Vernalization and Photoperiodism: A Symposium (1948)
by F. W. WENT
William G. Kerckhoff Laboratories of the Biological Sciences,
California Institute of Technology, Pasadena, Calif.

In the previous discussions the effects both of temperature and of periodic light-changes on plant development have been described. In each case direct effects and delayed or after-effects were observed.

In discussing vernalization of seeds it was shown that during the first thermophase the future development of the plant is strongly influenced by temperature, and this effect may be enhanced or counteracted to some extent by the second photophase. To obtain optimal development a marked temperature change has to occur between early stages of germination and later growth. This temperature change is a long-term process, measured in terms of months.

In photoperiodism the daily change from light to dark is essential to bring about developmental processes, and the effects of photoperiod are influenced or even determined by temperature. Theoretically this is interpreted by assuming that during the light and the dark periods different processes take place, which have to be balanced to obtain specific responses. Whereas in photoperiodism stress is laid on the direct effects of light and darkness, in thermoperiodicity the daily light cycle is given, and the effects of temperature during the light and dark periods are considered. Since development can be completely changed by varying temperatures during the dark period, and since optimal growth in most plants only occurs when the temperature is lower during night, stress should be laid on this daily cycle of optimal temperatures, and this is done by referring to this cycle as thermoperiodicity. Before discussing this latter phenomenon in greater detail, other cyclic temperature effects have to be mentioned, beginning with those having approximately a yearly cycle.

For a good understanding of temperature effects on the development of plants the fundamental work of BLAAUW and co-workers at the Laboratory for Plant Physiology at the Agricultural College in Wageningen, Netherlands, is essential. This work has mainly been carried out with bulbs. The latter have the great advantage, that during the greater part of their development they do not need any light, since the initiation and early growth of shoots, leaves and flowers occurs inside the apparently dormant bulb, while they are being stored at any desired temperature. Therefore the investigation of the temperature requirements during this apparent dormancy does not necessitate air-conditioned greenhouses, which were not available until recently. For this reason BLAAUW'S data are the most complete available at present on the optimal temperatures of the various stages in plant development.

Another reason for the importance of BLAAUW'S work are his microscopic observations of the meristem during and immediately after each temperature treatment, so that he could distinguish between the direct and indirect temperature effects, and so that he could identify each optimal temperature with a definite morphological stage.

After more than 10 years of work along these lines BLAAUW (1931) concludes: "It is hoped that the time has passed, when the development of buds and defoliation are described without any knowledge of what goes on inside the bud, and the development of the leaf primordia, of which only their macroscopical appearance is being observed." "Before talking about periodicity and dormancy, in connection with the later behavior of sprouting leaves or flowers, it is imperative that first the life history of these organs inside the bud is studied. For this precedes sprouting."

FIG. 1.—Optimal temperatures (ordinate, in degree centigrade) of the development of tulip bulbs (var. W. Copland) from the time of lifting from the ground to flowering (abscissa: time in weeks). Step-curve: experimentally determined optimal temperatures. Stippled curve: most likely actual optimal temperatures (from HARTSEMA, LUYTEN and BLAAUW 1930, p. 33).

In the experiments dry bulbs were placed in baskets and stored in temperature controlled rooms and incubators. All treatments were started immediately after the bulbs were lifted from the ground around July 1, when the current year's leaves had withered, and when the bulb seemed to have entered dormancy.

As a first example the development of the tulip bulb will be discussed. Figure 1 is taken from the publication of HARTSEMA, LUYTEN and BLAAUW (1930). Immediately after lifting, at the beginning of the experiment, next season's growing point has differentiated 3-4 leaf primordia, and is almost ready to initiate the flower. At that time the optimal temperature is highest, 20°C. When all flower part primordia have been initiated (about 3 weeks) the optimal temperature drops abruptly to 8°C., where it remains for 3 weeks, after which it stays around 9°C. During those weeks the flower parts develop into a complete flower, and the stem elongates slightly. When the direct effect of temperature is measured by actual stem elongation, the optimal temperature is higher, but such higher temperatures retard subsequent elongation, compared with the 9° temperature. Therefore BLAAUW speaks here of an indirect or inhibited optimum. It compares with the low vernalization temperatures, which inhibit germination, but accelerate later growth. By the time the leaves become visible from between the scales, the optimal temperature shifts to 13°C., which is optimal for actual stem elongation, and the optimum shifts still further to 17°C., when the leaves have emerged 3 cm. from the bulb. When they are 6 cm. long the optimal temperature shifts again, to 23°C. This means that each developmental stage has its own optimal temperature; organ initiation needs the highest temperature, stem elongation and unfolding of the flower a lower one, and preparation for elongation, a stage which has no morphological signature, occurs best at the lowest temperature. It is likely that the steps between 9°, 13° and 17° are not abrupt, but gradual, so that the stippled curve of figure 1 probably approximates the actual conditions closer than the step-curve. The drop from 20° to 8° on the other hand is abrupt, and any intermediate temperature interposed between these two delays development.

