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Vernalisation in Plants: Site, Requirements, Mechanism and Importance!
Temperature plays significant role in metabolic activities of plants. Temperature is one of the important factors determining the distribution of plants. Temperature also plays vital role in the germination of seeds and subsequent flowering of plants.
Plants of temperate zone, as expected, germinate at a relatively low temperature, whereas tropical plants germinate best at much higher temperature. Development and flowering in many temperate plants can be altered by subjecting moistened seeds to low temperature.
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Many plants do not come to flower before they experience a low temperature. These plants remain vegetative during warm season, experience low temperature during winter, grow further and then bear flowers and fruits. It was found by Lysenko (1928), a Russian scientists, that the cold- requiring biennial plants can be made to flower in one growing season by providing low temperature treatment to young plants or moistened seeds. He called the effect of this chilling treatment as vernalisation. Vernalisation is, therefore, a process of shortening of juvenile or vegetative phase and hastening flowering by a previous cold treatment.
An example of winter rye may be quoted here. When the seeds of this variety of rye were germinated at 1°C for four weeks, the plants flowered eleven weeks after planting, but at the same time seeds germinated at 18°C did not produce flowering shoot in the same duration (Fig. 5.5).
Another interesting fact about the effects of vernalisation came from the work on biennial varieties of Hyoscyamus niger (henbane) by Melchers and Lang (1948). This variety of henbane will flower only when vernalisation is followed by long day treatment, and vernalisation followed by short-day treatment fails to induce flowering (see figure 5.6).
Still more interesting is the fact that annual variety of henbane does not require cold treatment for flowering. The annual variety differs from biennial ones in the possession of a single dominant gene which functions as a substitute to vernalisation.
It is presumed that the said gene brings about direct production of the precursor of flowering substance, which in the biennial variety requires cold treatment. These findings indicate the possibility of conversion of certain hormone precursor into a flower-inducing active form which under the influence of appropriate day-length induces flowering. Common examples of plants requiring vernalisation are winter rye, winter wheat, winter oat, winter barley, pea, beet, cabbage, Henbane, Chrysanthemum, Viola, Clover, etc.
Site of Vernalisation (Site of Perception):
Site of perception of cold stimulus is different in different plants. It can be a germinating seed or metabolically active embryo (Secale cereale), shoot apical meristem (Chrysanthmum) or vegetative parts such as leaves (Hyoscyamus niger and other biennials).
Requirements of Vernalisation:
(i) Low temperature:
Vernalisation, unlike photoperiodism, is a cumulative process because plants become gradually more and more effectively vernalized with time upto as long as about two months. Full vernalisation requires up to about 50 days of treatment between – 2°C and about 12°C. If vernalisation is followed by high temperature treatment at about 40°C for a minimum of two days, the vernalizing stimulus is lost. This is known as devernalisation. Devernalised plants can, however, be vernalized again.
(ii) Actively Dividing Cells:
Vernalisation does not occur in dry seeds. The seeds must be germinated so that they contain an active embryo. For this the seeds are moistened before exposing them to low temperature. In a whole plant, an active meristem is required.
(iii) Water:
Proper protoplasmic hydration is must for perceiving the stimulus of vernalisation.
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(iv) Aerobic Respiration and
(v) Proper Nourishment
Mechanism of Vernalisation and Induction of Flowering:
The stimulus received by the actively dividing cells of shoot or embryo tip travels to all parts of the plant and prepare it to flower. The stimulus has been named as vernalin. Melchers (1936, 1937) demonstrated in henbane plants (Hyoscyamus niger) translocation of vernalisation stimulus takes place through a graft union (Fig. 5.7). If leaf or stem of a vernalised plant is grafted on an un-vernalised henbane plant the latter plant will flower.
The stimulus was found to be non-specific, i.e., can pass across a graft between plants of different species. It was Melchers who, for the first time, suggested that a substance which he called vernalin was produced during the process of vernalisation. Attempts to isolate and chemically identify vernalin have not succeeded as yet. However, Lang et. at. (1957) have demonstrated that treatment with Gibberellic acid (GA), a plant hormone, substitute for cold treatment in some species of plants.
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The formation of vernalin is not enough to bring about vernalisation. In addition, a suitable day length is also necessary. It is postulated that in the appropriate photoperiod, either vernalin is converted into florigen or vernalin regulates the synthesis of florigen from precursors. Florigen then induces the vegetative meristems to switch over to reproductive development. This initiates the process of flower differentiation.
Vernalisation simply prepares a plant to flower. It makes the plant perceptive to the stimulus; however, it itself does not function as a stimulus. Photoperiodism, on the contrary, not only provides the stimulus for flowering but also induces it.
Advantages/Importance of Vernalisation:
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(i) Vernalisation can help in shortening the juvenile or vegetative period of plant and induce early flowering. It is applicable to not only temperate plants but also to tropical plants, e.g., wheat, rice, millets, cotton.
(ii) It increases yield, resistance to cold and diseases.
(iii) ‘Kernel wrinkles’ of Triticale can be removed by vernalisation.
(iv) It enables the biennials to behave as annuals.
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(v) Plants can be grown in such regions where normally they do not grow.
