By GEORGE F. SPRAGUE
Assistant agronomist, Division of Cereal Crops and Diseases, Bureau of Plant Industry. United States Department of Agriculture
|1 Received for
publication Apr. 15, 1934; issued July 1934.
2 The writer acknowledges his indebtedness to H. H. McKinney and W. J. Sando for the use of temperature-control facilities and greenhouse space and for helpful suggestions during the progress of these investigations.
3 Reference is made by number (italic) to Literature Cited, p. 1120.
The methods of iarovization advocated by the Russian workers fall into two distinct groups: The low- and the high-temperature treatments. The low-temperature treatments have been known for a long time and are well established experimentally, as is evidenced by the work of Klippart (3, p. 757),3 Lysenko (4) McKinney and Sando (6), and others. The high-temperature treatments are less well established. They have been advocated as a very effective agent in the hastening of sexual maturity in the short-day plants. If the results reported by the Russians are universally confirmed, the process of iarovization might be expected to play a very important role in certain temperate regions. larovization of corn in the United States, however, even if effective, does not appear to offer any great commercial possibilities. On the other hand, it might have considerable value in certain physiologic and genetic experiments.
THE PROCESS OF IAROVIZATION
Originally the term "iarovization" (vernalization) was applied only to the low-temperature treatment of winter wheat to induce jointing and hasten sexual maturity. At the present time the term is generally applied to any treatment having as its object the hastening of sexual maturity.
The requirements for different crops differ m the duration of the treatment, the temperature, and the moisture content of the seeds. A summary table is presented by McKinney and Sando (7) listing the requirements for a few crops. All of the high-temperature treatments, according to the Russian workers, must take place in darkness.
MATERIALS AND METHODS
The experiments reported in this paper were conducted at the Arlington Experiment Farm, Rosslyn, Va., in 1933. Eight hybrids, 9 inbreds, and 1 variety of corn and 1 strain of teosinte were included. The variety of corn used was Gaspé, one of the earliest known. The hybrids and 7 of the inbreds are adapted to Corn Belt conditions. Of the remaining inbreds one is a derivative of the Garrick variety which is adapted to the South, and the other is a type called "Cuzcoid" because of its resemblance to the varieties from Cuzco, Peru. This last is a simple Mendelian recessive and in the segregating progenies so far tested has required about twice as long to reach the reproductive stage as its normal sibs. Sexual maturity in this strain has not been hastened by exposure to a 10-hour day.
A further indication of the range of season represented by the inbreds in this test may be obtained front the following comparison. The earliest strain, 616, shed pollen 60 days after planting. Cuzcoid, the latest strain, was killed by frost October 26, 160 days after planting, when less than one-tenth of the plants had tasseled. At this time it had an average of 38 nodes per plant.
All of the seed used was soaked in a 0.5-percent solution of Uspulun for 2 hours and then rinsed. They were then soaked for 9 hours in a weak salt solution of about the same molecular concentration as tap water and were then dried to approximately the moisture content (30 percent) recommended. This method was found to result in much more uniform germination than adding stated quantities of water as recommended by Lysenko (4). One complete set of the samples was then placed in light-proof bottles in a constant-temperature chamber and held at 75°F. for 14 days. Duplicates of some of the samples were subjected to the same temperature conditions, but exposed to the normal day or to continuous light (normal day plus artificial illumination). One lot was held at a temperature of 38°F. for the same period. At the end of the 14-day period all of the samples were planted in the field, in those samples in which germination had progressed the farthest the radicles were approximately 5 to 8 mm and the plumules 5 mm long. Molds, particularly Penicilllium, occurred in some seed lots, but no visibly infected seeds were planted.
The iarovized material was hill-checked in some cases in comparison with sprouted seed, and in other cases with dry seed of the same sort. In some instances, the difference due to the slight hastening of emergence because of the sprouted condition of the iarovized seed persisted and was reflected in a slightly earlier tasseling and silking (table 2, items 3 and 6). This should not be a serious source of error in experiments of this kind, as a slight advantage from this cause would be insignificant as compared with the marked acceleration which must result from iarovization if the method is to be commercially practical.
Dates of germination, pollen shedding, and silking were recorded for the plants in all perfect hills. In addition, percentage of germination, number of nodes, plant heights, and plant yields were obtained in most cases. All of the data reported are based on comparisons between plants of the same strain growing in the same hill.
The effect of the treatments on field germination and on emergence of the inbreds is shown in table 1. In every case iarovization resulted in a marked reduction in germination and in 13 of the 14 cases delayed emergence. The germination of the iarovized seed was so poor and seedling mortality following germination so great that no adequate data on maturity are available. For the three strains A, 324, and 540, which had the highest germination percentage and for which meager data are available, there was no indication of a hastened maturity following the treatments.