For the hyacinth a slightly different curve was obtained, as seen in figure 2 (from LUYTEN, VERSLUYS and BLAAUW 1932). The whole curve lies about 4° higher than that of the tulip. But most important is the different behavior during the early weeks. As in the tulip, the highest optimal temperature occurs immediately after lifting, and in this case is 34°C. By the time the first flowers on the raceme have been initiated the optimal temperature shifts to 25.5°, and when the highest flower are visible as primordia on the raceme meristem, the optimal temperature drops to 17°. Three weeks later the lowest optimal temperature of 13°C., is reached. When the high temperature of 34° is maintained throughout the period of flower initiation, subsequent flower development is abortive, and racemes with flowers of poor quality are produced. Therefore the intermediate temperature of 25.5° is a compromise between the optimal temperatures of at least two different processes, which proceed simultaneously inside the bulb: flower initiation with a very high temperature optimum, and preparation for further flower development, with a much lower optimum.

Before discussing these results any further, another paper from the same laboratory, by VERSLUYS (1927) must be mentioned. She studied the optimal temperatures for root initiation and root elongation throughout the whole development of the hyacinth bulb. It was found that the optimal temperature for root growth remained approximately constant at 27° C. During the part of the life cycle of the bulb which was investigated, few new roots were initiated, but the greatest number formed occurred again at about 27°C.

Viewing these results together, it can be said that each physiological process in the hyacinth bulb has its own optimal temperature, which differs from that of other processes. Therefore the over-all optimal temperature of the whole bulb is a compromise between the optima of the individual processes, as was clearly expressed in the flower-stand formation. Since not all flowers are initiated at the same time, some are already so far advanced that they need low temperatures for further development, while other flowers still are in the process of initiation, and consequently require high temperatures. While the later stages of elongation proceed best at 13°C., the temperature should be 27°C. for best root growth. Since root development does not seem to be controlling bulb growth, and sufficient roots are formed at the optimal temperature for elongation, no compromise between root and stem elongation temperatures is necessary.

FIG. 2.—Optimal temperatures (ordinate, in degree centigrade) during the development of hyacinth bulbs. Abscissa: time in weeks since lifting of bulbs from the ground (from LUYTEN, VERSLUYS and BLAAUW 1932, p. 51).

Whereas the curves shown in figures 1 and 2 show the shift in optimal temperatures, many diagrams showing the progress of individual processes at different temperatures are found in BLAAUW'S papers, and figure 3 (from BLAAUW, LUYTEN and HARTSEMA 1930) shows a graphical presentation of the flower development as a function of temperature. It shows the range of temperatures in which flower development can proceed with an optimum about 12° wide. The curves of figure 4 (from BLAAUW 1924) show the shift in optimum when observations are made in different intervals. From such curves it also would be possible to construct the curve of figure 2, but it would be less accurate.

For other bulbs the same type of an analysis was carried out. Daffodils (variety King Alfred) show a behavior similar to that of tulip and hyacinth, except that flower initiation already has taken place in the field, so that the optimal temperature starts as low as 13°C., lowering to 110 after eight weeks and shifting to 10° when leaves become visible. After 2 weeks the optimum increases to 17° and when leaves are 6 cm. to 20°C (see BLAAUW, HARTSEMA and HUISMAN 1932). In stark contrast with the bulbs from temperate climates the tropical Hippeastrum (BLAAUW 1931) has no obvious resting period but 2 to 3 times per year a whole cycle of leaf and flower formation is completed, at completely even temperatures.

