean temperatures are represented by the curves. The length of the daily photoperiods are indicated by the construction of the curve lines as shown in the figure legend.
In graph I sowings were made 102, 67 and 33 days before the plants were subjected to high temperatures and long days. In graph 11 sowings were made on one date only. Sixty days after sowing the experiment was divided into three parts, each part being subjected to the temperatures and photoperiods indicated. All parts received the high temperatures and long days 128 (lays after sowing. The broken line (h) is separated from line (H) to show that two photoperiods were maintained. Line (H) represents the true mean temperature of line (h).
Explanation of tabulated data.—The alphabetical symbols in the column refer to the same letters on the graphs. These headings denote the temperature and photoperiodic curves during the period of the experiment for the plants that headed at the times in days listed in the columns. The number of days required for heading were calculated from the time of planting. The asterisks indicate the earliest heading date for each variety or hybrid.
Harvest Queen plants from chilled germinated seeds will yield 75 seeds if grown at 70 to 75°F. 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.
Varieties show wide differences in their earliness reactions when the plants receive light daily during the period of low temperature. The tests cited in Figure 1 were conducted outdoors and in a greenhouse during the autumn and winter in order to test natural conditions.
|*In this paper 70°F. is taken as the dividing point in the range between low temperature and high temperature.|
It will be noted in Figure 1 that Red Winter spelt was the only variety to head earliest when grown at the higher temperatures throughout the life cycle, although heading was retarded only two days when exposed to a mean temperature of 57°F. for 33 days. Other tests indicate that earliness in Red Winter spelt is favored by a relatively short duration of low temperature* in the seedling stage. Purplestraw, a facultative variety, and Currell, a winter variety, were earliest when held near the 57°F. mean for 33 days. Harvest Queen, Turkey (C. I. No. 1558). Turkey (C. 1. No. 6175), Minhardi, and Sol winter vheats all headed earliest when exposed to the 57°F. mean for 67 days. Hybrid "X" was the only type in the series in which early heading was favored by exposure to the lower temperatures of the outdoors, and this relationship has been maintained in later tests. This wheat has been one of the earliest at the Arlington farm and even though it has been favored by the lowest temperature yet encountered in the present work, its stem elongates phenomenally fast after the plants are exposed to the high temperatures and to a long photoperiod.
Exposure to the lowest temperature did not produce earlier heading in Minhardi than did the mean temperature of 57°F., although Minhardi is outstandingly resistant to low temperatures lethal to the majority of winter wheats, these temperatures seem not to benefit earliness. A similar relationship seems to obtain in Buffum 17, another very hardy wheat.
It will be noted in Fig ure 1 that none of the exposures exceeding 67 days favored early heading in any of the varieties, nor was heading hastened by exposure in chambers to a mean of 24°F.
|†It is not implied that the data in Figure 1 represent the exact relative temperature requirements during the initial growth phase of the varieties listed. The exact relationships can determined only when tests include a larger number of temperatures and times of exposure these temperatures.|
The data in Table II suggest that Minliardi, Sol and Currell have relatively greater low-temperature requirements for earliness than do the data in Figure 1. It is believed these apparent inconsistencies can he explained on the basis that 30 to 35°F. is too far below the optimum in the case of these varieties as indicated for Sol and Currell in Table I.†
Although varieties of winter wheat differ in their temperature and length-of-day requirements for earliness, tests indicate that all the winter varieties tested complete their life cycles quite rapidly and produce good yields of seed when grown near 45 to 50°F. with a daily photoperiod of 8 to 10 hours for 43 days followed by temperatures near 70 to 75°F. 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.
It will be observed in the tabular matter in Figure 1 that certain varieties and Hybrid ''X'' reversed their relative order of heading, as for example, Red Winter spelt and Purplestraw; Harvest Queen and Hybrid "X"; Minhardi and Hybrid "X". Reversals in the order of heading occur also among certain spring wheats such as Bunyip and Prelude. When the daily photoperiod was varied and temperatures were maintained near 70 to 75°F. the order of heading was as follows: Bunyip, 53 days earlier with an 8-hour day, and 16 days earlier with an 11-hour day. Heading was simultaneous in a 16-hour day. Prelude was 15 days earlier than Bunyip in continuous light. The order of heading in these varieties was reversed in the field at different stations the phenomenon has been observed in other varieties by several workers in different parts of the world. Reversals seem to be tied up to a large extent with differences in the environment-response characteristics of the growth phases of the varieties.