TABLE 1.—Effects of iarovization on field germination and emergence in 14 inbred strains of corn
|Inbred strain no.||Condition of check||Germination||Mean difference
iarovized and check seed
lots in time to emergence a
|a Positive differences
indicate the iarovized lot emerged later than the check. In the column headed
a positive difference indicates that the iarovized lot required more days to emerge than the control.
b Iarovized in light (day only).
c Iarovized in light (continuous).
With the more vigorous material, which includes the hybrids and the variety Gaspé, there was a slight but consistent reduction in germination as a result of iarovization, as shown in table 2. The time required for germination shows no consistent differences resulting from the treatments. Only three of the mean differences are significant. In two of these, iarovization appears to have had an accelerating effect, but in both the check seed was dry and ungerminated. In one instance there is a significant retardation. In this paper, differences have been considered statistically significant when P is 0.02 or less. Such differences are italicized in table 2.
In 8 of the 15 comparisons, plants from iarovized seed shed pollen significantly before the checks, but in only 5 cases was there a significant difference in silking. This is in accord with general observations that pollen shedding is influenced to a greater extent by environment than is silking. There is a fairly high positive correlation between the days required for emergence and for pollen shedding. Substantially the same degree of correlation exists in both the iarovized and control lots. It should perhaps be emphasized that while in several cases hastening of sexual maturity is statistically significant, in no case is the acceleration of any significance agronomically.
The numbers of nodes visible at maturity were fewer in the plants from iarovized seed in all of the 11 comparisons involving iarovized v. noniarovized seed and of which counts were made. Only 7 of the 11 differences are statistically significant, but 11 deviations of like sign would be expected only once in 2,048 trials as a result of sampling error alone. Counting nodes visible at maturity is not satisfactory where the absolute number is wanted. In the present case, however, the interest lies in the relative difference between paired plants of successive hills, and the visible number of nodes is just as satisfactory a basis for comparison as the absolute number.
effects of iarovization on various agronomic characters in 8 hybrids and 1
variety of corn
[Differences for which P is less than 0.02 are italicized)
|Hybrid or variety||Condition
|Field germination||Mean differences between iarovized and control plantings in—a|
|Ears||Weight of shelled
grain per plant
|a Positive difference
indicates iarovized lot exceeds check.
b Sprouted, not iarovized.
c Iarovized, continuous light.
d Iarovized at 50 F.
There appears to be no consistent relationship between the reduction in number of nodes and plant height as a result of the treatment. In some strains the iarovized plants exhibit a significant reduction in number of nodes and yet are not significantly shorter than their controls.
Where any differences exist in number of ears, the iarovized material always has the lower number. None of the differences, however, is significant.
All of the iarovized material exhibits a reduced yield of shelled grain per plant, only five of the comparisons, however, being significant. The reductions in yield indicated in table 2 as being significant represent a reduction of approximately 15 to 30 percent. It is worthy of mention that the comparison exhibiting the least reduction in yield was significantly earlier in pollen shedding and silking than its controls and was iarovized under continuous light.
In the variety Gaspé iarovization at 75° F. or at 38° F. for a 14-day period was ineffective in hastening sexual maturity.
According to the theory advanced by Lysenko (4), short-day plants require light for processes of growth and the absence of light (darkness) for the initiation of reproduction. The necessary darkness may be supplied continuously during the early stages of the plant's development or as periods alternating with light, as in day and night, during a greater portion of the plant's development. larovization carried on in the dark is presumed to be effective, in the case of short-day plants, because it satisfies the plant's requirement for darkness. With this requirement satisfied, the plants are able to benefit from the long days, and hastened sexual maturity results.
The theory of a "darkness requirement" for the initiation of reproduction in corn, at least for some varieties of the temperate regions, is not in agreement with several facts. Corn has been grown in the greenhouse at the Arlington Farm during two winters under continuous illumination (normal day plus artificial illumination) without any material delay in the onset of flowering or maturity. In the winter of 1932-33 four inbred strains representing a considerable portion of the seasonal range of Corn Belt varieties were grown under continuous illumination from the dry seed to maturity. The plants were perfectly normal in their vegetative development and seasonal maturity.
As a further test for the necessity of a dark period, the same strains of corn were grown during the summer, one set of plants being exposed to the normal day, and a second set to continuous illumination (normal day plus artificial illumination). The results are presented in table 3.