PLATE 12 opposite page 149 perfectly illustrates the temperature effects in terms of bud development (magnification 14 X). This is a reproduction of figures 4-14 from LUYTEN, JOUSTRA and BLAAUW (1926), and it shows the state of the growing point of a tulip bulb after a four week storage at constant temperatures ranging from 1.5° C. to 35° C. The figures show what can be observed at 14 times magnification after all bulb-scales and enclosing leaf bases have been removed around the growing point. The scars of the removed foliage leaves are indicated and marked LL1, LL2, etc. The main vegetative growing point producing the shoot for the next year is marked VP, the lateral growing point which would have developed 2 years later is VPA (with its bulb scales R1 and R2). After the 4th or 5th foliage leaf (LL4 or LL5) was initiated, the growing point widened and the petals (TI and TII), stamens (M1 and M2) and carpels (VD) were initiated.

At 1.5°, 31° and 35° C. no change in the growing point had taken place during the 29 days storage; the growing points were in stage I (vegetative). At 5° and 28° the 4th or 5th foliage leaf had developed, and the growing point was just advanced to where it changes to the flowering condition. At 9° the growing point was in stage III (only petals initiated), at 25.5° it had reached stage VI (all flower parts, except carpels, well developed). and between 13° and 23° they were all in the same advanced stage of development (stage VII).

Another interesting fact can be observed as concerns the effect of temperature on flower initiation. When the temperature is high, the normal trimerous flower is formed (23° and 25.5°), but at low initiation temperatures (9°-13°) the flowers are predominantly tetramerous. At intermediate temperatures (17°-20°) intermediate numbers of flower parts are found. This was reported in detail by BLAAUW, LUYTEN and HARTSEMA (1932), where they showed that some tulips (like "Pride of Haarlem") were almost completely tetramerous at 13°C. initiation temperature, at which temperature the "Will. Copland" and other varieties of tulips were trimerous. At high temperatures (28°) tulip varieties initiated consistently a smaller number of flower parts.

The rhythmical development in tulip and hyacinth is controlled by variations in temperature, so that their strictly yearly cycle is synchronized with the progress of winter and summer in the temperate zones where they thrive. In Hippeastrum, an inhabitant of tropical regions with even temperatures throughout the year, an autonomous rhythm of the meristem causes a regular sequence of initiation of 3-4 leaves after which a flower stand is formed, after which, again, 3-4 leaves are produced, etc. This sequence can not be changed by temperature treatment.

FIG. 3.—Actual development (ordinate: condition of growing point, stage I being vegetative, VIII being complete flower initiation) of the growing point in tulip and hyacinth, as a function of storage temperature (abscissa, in degree centigrade). For the tulip the broken curve is recorded after 4 weeks, for the hyacinth the solid curve represents 8 weeks development at the different temperatures (from BLAAUW, LUYTEN and HARTSEMA 1930, p. 51).

One of the most interesting results of BLAAUW'S work is, that from the curves of figure 3 it can be deduced that development can be arrested by both low and high temperatures. The cessation of development near freezing is not amazing, since it occurs in most plants. But all growth can be stopped by keeping bulbs at 35°C., which temperature is not injurious at all to the bulbs. As soon as the temperature is lowered to a proper one for the stage of development, growth is resumed as if no interruption has occurred. In a series of experiments bulbs were inhibited for 6 months by either high or low temperatures and then shipped to the Southern hemisphere. In this way their rhythm was shifted half a year, and the bulbs kept time with the shifted sequence of seasons, enabling successful shipping to the opposite hemisphere, which had not been possible before (BLAAUW, LUYTEN and HARTSEMA 1930).

From a practical standpoint this spectacular success is not the most important. Of greater significance for the bulb industry is the fact that some of the most important stages in the development of a bulb are passed during its storage period. This had been realized by practical growers since DAMES in 1909, but it was BLAAUW who furnished the theoretical background and who rationalized the treatment. Adjustments to the treatment in storage are possible, which improve the later performance of the bulb in the field. If the bulbs have to be planted in a rather warm climate, part of the cold treatment they need (and usually receive in the field in colder climates) can be administered during storage. In this way performance in practically any climate can be guaranteed. This is actually put into practice now so that shipments of bulbs can be treated individually to insure best flowering at the point of destination (see e.g. VAN SLOGTEREN 1935, 1936).

Many more important results were obtained by BLAAUW and co-workers on development and temperature, but the previous review summarizes the results most important for interpretation of thermoperiodicity.

