Vernalization

• It is the acquisition of a plant’s ability to flower or germinate in the spring by exposure to the prolonged cold of winter. After vernalization, plants have acquired the ability to flower, but they may require additional seasonal cues or weeks of growth before they will actually flower.
• Chouard (1960) defined vernalization as the acquisition or acceleration of the ability to flower by a chilling treatment.’ Thus, a vernalizing cold treatment does not initiate flower primordia directly, but creates the capacity for subsequent flowering.
• Vernalization does not refer to the breaking of dormancy by cold, such as the release of pre-formed floral buds after chilling or the promotion of seed germination by cold (stratification). The vernalization response can be facultative or obligate.

Age and Site of Vernalization:

Ø Vernalization through cold treatment is very effective at the seed stage or seedling stage.

Ø In some cereals, even the embroyes can be successfully verbalized. However, in many cold recurring species, vernalization is not effective until and unless the plant possess at least few leaves.

Ø The requirement of few leaves for effective vernalization is called ‘Ripeness to Flowering’. This suggests that plants need certain degree of photosynthetic obtain to respond for cold treatment.

Ø Probably the role of carbohydrates in vernalization is to supply some energy.

Nevertheless, the most sensitive site which acts as the perceptive organ is the

meristematic region of the shoot apex.

Temperature Effect:

Ø For the normal growth and development, every plant requires on optimum temperature. But for vernalization the optimum temperature required is 3 0C to 17 0C, which varies depending upon the species involved

Ø However the efficiency of cold treatment in bringing about vernalization is determined by

the number of days shortened between germination and flowering stage.

Effect of water and oxygen:

Ø Along with the cold treatment plants also require water and oxygen for effective vernalization.

Ø The seeds or embryos should possess at least 40-50% water in their cells, without which

cold treatment has no effect.

Ø Similarly, oxygen is very essential; probably it is required for biological oxidation.Still, it is difficult to explain how cells use carbohydrates and oxygen for enzymatic oxidative process at such low temperature.

Vernalin:

Ø The presence of such substance has been demonstrated by grafting a vernalized plant to another non-vernalized plant at normal temperatures.

Ø Some substance is synthesized and such substance is now called ‘Vernalin” and it is capable of diffusion. Attempts to isolate and identify the components of vernalin have failed. Whether the vernalin is the same as florigin or a precursor of florigin is not known.

Devernalization:

Ø If vernalized seedlings or seeds are subjected to higher temperature like 35-40 0C the plants that develop from such treatment fail to flowers. Such a nullifying effect by higher temperatures is called Devernalization.

Ø Nevertheless, if the vernalized plants are maintained at sufficiently low temperatures for a long period of time, which has to be determined for every species, devernalization is not possible. This may be due to the putative vernalin have already acted upon the genetic material and committed it is flower formation.

Ø However, devernalized plants can be revernalized by subjecting the same seedling or seed again for another period of cold treatment by repetition of vernalization and devernalization cycles. Prolonged vernalization the effect decreases and seedlings loose their viability and potentiality to produce flowers.

Mechanism of floral induction in vernalized plants:

Ø It is clear from the earlier discussions that the plants with their specific genetic make up respond to different treatments like cold or photoperiods and produce flowers.

Ø Most of the cold requiring plants also require proper photoperiodic treatment. Gibberellins are known to overcome both cold treatment and photoperiodic treatment in long day plants, but it has no effect on short day plants. Synthesis of some unknown substance called vernalin during the period vernalization has been clearly demonstrated by grafting experiments.

Ø Furthermore, for proper vernalization, plants require sufficient amount of water, oxygen and some vegetation growth.

Ø Though all the above said factors are provided to the plant, flower inducing substance won’t be synthesized until and unless it is treated with proper cold condition at the stage of its development. It is during the cold treatment, the synthesis of the said flowering inducing factor is believed to be accelerated.

Importance of vernalization

• Vernalization activates a plant hormone called florigen present in the leaves which induces flowering at the end of the chilling treatment.
• It increases the cold and drought resistance in plants.
• It helps to reduce disease incidence in several plants.
• It helps to improve yield and remove wrinkledness of kernels and grains.
• Offseason flower and vegetable can be produced with its application
• It may resolve the adverse condition during the period of development of crops.
• Devernalization helps to control flowering in onion, garlic, potato and other plants.
• It shortens the vegetative period and hasten flowering period which will benefit to the farmers