Write the mechanism of vernalisation.

 

 Write the mechanism of vernalisation.

 

The word vernalisation is obtained from Latin word ‘vernus’ meaning of the spring. vernalisation may be defined as the acquisition of a plants ability to flower in the spring by exposer to the prolonged cold of winter. After vernalisation plants have acquired the ability to flower but they may require additional, seasonal cues or weeks of growth before they will actually flower.

History of vernalisation:

 It was observed by farmers that winter cereals whose seeds require chilling and spring cereals do not require spring. From this knowledge, a Russian scientist Lysenko described a chilling process for early flowering. He used to make the seeds of winter cereals behave like spring cereals. Later, a German scientist (a physiologist) Gassner properly describe the actual technique of vernalisation. Now a days molecular biologists try to focus on vernalisation aspects.

Age of the plant for vernalisation:

Vernalisation through cold treatment is very effective at the seed stage or seedling stage. In some cereals even the embryos can be successfully vernalised. However in many cold recurring species vernalised is not effective until and unless the plant possess atleast few leaves. Additional support to this view comes from an experiment on the embryo of ‘Petkus rye’. It was noted that the embryos are separated from the endosprem and then if the embryo alone is subjected to cold treatment embryos fail to be vernalised. But if such embryos are subjected to cold treatment alongwith carbohydrates like sucrose solution or endospermous tissues, vernalisaton is very effective.

Site of vernalisation:

The most sensitive site which act as the perceptive organ for  vernalisation is the meristematic region of the shoot apex. But the work of Willensiek (1964) on the vernalisation of the excised or isolated leaf and root of Lunaria biennis has shown that all dividing cells including those in roots or leaves, may be potential sites of vernalisation.

Temperature effect:

For the normal  growth and development, every plant requires on optimum temperature but for vernalisation the optimum temperature required is 3-17 o, which varies depending upon the species involve. Even the species to species. For example in ‘Petkus rye’ the most effective range of temperature is 3-7 o is optimul.

 

 

Effect of water and oxygen:

Along with the cold treatment plants also require water and oxygen for effective vernalisation.the seeds or embryos should posses atleast 40-50 % water in their cell, with out which cold treatment has no effect. Similarly oxygen is vary essential, probably it is required for biological oxidation.

Effect of hormone:

Various experiments of the past have revealed that during cold treatment the meristematic cells are stimulated to produce some substances. It was experimentally proved that a Gibberelin derived hormone is produced, which is named as Vernalin. It is a hypothetical hormone and it is capable of diffusion and transport. Attempts to isolate and proper identify the components of vernalin have failed. Whether the vernalin is the same as Florigen or a precursor of florigen is not known.

Mechanism of vernalisation:

The mechanism of vernalisation is obscure. The two main hypothetical theories of vernalisation are well accepted. These are-

1) Phasic developmental theory:

This theory was actually advanced by Lysenko. He gave a two phasic theory. According to him growth and development are two different phases of plant life and an exposor to low temperature treatment is necessary for the change over from the growth phase will take place only when the preceding has been completion such as light and temperature. Vernalisation accelerates the thermophase of development and after completion of thermophase, photophase starts during which shifting of vegetative phase to reproductive phase (flowering stage). Later it was modified with hormonal role by Lang. 1947 in this way.

Prcursor   →  Thermolabile substance   →   Thermo induced condition  →   Flowering

                                                                                                                                                                                                  Hormonal theory:

Many plants which require cold treatment, also require proper photoperiodic treatment for the induction of flowers, without which vernalisation does not have any effect.If such plants are treated with Gibberilins, they produce flowers without subjecting the plant to cold and photoperiodic treatments. For example Henbane is a rossate leaved long day plant which requires cold treatment for flowering. If such untreated plants are sprayed with Gibberelic acids the plant produce flowers. Thus Gibberelins are not only substitute of vernalisation, but also photoperiodic treatment.