See Temperature and Fertility bibliography

Another set of phenomena, which are closely related to vernalization, are the chilling requirements for development of buds of deciduous trees. In most of these plants the buds are dormant for a considerable part of the winter, and can be forced into growth only after having been subjected to freezing temperatures. In some cases the low temperatures may have no other effect than supplying a stimulus, so that a definite time after being subjected to a sudden drop in temperature, irrespective of the duration of this lower temperature, development occurs. The flower buds of the orchid Dendrobium crumenatum offer a clear-cut example (COSTER 1926, KUIJPER 1933). Nine days after a sufficiently rapid drop in temperature (usually associated with a heavy rainfall) the flowers of this orchid open, causing a sudden burst of flowering over a wide area. Some other orchids seem to behave in the same way, and probably other plants as well (gregarious flowering of Coffea liberica). In these cases the flower buds develop gradually up to a certain point, beyond which no growth is possible under the prevailing temperature conditions. The longer the temperature drop is delayed, the more flower buds will have reached the critical size, and the more abundant the flowering is after the temperature drop.

FIG. 4.—Length of the growing point and flower cluster (ordinate) in the hyacinth (var. Queen of the Blues), after they have been kept for various lengths of time at 11 different temperatures (abscissa, in degrees centigrade). Stippled line: original length; dots and broken line: after 3 weeks; triangles and solid line: after 5 weeks; squares and dash-dot line: after 8 weeks; crosses and broken line: after 12 1/2 weeks (from BLAAUW 1924, p. 35).

An intermediate case between the orchids and the deciduous trees is found in the lily of the valley, Convallaria majalis (HARTSEMA and LUYTEN, 1933). Their rootstocks become dormant in summer, when the current year's flowers and leaves have withered. Only after a one-week period of 0.5 to -2°C., or three weeks at 5° do the buds on these rootstocks start to grow. The optimal temperature for this effect seems to lie so close to freezing that it is hard to see what physiological process could be responsible for the breaking of the dormancy. A similar phenomenon is known for Gladiolus corms (DENNY 1942). When these are stored at warm soil temperatures, no development takes places. But short periods, of 24 hours or less, at 0-5°C., will break their dormancy. These cases may be comparable with the chilling requirements of some seeds, which will not germinate until they have been subjected to freezing (CROCKER 1916). In this case hard-seededness has been held responsible for the dormancy, the freezing softening the seed coat.

Most deciduous trees of the temperate zone pass through a period of dormancy, during which time the buds cannot, or only with great difficulty, be made to develop. This is a secondary induced dormancy. In spring, soon after the buds have broken, axillary buds enlarge on the young shoots, and in early summer they have reached full size. By that time these buds are not dormant, but defoliation, or placing cut branches in water in the greenhouse, will cause almost immediate development into shoots. Two months later the buds, when subjected to the same treatment, will not develop even under favorable conditions. At the time of leaf fall the buds have reached the stage of deepest dormancy. In trees growing out of doors this dormancy decreases, until it has completely disappeared in spring. When branches or small trees are kept in the greenhouse, the dormancy does not disappear. In nature the dormancy may not be broken by the time of spring when the winter has been exceptionally warm. This gives rise to the phenomenon of "delayed foliation" (CHANDLER et al. 1937). Consequently many of these deciduous plants requiring a cold winter cannot be grown in climates with an equitable climate, such as subtropical regions and higher altitudes in the tropics.

Precise laboratory experiments on the chilling requirements of deciduous trees have not been carried out as yet. Numerous observations in nature have led to the following conclusions:

Only temperatures below 5-8°C., seem to be effective in breaking dormancy. These low temperatures must last for a sufficient number of hours, so that for each species and variety of plant a minimal number of hours below 5°C., can be assigned, which are required before the tree will leaf out. These hours have not necessarily to be consecutive, but the effect is cumulative. Varieties native to colder climates have a longer chilling requirement than those from climates with warmer winters. Therefore northern varieties grown in the south usually show delayed foliation (late and erratic breaking of buds in spring), whereas southern varieties may be killed farther north when their chilling requirements have been met before the danger of late frosts is past. Although the purely factual description of chilling requirements is very incomplete as yet, the physiology of the buds is better investigated. No reference will be made to the numerous papers describing the chemical composition of dormant tissues and of tissues whose dormancy has been broken, naturally or by artificial means. Nor will the many artificial methods employed to break dormancy be enumerated.

BENNETT and SKOOG (1938) found that buds kept dormant by leaving pear trees during winter in a heated greenhouse, could be made to sprout by application of yeast extract. GUTHRIE (1940) suggested that the glutathione of yeast extract was the active agent breaking dormancy, but BENNETT, OSERKOWSKY and JACOBSON (1940) showed that the effect of yeast extract was due to a compound differing from glutathione.