Earliness characteristics of wheat varieties have been studied by several investigators in date-of-seeding experiments. By this method the effect of the total environmental complex is determined for different periods of the season or for the entire year. From such studies Lysenko4 devised a formula by which he attempts to preduct the heading date of a given variety when sown at any given time. However, since he considers that temperature is the controlling environmental factor in determining earliness, his formula can apply only to regions near the same latitude. To have wide application his formula should include factors for the influence of light. Date-of-seeding; tests have value in making the first general comparisons between varieties and serve as the starting point for more detailed studies to determine the relative influence of the several factors in the environment.
Plant Characteristics Concerned in Earliness
Earliness of sexual reproduction appears to depend on the interrelation of several characters, each of which may be measured when suitable methods are developed. It seems reasonable to believe that the study of these characters separately and in suitable combinations should aid in furthering an understanding of earliness.
Under the conditions of the writer's tests the stems did not elongate appreciably while primordial leaves and internodes were forming. Therefore it is expected that rapid elongation of the stem will be delayed as the number of internodes per culm is increased. When grown with continuous light and also with 16 hours of light daily the spring wheats that produced the fewest internodes headed much earlier than those which produced the greatest number as shown in Table III.
This relationship between internode number and earliness is reversed some varieties that differ but slightly in internode number. Also Prelude and Early Defiance grown in continuous light and with a 16-hour day headed several days apart, though each produced the same number of internodes per culm.
These apparent inconsistencies seem to be due to other characters that influence earliness. Preliminary tests suggest that varieties have different growth-rate characteristics when growing under optimum conditions for the earliest sexual reproduction. For example, the growth rate of the stem of Prelude was double that of Kinney wheat when both were grown near 70 to 75° F. with continuous light.
It will be observed in Table III that environmental conditions tending to favor earliness tend also to favor a reduced number of internodes per culm, though it is clearly evident that several varieties that were earlier in continuous light than they were in the 16-hour day showed no change in the number of internodes under these light conditions. Also Bunyip, in the 8-hour day, produced 14 internodes, whereas Prelude produced only 11, yet Bunyip headed much earlier than Prelude. This relationship seems to he due very largely to extreme retardation in the growth rate of the apical stem internode (peduncle) of Prelude, When grown near 70 to 75°F. with an 8-hour day, stem elongation started a week earlier in Prelude than in Bunyip. The flag leaf of Prelude extruded completely, but the apical internode elongated much more slowly than the lower ones and as a result Bunyip was in full head 53 days before Prelude. Examination of Prelude at the time Bunyip headed showed that its apical internode was less than one-eighth of an inch long and the head but one' half of an inch long.
While the present studies have not dealt with exact temperature optima for the earliest sexual reproduction it seems evident from the data presented that the temperature requirement for the most rapid completion of one growth phase or group of growth phases is not necessarily the same for another phase or group of phases of the same plant; also that the temperature requirement of a given growth phase or group of phases is not the same for all varieties. It seems reasonable to believe that these temperature-response characteristics of the growth phases are determined by internal mechanisms which constitute genetic characters, the inheritance of which seems to be more or less independent in the several growth phases of the plant. What has been said for temperature-response characteristics seems also to apply to the photoperiodic-response characteristics in winter wheats but uninterrupted light seems to be optimum for the most rapid completion of all growth phases in some of the spring wheats.
Although these environmental-response mechanisms cannot be measured directly at the present time, it appears that they may be measured indirectly in terms of internode number, growth rate, and finally in time of sexual reproduction over a range of temperature and light conditions.
|TABLE III—Number of lnternodes per culm in spring wheats in relation to the time of heading in days.|
|Temperatures and length of daily photoperiod|
|a 57 to 93°F.||b 70 to 75°F.|
|24 hours||16 hours||12 hours||8 hours|
|No. Ints.||Days Hdg.||No. Ints.||Days Hdg.||No. Ints.||Days Hdg.||No. Ints.||Days Hdg.|
The number of internodes was determined by all accurate count of all the
leaves, not including coleoptile, on the primary stalk. The leaves were
carefully marked during the process of growth errors in counting.
aGrown outdoors during midsummer.
bGrown in a greenhouse during the autumn, winter, and spring. Temperatures sometimes went above 75 F. on bright days.