TABLE 3.—Effect of day length on sexual maturity in 4 inbred strains of corn
|Inbred strain no.||Period from planting
till pollen shedding
when grown with a day length of—
|12 hours||16 hours||24 hours|
Continuous illumination resulted in approximately a week's delay in pollen shedding. For the one strain grown also with a 12-hour day, there was no significant difference in earliness between the 12-hour and 16-hour photoperiods.
The classification of corn as a short-day plant does not appear to be based on adequate experimental evidence. It is true that the work of Garner and Allard (2), Emerson (1), and McClelland (5) has shown that some varieties respond to a short day (10 to 12 hours of light). However, all of the varieties used by these investigators were tropical or semitropical sorts naturally adapted to short-day conditions. Varieties which are adapted to Corn Belt or more northern conditions and which normally bloom during a long day (15 to 18 hours of daylight) have not been adequately studied. In three strains grown in the greenhouse, augmenting the winter day (11 to 13 hours) by artificial illumination for 4.5 or 8.5 hours has not resulted in a significantly delayed sexual maturity.
|FIGURE 1.—Response of teosinte to darkness. The plant on the right was exposed to darkness for a 14-hour period daily. The one on the left was exposed to darkness continuously for 14 days followed by exposure to darkness for an 8- to 9-hour period daily. Photographed June 19, 31 days after planting.|
Teosinte has been shown by Emerson (1) to respond to a short day. It was thought that a comparison of the effectiveness of darkness supplied to this plant during the iarovization process and as a daily photoperiod would be instructive. Three lots of teosinte were planted May 19, one of them having been iarovized. The iarovized lot and one of the others were exposed to the natural day. There was no significant difference in sexual maturity of these two lots, both showing tassels September 16 and shedding pollen October 3. The second noniarovized lot was exposed to a 10-hour day. It showed tassels June 6 and shed pollen June 10, having been exposed to fewer hours of darkness (328) during this period than had the iarovized lot (336 hours) during the period of iarovization. It seems clear from this that length of day is much more important in determining the time of sexual maturity than is any darkness requirement that teosinte may possess (fig. 1).
McKinney and Sando (8) have shown that after the iarovization treatment the attainment of sexual maturity in winter wheat is greatly influenced by day length. To determine whether a similar condition exists in short-day crops, a second set of material, common millet and the corn hybrid AX164, were iarovized at 80ºF. at the moisture contents and for the periods recommended, one lot of seed of each crop in continuous darkness and a duplicate lot in continuous light (normal day plus artificial illumination). Following iarovization, the various lots, including dry and germinated checks, were grown in pots under a 16-hour and a 24-hour day.
The treatment given the millet was ineffective in hastening sexual maturity under either light treatment. The corn plants receiving the 16-hour day responded approximately as they had done in the field. For the plants grown under continuous illumination, iarovization with continuous light was superior to iarovization with continuous darkness in promoting early flowering, and both lots from iarovized seed were earlier than those from either the germinated or dry checks. The results are presented in table 4. There is nothing in the results obtained in these experiments with corn and teosinte to lend support to the darkness-requirement theory of Lysenko.
TABLE 4.—Influence of day length an the attainment of sexual maturity in corn
|Photoperiod (hours)||Period required to attain sexual maturity under indicated treatments|
The iarovization of certain corn hybrids resulted in a statistically significant hastening of sexual maturity. The difference, however, was so slight as to be of no importance agronomically. The general reduction in germination and vigor associated with the iarovization treatment appears to be a serious limitation of the method. In this respect these results depart rather drastically from those reported by the Russian investigators. The reason for the failure of agreement with their results is not entirely clear.
The Russians emphasize the fact that varieties differ markedly in their iarovization requirements. It is possible that all of the strains used in these tests require special conditions during iarovization, though this does not seem likely.
The experiments of McKinney and Sando (8) and the results reported here (table 4) are in agreement in indicating that the day length following iarovization has an important effect on the reaction. The plants in the present studies (tables 1 and 2) were grown under a daily photoperiod of approximately 15 hours, and those in the Russian work presumably under a longer daily photoperiod. It seems probable that at least part of the difference between the results in the two places may be ascribed to the day length under which the plants were grown. The claims for the necessity of darkness during the iarovization process, however, are riot substantiated by either the field or greenhouse tests.
larovization of corn, as practiced, consistently reduced the percentage of germination. It also resulted in a general reduction in the number of visible nodes, in plant height, number of ears, and weight of shelled grain per plant.
Pollen shedding and silking were significantly accelerated by iarovization in some strains but not to an extent to be of any agronomic importance.
There was no evidence of a darkness requirement for corn. Several strains of corn were grown to maturity under continuous light. larovization in continuous light was just as effective as that in continuous darkness.