Interpretation of these facts is hardly possible as yet. On the one hand it would appear that the dormancy induced in buds in the course of the summer is due to a growth inhibiting substance which accumulates in the fully developed buds. This substance is probably not identical with auxin, since auxin extractions at the time of deepest dormancy show the lowest auxin content. Yet it is possible to keep buds dormant beyond their normal seasonal loss of dormancy by spraying with auxin-like substances (GUTHRIE 1938). This auxin-induced dormancy therefore is different from normal dormancy.

The effectiveness of ethylene, ethylenechlorohydrin and similar substances in breaking bud dormancy could indicate destruction of inhibitors, as has been shown to be the case in the breaking of dormancy of potato tubers (MICHENER 1942). On the other hand the experiments of BENNETT and SKOOG (1938) and GUTHRIE (1940) indicate that application of materials which can be considered to contain growth promoting substances can break bud dormancy. From this one would be led to assume that dormancy is not due to accumulation of inhibitors, but to a lack of growth substances. It is conceivable that both mechanisms occur, but much work remains to be carried out before binding conclusions are possible.

Finally these findings will have to be correlated with the temperature effects. Not enough facts are at hand to even suggest a hypothesis how low temperatures could remove inhibitors or cause the production of growth promoting substances.

Already very early in the periodic development of plants with chilling requirements, periods of low temperature are necessary. In peach seedlings (LAMMERTS 1942) embryo-cultured seeds did grow immediately, but they soon became dormant. When these dormant seedlings were placed in cold storage at 5°C., 20-40 days sufficed to break their dormancy. The seeds of Convallaria majalis will start to germinate at medium temperature, but the epicotyl soon becomes dormant and will not continue development until exposed to low temperatures (BARTON and SCHROEDER 1942).

For a fairly complete discussion of all these facts we can go back to SACHS (1860). He was the first to study in detail (with a hardly adequate technique) the temperature relationships in the growth and development of plants. If we consider how primitive the methods were with which he had to obtain and maintain the various temperatures, his results and conclusions are truly remarkable. By using many different plants, he initiated a comparative growth physiology. The greater part of his work was carried out with seeds and seedlings, but in some further observations and considerations he extended his treatment of the temperature relationships of the growing plant until its maturity. SACHS realized, that each growth stage of the plant has its own temperature characteristics, which he considered to be adequately qualified by: minimal (z), optimal (y) and maximal (x) temperatures. As stages he considered seedling growth (S), vegetative growth (V), blossoming (B) and fruiting (F). He even distinguished a sub-stage: germination (G) which was somewhat different from seedling growth. Each plant could then be characterized as follows:

  x     x1     x2     x3     x4
G y   S y1   V y2   B y3   F y4
  z     z1     z2     z3     z4

As example (expressed in degree centigrade) he gave:

Corn G 46 S 46
34 34
9.5 14.5
Wheat G 42.5 S 42.5
29 29
5 10

For certain early spring flowers, such as Daphne, Galanthus and Hepatica he concluded that Z2 > Z3 < Z4.

The last few paragraphs of this paper of SACHS are worth quoting: "If we presume that all numerical values of the above scheme are known, it will be immediately possible to determine, whether a given climate offers the necessary growing conditions for a particular plant. In addition it would be necessary to add the specific time-relations, investigating for each x, y, and z how much time is required to complete phases G, S, V, B and F."

"Once all these data are known, we can hope that the law can be found, according to which temperature and development of a species are linked. The above scheme only serves to arrange the collected data in a logical manner. Using these known data it would be simple to discover the shortest possible time for development of the plant. This question cannot be answered without detailed analysis of the response of the plant, but offers much of interest in physiological respect."

The work of BLAAUW has answered some of the questions asked by SACHS, and has supplied the necessary data for V and B in a number of bulb species.

All previously mentioned facts show, that in the course of development of a plant there is a succession of processes, each of which may have a different temperature range. In some cases these ranges are so far apart, that under constant temperature conditions no continued development is possible, in other cases we find only a shift in optimal temperature from month to month. Such a shift in temperature characteristic can be expected whenever different processes succeed each other, no matter how short the duration of each process. Thus in photosynthesis the "Blackman" reaction has a Q10 of over 2, whereas the light reaction has a Q10 of about 1. In photoperiodism HAMNER and BONNER (1938) have shown that the dark reaction has a high temperature optimum, whereas the temperature during the photoperiod is of secondary importance in flower induction. This shows that the light and dark processes in flower induction can be separated by their temperature dependance. In general the photoperiodic response is greatly dependent upon temperature, and may be modified or even reversed by extreme temperatures (see pages 48-51).