Discussion and Conclusions
Important information on earliness in sexual reproduction can be obtained in many wheat varieties by dividing the life cycle of the plant into but two major gro wth phases and by determining their environmental characteristics. It seems likely, however, that a better understanding of earliness will result when the environmental characteristics are determined for a greater number of growth phases. Observations suggest that germination, the appearance of each of the leaves and the head, fertilization, and seed development may be considered roughly as indicators of the growth phases.
From the studies on seasonal growth habit it appears that wheat varieties should he classed for this cliaracteristic largely on the basis of their respective optimum temperature and length-of-day requirements for the earliest sexual reproduction possible in the variety. The terms ''winter'' and "spring' are used from the equator to the southern and northern limits of wheat culture and therefore do not represent definite temperatures or lengths of the daily photoperiod. They are relative terms and their value is largely provincial.
The typical spring wheats have relatively high-temperature and long-daylight optima during their entire life cycle, whereas the typical winter wheats have low-temperature and short-daylight optima during the initial growth phase and relatively high-temperature and long-daylight optima in the subsequent phases with respect to earliness. The low-temperature requirements of the initial growth phase of the winter group with an exposure of 43 clays and with a day eight to ten hours long appears to range from near 38 to 60°F.
A winter wheat when sown in the spring in most spring-wheat areas does not receive optimum temperature and light conditions during its initial growth phase, thus causing a degree of lateness which prevents heading or seed production before winter sets in However, if the season were sufficient ly long, and the temperatures not high, such plants should produce seed even though the plants received low temperatures or short days during their intial growth phase. This has been demonstrated by bringing such plants into the greenhouse in the late summer and by continuous culture in the greenhouse. On account of these relationships it is concluded that seasonal growth habit is a phase of earliness and lateness.
In addition to the low-temperature requirements of winter wheats for earliness, this group also differs from the spring group in resistance to subfreezing temperatures during the winter. There is no evidence, however, indicating that the exceedingly low temperatures which such vareties as Minhardi can survive are of specific value in stimulating earliness.
On the basis of the extreme reversals in the order of heading in certain pairs of varieties when grown in different environments, for example Bunyip and Prelude, it is concluded that segregating populations from certain parent crosses will not give constant segregating ratios for earliness under all conditions of temperature and day length. This is also to be expected on the basis of the conclusion that earliness is influenced by several characters. It is concluded, therefore, that populations which are segregating for earliness and lateness should be tested and classified as for as practicable under several suitable conditions of temperature and day length. This should facilitate the selection of genotypes homozygous for the several characters influencing earliness and lateness. Such types will have far greater value than most of the existing types for making detailed genetical, environmental, and biochemical studies and interpretations.
Sexual reproduction can be greatly accelerated in winter wheats by first subjecting, the slightly germinated seeds to low temperatures (near freezing) in the dark for 50 to 65 days before sowing. When so treated winter wheats sown in the field in the spring will behave as spring wheats. The principles of this method were reported from Ohio in 1858.
By chilling germinated seeds preparatory to planting at higher growing temperatures in a long day, it is possible for the experimenter to obtain two or more crops of winter wheat in a year, depending on the variety.
Seasonal growth habit in wheat is considered an aspect of earliness and lateness. So far as earliness is concerned the winter varieties have low-temperature and short-day optima during the initial growth phase, whereas spring varieties have optima at the higher temperatures and the longer photoperiods.
Earliness and lateness of sexual reproduction appear to depend on the interrelation of several characters, as the number of internodes per culm, the growth rate of the internodes. the duration of the elongation of the internodes, and environmental-response characters winch influence the expression of these several characters.
It is worth noting that Klippart was not the first. Allen (1847, 1849) was earlier, and Col. W. Abbott's similar results were reported in 1838. However, these experimenters either froze the seeds in water, or germinated them before freezing. Lysenko's method, by contrast, was described by Chouard (1960):
Finally, in 1928, Lysenko established that slight imbibition (e.g., 50 parts water to 100 parts dry matter) makes the cereal seed susceptible to this action of cold without inducing the excessive germination that could prevent use of a sowing machine.