Introduction to the Botany of Field Crops: Non-cereals pp. 138-143 (1936)
James Masson Hector
PHOTOPERIODISM AND VERNALIZATION IN RELATION TO HABIT
Wheat and similar cereals, such as oats, barley and rye, are generally spoken of as long-day plants, flowering and fruiting in the summer days (or their equivalents) of temperate climates. They are, however, by no means equivalent in their reactions either to daylight or to temperature.
In the first instance wheat has been divided into two cultural classes-winter wheats and spring wheats. Winter wheats sown in autumn germinate at moderate temperatures, produce small, compact tillered plants which can survive low temperatures, and only commence to flower with increasing length of day the following season. Nevertheless their life period is annual. Sown in spring their behaviour is that of a biennial. During the first season they make vigorous vegetative growth and generally fail to ear; flowering takes place in the subsequent season. Spring wheats on the other hand are always typical annuals. They germinate at moderate temperatures, make slow vegetative growth for a short period and thereafter ear rapidly with lengthening day. If sown in autumn the majority are killed by lack of hardiness to winter conditions.
The distinction is by no means absolute. Yet in the more advanced agricultural countries, the general reaction of most of the varieties is known with such accuracy that their behaviour in certain districts can be relied on. Introductions from other districts or from other countries, however, may behave in a manner contrary to expectation. In other words the reactions appear to be fixed for certain areas, unpredictable for others.
Attempts have been made to differentiate winter and spring wheats on grounds other than physiological, but none of the criteria has proved decisive. Thus all winter forms are said to possess small dome-shaped growing points which do not elongate for a considerable period, whilst the growing points of spring wheats elongate very rapidly (15 to 20 days). Also, soft spring wheats are said to have longer and more pubescent leaves, accompanied by a relative lower chlorophyll content, than winter wheats.
As regards "winter hardiness," methods have been devised by which resistance to cold can now be estimated with considerable accuracy.
In a series of papers, Lysenko and his associates analysed the reaction of wheat and other cereals to temperature and daylight. In terms of their experiments, Lysenko advanced an explanation which differs markedly from that of earlier investigators.
The flowering of an annual plant is regarded by Lysenko as the last stage of a developmental sequence which is independent of the growth of the plant. Growth, as such, is quantitative: flowering is the result of a sequence of changes which follow in serial order, the changes being "qualitative," not quantitative in nature. The quantitative and qualitative stages may or may not coincide.
Each stage is conditioned by a definite set of external conditions such as temperature, light, moisture, aeration, etc., and must be completed before the succeeding stage can be initiated. If any one stage be omitted, the following stage is inhibited. Thus a winter cereal sown in spring lacks the essential conditions necessary for the initial developmental stage. As a consequence the plants remain for an indefinite period at the tillering stage, in spite of the fact that the conditions necessary for the subsequent stage are in operation. Interaction of the conditions may thus lead to the production of plants exhibiting (a) rapid growth and slow development; (b) slow growth and rapid development; (c) rapid growth and rapid development.
Two stages—perhaps three—pre-requisite for flowering have been recognized:
1. THE THERMO-STAGE.—The conditions here required are low temperatures, ranging from 0° to 20°C, according to the species or variety, and suitable conditions of moisture and aeration. Light and darkness play no part. This thermo-stage must be in operation and completed before the initials of the reproductive stage can be laid down. The time required for its completion does not depend on the size and age of the plant (i.e., on its growth), but only upon its genotype and the environmental conditions. It can therefore be initiated not only in the growing plant but also in a seed which has just commenced development, but whose seed-coat is still unbroken. Upon this fact Lysenko has based his process of "vernalization."
For the completion of the first or thermo-stage, the different wheat varieties require different temperatures. In this respect they may be divided into three classes:—winter, semi-winter and spring forms. Winter forms require, together with other suitable conditions, temperatures not lower than -2 and not higher than +10°; semi-winter forms, temperatures not lower than +3° and not higher than +15°; spring forms, temperatures from +5 to +20°C. Within each group, however, there is considerable variation, and the grouping is accordingly only approximate.
2. THE PHOTO-STAGE.—If the changes induced by the thermo-stage are complete, a further stage is necessary before reproduction can take place. This further stage is the "photo-stage." Here higher temperatures and either long-day conditions or short-day conditions for varying periods are essential.
In the case of wheat, the conditions are high temperature, plus long-day conditions, or better still, continuous illumination. Such plants could indeed be termed "plants of continuous illumination," for in the second stage they require continuous light for a definite period and only tolerate an alternation of day and night if the dark period be relatively short. "Short-day" plants, such as maize and millet (Panicum), behave quite differently. Following upon the thermoperiod, they require a period of 10 to 15 days of continuous darkness in order to flower. Thereafter they can develop equally well in either a long or a short day. According to Lysenko, such "short-day" plants could be termed "plants which in one of their developmental stages require darkness or low light intensity."
3. THE THIRD STAGE.—According to Kraevŏi and Kiričenko, a third stage, not yet fully elucidated, must follow the second. During this third stage, the changes essential to the formation of functional gametes are initiated. In wheat it appears to be dependent upon a length of day equivalent to at least 8 to 12 hours. Hence in the later stages, a shortening day is all that is required.