1 On consideration of priority the term photoperiodism is now favored over photoperiodicity. On the same basis we should accept thermoperiodicity (WENT 1944). Besides the word periodism does not occur in Webster so that periodicity is preferable on linguistic grounds.

Whereas in many plants the growth rate stays rather constant from day to night, in others the greater part of stem elongation occurs during night. Therefore, the temperature during night can be expected to have a marked influence on the growth rate of the plant as a whole. During day photosynthesis most likely does not have the same temperature optimum. This consideration shows already that we can expect different optimal temperatures during day and night. This is the basis for thermoperiodicity.1 Actually in most plants investigated thus far optimal growth and development occurs when day temperatures are considerably higher than night temperatures. For future discussion it seems advisable to refer to the light period, which usually, but not necessarily, coincides with the daytime, as the photoperiod, and to the temperature prevailing during this period as phototemperature. Since in thermoperiodicity the dark period is equally important as the photoperiod, and has to be referred to often, the term nyctoperiod is suggested. The temperature during the nyctoperiod is the nyctotemperature. This same term can be used in photoperiodism discussions.

FIG. 5. — Relationship between stem growth rate (ordinate, in mm./day) and temperature (abscissa, in degree Centigrade) of tomato plants. The circles represent plants kept both day and night at the indicated temperature. The crosses show growth rates of plants kept during eight day hours at 26.5°C. and during night at the temperatures indicated on abscissa. Squarest plants kept during day at 19-20°, during night at 26.5°C. (from WENT 1944, p. 140).

In work designed to find the optimal growing conditions for tomato plants (WENT 1944) the optimal nyctotemperature was determined as 17-18°C., whereas the optimal phototemperature was closer to 26°C. than to 17° (see figure 5). These temperature relations were worked out in greater detail in later publications (WENT 1944a, 1945a). The nyctotemperature optimum was high in the seedling stage (about 30°C.), and during the course of development gradually fell to 18° for the San Jose Canner, and to 130 for the Illinois-19 tomato. This optimum was also influenced by the light intensity during the photoperiod, being lower for lower light intensities. The previous temperature treatment equally determined the response to the nyctotemperature. The conditions optimal for stem elongation were also optimal for fruit set and fruit growth.

An analysis of the temperature response of the chili pepper (Capsicum annuum) gave essentially the same results (DORLAND and WENT, 1947). Like the tomato, optimal growth was obtained at a phototemperature of 26°, and the optimal nyctotemperature dropped from 30° in young plants to 8° for full grown plants; the same gradual decrease in optimal nyctotemperatures was found in blossoming, fruit set and fruit development.

In botanical literature practically no references are found to thermoperiodicity. SCHIMPER (1898) refers to observations of a peach grower, showing that optimal development of peach fruits requires a gradual rise in temperature from blossoming to fruit ripening, and a daily temperature drop from day to night, with an amplitude of about 3-5°C.

BONNER (1943) presented data, from which it can be concluded that Cosmos grows to about twice the weight at an 18° nyctotemperature and 26° phototemperature, when compared with constant 18° or 26°C. temperatures.

Not only growth but also other processes such as rubber formation in guayule (Parthenium argentatum) are strongly thermoperiodic (BONNER 1944). Rubber is formed at the fastest rate at nyctotemperatures between 5 and 10°C., and is very slight above 15°C. But rubber formation only occurs provided the phototemperatures are fairly high (18-26°C.).

Many plants do not grow or even die when the nyctotemperature is 26° or higher. LEWIS and WENT (1945) showed that Baeria chrysostoma and various other California spring annuals do not germinate, and young or older plants die when subjected to 26° nyctotemperatures. This is not due to diseases or pests, for Loo (1946) showed that even under sterile conditions Baeria plants die at such high nyctotemperatures. Since the lethal effect of high nyctotemperatures can be counteracted by lengthening the photoperiod, it seems possible that excessive respiration, coupled with deficient carbohydrate supply of the growing regions, is responsible for death.

ROBERTS (1943) in a short note reported on experiments with a wide variety of plants grown at either 24 or 13°C., at day or at night. He concluded that "The temperature during the dark period of the day is an important factor affecting bloom induction as well as some other reactions."

In commercial greenhouse culture it is well known that during day and night the temperatures should be kept at different levels. In LAURIE and KIPLINGER (1944) the following optimal temperatures in degree Centigrade are given:

Violet 8.5-14 4.5-10
Snapdragon 14 -16 7 -9
Lathyrus 13 -15.5 9 -10
Roses 21 -23 14.5-16.5


Seedlings in general 21-29
Seedlings of Odontoglossum 13.5-15.5
Mature plants of Cattleya 15.5-18.5
Mature plants of Odontoglossum 10

In many instances only optimal nyctotemperatures are given, apparently because they are considered more important, and since day temperatures are so hard to control. In some cases special mention is made of difference in optimal phototemperature according to light intensity (Lathyrus 13° on cloudy, 15.5° on sunny days); in another instance a differentiation of the optimal nyctotemperature according to light is recorded (for Phalaenopsis 15.5°-18.5° during winter and 21° during summer).

Many more quotations could be made from the published experience of greenhouse growers, but in the botanical literature little more has appeared concerning thermoperiodicity.

FIG. 6. — Relations between (night) temperature (abscissa in degree Centigrade) and (1) total growth of intact plants (circles), (2) direct effect on stem elongation (triangles), (3) growth of isolated roots (crosses), and (4) translocation (squares). (from WENT 1944a, p. 612).

The explanation of various phenomena observed in thermoperiodicity can be found in WENT (1944a, 1945a). During daytime photosynthesis seems to be the limiting factor for development. This is only true when the nyctotemperature is within the optimal range so that the assimilates can be utilized to best advantage. When the nyctotemperatures are too low, photosynthesis does not limit development any more (WENT 1945).

In darkness the fairly low optimum temperature in tomatoes is caused by the competition between two individual processes (see figure 6). Most of the stem elongation occurs during night. The growth process has a temperature coefficient Q10> 2, its optimum lies around 30°. The rate of food translocation, however, has a temperature coefficient Q10< 1, so that at higher nyctotemperatures, less sugar reaches the growing region, and the food supply becomes limiting. Thus in young small tomato plants, where food translocation takes place over short distances only, the optimal nyctotemperature coincides with the optimal temperature of the growth process (30°). As the tomato plant becomes taller, and the assimilating and growing regions become separated by longer distances, the food translocation becomes more and more limiting at the higher temperatures so that the optimal nyctotemperature drops to lower and lower levels (13-18°) in different tomato varieties. This same phenomenon can be seen in LAURIE and KIPLINGER'S data on page 155 for orchids. Seedlings must be kept 4-10° warmer than mature plants. This is also the reason why in spring so many garden crops and flowers are germinated in greenhouses, where they are kept warm. By the time that their temperature requirements have been lowered, the outside temperatures have sufficiently risen to insure good growth of the seedlings when brought into the open.

The shift in optimal photo- and nyctotemperature according to the light intensity during the photoperiod, which was found in the tomato experiments in the air-conditioned greenhouses is well known in commercial practice, as indicated previously. In future work a further differentiation of the nyctotemperature effect must be made, and a more precise localization of the temperature effect has to be achieved.

The seeds of many plants germinate well only when they are subjected to a daily fluctuation in temperature (HARRINGTON 1923). This does not seem to be a case of thermoperiodicity in the sense that processes with different optimal temperatures have to alternate to cause germination. This is perhaps best demonstrated by the experiments of MORINAGA (1926) who found that seeds of Cynodon dactylon require alternation of temperature for best germination, but that scarification for 3-9 minutes with concentrated sulphuric acid made optimal germination possible without alternation of temperature. TOOLE (1940) found the same effect for Oryzopsis. It is also demonstrated by the fact that not the actual temperatures employed, but the alternation of temperatures as such determine germination (HARRINGTON 1923). Therefore the seat of response to alternating temperatures seems to be in the seed coat and not in the embryo. The alternating temperatures seem to affect the "encasing structures interfering with oxygen absorption by the embryo and perhaps carbon dioxide elimination from it, resulting in the limitation of the processes dependent upon these" (case 4 of CROCKER 1916). This conclusion differs from the one reached by MORINAGA (1926), who concluded that "alternating temperatures have their effects on the embryos."


Additional References:—