Summarizing the situation, the analysis indicates that a winter wheat is one which has the capacity to resist low temperatures, requires an initial developmental period of 40 to 50 days during which the temperature range varies from -2 to 10° C, whilst light is of little or no importance; thereafter requires a period of higher temperature during which the ideal condition would be continuous daylight but in nature consists of what is termed "long-day"; and finally requires a light period equivalent to at least an 8-hour day for the formation of fertile gametes. On the other hand, a typical spring wheat has not the capacity to resist low temperatures. It requires for reproduction a sequence which is (1) a minimum period of 10 to 20 days, during which the temperature must range between 5 to 20°C, the light therein being unimportant; (2) a subsequent period at higher temperatures under continuous illumination or long-day conditions; and (3) a final period of illumination equivalent to at least an 8-hour day.
The distinctions, however, are not absolute. When a large number of varieties are studied, they form a graded series.
As already noted, Lysenko has based his process of "vernalization" ("Jarovizacija") on the fact that the "thermo-stage" can be induced in seeds. which have just commenced their growth. Such seeds have—in so far as the embryo is concerned—ceased to be seeds in the physiological sense. Consequently, the first of the qualitative changes pre-requisite to development may then be induced in spite of the fact that practically no growth (quantitative change) has taken place.
In the case of "long-day” cereals, such as wheat, oat, barley and rye, the process is as follows. The seed must be first be induced to germinate without the radicle breaking the seed-coat. This is done by adding water and keeping the seed for 3 to 5 days at a temperature of 5 to 10°C. The amount of water to be added varies with the type for winter wheats, 37 litres per 100 km. of wheat for late spring wheats, 33 litres; and for early spring wheats, 31 litres. Further, the water should be applied in three successive applications in order to ensure complete absorption, and thorough mixing is also essential.
When 3 to 5 per cent. of the seeds have burst their grain coats, vernalization commences, and the process is continued for 40 to 50 days in the case of winter wheats, 10 to 15 days for late spring wheats, and 5 to 6 days for early spring wheats. During these intervals, the seed must be maintained at the stage at which vernalization commenced. No further growth should be permitted, but the water content and a suitable temperature must be maintained throughout. If the grains become too dry, more water should be added; if too moist, more grain. The most important factor, however, is the maintenance throughout the period of the correct temperature for the variety in question. These temperatures are 0° to 2 to 3°C for winter wheats, 3 to 5 to 6°C for late spring wheats, and 8 to 10° C for early spring wheats. The optimum temperature for any one variety within these classes—if not already known—must be determined by experiment.
During these respective periods, the embryos within the grain, being maintained at the correct temperature, undergo the whole of the qualitative changes of the thermo-stage of development. If sown immediately thereafter in spring, the subsequent developmental stages determined by light follow naturally, and the ears will form and ripen in advance of their normal period, the actual shortening of the period in each instance being theoretically equal to the time during which they were vernalized.
It is now claimed that the changes induced in the protoplasm by the main factors of each stage can be detected micro-chemically. Thus, if the thermostage be completed by vernalization, the growing points of such seeds stain an intense blue when treated with 5 per cent. ferric chloride followed by 5 per cent. potassium ferrocyanide. The growing points of untreated seeds or seeds that have only been partially vernalized remain unstained or become yellow or green. As the vernalization process passes towards completion, it is stated that blue-stained cells are found in increasing numbers scattered amongst the unstained cells.
By staining the epidermal cells with certain dyes, it is possible to detect the completion of the photo-stage. Thus the lower epidermis of wheat stained with a mixture of methylene blue and methyl red gave the following reactions: (a) pink—plants in which the thermo-stage was incomplete; (b) blue—plants in which the thermo-stage was complete; (c) mauve-pink—plants in which the photo-stage was complete.
These and similar experiments on the effects of light and temperature, however, have been variously interpreted. For example, Ljubimenko (1933) regards the changes produced as an "induction," which may, therefore, be retarded or accelerated and even reversed. Also, thermal and photoperiodic induction consist not in the specific preparation of the plant for reproduction, but in an acceleration of the entire developmental process. By maintaining the external factors at the requisite levels, both the reproductive and also the vegetative characters may be accelerated or retarded together. Vasiljev (1934) and Cailahjan (1934) consider that vernalization can be induced by light alone. Lebedinceva studied the effect of chilling plus length of day and came to the conclusion that the usual distinction between long- and short-day plants is erroneous. Thus spring wheat and rye are true long-day plants; winter wheats on the other hand are only conditionally long-day plants, i.e., they behave as long-day plants only after vernalization.
This viewpoint is similar to that advanced by Purvis (1934) who investigated the effect of day length on winter and spring rye after germination at 1°C and 18°C. In assigning the material to its photoperiodic category, she considered not merely the time of emergence of the inflorescence but in particular the time of formation of the flower primordia.
In winter rye, the differentiation of flower primordia was found to be subject to an interaction between day length and the temperature during germination, these factors determining both the minimal number of leaves formed before differentiation of flowers began, and the rate of growth of the meristematic tissue. As a consequence of this interaction, low temperature germination hastened flower inception in winter rye under long days, but produced no effect under short days. At the same time, after germination at high temperatures, short-day treatment led to earlier differentiation of flower primordia than did long-day treatment. No temperature after-effect was found in spring rye, but short days retarded the differentiation of flower primordia. Once the differentiation of primordia had taken place, however, further development was always hastened by long days. Hence if ear emergence be taken as the criterion of flowering, all types of rye may be regarded as "long-day" plants; but if differentiation of flower primordia be taken into consideration, winter rye germinated at high temperatures behaves as a short-day plant; germinated at low temperatures, it behaves as does spring rye.